Evolution of Sexual Reproduction

The genetic cascades regulating sex determination of the housefly, Musca domestica, and the fruitfly, Drosophila melanogaster, appear strikingly different. The bifunctional switch gene doublesex, however, is present at the bottom of the regulatory cascades of both species, and so is transformer-2, one of the genetic elements required for the sex-specific regulation of doublesex. The upstream regulators are different: Drosophila utilizes Sex-lethal to coordinate the control of sex determination and dosage compensation, i.e., the process that equilibrates the difference of two X chromosomes in females versus one X chromosome in males. In the housefly, Sex-lethal is not involved in sex determination, and dosage compensation, if existent at all, is not coupled with sexual differentiation. This allows for more adaptive plasticity in the housefly system. Accordingly, natural housefly populations can vary greatly in their mechanism of sex determination, and new types can be generated in th...

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Sexual reproduction is a nearly universal feature of eukaryotic organisms. Given its ubiquity and shared core features, sex is thought to have arisen once in the last common ancestor to all eukaryotes. Using the perspectives of molecular genetics and cell biology, we consider documented and hypothetical scenarios for the instantiation and evolution of meiosis, fertilization, sex determination, uniparental inheritance of organelle genomes, and speciation.

Sexual reproduction is a nearly universal feature of eukaryotic organisms. Given its ubiquity and shared core features, sex is thought to have arisen once in the last common ancestor to all eukaryotes. Using the perspectives of molecular genetics and cell biology, we consider documented and hypothetical scenarios for the instantiation and evolution of meiosis, fertilization, sex determination, uniparental inheritance of organelle genomes, and speciation.

Coevolution occurs when species interact to influence one another's fitness, resulting in reciprocal evolutionary change. In many coevolving lineages, trait expression in one species is modified by the genotypes and phenotypes of the other, forming feedback loops reminiscent of models of intraspecific social evolution. Here, we adapt the theory of within-species social evolution, characterized by indirect genetic effects and social selection imposed by interacting individuals, to the case of interspecific interactions. In a trait-based model, we derive general expressions for multivariate evolutionary change in two species and the expected between-species covariance in evolutionary change when selection varies across space. We show that reciprocal interspecific indirect genetic effects can dominate the coevolutionary process and drive patterns of correlated evolution beyond what is expected from direct selection alone. In extreme cases, interspecific indirect genetic effects can lead to coevolution when selection does not covary between species or even when one species lacks genetic variance. Moreover, our model indicates that interspecific indirect genetic effects may interact in complex ways with cross-species selection to determine the course of coevolution. Importantly, our model makes empirically testable predictions for how different forms of reciprocal interactions contribute to the coevolutionary process.

This essay presents a novel framework for understanding evolution not as a progressive or teleological force, but as an entropy-driven consequence of life's fundamental imperative: stabilization through reproduction. Drawing from evolutionary biology, thermodynamics, and systems theory, it argues that the true "goal" of biological systems is to resist entropy through homeostasis, genetic integrity, and sexual reproduction. Evolution, under this view, is not adaptive by nature but an emergent cost of imperfect stabilization mechanisms in an entropic universe. This reorientation carries significant implications for both biological theory and sociocultural dynamics.

The occurrence and form of sex chromosomes were investigated with the aid of C-banding and 4'-6-diamidino-2-phenylindole (DAPI) staining in 13 species of lacerrid lizards. The results obtained show the presence in five species of a female heterogamety in which the two sex chromosomes have the same shape and size, but the W differs from the Z in being almost entirely heterochromatic. This condition is clearly similar to that found in some snakes and considered to be an early stage of differentiation of sex chromosomes by Singh et al. (1976Singh et al. ( , 1980)). A more evolved condition may be that found in three other species in which the W is distinctly smaller than the Z. A third situation is that found in all Podarcis species which, even though they are considered to be among the more evolved species in the family, possess two sex chromosomes that are indistinguishable. In general, the situation in lacertids may be compatible with the hypothesis of sex chromosome evolution put forward by Singh et al. (1976Singh et al. ( , 1980)). However a differentiation mechanism of this kind does not seem to be well established in lacertids, and is probably not the only mechanism that is in operation in this family.

Sexual reproduction is widespread among eukaryotes, and the sex-determining processes vary greatly among species. While genetic sex determination (GSD) has been intensively described in bilaterian species, no example has yet been recorded among non-bilaterians. However, the quasi-ubiquitous repartition of GSD among multicellular species suggests that similar evolutionary forces can promote this system, and that these forces could occur also in non-bilaterians. Studying sex determination across the range of Metazoan diversity is indeed important to understand better the evolution of this mechanism and its lability. We tested the existence of sexlinked genes in the gonochoric red coral (Corallium rubrum, Cnidaria) using restriction site-associated DNA sequencing. We analysed 27 461 single nucleotide polymorphisms (SNPs) in 354 individuals from 12 populations including 53 that were morphologically sexed. We found a strong association between the allele frequencies of 472 SNPs and the sex of individuals, suggesting an XX/XY sex-determination system. This result was confirmed by the identification of 435 male-specific loci. An independent test confirmed that the amplification of these loci enabled us to identify males with absolute certainty. This is the first demonstration of a GSD system among non-bilaterian species and a new example of its convergence in multicellular eukaryotes.

Choanoflagellate species have been taxonomically divided upon the morphological and developmental basis of their extracellular coat (periplast). Species within the order Craspedida possess a purely organic periplast, whereas taxa of the order Acanthoecida have an additional silica based periplast termed the lorica. Whilst smallscale phylogenetic studies have recovered the two orders as monophyletic, recent phylogenomic analyses have rejected the monophyly of the craspedids. These analyses suggest that the freshwater craspedid Codosiga hollandica falls within the earliest branching choanoflagellate lineage; however, it has also been noted that phylogenomic studies result in inconsistent phylogenies, with unstable long-branched species being recovered as basal choanoflagellates. Presented here are phylogenetic analyses, based upon slowly evolving ribosomal proteins, which consistently recover both craspedid monophyly and a derived placement of C. hollandica. The addition of further, long-branched, ribosomal proteins to phylogenetic analyses are shown to generate longer terminal branches and a weakening of the support for Craspedida. The analyses highlight the requirement for an increase in data, in terms of both taxa and gene coverage, to successfully resolve the earliest craspedid branches. In addition, phylogenetically unstable species are identified that are recommended to be omitted from phylogenomic studies as they have the potential to disrupt recovered relationships.

Sex determination is the developmental process by which organismal sex is established. Sex determination evolves fast, often due to changes in the master regulators at the top of the pathway. In addition, some species are polymorphic for multiple different master regulators within natural populations. Understanding the forces that maintain this polygenic sex determination can be informative of the factors that drive the evolution of sex determination. The house fly,Musca domestica, is a well-suited model to those ends because natural populations harbor male-determining loci on each of the six chromosomes and a bi-allelic female-determiner. Multiple lines of evidence suggest that natural selection maintains polygenic sex determination in house fly. However, previous work found that there are very few sequence differences between proto-Y chromosomes and their homologous X chromosomes. This suggests that there is not much genetic variation upon which natural selection could act to main...

Intralocus sexual conflict and intragenomic conflict both affect sex chromosome evolution and can in extreme cases even cause the complete turnover of sex chromosomes. Additionally, established sex chromosomes often become the focus of heightened conflict. This creates a tangled relationship between sex chromosomes and conflict with respect to cause and effect. To further complicate matters, sexual and intragenomic conflict may exacerbate one another and thereby further fuel sex chromosome change. Different magnitudes and foci of conflict offer potential explanations for lineage-specific variation in sex chromosome evolution and answer long-standing questions as to why some sex chromosomes are remarkably stable, whereas others show rapid rates of evolutionary change.

Genetic sex determination by W and Z chromosomes has developed independently in different groups of organisms. To better understand the evolution of sex chromosomes and the plasticity of sex-determination mechanisms, we sequenced the whole genomes of a male (ZZ) and a female (ZW) half-smooth tongue sole (Cynoglossus semilaevis). In addition to insights into adaptation to a benthic lifestyle, we find that the sex chromosomes of these fish are derived from the same ancestral vertebrate protochromosome as the avian W and Z chromosomes. Notably, the same gene on the Z chromosome, dmrt1, which is the male-determining gene in birds, showed convergent evolution of features that are compatible with a similar function in tongue sole. Comparison of the relatively young tongue sole sex chromosomes with those of mammals and birds identified events that occurred during the early phase of sex-chromosome evolution. Pertinent to the current debate about heterogametic sex-chromosome decay, we find that massive gene loss occurred in the wake of sex-chromosome 'birth'.

The notion that eukaryotes are ancestrally sexual has been gaining attention. This idea comes in part from the discovery of sets of “meiosis‐specific genes” in the genomes of protists. The existence of these genes has persuaded many that these organisms may be engaging in sex, even though this has gone undetected. The involvement of sex in protists is supported by the view that asexual reproduction results in the accumulation of mutations that would inevitably result in the decline and extinction of such lineages. It is argued that this phenomenon can be obviated by polyploidy and that the “meiosis‐specific genes” are used in other processes, including polyploidy control and homologous recombination, independent of meiosis. These phenomena account for the finding that these genes are expressed in cultures devoid of apparent cell fusion events. Hence, it is also proposed that asexual, and not sexual, reproduction is the ancestral condition.

Intralocus sexual conflict and intragenomic conflict both affect sex chromosome evolution and can in extreme cases even cause the complete turnover of sex chromosomes. Additionally, established sex chromosomes often become the focus of heightened conflict. This creates a tangled relationship between sex chromosomes and conflict with respect to cause and effect. To further complicate matters, sexual and intragenomic conflict may exacerbate one another and thereby further fuel sex chromosome change. Different magnitudes and foci of conflict offer potential explanations for lineage-specific variation in sex chromosome evolution and answer long-standing questions as to why some sex chromosomes are remarkably stable, whereas others show rapid rates of evolutionary change.

Deleterious mutations can have a strong influence on the outcome of evolution. The nature of this influence depends on how mutations combine together to affect fitness. ''Negative epistasis'' occurs when a new deleterious mutation causes the greatest loss in fitness in a genome that already contains many deleterious mutations. Negative epistasis is a key ingredient for some of the leading hypotheses regarding the evolution of recombination, the evolution of sex, and a variety of other phenomena. In general, laboratory studies have not supported the idea that negative epistasis is ubiquitous, and this has led to doubts about its importance in biological evolution. Here, we show that these experimental results may be misleading, because negative epistasis can produce evolutionary advantages for sex and recombination while simultaneously being almost impossible to detect using current experimental methods. Under asexual reproduction, such hidden epistasis influences evolutionary outcomes only if the fittest individuals are present in substantial numbers, while also forming a very small proportion of the population as a whole. This implies that our results for asexuals will apply only for very large populations, and also limits the extent of the fitness benefits that hidden epistasis can provide. Despite these caveats, our results show that the fitness consequences of sex and recombination cannot always be inferred from observable epistasis alone.

FeO-containing K-Na-rich silicate (`KNSF': SiO 2 64.3 wt%, K 2 O 18.6 wt%, Na 2 O 12.4 wt%, FeO 4.6 wt%) with a low melting temperature. The experiments at the Australian National University (ANU) used Pd 90 Fe 10 alloys made by melting mixes of Pd and Fe powders. These alloys were also fabricated into capsules and equilibrated with the FeOcontaining K-Na-rich silicate. The silicate material was prepared from a mixture of reagent grade oxides and carbonates (K, Na), melted under air, and quenched to glass. Small cylinders were drilled out of the KNSF silicate glass and were inserted into capsules fabricated from the Pd 90 Fe 10 and Pt 90 Fe 10 alloys (BGI experiments). In the case of ANU experiments, silicate glass powders were loaded into the capsules. The metal phases of interest were used as capsule material to avoid contamination of the sample with capsule material in the metal±silicate partition experiments; for example, contamination by MgO or Al 2 O 3 of silicate phases by using MgO-or Al 2 O 3-capsules as shown in, for example, ref. 23. Graphite was used as the heater in piston-cylinder furnace assemblies and LaCrO 3 as the heater in multi-anvil furnaces. The entire assemblies were placed in a vacuum oven at 250 8C overnight before the experiment. Details of the experimental design and method can be found in refs 24 (piston cylinder) and 25 (multi-anvil). After reaching the desired pressure, samples were heated to the desired temperatures (1,100±1,600 8C). Experimental conditions were chosen so that solid metal and liquid silicate coexisted during the experiments. The low-melting KNSF-silicate remained above its liquidus within the investigated P±T-range. Run durations were up to 36 h, and the experiments were terminated by turning off the power to the heater. The temperature drop was suf®ciently rapid to quench the silicate liquids into glasses. Metal compositions and major-element silicate compositions were determined by electron microprobe (Cameca Camebax, University of Cologne; 10 nA, 20 kV, counting time: 40 s). Palladium and Pt concentrations in silicates were analysed by an ultraviolet (quadrupled Nd-YAG) laser ablation-PlasmaQuad PQ-2+ inductively coupled plasma-mass spectrometer (ICP-MS) system at Memorial University, Newfoundland, Canada, using procedures described in ref. 26. The beam spot was 100 mm diameter, laser repetition rate 10 Hz, laser energy density 40 J cm-2 , and counting time 60 s. The Pd and Pt standards were homogeneous synthetic silicate glasses whose Pd and Pt concentrations had been measured previously by instrumental neutron activation analysis. The Si content of each individual silicate sample was used as an internal standard for the laser ablation ICP-MS analyses. To enhance the comparison between the high-pressure results and 1-atm data, additional 1-atm experiments with pure Pd and Pt 90 Fe 10-metal and an FeO-containing melt (`BK': SiO 2 49.1 wt%, CaO 19.2 wt%, MgO 10.6 wt%, Al 2 O 3 14.1 wt%, FeO 7.0 wt%) as silicate starting material were performed under controlled temperature and oxygen fugacity in gas mixing furnaces at the University of Cologne using the loop technique as described in ref. 27. The Pd and Pt contents in these silicate samples were analysed by instrumental neutron activation analysis at the University of Cologne.

In many species, most (or all) offspring are produced by sexual means. However, theory suggests that selection should often favour the evolution of species in which a small fraction of offspring are produced sexually, and the rest are produced asexually. Here, we present the analysis of a model that may help to resolve this paradox. We show that, when heterozygote advantage is in force, members of species in which sex is rare will tend to produce poorly adapted offspring when they mate. This problem should be less severe in species where most offspring are produced by sexual means. As a consequence, once the rate of sexual reproduction becomes sufficiently rare, the benefits of sex may vanish, leading to the evolution of obligate asexuality. Substantial benefits of sexual reproduction may tend to accrue only if a large proportion of offspring are produced sexually. We suggest that similar findings are likely in the case of epistatic interactions between loci.

The maintenance of sex is an unresolved paradox in evolutionary biology, given the inherent twofold tness advantage for asexuals. Parasitic helminths offer a unique opportunity to address this enigma. Parasites that can create novel antigenic strains are able to escape pre-existing host immunity. Viruses produce diversity through mutation with rapid clonal proliferation. The long generation times of helminth parasites prevent them from adopting this strategy. Instead, we argue that sexual reproduction enables parasitic helminths to rapidly generate strain diversity. We use both a stochastic, individual-based model and a simple analytical model to assess the selective value of sexual versus asexual reproduction in helminth parasites. We demonstrate that sexual reproduction can more easily produce and maintain strain diversity than asexual reproduction for long-lived parasites. We also show that sexual parasite populations are resistant to invasion by rare asexual mutants. These results are robust to high levels of cross-immunity between strains. We suggest that the enhancement of strain diversity, despite stochastic extinction of strains, may be critical to the evolutionary success of sex in long-lived parasites.

Genetic sex determination by W and Z chromosomes has developed independently in different groups of organisms. To better understand the evolution of sex chromosomes and the plasticity of sex-determination mechanisms, we sequenced the whole genomes of a male (ZZ) and a female (ZW) half-smooth tongue sole (Cynoglossus semilaevis). In addition to insights into adaptation to a benthic lifestyle, we find that the sex chromosomes of these fish are derived from the same ancestral vertebrate protochromosome as the avian W and Z chromosomes. Notably, the same gene on the Z chromosome, dmrt1, which is the male-determining gene in birds, showed convergent evolution of features that are compatible with a similar function in tongue sole. Comparison of the relatively young tongue sole sex chromosomes with those of mammals and birds identified events that occurred during the early phase of sex-chromosome evolution. Pertinent to the current debate about heterogametic sex-chromosome decay, we find that massive gene loss occurred in the wake of sex-chromosome 'birth'.

The dynamics of the physical and chemical factors that regulate oceanographic processes on the continental shelf off the state of Maranhão (northeastern Brazil) was evaluated using a transect along São Marcos Bay (01°41'S-02°28'S and 43°47'W-44°13'W) in January, March, May, July and September 2014, with a total of seven sampling stations. Water samples were collected from the surface using a Van Dorn water sampler. The following hydrochemical variables were analyzed: water transparency (m), temperature (°C), salinity, conductivity (mS cm-¹), total dissolved solids (TDS, g L-1), pH, dissolved oxygen (mg L-1), turbidity (NTU), total suspended solids (TSS) and dissolved nutrients (phosphate, nitrite and silicate). The relationship between these variables and seasonality in the region [rainy season (January to June) and dry season (July to December)] were also evaluated. For data with normality and equal variances, a one-way analysis of variance (ANOVA) was used for the spatial and temporal comparisons of the physicochemical variables. Results showed that the spatial and temporal variability of the physicochemical variables analyzed in this study is associated with local dynamics governed by river discharge, tidal movements, currents and meteorological events. The fluvial transport from the rivers of the state of Maranhão and other freshwater sources in the Amazon region are apparently the major contributors responsible for the maintenance of nutrient availability on the Maranhão continental shelf.

Running title: Rapid evolution of sex role specialization Acknowledgements: We thank Lukas Schärer for important feedback when planning the study. We also thank Tammy Ho who helped with lab work during the fitness assay and Dita Vizoso for laboratory protocols and methods. This project was funded by Maja och Erik Lindqvists forskningsstiftelse, Nilsson-Ehle

Many separate-sexed organisms have sex chromosomes controlling sex determination. Sex chromosomes often have reduced recombination, specialized (frequently sex-specific) gene content, dosage compensation and heteromorphic size. Research on sex determination and sex chromosome evolution has increased over the past decade and is today a very active field. However, some areas within the field have not received as much attention as others. We therefore believe that a historic overview of key findings and empirical discoveries will put current thinking into context and help us better understand where to go next. Here, we present a timeline of important conceptual and analytical models, as well as empirical studies that have advanced the field and changed our understanding of the evolution of sex chromosomes. Finally, we highlight gaps in our knowledge so far and propose some specific areas within the field that we recommend a greater focus on in the future, including the role of ecology in sex chromosome evolution and new multilocus models of sex chromosome divergence. The accepted theory of the evolution of heteromorphic sex chromosomes (figure 1) starts with a pair of homologous autosomes that gain a major sexdetermining function through one or several genes [2,3,8]. This can happen in a system that already has a sex chromosome pair (and in that case it results in

Antagonistic interactions between the sexes are important drivers of evolutionary divergence. Interlocus sexual conflict is generally described as a conflict between alleles at two interacting loci whose identity and genomic location are arbitrary. Here we build on previous theory and suggest that when these two loci are located on the sex chromosomes it can lead to cycles of antagonistic coevolution between them, and therefore between the sexes. We tested this hypothesis by performing experimental crosses using Drosophila melanogaster in which we reciprocally exchanged the sex chromosomes between five wild-type populations in a round-robin design. Disrupting putatively coevolved sex chromosome pairs resulted in increased male reproductive success in 16 out of 20 experimental populations (10 of which were significant), but also resulted in lower offspring egg-to-adult viability that affected both male and female fitness. After 25 generations of experimental evolution these sexually antagonistic fitness effects appeared to have been resolved. To help formalise our hypothesis, we developed population genetic models of antagonistic coevolution using fitness expressions based on our empirical results. Our models support the conclusion that antagonistic coevolution between the sex chromosomes is plausible under the fitness effects observed in our experiments. Together, our results lend both empirical and theoretical support to the idea that a cycle of antagonistic coevolution can occur between sex chromosomes and illustrates how this process may drive genetic and phenotypic divergence between populations. .

Sex chromosomes are typically viewed as having originated from a pair of autosomes, and differentiated as the sex-limited chromosome (e.g. Y) has degenerated by losing most genes through cessation of recombination. While often thought that degenerated sex-limited chromosomes primarily affect traits involved in sex determination and sex cell production, accumulating evidence suggests they also influence traits not sex-limited or directly involved in reproduction. Here, we provide an overview of the effects of sex-limited chromosomes on non-reproductive traits in XY, ZW or UV sex determination systems, and discuss evolutionary processes maintaining variation at sex-limited chromosomes and molecular mechanisms affecting non-reproductive traits.

The evolution of separate sexes from hermaphroditism is thought to have occurred independently many times, and may be linked to the evolution of sex chromosomes. Even though we have a good understanding of the theoretical steps in the evolution of sex chromosomes from a hermaphrodite ancestor, the initial stages are still hard to study in animals because many well-studied animal sex chromosome systems are old. We addressed this problem by experimentally selecting a hermaphrodite via sex-limited experimental evolution for several generations, simulating the early stages in the evolution of a sex chromosome. After 14 generations, a fitness assay revealed evidence of incipient sex role specialization in the female-selected lines, presumably reflecting the release from constraints usually imposed by selection on the other sex role. Importantly, however, this was not simply explained by morphology because testis and ovary sizes did not diverge among treatments. There was no evidence of a...

Antagonistic interactions between the sexes are important drivers of evolutionary divergence. Interlocus sexual conflict is generally theorised as a conflict between alleles at two interacting loci whose identity and genomic location are arbitrary. Here we build on previous theory and suggest that when these two loci are located on the X and Y (or Z and W) chromosomes, it can lead to cycles of antagonistic coevolution between the sex chromosomes and the two sexes. To test this hypothesis, we performed experimental crosses using Drosophila melanogaster where we reciprocally exchanged the sex chromosomes between five wild-type populations in a round-robin design. Disrupting putatively coevolved sex chromosome pairs resulted in increased male reproductive success in 16 out of 20 experimental populations (10 of which were significant), but also resulted in lower offspring egg-to-adult viability that affected both male and female fitness. However, after 25 generations of experimental evo...

Many separate-sexed organisms have sex chromosomes controlling sex determination. Sex chromosomes often have reduced recombination, specialized (frequently sex-specific) gene content, dosage compensation and heteromorphic size. Research on sex determination and sex chromosome evolution has increased over the past decade and is today a very active field. However, some areas within the field have not received as much attention as others. We therefore believe that a historic overview of key findings and empirical discoveries will put current thinking into context and help us better understand where to go next. Here, we present a timeline of important conceptual and analytical models, as well as empirical studies that have advanced the field and changed our understanding of the evolution of sex chromosomes. Finally, we highlight gaps in our knowledge so far and propose some specific areas within the field that we recommend a greater focus on in the future, including the role of ecology ...

Recent genetic and genomic studies have revealed tremendous diversity in sex chromosomes across diverse taxa. Although theoretical studies predict that sex chromosome evolution can drive the evolution of sexual dimorphism, empirical studies are still limited. A taxonomic group that shows diversity in both sex chromosomes and sexual dimorphism provides excellent opportunity to investigate the link between the evolution of sex chromosomes and sexual dimorphism. The medaka fishes (family Adrianichthyidae) exhibit both rapid sex chromosome turnovers and diversification of sexually dimorphic traits. In the present study, we investigated the sex chromosomes of 13 Oryzias species from Sulawesi, Indonesia, that have diversified in sexual dichromatism. Using pooled sequencing we found sex chromosomes in 9 species that all had XY systems, with a few species being possibly modified by multiple loci. Seven species (O. woworae, O. asinua, O. wolasi, O. matanensis, O. celebensis, O. hadiatyae, and O. dopingdopingensis) share linkage group (LG) 24 as sex chromosomes; however, they differed in the length and magnitude of sequence divergence between the X and Y chromosomes. The sex chromosome of O. eversi was LG4, which has not been reported as a sex chromosome in any other medaka species. In O. sarasinorum, LG16 and LG22 are associated with sex. Although LG16 was found to be sex-linked in another medaka species previously examined, the sex-determining regions did not overlap. Frequent turnovers and the great diversity of the sex chromosomes make Sulawesian medaka species a model system for investigating the roles of sex chromosome evolution in the diversification of sexual dimorphism.

The predominance of sexual reproduction despite its costs indicates that sex provides substantial benefits, which are usually thought to derive from the direct genetic consequences of recombination and syngamy. While genetic benefits of sex are certainly important, sexual and asexual individuals, lineages, or populations may also differ in physiological and life history traits that could influence outcomes of competition between sexuals and asexuals across environmental gradients. Here, we address possible phenotypic costs of a very common correlate of asexuality, polyploidy. We suggest that polyploidy could confer resource costs related to the dietary phosphorus demands of nucleic acid production; such costs could facilitate the persistence of sex in situations where asexual taxa are of higher ploidy level and phosphorus availability limits important traits like growth and reproduction. We outline predictions regarding the distribution of diploid sexual and polyploid asexual taxa across biogeochemical gradients and provide suggestions for study systems and empirical approaches for testing elements of our hypothesis.

Sexual reproduction and clonality in eukaryotes are mostly seen as exclusive, the latter being rather exceptional. This view might be biased by focusing almost exclusively on metazoans. We analyze and discuss reproduction in the context of extant eukaryotic diversity, paying special attention to protists. We present results of phylogenetically extended searches for homologs of two proteins functioning in cell and nuclear fusion, respectively (HAP2 and GEX1), providing indirect evidence for these processes in several eukaryotic lineages where sex has not been observed yet. We argue that (i) the debate on the relative significance of sex and clonality in eukaryotes is confounded by not appropriately distinguishing multicellular and unicellular organisms; (ii) eukaryotic sex is extremely widespread and already present in the last eukaryotic common ancestor; and (iii) the general mode of existence of eukaryotes is best described by clonally propagating cell lines with episodic sex trigg...

Sex determination, the developmental process by which organismal sex is established, evolves fast, often due to changes in the master regulators at the top of the pathway. Additionally, in species with polygenic sex determination, multiple different master regulators segregate as polymorphisms. Understanding the forces that maintain polygenic sex determination can be informative of the factors that drive the evolution of sex determination. The house fly, Musca domestica, is a well-suited model to those ends because natural populations harbor male-determining loci on each of the six chromosomes and a biallelic female determiner. To investigate how natural selection maintains polygenic sex determination in the house fly, we assayed the phenotypic effects of proto-Y chromosomes by performing mRNA-sequencing experiments to measure gene expression in house fly males carrying different proto-Y chromosomes. We find that the proto-Y chromosomes have similar effects as a nonsex-determining autosome. In addition, we created sex-reversed males without any proto-Y chromosomes and they had nearly identical gene expression profiles as genotypic males. Therefore, the proto-Y chromosomes have a minor effect on male gene expression, consistent with previously described minimal X-Y sequence differences. Despite these minimal differences, we find evidence for a disproportionate effect of one proto-Y chromosome on male-biased expression, which could be partially responsible for fitness differences between males with different proto-Y chromosome genotypes. Therefore our results suggest that, if natural selection maintains polygenic sex determination in house fly via gene expression differences, the phenotypes under selection likely depend on a small number of genetic targets.

Parthenogenesis is suppressed in rice egg cells to avoid precocious development before fertilization. We found that cold treatment released cell cycle arrest in egg cells and triggered mitosis. Egg cells isolated from japonica (Nipponbare; NB) and aus (Kasalath; KS) subspecies divided and regenerated into mature plants after cold treatment. The egg-derived plants showed variety of ploidy levels, including haploid (n), diploid (2n), and tetraploid (4n). Nuclear DNA quantification showed that genome duplication occurred during early parthenogenetic development. Owing to the presence of single nucleotide polymorphisms (SNPs) between NB and KS, inter-subspecific hybrid plants (NB-KS hybrids) were created via electrofusion. Egg cells from the NB-KS hybrid developed parthenogenetically into polyploid plants. 2n and 4n plants originating from the same NB-KS egg cell displayed the same homozygous SNP patterns throughout the genome, indicating that these plants were doubled and quadrupled haploids. Transcriptome analyses of cold-treated egg cells demonstrated that parthenogenesis-related candidate genes, including OsBBML1, were upregulated.

The dynamics of the physical and chemical factors that regulate oceanographic processes on the continental shelf off the state of Maranhão (northeastern Brazil) was evaluated using a transect along São Marcos Bay (01°41'S-02°28'S and 43°47'W-44°13'W) in January, March, May, July and September 2014, with a total of seven sampling stations. Water samples were collected from the surface using a Van Dorn water sampler. The following hydrochemical variables were analyzed: water transparency (m), temperature (°C), salinity, conductivity (mS cm-¹), total dissolved solids (TDS, g L-1), pH, dissolved oxygen (mg L-1), turbidity (NTU), total suspended solids (TSS) and dissolved nutrients (phosphate, nitrite and silicate). The relationship between these variables and seasonality in the region [rainy season (January to June) and dry season (July to December)] were also evaluated. For data with normality and equal variances, a one-way analysis of variance (ANOVA) was used for the spatial and temporal comparisons of the physicochemical variables. Results showed that the spatial and temporal variability of the physicochemical variables analyzed in this study is associated with local dynamics governed by river discharge, tidal movements, currents and meteorological events. The fluvial transport from the rivers of the state of Maranhão and other freshwater sources in the Amazon region are apparently the major contributors responsible for the maintenance of nutrient availability on the Maranhão continental shelf.

The preliminary aim of the study before future breeding program was to reveal usability of ISSR markers to assess genetic diversity and relatedness within raspberry genotypes wildly grown in Northern Turkey. In this study, 19 wild raspberry genotypes collected from Black Sea region in Turkey were used. ISSR PCR analyses were carried on the DNA of genotypes isolated using mini CTAB extraction method. The levels of polymorphism between genotypes were determined using the UBC ISSR primers. A total of 111 bands were obtained from 15 UBC ISSR primers. Out of 111 bands, 101 bands were polymorphic. The number of bands obtained per primer ranged between 3 and 10, the average number of bands were determined as 7.4. The average number of polymorphic bands per primer was 6.73. Similarities and differences between genotypes have been studied at the molecular level. The data used for statistical analysis were obtained by the evaluation of ISSR bands. Similarity coefficient and UPGMA dendrogram w...

Here we introduce a new model species, Silene colpophylla, that could facilitate research of sex chromosome evolution and sex-determining systems. This species is related to the well-established dioecious plant model Silene latifolia. Our results show that S. colpophylla is, similarly to S. latifolia, a male heterogametic species, but its sex chromosomes have evolved from a different pair of autosomes than in S. latifolia. The results of our phylogenetic study and mapping of homologs of S. latifolia X-linked genes indicate that the sex determination system in S. colpophylla evolved independently from that in S. latifolia. We assert that this model species pair will make it possible to study two independent patterns of sex chromosome evolution in related species.

Oomycete infection of various freshwater animals, including salmonid eggs, causes significant economic damage to aquaculture worldwide. In this study, we detected oomycetes in infected chum salmon Oncorhynchus keta eggs at two hatcheries in northern Japan, in the source water used for egg incubation, and in the air at the hatcheries to clarify the source(s) of oomycete transmission using a DNA molecular marker. Seven oomycete taxa, belonging to Saprolegniaceae and Pythiaceae, were detected from the infected eggs. From the source water used for egg incubation and the air at the hatcheries, nine oomycete taxa, including those found on infected eggs, were detected, suggesting that both water and air are potential sources of oomycete transmission. There is no report of airborne transmission of these oomycetes detected in this study so far. Regarding protection and sterilization against oomycete infection in aquaculture hatcheries, not only water used at hatcheries but also the air in hatcheries may need to be considered hereafter.

In recent years, several lines of evidence have questioned the asexual nature of Aspergillus fumigatus , showing that this fungus possesses a fully functional sexual reproductive cycle that leads to the production of cleistothecia and ascospores. The presence of a sexual cycle in A. fumigatus could have signifi cant medical implications, as sexual reproduction might contribute to increased virulence or resistance to antifungal agents. In the present work, we studied the relationship between mating type and invasiveness in A. fumigatus. Statistical analysis of the results showed a signifi cant association between the mating type MAT1-1 and an invasive origin of the isolates. Similarly, when the clinical or environmental origin of isolates was considered instead of their invasive or non-invasive origin, a signifi cant association between the mating type MAT1-1 and clinical origin was observed. Finally, the association between mating type MAT1-1 and pathogenicity, measured by an Elastase Activity Index Ն1, was signifi cant. Our results suggest a possible association between the MAT1-1 mating type and A. fumigatus invasiveness.

Wolbachia are endosymbiotic bacteria known to manipulate the reproduction of their hosts. Some populations of the parasitoid wasp Asobara japonica are infected with Wolbachia and reproduce parthenogenetically, while other populations are not infected and reproduce sexually. Wolbachia-infected A. japonica females regularly produce small numbers of male offspring. Because all females in the field are infected and infected females are not capable of sexual reproduction, male production seems to be maladaptive. We investigated why these females nevertheless produce males. We tested three hypotheses: high rearing temperatures could result in higher offspring sex ratios (more males), low Wolbachia titer of the mother could lead to higher offspring sex ratios and/or the Wolbachia infection is of relatively recent origin and not enough time has passed to allow complete coadaptation between Wolbachia and host. In all, 33% of the Wolbachia-infected females produced males and 56% of these males were also infected with Wolbachia. Neither offspring sex ratio nor male infection frequency was significantly affected by rearing temperature or Wolbachia concentration of the mother. The mitochondrial DNA sequence of one of the uninfected populations was identical to that of two of the infected populations. Therefore, the initial Wolbachia infection of A. japonica must have occurred recently. Mitochondrial sequence variation among the infected populations suggests that the spread of Wolbachia through the host populations involved horizontal transmission. We conclude that the occasional male production by Wolbachia-infected females is most likely a maladaptive side effect of incomplete coevolution between symbiont and host in this relatively young infection.

Sex chromosomes are a great example of a convergent evolution at the genomic level, having evolved dozens of times just within amniotes. An intriguing question is whether this repeated evolution was random, or whether some ancestral syntenic blocks have significantly higher chance to be co-opted for the role of sex chromosomes owing to their gene content related to gonad development. Here, we summarize current knowledge on the evolutionary history of sex determination and sex chromosomes in amniotes and evaluate the hypothesis of non-random emergence of sex chromosomes. The current data on the origin of sex chromosomes in amniotes suggest that their evolution is indeed non-random. However, this non-random pattern is not very strong, and many syntenic blocks representing putatively independently evolved sex chromosomes are unique. Still, repeatedly co-opted chromosomes are an excellent model system, as independent co-option of the same genomic region for the role of sex chromosome of...

Until recently, the field of sex chromosome evolution has been dominated by the canonical unidirectional scenario, first developed by Muller in 1918. This model postulates that sex chromosomes emerge from autosomes by acquiring a sex-determining locus. Recombination reduction then expands outwards from this locus, to maintain its linkage with sexually antagonistic/advantageous alleles, resulting in Y or W degeneration and potentially culminating in their disappearance. Based mostly on empirical vertebrate research, we challenge and expand each conceptual step of this canonical model and present observations by numerous experts in two parts of a theme issue ofPhil. Trans. R. Soc. B.We suggest that greater theoretical and empirical insights into the events at the origins of sex-determining genes (rewiring of the gonadal differentiation networks), and a better understanding of the evolutionary forces responsible for recombination suppression are required. Among others, crucial question...

Amniotes possess variability in sex determination, from environmental sex determination (ESD), where no sex chromosomes are present, to genotypic sex determination (GSD) with highly differentiated sex chromosomes. Some evolutionary scenarios postulate high stability of differentiated sex chromosomes and rare transitions from GSD to ESD. However, sex chromosome turnovers and two independent transitions from highly differentiated ZZ/ZW sex chromosomes to ESD were previously reported in the lacertid lizards. Here, we examined the homology of sex chromosomes in the wide phylogenetic spectrum of lacertids and their outgroups by comparing gene copy numbers between sexes in genes previously found to be Z-specific in some lacertids. Our current sampling covers 45 species from 26 genera including lineages supposed to possess a derived sex determining systems. We found that all tested lacertids share homologous differentiated ZZ/ZW sex chromosomes, which were present already in their common a...

In many mammalian species, females live on average longer than males. In humans, women have consistently longer telomeres than men, and this has led to speculation that sex differences in telomere length (TL) could play a role in sex differences in longevity. To address the generality and drivers of patterns of sex differences in TL across vertebrates, we performed metaanalyses across 51 species. We tested two main evolutionary hypotheses proposed to explain sex differences in TL, namely the heterogametic sex disadvantage and the sexual selection hypotheses. We found no support for consistent sex differences in TL between males and females among mammal, bird, fish and reptile species. This absence of sex differences in TL across different classes of vertebrates does not support the heterogametic sex disadvantage hypothesis. Likewise, the absence of any negative effect of sexual size dimorphism on male TL suggests that sexual selection is not likely to mediate the magnitude of sex differences in TL across vertebrates. Finally, the comparative analyses we conducted did not detect any association between sex differences in TL and sex differences in longevity, which does not support the idea that sex differences in TL could explain the observed sex differences in longevity.

We present a draft genome assembly for the tropical liverwort, Marchantia inflexa, which adds to a growing body of genomic resources for bryophytes and provides an important perspective on the evolution and diversification of land plants. We specifically address questions related to sex chromosome evolution, sexual dimorphisms, and the genomic underpinnings of dehydration tolerance. this assembly leveraged the recently published genome of related liverwort, M. polymorpha, to improve scaffolding and annotation, aid in the identification of sex-linked sequences, and quantify patterns of sequence differentiation within Marchantia. We find that genes on sex chromosomes are under greater diversifying selection than autosomal and organellar genes. Interestingly, this is driven primarily by divergence of male-specific genes, while divergence of other sex-linked genes is similar to autosomal genes. Through analysis of sex-specific read coverage, we identify and validate genetic sex markers for M. inflexa, which will enable diagnosis of sex for non-reproductive individuals. To investigate dehydration tolerance, we capitalized on a difference between genetic lines, which allowed us to identify multiple dehydration associated genes two of which were sex-linked, suggesting that dehydration tolerance may be impacted by sex-specific genes. Bryophytes (mosses, liverworts, and hornworts) are living representatives of an early diverging land plant lineage 1,2 and they provide an important landmark for comparative phylogenetics. Although the exact relationships among bryophyte lineages are currently contested, the earliest fossil evidence assigned to a bryophyte is liverwort-like with dorsiventral complex thallus morphology and a leafless gametophyte 3-5 , suggesting that liverworts retain a large suite of ancestral characters not conserved in other land plants. Importantly, building a fundamental understanding of genomic patterns can be readily accomplished by working with bryophytes because of their small genomes 6 , many of which contain comparatively few paralogous duplications of regulatory genes 2. Thus far, genome assemblies have been developed for a number of bryophyte species (including the mosses Physcomitrella patens 7 , and Sphagnum fallax 8 , the liverwort Marchantia polymorpha 2 , and the hornwort Anthoceros agrestis 9), and more are underway (Takakia lepidozioides, Ceratodon purpureus, Funaria hygrometrica 10 and Syntricia caninervis). Relatively few assemblies of bryophyte mitochondria 11-15 and chloroplasts 16-18 are available compared to other plant lineages, yet mitochondrial genomes of bryophytes tend to be less complex than those of tracheophytes, having no large repeated sequences and limited recombination 19. Genome sequencing efforts of additional bryophyte taxa will provide critical insight into more recent evolutionary changes within these lineages and may help to better resolve ancestral states. In this context, we targeted the tropical liverwort, Marchantia inflexa (Nees & Mont) for sequencing and assembly. Marchantia inflexa is a New World liverwort that is distributed throughout Central America and the Caribbean, from northern Venezuela to the southern United States 20. Marchantia inflexa is 68-126 million years diverged from the well-studied sister species M. polymorpha 21. Marchantia inflexa is dioecious (has unisexual

Maternally inherited bacterial endosymbionts that affect host fitness are common in nature. Some endosymbionts colonise host populations by reproductive manipulations (such as cytoplasmic incompatibility; CI) that increase the reproductive fitness of infected over uninfected females. Theory predicts that CI-inducing endosymbionts in haplodiploid hosts may also influence sex allocation, including in compatible crosses, however, empirical evidence for this is scarce. We examined the role of two common CIinducing endosymbionts, Cardinium and Wolbachia, in the sex allocation of Pezothrips kellyanus, a haplodiploid thrips species with a split sex ratio. In this species, irrespective of infection status, some mated females are constrained to produce extremely malebiased broods, whereas other females produce extremely female-biased broods. We analysed brood sex ratio of females mated with males of the same infection status at two temperatures. We found that at 20°C the frequency of constrained sex allocation in coinfected pairs was reduced by 27% when compared to uninfected pairs. However, at 25°C the constrained sex allocation frequency increased and became similar between coinfected and uninfected pairs, resulting in more male-biased population sex ratios at the higher temperature. This temperature-dependent pattern occurred without changes in endosymbiont densities and compatibility. Our findings indicate that endosymbionts affect sex ratios of haplodiploid hosts beyond the commonly recognised reproductive manipulations by causing female-biased sex allocation in a temperature-dependent fashion. This may contribute to a higher transmission efficiency of CI-inducing endosymbionts and is consistent with previous models that predict that CI by itself is less efficient in driving endosymbiont invasions in haplodiploid hosts.

Most explanations for the existence of species involve a role for sex. A new study of a group of asexual rotifers supports the idea that selection for a common ecological niche can produce a pattern that mimics sexual species, even in the absence of sex.