Sexual selection in females and the evolution of polyandry.

Over the last decades, the field of sexual selection underwent a paradigm shift from sexual-stereotype thinking of "eager" males and "coy" females towards a more nuanced perspective acknowledging that not only males but also females can benefit from multiple mating and compete for mating partners. Yet, sexual selection in females is still considered a peculiarity, and the evolution of polyandry is often viewed to result from a higher mating interest of males. Here, we present meta-analytic evidence from 77 species across a broad range of animal taxa to demonstrate that female reproductive success is overall positively correlated with mating success, suggesting that females typically benefit from multiple mating. Importantly, we found that these fitness gains likely promote the evolution of polyandry. Our findings offer support for the idea that sexual selection is widespread in females and to play a key role for the evolution of animal mating systems. Thereby, our results extend our understanding of the evolutionary consequences of sexual reproduction and contribute to a more balanced view of how sexual selection operates in males and females.

Mating system and speciation I: Accumulation of genetic incompatibilities in allopatry.

Self-fertilisation is widespread among hermaphroditic species across the tree of life. Selfing has many consequences on the genetic diversity and the evolutionary dynamics of populations, which may in turn affect macroevolutionary processes such as speciation. On the one hand, because selfing increases genetic drift and reduces migration rate among populations, it may be expected to promote speciation. On the other hand, because selfing reduces the efficacy of selection, it may be expected to hamper ecological speciation. To better understand under which conditions and in which direction selfing affects the build-up of reproductive isolation, an explicit population genetics model is required. Here, we focus on the interplay between genetic drift, selection and genetic linkage by studying speciation without gene flow. We test how fast populations with different rates of selfing accumulate mutations leading to genetic incompatibilities. When speciation requires populations to pass through a fitness valley caused by underdominant and compensatory mutations, selfing reduces the depth and/or breadth of the valley, and thus overall facilitates the fixation of incompatibilities. When speciation does not require populations to pass through a fitness valley, as for Bateson-Dobzhanzky-Muller incompatibilities (BDMi), the lower effective population size and higher genetic linkage in selfing populations both facilitate the fixation of incompatibilities. Interestingly, and contrary to intuitive expectations, local adaptation does not always accelerate the fixation of incompatibilities in outcrossing relative to selfing populations. Our work helps to clarify how incompatibilities accumulate in selfing vs. outcrossing lineages, and has repercussions on the pace of speciation as well as on the genetic architecture of reproductive isolation.

Rapid evolution of post-zygotic reproductive isolation is widespread in Arctic plant lineages.

BACKGROUND AND AIMS: The Arctic tundra, with its extreme temperatures and short growing season, is evolutionarily young and harbours one of the most species-poor floras on Earth. Arctic species often show little phenotypic and genetic divergence across circumpolar ranges. However, strong intraspecific post-zygotic reproductive isolation (RI) in terms of hybrid sterility has frequently evolved within selfing Arctic species of the genus Draba. Here we assess whether incipient biological species are common in the Arctic flora. METHODS: We conducted an extensive crossing experiment including six species representing four phylogenetically distant families collected across the circumpolar Arctic. We crossed conspecific parental populations representing different spatial scales, raised 740 F1 hybrids to maturity and measured fertility under laboratory conditions. We examined genetic divergence between populations for two of these species (Cardamine bellidifolia and Ranunculus pygmaeus). KEY RESULTS: In five of the six species, we find extensive reduction in pollen fertility and seed set in F1 hybrids; 219 (46 %) of the 477 F1 hybrids generated between parents separated by ≥427 km had <20 % pollen fertility. Isolation with migration (IM) and *BEAST analyses of sequences of eight nuclear genes in C. bellidifolia suggests that reproductively isolated populations of this species diverged during, or even after, the last glaciation. Likewise, Arctic populations of R. pygmaeus were genetically very similar despite exhibiting strongly reduced fertility in crosses, suggesting that RI evolved recently also in this species. CONCLUSION: We show that post-zygotic RI has developed multiple times within taxonomically recognized Arctic species belonging to several distantly related lineages, and that RI may have developed over just a few millennia. Rapid and widespread evolution of incipient biological species in the Arctic flora might be associated with frequent bottlenecks due to glacial cycles, and/or selfing mating systems, which are common in the harsh Arctic environment where pollinators are scarce.

Assortative Mating in Animals and Its Role for Speciation.

Evolutionary theory predicts that positive assortative mating-the tendency of similar individuals to mate with each other-plays a key role for speciation by generating reproductive isolation between diverging populations. However, comprehensive tests for an effect of assortative mating on species richness at the macroevolutionary scale are lacking. We used a meta-analytic approach to test the hypothesis that the strength of assortative mating within populations is positively related to species richness across a broad range of animal taxa. Specifically, we ran a phylogenetically independent meta-analysis using an extensive database of 1,447 effect sizes for the strength of assortative mating, encompassing 307 species from 130 families and 14 classes. Our results suggest that there is no relationship between the strength of assortative mating and species richness across and within major taxonomic groups and trait categories. Moreover, our analysis confirms an earlier finding that animals typically mate assortatively (global Pearson correlation coefficient: r=0.36; 95% confidence interval: 0.19-0.52) when accounting for phylogenetic nonindependence. We argue that future advances will rely on a better understanding of the evolutionary causes and consequences of the observed intra- and interspecific variation in the strength of assortative mating.

Sexual selection predicts species richness across the animal kingdom.

Our improving knowledge of the animal tree of life consistently demonstrates that some taxa diversify more rapidly than others, but what contributes to this variation remains poorly understood. An influential hypothesis proposes that selection arising from competition for mating partners plays a key role in promoting speciation. However, empirical evidence showing a link between proxies of this sexual selection and species richness is equivocal. Here, we collected standardized metrics of sexual selection for a broad range of animal taxa, and found that taxonomic families characterized by stronger sexual selection on males show relatively higher species richness. Thus, our data support the hypothesis that sexual selection elevates species richness. This could occur either by promoting speciation and/or by protecting species against extinction.

Fluorescent sperm in a transparent worm: validation of a GFP marker to study sexual selection.

BACKGROUND: Sexual selection has initially been thought to occur exclusively at the precopulatory stage in terms of contests among males and female mate choice, but research over the last four decades revealed that it often continues after copulation through sperm competition and cryptic female choice. However, studying these postcopulatory processes remains challenging because they occur internally and therefore are often difficult to observe. In the transparent free-living flatworm Macrostomum lignano, a recently established transgenic line that expresses green fluorescent protein (GFP) in all cell types, including sperm, offers a unique opportunity to non-invasively visualise and quantify the sperm of a GFP-expressing donor inside the reproductive tract of wild-type recipients in vivo. We here test several aspects of the reproductive performance of the transgenic individuals and the accuracy of the techniques involved in assessing the GFP-expressing worms and their sperm. We then show the usefulness of these methods in a study on sperm displacement. RESULTS: GFP-expressing worms do not differ from wild-type worms in terms of morphology, mating rate and reproductive success. In addition, we show that the GFP signal is reliably and unequivocally expressed by all GFP-expressing individuals observed under epifluorescence illumination. However, the intensity of the GFP signal emitted by sperm of GFP expressing donors can vary (which we show to be at least in part due to sperm ageing) and the GFP marker is inherited according to Mendel's laws in most, but not all, of the individuals. Nevertheless, we argue these two issues can be addressed with an appropriate experimental design. Finally, we demonstrate the value of the GFP-techniques by comparing the number of GFP-expressing sperm in a wild-type recipient before and after mating with a competing sperm donor, providing clear experimental evidence for sperm displacement in M. lignano. This result suggests that sperm donors can displace previously stored sperm and replace it with their own. CONCLUSION: The availability of the GFP-techniques in a transparent organism provide unique opportunities to visualise and quantify internal processes in the female reproductive tract after mating, which opens new avenues in the study of sexual selection.

Contrasting the form and strength of pre- and postcopulatory sexual selection in a flatworm.

Sexual traits may be selected during multiple consecutive episodes of selection, occurring before, during, or after copulation. The overall strength and form of selection acting on traits may thus be determined by how selection (co-)varies along different episodes. However, it is challenging to measure pre- and postcopulatory phenotypic traits alongside variation in fitness components at each different episode. Here, we used a transgenic line of the transparent flatworm Macrostomum lignano expressing green fluorescent protein (GFP) in all cell types, including sperm cells, enabling in vivo sperm tracking. We assessed the mating success, sperm-transfer efficiency, and sperm fertilizing efficiency of GFP(+) focal worms in which we measured 13 morphological traits. We found linear selection on sperm production rate arising from pre- and postcopulatory components and on copulatory organ shape arising from sperm fertilizing efficiency. We further found nonlinear (mostly concave) selection on combinations of copulatory organ and sperm morphology traits arising mostly from sperm-transfer efficiency and sperm fertilizing efficiency. Our study provides a fine-scale quantification of sexual selection, showing that both the form and strength of selection can change across fitness components. Quantifying how sexual selection builds up along episodes of selection allows us to better understand the evolution of sexually selected traits.

How Does Selfing Affect the Pace and Process of Speciation?

Surprisingly little attention has been given to the impact of selfing on speciation, even though selfing reduces gene flow between populations and affects other key population genetics parameters. Here we review recent theoretical work and compile empirical data from crossing experiments and genomic and phylogenetic studies to assess the effect of mating systems on the speciation process. In accordance with theoretical predictions, we find that accumulation of hybrid incompatibilities seems to be accelerated in selfers, but there is so far limited empirical support for a predicted bias toward underdominant loci. Phylogenetic evidence is scarce and contradictory, including studies suggesting that selfing either promotes or hampers speciation rate. Further studies are therefore required, which in addition to measures of reproductive barrier strength and selfing rate should routinely include estimates of demographic history and genetic divergence as a proxy for divergence time.

Cosegregation of recombinant chromatids maintains genome-wide heterozygosity in an asexual nematode.

In asexual animals, female meiosis is modified to produce diploid oocytes. If meiosis still involves recombination, this is expected to lead to a rapid loss of heterozygosity, with adverse effects on fitness. Many asexuals, however, have a heterozygous genome, the underlying mechanisms being most often unknown. Cytological and population genomic analyses in the nematode Mesorhabditis belari revealed another case of recombining asexual being highly heterozygous genome-wide. We demonstrated that heterozygosity is maintained despite recombination because the recombinant chromatids of each chromosome pair cosegregate during the unique meiotic division. A theoretical model confirmed that this segregation bias is necessary to account for the observed pattern and likely to evolve under a wide range of conditions. Our study uncovers an unexpected type of non-Mendelian genetic inheritance involving cosegregation of recombinant chromatids.

The repeatable opportunity for selection differs between pre- and postcopulatory fitness components.

In species with multiple mating, intense sexual selection may occur both before and after copulation. However, comparing the strength of pre- and postcopulatory selection is challenging, because (i) postcopulatory processes are generally difficult to observe and (ii) the often-used opportunity for selection (I) metric contains both deterministic and stochastic components. Here, we quantified pre- and postcopulatory male fitness components of the simultaneously hermaphroditic flatworm, Macrostomum lignano. We did this by tracking fluorescent sperm-using transgenics-through the transparent body of sperm recipients, enabling to observe postcopulatory processes in vivo. Moreover, we sequentially exposed focal worms to three independent mating groups, and in each assessed their mating success, sperm-transfer efficiency, sperm fertilizing efficiency, and partner fecundity. Based on these multiple measures, we could, for each fitness component, combine the variance (I) with the repeatability (R) in individual success to assess the amount of repeatable variance in individual success-a measure we call the repeatable opportunity for selection (IR ). We found higher repeatable opportunity for selection in sperm-transfer efficiency and sperm fertilizing efficiency compared to mating success, which clearly suggests that postcopulatory selection is stronger than precopulatory selection. Our study demonstrates that the opportunity for selection contains a repeatable deterministic component, which can be assessed and disentangled from the often large stochastic component, to provide a better estimate of the strength of selection.

Social effects on fruit fly courtship song.

Courtship behavior in Drosophila has often been described as a classic innate behavioral repertoire, but more recently extensive plasticity has been described. In particular, prior exposure to acoustic signals of con- or heterspecific males can change courtship traits in both sexes that are liable to be important in reproductive isolation. However, it is unknown whether male courtship song itself is socially plastic. We examined courtship song plasticity of two species in the Drosophila melanogaster subgroup. Sexual isolation between the species is influenced by two male song traits, the interpulse interval (IPI) and sinesong frequency (SSF). Neither of these showed plasticity when males had prior experience of con- and heterospecific social partners. However, males of both species produced longer bursts of song during courtship when they were exposed to social partners (either con- or heterospecific) than when they were reared in isolation. D. melanogaster carrying mutations affecting short- or medium-term memory showed a similar response to the social environment, not supporting a role for learning. Our results demonstrate that the amount of song a male produces during courtship is plastic depending on the social environment, which might reflect the advantage of being able to respond to variation in intrasexual competition, but that song structure itself is relatively inflexible, perhaps due to strong selection against hybridization.

Indirect genetic effects and sexual conflicts: Partner genotype influences multiple morphological and behavioral reproductive traits in a flatworm.

The expression of an individual's phenotypic traits can be influenced by genes expressed in its social partners. Theoretical models predict that such indirect genetic effects (IGEs) on reproductive traits should play an important role in determining the evolutionary outcome of sexual conflict. However, empirical tests of (i) whether reproductive IGEs exist, (ii) how they vary among genotypes, and (iii) whether they are uniform for different types of reproductive traits are largely lacking. We addressed this in a series of experiments in the simultaneously hermaphroditic flatworm Macrostomum lignano. We found strong evidence for IGEs on both morphological and behavioral reproductive traits. Partner genotype had a significant impact on the testis size of focal individuals-varying up to 2.4-fold-suggesting that IGEs could mediate sexual conflicts that target the male sex function. We also found that time to first copulation was affected by a genotype × genotype interaction between mating partners, and that partner genotype affected the propensity to copulate and perform the postcopulatory suck behavior, which may mediate conflicts over the fate of received ejaculate components. These findings provide clear empirical evidence for IGEs on multiple behavioral and morphological reproductive traits, which suggests that the evolutionary dynamics of these traits could be altered by genes contained in the social environment.

Quantifying episodes of sexual selection: Insights from a transparent worm with fluorescent sperm.

Sexual selection operates through consecutive episodes of selection that ultimately contribute to the observed variance in reproductive success between individuals. Understanding the relative importance of these episodes is challenging, particularly because the relevant postcopulatory fitness components are often difficult to assess. Here, we investigate different episodes of sexual selection on the male sex function, by assessing how (precopulatory) mating success, and (postcopulatory) sperm-transfer efficiency and sperm-fertilizing efficiency contribute to male reproductive success. Specifically, we used a transgenic line of the transparent flatworm, Macrostomum lignano, which expresses green fluorescent protein (GFP) in all cell types, including sperm cells, enabling in vivo sperm tracking and paternity analysis. We found that a large proportion of variance in male reproductive success arose from the postcopulatory episodes. Moreover, we also quantified selection differentials on 10 morphological traits. Testis size and seminal vesicle size showed significant positive selection differentials, which were mainly due to selection on sperm-transfer efficiency. Overall, our results demonstrate that male reproductive success in M. lignano is not primarily limited by the number of matings achieved, but rather by the ability to convert matings into successful fertilizations, which is facilitated by producing many sperm.

Sex allocation adjustment to mating group size in a simultaneous hermaphrodite.

Sex allocation theory is considered as a touchstone of evolutionary biology, providing some of the best supported examples for Darwinian adaptation. In particular, Hamilton's local mate competition theory has been shown to generate precise predictions for extraordinary sex ratios observed in many separate-sexed organisms. In analogy to local mate competition, Charnov's mating group size model predicts how sex allocation in simultaneous hermaphrodites is affected by the mating group size (i.e., the number of mating partners plus one). Until now, studies have not directly explored the relationship between mating group size and sex allocation, which we here achieve in the simultaneously hermaphroditic flatworm Macrostomum lignano. Using transgenic focal worms with ubiquitous expression of green-fluorescent protein (GFP), we assessed the number of wild-type mating partners carrying GFP+ sperm from these focal worms when raised in different social group sizes. This allowed us to test directly how mating group size was related to the sex allocation of focal worms. We find that the proportion of male investment initially increases with increasing mating group size, but then saturates as predicted by theory. To our knowledge, this is the first direct test of the mating group size model in a simultaneously hermaphroditic animal.