Postcopulatory, prezygotic reproductive isolation

Despite genetic isolation in sympatry, post-copulatory reproductive barriers have not evolved between bat- and human-associated common bedbugs (Cimex lectularius L.)

BACKGROUND

The common bedbug Cimex lectularius is a widespread ectoparasite on humans and bats. Two genetically isolated lineages, parasitizing either human (HL) or bat (BL) hosts, have been suggested to differentiate because of their distinct ecology. The distribution range of BL is within that of HL and bedbugs live mostly on synanthropic bat hosts. This sympatric co-occurrence predicts strong reproductive isolation at the post-copulatory level.

RESULTS

We tested the post-copulatory barrier in three BL and three HL populations in reciprocal crosses, using a common-garden blood diet that was novel to both lineages. We excluded pre-copulation isolation mechanisms and studied egg-laying rates after a single mating until the depletion of sperm, and the fitness of the resulting offspring. We found a higher sperm storage capability in BL, likely reflecting the different seasonal availability of HL and BL hosts. We also observed a notable variation in sperm function at the population level within lineages and significant differences in fecundity and offspring fitness between lineages. However, no difference in egg numbers or offspring fitness was observed between within- and between-lineage crosses.

CONCLUSIONS

Differences in sperm storage or egg-laying rates between HL and BL that we found did not affect reproductive isolation. Neither did the population-specific variation in sperm function. Overall, our results show no post-copulatory reproductive isolation between the lineages. How genetic differentiation in sympatry is maintained in the absence of a post-copulatory barrier between BL and HL remains to be investigated.

Interplay between male quality and male-female compatibility across episodes of sexual selection

The processes underlying mate choice profoundly influence the dynamics of sexual selection and the evolution of male sexual traits. Consistent preference for certain phenotypes may erode genetic variation in populations through directional selection, whereas divergent preferences (e.g., genetically compatible mates) provide one mechanism to maintain such variation. However, the relative contributions of these processes across episodes of selection remain unknown. Using Drosophila melanogaster, we followed the fate of male genotypes, previously scored for their overall reproductive value and their compatibility with different female genotypes, across pre- and postmating episodes of selection. When pairs of competitor males differed in their intrinsic quality and their compatibility with the female, both factors influenced outcomes from mating success to paternity but to a varying degree between stages. These results add further dimensions to our understanding of how the interactions between genotypes and forms of selection shape reproductive outcomes and ultimately reproductive trait evolution.

Genetic Contributions to Ectopic Sperm Cell Migration in Caenorhabditis Nematodes

Reproductive barriers involving gametic incompatibilities can act to enhance population divergence and promote the persistence of species boundaries. Observing gametic interactions in internal fertilizing organisms, however, presents a considerable practical challenge to characterizing mechanisms of such gametic isolation. Here we exploit the transparency of Caenorhabditis nematodes to investigate gametic isolation mediated by sperm that can migrate to ectopic locations, with this sperm invasion capable of inducing female sterility and premature death. As a step toward identifying genetic factors and mechanisms associated with female susceptibility to sperm invasion, we characterized a panel of 25 C. elegans genetic mutants to test for effects on the incidence and severity of sperm invasion in both conspecific and inter-species matings. We found genetic perturbations to contribute to distinct patterns of susceptibility that identify ovulation dynamics and sperm guidance cues as modulators of ectopic sperm migration incidence and severity. Genotypes confer distinctive phenotypic sensitivities to the sperm from conspecific C. elegans males vs. heterospecific C. nigoni males, implicating evolution of functional divergence in the history of these species for components of sperm-reproductive tract interactions. Sexually-antagonistic co-evolution within species that drives divergent trait and molecular evolution between species provides a working model to explain mismatched species-specific gametic interactions that promote or mitigate ectopic sperm migration.

Persistent postmating, prezygotic reproductive isolation between populations

Studying reproductive barriers between populations of the same species is critical to understand how speciation may proceed. Growing evidence suggests postmating, prezygotic (PMPZ) reproductive barriers play an important role in the evolution of early taxonomic divergence. However, the contribution of PMPZ isolation to speciation is typically studied between species in which barriers that maintain isolation may not be those that contributed to reduced gene flow between populations. Moreover, in internally fertilizing animals, PMPZ isolation is related to male ejaculate-female reproductive tract incompatibilities but few studies have examined how mating history of the sexes can affect the strength of PMPZ isolation and the extent to which PMPZ isolation is repeatable or restricted to particular interacting genotypes. We addressed these outstanding questions using multiple populations of Drosophila montana. We show a recurrent pattern of PMPZ isolation, with flies from one population exhibiting reproductive incompatibility in crosses with all three other populations, while those three populations were fully fertile with each other. Reproductive incompatibility is due to lack of fertilization and is asymmetrical, affecting female fitness more than males. There was no effect of male or female mating history on reproductive incompatibility, indicating that PMPZ isolation persists between populations. We found no evidence of variability in fertilization outcomes attributable to different female × male genotype interactions, and in combination with our other results, suggests that PMPZ isolation is not driven by idiosyncratic genotype × genotype interactions. Our results show PMPZ isolation as a strong, consistent barrier to gene flow early during speciation and suggest several targets of selection known to affect ejaculate-female reproductive tract interactions within species that may cause this PMPZ isolation.

Possible glimpses into early speciation: the effect of ovarian fluid on sperm velocity accords with post-copulatory isolation between two guppy populations

Identifying mechanisms of reproductive isolation is key to understanding speciation. Among the putative mechanisms underlying reproductive isolation, sperm-female interactions (post-mating-prezygotic barriers) are arguably the hardest to identify, not least because these are likely to operate at the cellular or molecular level. Yet sperm-female interactions offer great potential to prevent the transfer of genetic information between different populations at the initial stages of speciation. Here, we provide a preliminary test for the presence of a putative post-mating-prezygotic barrier operating between three populations of Trinidadian guppies (Poecilia reticulata), an internally fertilizing fish that inhabits streams with different levels of connectivity across Trinidad. We experimentally evaluate the effect of female ovarian fluid on sperm velocity (a predictor of competitive fertilization success) according to whether males and females were from the same (native) or different (foreign) populations. Our results reveal the potential for ovarian fluid to act as a post-mating-prezygotic barrier between two populations from different drainages, but also that the strength of this barrier is different among populations. This result may explain the previous finding that, in some populations, sperm from native males have precedence over foreign sperm, which could eventually lead to reproductive isolation between these populations.

Divergence in style length and pollen size leads to a postmating-prezygotic reproductive barrier among populations of Silene latifolia

A central tenet of speciation research is the need to identify reproductive isolating barriers. One approach to this line of research is to identify the phenotypes that lead to reproductive isolation. Several studies on flowering plants have shown that differences in style length contribute to reproductive isolation between species, leading us to consider whether style length could act as a reproductive barrier among populations of a single species. This could occur if style length varied sufficiently and pollen size covaried with style length. Populations of Silene latifolia exhibit variation in flower size, including style length, that is negatively correlated with annual precipitation. We show that this divergence in style length has a genetic basis and acts as a reproductive barrier: males from small-flowered populations produced relatively small pollen grains that were poor at fertilizing ovules when crossed to females from large-flowered populations, leading to a significant reduction in seed production. Manipulating the distance pollen tubes had to travel revealed that this failure was purely mechanical and not the result of other incompatibilities. These results show that style length acts as a postmating-prezygotic reproductive barrier and indicate a potential link between ecotypic differentiation and reproductive isolation within a species.

The contribution of post-copulatory mechanisms to incipient ecological speciation in sticklebacks

Ecology can play a major role in species diversification. As individuals are adapting to contrasting habitats, reproductive barriers may evolve at multiple levels. While pre-mating barriers have been extensively studied, the evolution of post-mating reproductive isolation during early stages of ecological speciation remains poorly understood. In diverging three-spined stickleback ecotypes from two lakes and two rivers, we observed differences in sperm traits between lake and river males. Interestingly, these differences did not translate into ecotype-specific gamete precedence for sympatric males in competitive in vitro fertilization experiments, potentially owing to antagonistic compensatory effects. However, we observed indirect evidence for impeded development of inter-ecotype zygotes, possibly suggesting an early stage of genetic incompatibility between ecotypes. Our results show that pre-zygotic post-copulatory mechanisms play a minor role during this first stage of ecotype divergence, but suggest that genetic incompatibilities may arise at early stages of ecological speciation.

Reproduction barrier between two lineages of bed bug (Cimex lectularius) (Heteroptera: Cimicidae)

Populations of bed bugs, Cimex lectularius, have increased in recent years spreading into numerous urban areas across the Western world and making them an increasingly important pest of the twenty-first century. Research into hybridization within and between different lineages of bed bugs can help us to understand processes of micro- and macro-evolution in these ectoparasites and may inform the control of this pest species. Hybridization experiments between two host lineages of bed bug (C. lectularius) from Central Europe (Czech Republic), those associated with humans and those with bats, were conducted under laboratory conditions. Number of eggs and early instars were compared between crosses of mixed host lineages (interspecific mating) with pairs from the same host lineage, those from the same locality and same lineage from different localities (intraspecific mating). While crosses within host lineages resulted in egg production and later instars, crosses between different host lineages were unsuccessful, although of the mated females possessed sperm in their mesospermaleges and/or seminal conceptacles. These crosses did not even result in egg production. Moreover, in the mixed lineage crosses, mortality rates in adults were higher (51 and 50% higher in bat and human lineage, respectively) than in those animals from the same lineage. Survival of adults was in pairs from the same locality slightly higher than in pairs from different localities and differed statistically. These results support the existence of post-mating barriers and show reproductive isolation between two lineages of C. lectularius. Bat and human host adaptations can promote evolving of such barriers and can be product of alloxenic speciation.

Comparison of sexual compatibility in crosses between the southern and northern populations of the cabbage beetle Colaphellus bowringi

It is widely accepted that the genetic divergence and reproductive incompatibility between closely related species and/or populations is often viewed as an important step toward speciation. In this study, sexual compatibility in crosses between the southern XS population and the northern TA population of the polyandrous cabbage beetle Colaphellus bowringi was investigated by testing their mating preferences, mating latency, copulation duration, and reproductive performances of post-mating. In choice mating experiments, the percentages of matings were significantly higher in intra-population crosses than in inter-population crosses. Both isolation index (I) and index of pair sexual isolation (IPSI ) indicated partial mating incompatibility or assortative mating in crosses between the two different geographical populations. In single pair mating experiments, XS females in inter-population crosses mated significantly later and copulated significantly shorter than those in intra-population crosses. However, TA females in inter-population crosses mated significantly earlier and copulated longer than those in intra-population crosses, suggesting that larger XS males may enhance heterotypic mating. The lifetime fecundity was highest in XS homotypic matings, lowest in TA homotypic matings, and intermediate in heterotypic matings between their parents. The inter-population crosses resulted in significantly lower egg hatching rate and shorter female longevity than intra-population crosses. These results demonstrated that there exist some incompatibilities in premating, postmating-prezygotic, and postzygotic stages between the southern XS population and northern TA population of the cabbage beetle Colaphellus bowringi.

Rapid evolution of asymmetric reproductive incompatibilities in stalk-eyed flies

The steps by which isolated populations acquire reproductive incompatibilities remain poorly understood. One potentially important process is postcopulatory sexual selection because it can generate divergence between populations in traits that influence fertilization success after copulation. Here we present a comprehensive analysis of this form of reproductive isolation by conducting reciprocal crosses between variably diverged populations of stalk-eyed flies (Teleopsis dalmanni). First, we measure seven types of reproductive incompatibility between copulation and fertilization. We then compare fertilization success to hatching success to quantify hybrid inviability. Finally, we determine if sperm competition acts to reinforce or counteract any incompatibilities. We find evidence for multiple incompatibilities in most crosses, including failure to store sperm after mating, failure of sperm to reach the site of fertilization, failure of sperm to fertilize eggs, and failure of embryos to develop. Local sperm have precedence over foreign sperm, but this effect is due mainly to differences in sperm transfer and reduced hatching success. Crosses between recently diverged populations are asymmetrical with regard to the degree and type of incompatibility. Because sexual conflict in these flies is low, postcopulatory sexual selection, rather than antagonistic coevolution, likely causes incompatibilities due to mismatches between male and female reproductive traits.

Rapid diversification of sperm precedence traits and processes among three sibling Drosophila species

Postcopulatory sexual selection is credited with driving rapid evolutionary diversification of reproductive traits and the formation of reproductive isolating barriers between species. This judgment, however, has largely been inferred rather than demonstrated due to general lack of knowledge about processes and traits underlying variation in competitive fertilization success. Here, we resolved processes determining sperm fate in twice-mated females, using transgenic Drosophila simulans and Drosophila mauritiana populations with fluorescently labeled sperm heads. Comparisons among these two species and Drosophila melanogaster revealed a shared motif in the mechanisms of sperm precedence, with postcopulatory sexual selection potentially occurring during any of the three discrete stages: (1) insemination; (2) sperm storage; and (3) sperm use for fertilization, and involving four distinct phenomena: (1) sperm transfer; (2) sperm displacement; (3) sperm ejection; and (4) sperm selection for fertilizations. Yet, underlying the qualitative similarities were significant quantitative differences in nearly every relevant character and process. We evaluate these species differences in light of concurrent investigations of within-population variation in competitive fertilization success and postmating/prezygotic reproductive isolation in hybrid matings between species to forge an understanding of the relationship between microevolutionary processes and macroevolutionary patterns as pertains to postcopulatory sexual selection in this group.

Divergence in genital morphology may contribute to mechanical reproductive isolation in a millipede

Genitalia appear to evolve rapidly and divergently in taxa with internal fertilization. The current consensus is that intense directional sexual selection drives the rapid evolution of genitalia. Recent research on the millipede Antichiropus variabilis suggests that the male genitalia are currently experiencing stabilizing selection – a pattern of selection expected for lock-and-key structures that enforce mate recognition and reproductive isolation. Here, we investigate how divergence in genital morphology affects reproductive compatibility among isolated populations of A. variabilis. Females from a focal population were mated first to a male from their own population and, second, to a male from one of two populations with divergent genital morphology. We observed variation in mating behavior that might indicate the emergence of precopulatory reproductive barriers: males from one divergent population took significantly longer to recognize females and exhibited mechanical difficulty in genital insertion. Moreover, we observed very low paternity success for extra-population males who were successful in copulating. Our data suggest that divergence in genital shape may be contributing to reproductive isolation, and incipient speciation among isolated populations of A. variabilis.

Post-mating prezygotic barriers to gene exchange between hybridizing field crickets

Studies of sexual selection in speciation have traditionally focused on mate preference, with less attention given to traits that act between copulation and fertilization. However, recent work suggests that post-mating prezygotic barriers may play an important role in speciation. Here, we evaluate the role of such barriers in the field crickets, Gryllus firmus and Gryllus pennsylvanicus. Gryllus pennsylvanicus females mated with G. firmus males produce viable, fertile offspring, but when housed with both species produce offspring sired primarily by conspecifics. We evaluate patterns of sperm utilization in doubly mated G. pennsylvanicus females and find no evidence for conspecific sperm precedence. The reciprocal cross (G. firmus female × G. pennsylvanicus male) produces no progeny. Absence of progeny reflects a barrier to fertilization rather than reduced sperm transfer, storage or motility. We propose a classification scheme for mechanisms underlying post-mating prezygotic barriers similar to that used for premating barriers.

Natural selection hampers divergence of reproductive traits in a seed beetle

Speciation is thought to often result from indirect selection for reproductive isolation. This will occur when reproductive traits that cause reproductive isolation evolve (i) as a by-product of natural selection on traits with which they are genetically correlated or (ii) as an indirect result of diversifying sexual selection. Here, we use experimental evolution to study the degree of divergent evolution of reproductive traits by manipulating the intensity of natural and sexual selection in replicated selection lines of seed beetles. Following 40 generations of selection, we assayed the degree of divergent evolution of reproductive traits between replicate selection lines experiencing the same selection regime. The evolution of reproductive traits was significantly divergent across selection lines within treatments. The evolution of reproductive traits was both slower and, more importantly, significantly less divergent among lines experiencing stronger directional natural selection. This suggests that reproductive traits did not evolve as an indirect by-product of adaptation. We discuss several ways in which natural selection may hamper divergent evolution among allopatric populations.

Clinal variation in post-winter male fertility retention; an adaptive overwintering strategy in Drosophila melanogaster

In insects including Drosophila melanogaster, females can overwinter at the adult stage by adopting a shallow reproductive diapause, but almost nothing is known about male reproductive diapause. In this study, we test for the maintenance of fertility in overwintering males from the eastern Australian D. melanogaster cline. Males from southern temperate populations maintained in field cages in temperate Melbourne had a higher fertility in spring compared with males from tropical locations. Temperate males successfully inseminated more females, and there were also more offspring produced per inseminated female. The resulting linear post-winter fertility clines were unrelated to male body size. In contrast, there was no clinal variation for fertility in nonoverwintering males held in the laboratory. The cline in overwintering male fertility is likely to have evolved as an adaptive response to latitudinal climatic variation over the last 100 years.

Complex interactions with females and rival males limit the evolution of sperm offence and defence

Postcopulatory sexual selection favours males which are strong offensive and defensive sperm competitors. As a means of identifying component traits comprising each strategy, we used an experimental evolution approach. Separate populations of Drosophila melanogaster were selected for enhanced sperm offence and defence. Despite using a large outbred population and evidence of substantive genetic variation for each strategy, neither trait responded to selection in the two replicates of this experiment. Recent work with fixed chromosome lines of D. melanogaster suggests that complex genotypic interactions between females and competing males contribute to the maintenance of this variation. To determine whether such interactions could explain our lack of response to selection on sperm offence and defence, we quantified sperm precedence across multiple sperm competition bouts using an outbred D. melanogaster population exhibiting continuous genetic variation. Both offensive and defensive sperm competitive abilities were found to be significantly repeatable only across matings involving ejaculates of the same pair of males competing within the same female. These repeatabilities decreased when the rival male stayed the same but the female changed, and they disappeared when both the rival male and the female changed. Our results are discussed with a focus on the complex nature of sperm precedence and the maintenance of genetic variation in ejaculate characteristics.

Chemically Mediated Species Recognition in Closely Related Podarcis Wall Lizards

In many animals, chemical signals play an important role in species recognition and may contribute to reproductive isolation and speciation. The Iberian lizards of the genus Podarcis, with up to nine currently recognized lineages that are often sympatric, are highly chemosensory and provide an excellent model for the study of chemically mediated species recognition in closely related taxa. In this study, we tested the ability of male and female lizards of two sister species with widely overlapping distribution ranges (Podarcis bocagei and P. hispanica type 1) to discriminate between conspecific and heterospecific mates by using only substrate-borne chemical cues. We scored the number of tongue flicks directed at the paper substrate by each individual in a terrarium previously occupied by a conspecific or a heterospecific lizard of the opposite sex. Results show that males of P. bocagei and P. hispanica type 1 are capable of discriminating chemically between conspecifics and heterospecifics of the opposite sex, but females are not. These results suggest that differences in female, but not male, chemical cues may underlie species recognition and contribute to reproductive isolation in these species. The apparent inability of females to discriminate conspecific from heterospecific males, which is not because of reduced baseline exploration rates, is discussed in the context of sexual selection theory and species discrimination.

Divergence in replicated phylogenies: the evolution of partial post-mating prezygotic isolation in bean weevils

By tradition, speciation research has been focused on processes leading to either premating or post-zygotic reproductive isolation. The processes which generate isolation after mating but before zygote formation are less well understood. Here, we study divergence in characters which contribute to post-mating prezygotic isolation, such as egg production and remating rate. We propose that 'replicated' laboratory phylogenies with known histories can be used to yield insights into the processes of divergence. We performed a series of cross-matings between populations within two strains of the bean weevil Callosobruchus maculatus. Each strain has a unique and independent origin and both have been kept in the same set of laboratories during the last few decades. Our results show that divergence has occurred between laboratory populations within strains with regards to the effects that mating has on female reproductive behaviour, showing that the evolution of partial post-mating prezygotic isolation can be rapid. More importantly, the pattern of divergence across populations was distinct in the two strains, suggesting that coevolutionary trajectories are not determined by environmental factors but are to some extent arbitrary. We discuss the limitations of the novel empirical strategy employed here, and conclude that our results lend support to the hypothesis that post-mating sexual selection is capable of rapidly generating post-mating prezygotic isolation.

Discriminating males alter sperm production between species

Prezygotic reproductive isolation and its importance in speciation is traditionally approached from the viewpoint of those events that occur before mating. However, recent interest in sperm competition theory has shown that prezygotic isolation can be affected by mechanisms that occur after mating but before fertilization. One neglected aspect of these studies is how the cost of sperm production might play a role in species isolation. We examined differential sperm production in a species whose males are sexually parasitized by a unisexual gynogenetic species. Gynogens are clonal females that require sperm from males of closely related bisexual species to initiate embryogenesis. We tested for differential sperm production by male sailfin mollies (Poecilia latipinna) when they were in the presence of either a heterospecific, gynogenetic female (Poecilia formosa, Amazon molly) or a conspecific female. We found that previously demonstrated male mate choice for conspecific over heterospecific females also is revealed in sperm production. Males from both an allopatric and a sympatric population produce more sperm when in the presence of a conspecific female than when in the presence of a heterospecific female. We suggest that differential sperm production also could play a role in prezygotic reproductive isolation in bisexual species complexes that occur in sympatry.

Ejaculate-female coevolution in Drosophila mojavensis

Interspecific studies indicate that sperm morphology and other ejaculatory traits diverge more rapidly than other types of character in Drosophila and other taxa. This pattern has largely been attributed to postcopulatory sexual selection involving interaction between the sexes. Such divergence has been suggested to lead rapidly to reproductive isolation among populations and thus to be an 'engine of speciation.' Here, we test two critical predictions of this hypothesis: (i) there is significant variation in reproductive traits among incipient species; and (ii) divergence in interacting sex-specific traits exhibits a coevolutionary pattern among populations within a species, by examining geographical variation in Drosophila mojavensis, a species in the early stages of speciation. Significant among-population variation was identified in sperm length and female sperm-storage organ length, and a strong pattern of correlated evolution between these interacting traits was observed. In addition, crosses among populations revealed coevolution of male and female contributions to egg size. Support for these two important predictions confirms that coevolving internal characters that mediate successful reproduction may play an important part in speciation. The next step is to determine exactly what that role is.

Sperm-female coevolution in Drosophila

Rapid evolution of reproductive traits has been attributed to sexual selection arising from interaction between the sexes. However, little is known about the nature of selection driving the evolution of interacting sex-specific phenotypes. Using populations of Drosophila melanogaster selected for divergent sperm length or female sperm-storage organ length, we experimentally show that male fertilization success is determined by an interaction between sperm and female morphology. In addition, sperm length evolution occurred as a correlated response to selection on the female reproductive tract. Giant sperm tails are the cellular equivalent of the peacock's tail, having evolved because females evolved reproductive tracts that selectively bias paternity in favor of males with longer sperm.

The rapid evolution of reproductive proteins

Many genes that mediate sexual reproduction, such as those involved in gamete recognition, diverge rapidly, often as a result of adaptive evolution. This widespread phenomenon might have important consequences, such as the establishment of barriers to fertilization that might lead to speciation. Sequence comparisons and functional studies are beginning to show the extent to which the rapid divergence of reproductive proteins is involved in the speciation process.

Functional incompatibility between the fertilization systems of two allopatric populations of Callosobruchus maculatus (Coleoptera: Bruchidae)

Recent studies indicate that postcopulatory sexual selection may represent an important component of the speciation process by initiating reproductive isolation via the evolutionary divergence of fertilization systems. Using two geographically isolated populations of the polyandrous beetle Callosobruchus maculatus, we investigated divergence in fertilization systems by determining the extent of postcopulatory functional incompatibility. Through reciprocal, cross-population matings we were able to separately estimate the effects of male and female population origin and their interaction on the extent of last-male sperm precedence, female receptivity to further copulation and female oviposition. Our results indicate partial incompatibility between the fertilization systems of the two populations at all three functional levels. Males derived from the same population as females outcompete rival, allopatric males with respect to sperm preemption, sperm protection, and ability to stimulate female oviposition. This pattern is reciprocated in both populations indicating that postcopulatory, prezygotic events represent important mechanisms by which between-population gene flow is reduced. We suggest the partial gametic isolation observed is a by-product of the coevolution of male and female fertilization systems by a process of cryptic female choice. Our results are consistent with a mechanism akin to conventional mate choice models although they do not allow us to reject antagonistic sexual coevolution as the mechanism of cryptic female choice.