Reinforcement shapes clines in female mate discrimination in Drosophila subquinaria.
ABSTRACT: Reinforcement of species boundaries may alter mate recognition in a way that also affects patterns of mate preference among conspecific populations. In the fly Drosophila subquinaria, females sympatric with the closely related species D. recens reject mating with heterospecific males as well as with conspecific males from allopatric populations. Here, we assess geographic variation in behavioral isolation within and among populations of D. subquinaria and use cline theory to understand patterns of selection on reinforced discrimination and its consequences for sexual isolation within species. We find that selection has fixed rejection of D. recens males in sympatry, while significant genetic variation in this behavior occurs within allopatric populations. In conspecific matings sexual isolation is also asymmetric and stronger in populations that are sympatric with D. recens. The clines in behavioral discrimination within and between species are similar in shape and are maintained by strong selection in the face of gene flow, and we show that some of their genetic basis may be either shared or linked. Thus, while reinforcement can drive extremely strong phenotypic divergence, the long-term consequences for incipient speciation depend on gene flow, genetic linkage of discrimination traits, and the cost of these behaviors in allopatry.
Project description:Interactions between species can alter selection on sexual displays used in mate choice within species. Here we study the epicuticular pheromones of two Drosophila species that overlap partially in geographic range and are incompletely reproductively isolated. Drosophila subquinaria shows a pattern of reproductive character displacement against Drosophila recens, and partial behavioral isolation between conspecific sympatric versus allopatric populations, whereas D. recens shows no such variation in mate choice. First, using manipulative perfuming experiments, we show that females use pheromones as signals for mate discrimination both between species and among populations of D. subquinaria. Second, we show that patterns of variation in epicuticular compounds, both across populations and between species, are consistent with those previously shown for mating probabilities: pheromone compositions differ between populations of D. subquinaria that are allopatric versus sympatric with D. recens, but are similar across populations of D. recens regardless of overlap with D. subquinaria. We also identify differences in pheromone composition among allopatric regions of D. subquinaria. In sum, our results suggest that epicuticular compounds are key signals used by females during mate recognition, and that these traits have diverged among D. subquinaria populations in response to reinforcing selection generated by the presence of D. recens.
Project description:Reinforcement refers to the evolution of increased mating discrimination against heterospecific individuals in zones of geographic overlap and can be considered a final stage in the speciation process. One the factors that may affect reinforcement is the degree to which hybrid matings result in the permanent loss of genes from a species' gene pool. Matings between females of Drosophila subquinaria and males of D. recens result in high levels of offspring mortality, due to interspecific cytoplasmic incompatibility caused by Wolbachia infection of D. recens. Such hybrid inviability is not manifested in matings between D. recens females and D. subquinaria males. Here we ask whether the asymmetrical hybrid inviability is associated with a corresponding asymmetry in the level of reinforcement. The geographic ranges of D. recens and D. subquinaria were found to overlap across a broad belt of boreal forest in central Canada. Females of D. subquinaria from the zone of sympatry exhibit much stronger levels of discrimination against males of D. recens than do females from allopatric populations. In contrast, such reproductive character displacement is not evident in D. recens, consistent with the expected effects of unidirectional cytoplasmic incompatibility. Furthermore, there is substantial behavioral isolation within D. subquinaria, because females from populations sympatric with D. recens discriminate against allopatric conspecific males, whereas females from populations allopatric with D. recens show no discrimination against any conspecific males. Patterns of general genetic differentiation among populations are not consistent with patterns of behavioral discrimination, which suggests that the behavioral isolation within D. subquinaria results from selection against mating with Wolbachia-infected D. recens. Interspecific cytoplasmic incompatibility may contribute not only to post-mating isolation, an effect already widely recognized, but also to reinforcement, particularly in the uninfected species. The resulting reproductive character displacement not only increases behavioral isolation from the Wolbachia-infected species, but may also lead to behavioral isolation between populations of the uninfected species. Given the widespread occurrence of Wolbachia among insects, it thus appears that there are multiple ways by which these endosymbionts may directly and indirectly contribute to reproductive isolation and speciation.
Project description:When two species are incompletely isolated, strengthening premating isolation barriers in response to the production of low fitness hybrids may complete the speciation process. Here, we use the sister species Drosophila subquinaria and Drosophila recens to study the conditions under which this reinforcement of species boundaries occurs in natural populations. We first extend the region of known sympatry between these species, and then we conduct a fine-scale geographic survey of mate discrimination coupled with estimates of gene flow within and admixture between species. Within D. subquinaria, reinforcement is extremely effective: we find variation in mate discrimination both against D. recens males and against conspecific allopatric males on the scale of a few kilometres and in the face of gene flow both from conspecific populations and introgression from D. recens. In D. recens, we do not find evidence for increased mate discrimination in sympatry, even where D. recens is rare, consistent with substantial gene flow throughout the species' range. Finally, we find that introgression between species is asymmetric, with more from D. recens into D. subquinaria than vice versa. Within each species, admixture is highest in the geographic region where it is rare relative to the other species, suggesting that when hybrids are produced they are of low fitness. In sum, reinforcement within D. subquinaria is effective at maintaining species boundaries, but even when reinforcing selection is strong it may not always result in a pattern of strong reproductive character displacement due to variation in the frequency of hybridization and gene flow from neighbouring populations.
Project description:The reinforcement of premating barriers due to reduced hybrid fitness in sympatry may cause secondary sexual isolation within a species as a by-product. Consistent with this, in the fly Drosophila subquinaria, females that are sympatric with D. recens mate at very low rates not only with D. recens, but also with conspecific D. subquinaria males from allopatry. Here, we ask if these effects of reinforcement cascade more broadly to affect sexual isolation with other closely related species. We assay reproductive isolation of these species with D. transversa and find that choosy D. subquinaria females from the region sympatric with D. recens discriminate strongly against male D. transversa, whereas D. subquinaria from the allopatric region do not. This increased sexual isolation cannot be explained by natural selection to avoid mating with this species, as they are allopatric in geographic range and we do not identify any intrinsic postzygotic isolation between D. subquinaria and D. transversa. Variation in epicuticular hydrocarbons, which are used as mating signals in D. subquinaria, follow patterns of premating isolation: D. transversa and allopatric D. subquinaria are most similar to each other and differ from sympatric D. subquinaria, and those of D. recens are distinct from the other two species. We suggest that the secondary effects of reinforcement may cascade to strengthen reproductive isolation with other species that were not a target of selection. These effects may enhance the divergence that occurs in allopatry to help explain why some species are already sexually isolated upon secondary contact.
Project description:Most animal species use distinctive courship patterns to choose among potential mates. Over time, the sensory signaling and preferences used during courtship can diverge among groups that are reproductively isolated. This divergence of signal traits and preferences is thought to be an important cause of behavioral isolation during the speciation process. Here, we examine the sensory modalities used in courtship by two closely related species, Drosophila subquinaria and Drosophila recens, which overlap in geographic range and are incompletely reproductively isolated. We use observational studies of courtship patterns and manipulation of male and female sensory modalities to determine the relative roles of visual, olfactory, gustatory, and auditory signals during conspecific mate choice. We find that sex-specific, species-specific, and population-specific cues are used during mate acquisition within populations of D. subquinaria and D. recens. We identify shifts in both male and female sensory modalities between species, and also between populations of D. subquinaria. Our results indicate that divergence in mating signals and preferences have occurred on a relatively short timescale within and between these species. Finally, we suggest that because olfactory cues are essential for D. subquinaria females to mate within species, they may also underlie variation in behavioral discrimination across populations and species.
Project description:Inferring evolutionary relationships among recently diverged lineages is necessary to understand how isolating barriers produce independent lineages. Here, we investigate the phylogenetic relationships between three incompletely isolated and closely related mushroom-feeding Drosophila species. These species form the Drosophila subquinaria species complex and consist of one Eurasian species (D. transversa) and two widespread North American species (D. subquinaria and D. recens) that are sympatric in central Canada. Although patterns of pre- and post-mating isolation among these species are well characterized, previous work on their phylogenetic relationships is limited and conflicting. In this study, we generated a multi-locus data set of 29 loci from across the genome sequenced in a population sample from each species, and then, we inferred species relationships and patterns of introgression. We find strong statistical support that D. subquinaria is paraphyletic, showing that samples from the geographic region sympatric with D. recens are most closely related to D. recens, whereas samples from the geographic region allopatric with D. recens are most closely related to D. transversa. We present several lines of evidence that both incomplete lineage sorting and gene flow are causing phylogenetic discordance. We suggest that ongoing gene flow primarily from D. recens into D. subquinaria in the sympatric part of their ranges causes phylogenetic uncertainty in the evolutionary history of these species. Our results highlight how population genetic data can be used to disentangle the sources of phylogenetic discordance among closely related species.
Project description:Reinforcement occurs when selection against hybrid offspring strengthens behavioral isolation between parental species and may be an important factor in speciation. Theoretical models and experimental evidence indicate that both female and male preferences can be strengthened upon secondary contact via reinforcement. However, the question remains whether this process is more likely to affect the preferences of one sex or the other. Males of polygynous species are often predicted to exhibit weaker preferences than females, potentially limiting the ability for reinforcement to shape male preferences. Yet, in darters (Percidae: Etheostoma), male preference for conspecific mates appears to arise before female preferences during the early stages of allopatric speciation, and research suggests that male, but not female, preferences become reinforced upon secondary contact. In the current study, we aimed to determine whether the geographically widespread darter species Etheostoma zonale exhibits a signature of reinforcement, by comparing the strength of preference for conspecific mates between populations that are sympatric and allopatric with respect to a close congener, E. barrenense. We examined the strength of preference for conspecifics for males and females separately to determine whether the preferences of one or both sexes have been strengthened by reinforcement. Our results show that both sexes of E. zonale from sympatric populations exhibit stronger conspecific preferences than E. zonale from allopatric populations, but that female preferences appear to be more strongly reinforced than male preferences. Results therefore suggest that reinforcement of female preferences may promote behavioral isolation upon secondary contact, even in a genus that is characterized by pervasive male mate choice.
Project description:Reproductive character displacement is the adaptive evolution of traits that minimize deleterious reproductive interactions between species. When arising from selection to avoid hybridization, this process is referred to as reinforcement. Reproductive character displacement generates divergence not only between interacting species, but also between conspecific populations that are sympatric with heterospecifics versus those that are allopatric. Consequently, such conspecific populations can become reproductively isolated. We compared female mate preferences in, and evaluated gene flow between, neighbouring populations of spadefoot toads that did and did not occur with heterospecifics (mixed- and pure-species populations, respectively). We found that in mixed-species populations females significantly preferred conspecifics. Such females also tended to prefer a conspecific call character that was dissimilar from heterospecifics. By contrast, females from pure-species populations did not discriminate conspecific from heterospecific calls. They also preferred a more exaggerated conspecific call character that resembles heterospecific males. Moreover, gene flow was significantly reduced between mixed- and pure-species population types. Thus, character displacement (and, more specifically, reinforcement) may initiate reproductive isolation between conspecific populations that differ in interactions with heterospecifics.
Project description:Selection against hybridization can cause mating traits to diverge between species in sympatry via reproductive character displacement (RCD). Additionally, selection against interspecific fighting can cause aggressive traits to diverge between sympatric species via agonistic character displacement (ACD). By directly affecting conspecific recognition traits, RCD and ACD between species can also incidentally cause divergence in mating and fighting traits among populations within a species [termed cascade RCD (CRCD) and cascade ACD]. Here, we demonstrate patterns consistent with male-driven RCD and ACD in 2 groups of darters (orangethroat darter clade Ceasia and rainbow darter Etheostoma caeruleum). In both groups, males that occur in sympatry (between Ceasia and E. caeruleum) have higher levels of preference for mating and fighting with conspecifics over heterospecifics than do males from allopatry. This is consistent with RCD and ACD. We also found patterns consistent with CRCD and cascade ACD among species of Ceasia. Ceasia males that are sympatric to E. caeruleum (but allopatric to one another) also have heightened preferences for mating and fighting with conspecific versus heterospecific Ceasia. In contrast, Ceasia males that are allopatric to E. caeruleum readily mate and fight with heterospecific Ceasia. We suggest that RCD and ACD between Ceasia and E. caeruleum has incidentally led to divergence in mating and fighting traits among Ceasia species. This study is unique in that male preferences evolve via both RCD (male preference for conspecific females) and ACD (male preference to fight conspecific males) which leads to subsequent divergence among allopatric lineages.
Project description:Identifying the traits causing reproductive isolation and the order in which they evolve is fundamental to understanding speciation. Here, we quantify prezygotic and intrinsic postzygotic isolation among allopatric, parapatric, and sympatric populations of the butterflies Heliconius elevatus and Heliconius pardalinus. Sympatric populations from the Amazon (H. elevatus and H. p. butleri) exhibit strong prezygotic isolation and rarely mate in captivity; however, hybrids are fertile. Allopatric populations from the Amazon (H. p. butleri) and Andes (H. p. sergestus) mate freely when brought together in captivity, but the female F1 hybrids are sterile. Parapatric populations (H. elevatus and H. p. sergestus) exhibit both assortative mating and sterility of female F1s. Assortative mating in sympatric populations is consistent with reinforcement in the face of gene flow, where the driving force, selection against hybrids, is due to disruption of mimicry and other ecological traits rather than hybrid sterility. In contrast, the lack of assortative mating and hybrid sterility observed in allopatric populations suggests that geographic isolation enables the evolution of intrinsic postzygotic reproductive isolation. Our results show how the types of reproductive barriers that evolve between species may depend on geography.