Testing for mating isolation between ecotypes: laboratory experiments with lake, stream and hybrid stickleback

Mating isolation is a frequent contributor to ecological speciation – but how consistently does it evolve as a result of divergent selection? We tested for genetically based mating isolation between lake and stream threespine stickleback (Gasterosteus aculeatus L.) from the Misty watershed, Vancouver Island, British Columbia. We combined several design elements that are uncommon in the studies of stickleback mate choice: (i) we used second‐generation laboratory‐reared fish (to reduce environmental and maternal effects), (ii) we allowed for male–male competitive interactions (instead of the typical no‐choice trials) and (iii) we included hybrids along with pure types. Males of different types (Lake, Inlet, hybrid) were paired in aquaria, allowed to build nests and then exposed sequentially to females of all three types. We found that Lake and Inlet males differed in behaviours thought to influence stickleback mate choice (inter‐ and intra‐sexual aggression, display and nest activities), whereas hybrids were either intermediate or apparently ‘inferior’ in these behaviours. Despite these differences, Lake and Inlet fish did not mate assortatively and hybrid males did not have a mating disadvantage. Our study reinforces the noninevitability of mating isolation evolving in response to ecological differences and highlights the need to further investigate the factors promoting and constraining progress towards ecological speciation.     

The association between the caprellid Pariambus typicus Krøyer (Crustacea,Amphipoda) and ophiuroids

The work of Kazutsugu Hirayama over the past 25 yearspromoted the wide use of the marine rotifer Brachionusplicatilis as an experimental model in zooplanktonecology. His reports about the nature of geneticvariation in the B. plicatilis complex stimulated usto investigate how mate recognition maintains speciesboundaries. For the past several years, we haveexamined chemical communication between female andmale B. plicatilis. Here we report on the comparativebinding of polyclonal antibody against the materecognition pheromone (MRP) to three B. plicatilisstrains and three B. rotundiformis strains.Quantification of anti-MRP binding permitsinvestigation of how the female mating signal differsamong closely related Brachionus species and strains.Antibody binding reflects differentiation independentof the male receptor which has been describedelsewhere. Anti-MRP bound to females of all sixstrains and was localized in the corona. Antibodybinding greatly reduced mating in all three B. plicatilis strains. However, antibody bindingsignificantly reduced mating in only one of the B. rotundiformis strains. The MRP of both species has asimilar molecular weight, but the differential bindingsuggests that the mate recognition pheromone onfemales has differentiated in B. plicatilis and B. rotundiformis.     

Species recognition by male swordtails via chemical cues

Species recognition can often play a key role in female matingpreferences. Far less is known about conspecific mate recognitionfrom the male perspective. In many closely related taxa, femalesexhibit few obvious visual differences and males might haveto attend to chemical cues in mate recognition, a possibilitythat has rarely been explored in vertebrates. Here, we examinemale species recognition via odor cues in the swordtail fish,Xiphophorus birchmanni. In dichotomous choice experiments wefirst tested whether males respond to female odor cues. We foundthat males were attracted to conspecific female odor and thoseof a related allopatric congener, Xiphophorus malinche, overa water control. Males did not, however, respond to the femaleodor of the more distantly related sympatric platyfish, Xiphophorusvariatus. We then gave male X. birchmanni the choice betweenconspecific and heterospecific female stimuli. Males, in thisscenario, significantly preferred the conspecific odor whenthe alternative was platyfish. However, when offered odor cuesof X. malinche, male X. birchmanni actually preferred the heterospecificfemale cue. The complex array of preferences reported here,previously documented only in females, underscores the needto consider the behavior of both sexes in dictating actual matingoutcomes.     

Divergent patterns of diversification in courtship and genitalic characters of Timema walking‐sticks

Understanding the patterns of diversification in sexual traits and the selection underlying such diversification represents a major unresolved question in evolutionary biology. We examined the phylogenetic diversification for courtship and external genitalic characters across ten species of Timema walking‐sticks, to infer the tempos and modes of character change in these sexual traits and to draw inferences regarding the selective pressures underlying speciation and diversification in this clade. Rates of inferred change in male courtship behaviours were proportional to speciation events, but male external genitalic structures showed a pattern of continuous change across evolutionary time, with divergence proportional to branch lengths. These findings suggest that diversification of courtship behaviour is mediated by processes that occur in association with speciation, whereas diversification of genitalia occurs more or less continuously, most likely driven by forces of sexual selection.     

Sexual imprinting on ecologically divergent traits leads to sexual isolation in sticklebacks

During sexual imprinting, offspring learn parental phenotypes and then select mates who are similar to their parents. Imprinting has been thought to contribute to the process of speciation in only a few rare cases; this is despite imprinting's potential to generate assortative mating and solve the problem of recombination in ecological speciation. If offspring imprint on parental traits under divergent selection, these traits will then be involved in both adaptation and mate preference. Such 'magic traits' easily generate sexual isolation and facilitate speciation. In this study, we show that imprinting occurs in two ecologically divergent stickleback species (benthics and limnetics: Gasterosteus spp.). Cross-fostered females preferred mates of their foster father's species. Furthermore, imprinting is essential for sexual isolation between species; isolation was reduced when females were raised without fathers. Daughters imprinted on father odour and colour during a critical period early in development. These traits have diverged between the species owing to differences in ecology. Therefore, we provide the first evidence that imprinting links ecological adaptation to sexual isolation between species. Our results suggest that imprinting may facilitate the evolution of sexual isolation during ecological speciation, may be especially important in cases of rapid diversification, and thus play an integral role in the generation of biodiversity.     

Male fruit flies learn to avoid interspecific courtship

Experimental data suggest, and theoretical models typicallyassume, that males of many fruit flies (Drosophila spp) areat least partially indiscriminate while searching for mates,and that it is mostly the females who exert selective mate choice,which can lead to incipient speciation. Evidence on learningby male D. melanogaster in the context of courtship, however,raises the possibility that the initially indiscriminate malesbecome more selective with experience. I tested this possibilityby comparing the courtship behavior of male D. melanogasterexperienced at courting females of the closely related species,D. simulans, and inexperienced males. I found that comparedwith the inexperienced males, the males experienced with courtingD. simulans females showed significantly lower courtship towardfemale D. simulans. Both male treatments, however, showed virtuallyidentical courtship durations with female D. melanogaster. Theseresults indicate that male fruit flies adaptively refine theircourtship behavior with experience and suggest that the malescontribute more to assortative mating and incipient speciationthan is commonly assumed.