Assortative mating between two distinct micro-allopatric populations of Littorina saxatilis (Olivi) on the northeast coast of England

Size assortative mating is a common invertebrate mating pattern and is usually accompanied by male and female sexual selection, and these three behaviours can contribute to reproductive isolation. Two distinct populations of the marine prosobranch Littorina saxatilis, H and M, occur within 15 m of each other on the same shore. Previous studies have demonstrated that these two forms have different reproductive strategies and that the rare hybrids between the two forms show evidence of reproductive dysfunction and hence are less fit than the assumed parental forms. In both populations, female shell height was shown to be a predictor of the number of embryos contained within the brood pouch. The mean shell height of the M population was significantly larger than that of the H population, and the M population matures at a larger shell size than the H population. The two populations show complete assortative mating to type in the field, and occupy different microhabitats on the same shore. Therefore, laboratory-based experiments were performed to determine if assortative mating was maintained in sympatry and also to determine the effect of population density on mate choice. The males of both populations showed sexual selection for female size, choosing to mate with females approximately 10% larger than themselves from an assortment of female sizes. The M population showed complete assortative mating to type, irrespective of the density of H and M females, whereas at low densities the H males did occasionally mate with M females. The role of assortative mating and reinforcement (due to natural selection acting against the less fit hybrids), in maintaining the partial reproductive barrier between the two populations is discussed.     

Frequency- and density-dependent sexual selection in natural populations of Galician Littorina saxatilis Olivi

Galician exposed shore populations of the direct developing periwinkle Littorina saxatilis are strikingly polymorphic, with an ornamented and banded upper shore form and a smooth and unbanded lower shore form. Intermediates between the two pure forms occur in a narrow mid shore zone together with the parental forms. We have previously shown that the two pure forms share the same gene pool but that mating between them is non-random. This is due to a non-random microdistribution in the zone of overlap, and also to assortative mating. In this study we present data which show that intermediate (hybrid) females mate less often than pure females in micropatches dominated by either of the pure forms, but not in micropatches in which the two pure forms are equally common. Thus, sexual fitness in intermediate females depends on the frequency of both pure morphs. Furthermore, sexual selection against intermediate females also varies with the densities of snails within each micro patch. The biological mechanisms which may explain this particular reduction of female hybrid fitness are discussed. Assortative mating between the pure morphs is sometimes almost complete, while both morphs do not mate the intermediates assortatively. In the light of this, sexual selection against intermediate females may contribute considerably to restrict gene flow between the pure forms.     

Sexual size dimorphism and assortative mating in Carolina Wrens

ABSTRACT.   Sexual size dimorphism (SSD) may be due to sexual and natural selection, but identifying specific mechanisms that generate such dimorphism in a species is difficult. I examined SSD in Carolina Wrens ( Thryothorus ludovicianus ) by examining (1) the degree of SSD in the population and between pairs using five morphometrics, (2) assortative mating patterns based on size and age, and (3) relationships between size and longevity. Analysis revealed that males were significantly larger than females in all body measurements. For example, mass, bill, and wing measurements yielded a canonical variable that permitted separation of the sexes and linear classification functions correctly determined the sex of 95% (238/250) of all wrens measured. No evidence was found to suggest that SSD was related to resource partitioning. However, assortative mating trends based on morphometrics (e.g., wing length), positive associations between longevity and morphometrics (e.g., wing length in females and body size in males), and intense male-male contests for territorial resources year-round provide evidence that sexual selection may contribute to SSD in Carolina Wrens.     

Sexual selection on phenotypic traits in a hybrid zone of Littorina saxatilis (Olivi)

Step clinal transitions in inherited character(s) between genetically distinct populations are usually referred to as hybrid zones. An example is found in the population of the intertidal snail Littorina saxatilis in Galicia (NW Spain). We studied the shape of the overall fitness surface for sexual selection in this hybrid zone, and the position of hybrids and pure morphs on this surface. We found that sexual divergent selection acted on a combination of phenotypic traits separating the pure morphs, and therefore that sexual selection contributed to morph differentiation. The average fitness of hybrids as a group was not significantly different from that of the pure morphs, but they did show divergent sexual selection in some traits. These results are in agreement with a model of divergent selection favouring both the pure morph as well as those hybrids most resembling each morph. The finding of divergent selection is remarkable because quadratic selection gradients are usually weak in nature.     

Mechanisms of incomplete prezygotic reproductive isolation in an intertidal snail: testing behavioural models in wild populations

Two morphs (ecotypes) of the marine snail Littorina saxatilis coexist along Galician exposed rocky shores. They hybridize, but gene flow is impeded by a partial prezygotic reproductive barrier, and we have earlier suggested that this is a case of incipient sympatric speciation. To assess the mechanisms of prezygotic reproductive isolation, we estimated deviations from random mating (sexual selection and sexual isolation) of sympatric snails in 13 localities on the shore, and performed mate choice experiments in the laboratory. We also investigated the microdistribution of both morphs over patches of barnacles and blue mussels in the hybridization zone. We used computer simulations to separate the mechanisms contributing to reproductive isolation. On the shores sampled, male–female pairs were strongly assortative both with respect to morphs (mean Yule's V = 0.77) and size (mean Pearson's r = 0.47). In the laboratory, males of both morphs mounted other snails and mated other males and juveniles at random. However, mature females of equal sizes mated assortatively with respect to morph. The two morphs were nonrandomly distributed over barnacle and mussel patches in the hybridization zone. Monte Carlo simulations showed that this microdistribution could explain about half the morph and size relationships in male–female pairs, while a simple rejection mechanism, rejecting the first 1–3 mates if they were of contrasting morphs, accounted for the remaining part of the reproductive isolation, and for parts of the size relationships found between mates. A size discriminant mate choice mechanism may also, to a lesser extent, contribute to the sexual isolation. Sexual selection was observed for female size (larger ones being favoured) and among certain morphs, but distinct biological mechanisms may cause these processes.     

Seasonal migration promoting assortative mating in Littorina brevicula on a boulder shore in Japan

Littorina brevicula Philippi is one of the most common snails found in the upper intertidal zone of Japan. In Amakusa, some of the population of L. brevicula migrate to the lower zone in the winter, while the rest stay in the upper zone. Thus, during the winter, which is its reproductive season, the population of L. brevicula divides into two sub-populations. This leads to a hypothesis that the migration pattern in winter is genetically controlled and this behavioural dimorphism is maintained by reproductive isolation between the two sub-populations. In order to test this hypothesis, the following three points were investigated: (1) whether the same snails migrate in a similar way every winter, (2) whether there is a significant tidal level preference in snails, and (3) whether reproductive isolation occurs between the two sub-populations. The results showed (1) the migration behaviour of each snail was consistent over two successive winters, i.e. the same group of snails migrated downward every winter and the same group of snails stayed in the upper zone every winter, (2) transplanted snails moved toward the original zones where they were caught, suggesting that the snails actively selected their tidal zone in winter, and (3) most of the snails copulated within each sub-population. Therefore, reproductive isolation between the two sub-populations was considered to be established to some extent by the dimorphic migration behaviour. In conclusion, the migratory behaviour of L. brevicula is determined separately for each individual and might be genetically controlled, and the behavioural dimorphism may be maintained by partial reproductive isolation between the two sub-populations.     

The evolutionary forces maintaining a wild polymorphism of Littorina saxatilis: model selection by computer simulations

Two rocky shore ecotypes of Littorina saxatilis from north-west Spain live at different shore levels and habitats and have developed an incomplete reproductive isolation through size assortative mating. The system is regarded as an example of sympatric ecological speciation. Several experiments have indicated that different evolutionary forces (migration, assortative mating and habitat-dependent selection) play a role in maintaining the polymorphism. However, an assessment of the combined contributions of these forces supporting the observed pattern in the wild is absent. A model selection procedure using computer simulations was used to investigate the contribution of the different evolutionary forces towards the maintenance of the polymorphism. The agreement between alternative models and experimental estimates for a number of parameters was quantified by a least square method. The results of the analysis show that the fittest evolutionary model for the observed polymorphism is characterized by a high gene flow, intermediate-high reproductive isolation between ecotypes, and a moderate to strong selection against the nonresident ecotypes on each shore level. In addition, a substantial number of additive loci contributing to the selected trait and a narrow hybrid definition with respect to the phenotype are scenarios that better explain the polymorphism, whereas the ecotype fitnesses at the mid-shore, the level of phenotypic plasticity, and environmental effects are not key parameters.     

Shell colour polymorphism associated with substrate colour in the intertidal snail Littorina saxatilis Olivi (Prosobranchia: Littorinidae)

Littorina saxatilis Olivi (1792), the rough winkle, is highly polymorphic in shell colour. Shell colour frequencies were studied at six locations in south-western Wales, U.K., each at a geological contact between red sandstone and grey limestone or volcanic rock. At each site shell colour frequencies were determined in samples from the contact zone and on red or grey rock on either side. Highly significant associations were found between shell colour frequencies and substrate colour. Grey shells were always more common on grey rock than on red rock, and brown shells were usually more common on red than on grey rock, suggesting selection for cryptic colouration. Shell colour frequency differences were also found between replicate samples taken only 5 m apart from the same kind of rock, and between samples from the same kind of rock at the six study sites. These latter differences suggest that selection for camouflage is not the only factor involved in maintaining shell colour polymorphism in this species.     

A striking example of the founder effect in the mollusc Littorina saxatilis

Two South African populations of Littorina saxatilis were examined by starch-gel electrophoresis at 16 enzyme loci and compared with 13 populations of North Atlantic saxatilis from both American and European coasts, and with six British populations of the closely related species Littorina arcana. The South African animals showed a severely reduced heterozygosity (– 0.052) compared with Atlantic populations of saxatilis ( = 0.181), and the mean genetic distance between the two areas was high ( = 0.203) compared with distances within the North Atlantic saxatilis populations (D = 0.034). In fact, the saxatilis from South Africa were genetically more distant from the North Atlantic samples of L. saxatilis than were the arcana from British shores. The reduced genetic heterozygosity and genetic divergence of the South African populations is attributed to founder effects following a postulated recent introduction by man.     

Sexual size dimorphism and assortative mating for morphological traits in Passer domesticus

In this study, assortative mating for different morphological traits was studied in a captive population of house sparrows (Passer domesticus). Males were larger than females. Assortative mating was found for tail length, wing length and general body size. Males with larger badge size mated with females with longer tails. The strongest assortative mating occurred for tail length (r=0.77), and this assortative mating remained significant after controlling for wing length, mass and tarsus length, suggesting that it was not an artefact of assortative mating for body size. The possibility of sexual selection for tail length in the house sparrow is discussed.