UNISEXUAL/BISEXUAL BREEDING COMPLEXES IN POECILIIDAE: WHY DO MALES COPULATE WITH UNISEXUAL FEMALES?

Persistent Copulation in Asexual Female Potamopyrgus antipodarum: Evidence for Male Control with Size-Based Preferences

Transitions from sexual to asexual reproduction provide a useful context for investigating the evolutionary loss of nonfunctional traits. It is often assumed that useless behaviors or structures will degrade, but this process is poorly understood. Potamopyrgus antipodarum is an ancestrally sexual New Zealand freshwater snail characterized by numerous independent transitions to asexual all-female lineages. The availability of multiple independently-derived asexual lineages of various time since derivation from sexual ancestors means that the P. antipodarum system is well-suited for the study of trait loss related to mating behavior and copulation. Here, we asked whether mating behavior in asexual female P. antipodarum degrades with increasing asexual lineage age. While copulation frequency did not differ in females from old versus young asexual lineages, post hoc analyses indicated that it was instead positively associated with mean lineage female size. We observed that female P. antipodarum take a passive physical role in copulatory interactions, indicating that female behavior may not be a useful variable for detection of sex-related vestigialization in this system. Instead, males seem to be in proximate control of copulation frequencies, meaning that male mating behavior may be a primary determinant of the expression of mating behavior in asexual female P. antipodarum.

Feeding rates in the sailfin molly Poecilia latipinna and its coexisting sexual parasite, the gynogenetic Amazon molly Poecilia formosa

Feeding rates of the gynogenetic Amazon molly Poecilia formosa and one of its sexual hosts, the sailfin molly Poecilia latipinna, were measured under winter and summer temperature conditions. Food consumption of the unisexual P. formosa in winter conditions was significantly higher than that of P. latipinna, and it is hypothesized that the resulting food stress might have an important influence on the population composition of these closely related fishes via higher winter mortality in P. formosa.

How populations persist when asexuality requires sex: the spatial dynamics of coping with sperm parasites

The twofold cost of sex implies that sexual and asexual reproduction do not coexist easily. Asexual forms tend to outcompete sexuals but may eventually suffer higher extinction rates, creating tension between short- and long-term advantages of different reproductive modes. The 'short-sightedness' of asexual reproduction takes a particularly intriguing form in gynogenetic species complexes, in which an asexual species requires sperm from a related sexual host species to trigger embryogenesis. Asexuals are then predicted to outcompete their host, after which neither species can persist. We examine whether spatial structure can explain continued coexistence of the species complex, and assess the evidence based on data on the Amazon molly (Poecilia formosa). A modification of the Levins metapopulation model creates two regions of good prospects for coexistence, connected by a region of poorer patch occupancy levels. In the first case, mate discrimination and/or niche differentiation keep local extinction rates low, and most patches contain both species; the other possibility resembles host-parasite dynamics where parasites frequently drive the host locally extinct. Several dynamical features are counterintuitive and relate to the parasitic nature of interactions in the species complex: for example, high local extinction rates of the asexual species can be beneficial for its own persistence. This creates a link from the evolution of sexual reproduction to that of prudent predation.

The Effect of Assortative Mating on the Coexistence of a Hybridogenetic Waterfrog and Its Sexual Host

In central Europe, the hybridogenetic waterfrog Rana esculenta, a hybrid between Rana ridibunda and Rana lessonae, lives in sympatry with one of its parental species, the poolfrog Rana lessonae. As R. esculenta has to backcross constantly with R. lessonae in order to produce viable offspring, this coexistence is obligatory for R. esculenta. Since R. esculenta has a higher primary fitness than R. lessonae, a mechanism is required that prevents the hybrid from driving the parental species, and hence itself, to extinction. Here, we present an analytical model and a computer simulation that investigate whether assortative mating can operate as a such a control mechanism. Our results show that assortative mating is very effective in regulating coexistence in such a hybrid-host system. This is particularly true when choice is affected by the proportion of the two male types in the population. Furthermore, we could show that even if the species composition in a mixed hybrid-host population may be largely influenced by differences in life-history parameters, assortative mating still plays a very important role by stabilizing coexistence. Thus, mating behavior turns out to be more important for the populations dynamics of hybridogenetic waterfrog systems than previously assumed.