Life-History Changes That Accompany the Transition From Sexual to Parthenogenetic Reproduction in Drosophila Mercatorum

Abstract

In spite of the predicted genetic and ecological costs of sex, most natural populations maintain sexual reproduction, even those capable of facultative parthenogenesis. Unfertilized eggs from natural populations of Drosophila mercatorum occasionally develop into viable adults, but obligately parthenogenetic populations are unknown in this species. To evaluate the microevolutionary forces that both favor and constrain the evolution of parthenogenesis in D. mercatorum, we have measured parthenogenetic rates across a natural, sexually reproducing population and characterized the life-history changes that accompany the transition from sexual to parthenogenetic reproduction in laboratory strains. A highly significant difference in parthenogenetic rate was found between two populations in close geographic proximity, with increased rate found with lower population density. Laboratory strains of parthenogenetic females suffered increased mortality and reduced egg viability relative to their virgin counterparts from a sexual strain. Lifetime egg production was similar across all strains, but a shift in peak egg production to an earlier age also occurred. The combination of these life-history traits resulted in a higher net reproductive value for sexual females, but because they also had a longer generation time, intrinsic rate of increase was not as dramatically different from parthenogenetic females. In environments with high early mortality, there may be no fitness disadvantage to parthenogenesis, but the predicted ecological advantage of a twofold increase in intrinsic rate of increase was not realized. These results support the theory of Stalker (1956) that parthenogenesis is favored in environments in which sexual reproduction is difficult or impossible.

What does the geography of parthenogenesis teach us about sex?

“…[99] in mixed populations of sexual and asexual Drosophila). The disproportionate presence of asexual plants in newly deglaciated areas compared with their sexual parents [90], suggests that the wave of recolonization from glacial refugia was largely dominated by asexuals [3].…”

Transitioning from egg to embryo: Triggers and mechanisms of egg activation

“…Less understood is occasional or sporadic parthenogenesis (thycoparthenogenesis), in which unfertilized eggs laid by virgin females occasionally give rise to progeny. For example, in some strains of Drosophila mercatorum, 8 -10% of unfertilized eggs are able to develop into viable females (Templeton, 1979;Kramer and Templeton, 2001).…”

Gene flow between arrhenotokous and thelytokous populations of Venturia canescens (Hymenoptera)

“…Although empirical support for these explanations is not overwhelming (but see: Lamb and Willey, 1979;Leslie and Vrijenhoek, 1980;Lively, 1987;Lively et al, 1990;Jokela et al, 1997;Corley and Moore, 1999;Storhas et al, 2000;Kramer and Templeton, 2001); the robustness of sexual reproduction against asexual competitors at evolutionary and ecological time scales is at least theoretically underpinned (Lynch, 1984;Kondrashov, 1993;Charlesworth et al, 1993;Hurst and Peck, 1996;Peck et al, 1997;Peck et al, 1998;Barton and Charlesworth, 1998;West et al, 1999).…”