How Does the Alteration of Meiosis Evolve to Parthenogenesis?- Case Study in a Water Flea, Daphnia pulex -

Most daphnid species reproduce parthenogenetically as well as sexually, resulting in the production of diploid progenies in both cases. In natural populations, parthenogenesis is the common mode of reproduction, and parthenogenetic offspring are all female. However, in response to certain environmental conditions, such as crowding or seasonal change, male offspring are also produced parthenogenetically, and sexual reproduction occurs (Hebert, 1978). Although they switch between parthenogenesis and sexual reproduction in response to environmental conditions, little is known about the molecular and cytological mechanisms switching and governing each reproductive mode. It can be interpreted that D. pulex develops a reproductive strategy utilizing ‘parthenogenesis’, which has high reproductive power, and ‘sexual reproduction’, which generates genetic diversity, in response to different environments. These theoretical studies have been made on evolutionary mechanism of reproductive modes (Decaestecker et al., 2009); however, practically no study analyzing the evolutionary mechanism while taking the developmental constraints into consideration has so far been conducted. Our understanding of the evolution of reproductive strategy would increase once we precisely clarify the developmental gene programs operating there. We have recently started to develop D. pulex as an experimental model for studying oogenesis and developmental mechanisms during evolution.