Abortive meiosis in the oogenesis of parthenogenetic Daphnia pulex
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Most daphnid species adopt parthenogenesis and sexual reproduction differentially in response to varied environmental cues, resulting in the production of diploid progenies in both cases. Previous studies have reportedly suggested that daphnids produce their parthenogenetic eggs via apomixis; the nuclear division of mature oocytes should be an equational division similar to somatic mitosis. However, it seems premature to conclude that this has been unequivocally established in any daphnids. Therefore, the objective of our research was to precisely reveal the process and mechanism of parthenogenetic oogenesis and maintenance of diploidy in Daphnia pulex through histology, karyology, and immunohistochemistry. We found that, when a parthenogenetic egg entered the first meiosis, division was arrested in the early first anaphase. Then, two half-bivalents, which were dismembered from each bivalent, moved back to the equatorial plate and assembled to form a diploid equatorial plate. Finally, the sister chromatids were separated and moved to opposite poles in the same manner as the second meiotic division followed by the extrusion of one extremely small daughter cell (resembling a polar body). These results suggest that parthenogenetic D. pulex do not adopt typical apomixis. We hypothesize that D. pulex switches reproductive mode depending on whether the egg is fertilized or not.
KeywordsDaphnia pulex Parthenogenesis Automixis Diploidy maintenance
We are grateful to Dr. Masakane Yamashita and his team, Faculty of Advanced Life Science, Hokkaido University, and Dr. Toru Miura and his team, Graduate School of Environmental Sciences, Hokkaido University, for helpful comments and suggestions on the study. We also thank the members of the Tochinai laboratory for their helpful advice and discussions. Chizue Hiruta was supported by a JSPS Research Fellowship for Young Scientists. This work was supported by a Grant-in-Aid for Exploratory Research, financed by the Ministry of Education, Culture, Sports, Science and Technology, Japan, to Shin Tochinai.
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