Abstract
In favorable conditions, Daphnia magna undergoes parthenogenesis to increase progeny production in a short time. However, in unfavorable conditions, Daphnia undergoes sexual reproduction instead and produces resting eggs. Here, we report live observations of the oviposition process in Daphnia magna. We observed that the cellular contents flowed irregularly through the narrow egg canal during oviposition. Amorphous ovarian eggs developed an oval shape immediately after oviposition and, eventually, a round shape. Oviposition of resting eggs occurred in a similar way. Based on the observations, we propose that, unlike Drosophila eggs, Daphnia eggs cannot maintain cytoplasmic integrity during oviposition. We also determined that the parthenogenetic eggs were activated within 20 min, as demonstrated by vitelline envelope formation. Therefore, it is plausible that the eggs of Daphnia magna may be activated by squeezing pressure during oviposition.
Highlights
Crustaceans of the genus Daphnia are used as model organisms in the fields of toxicology, ecology, and evolutionary biology [1]
The results showed that the contents of the resting eggs passed through a narrow egg canal and accumulated outside the ovary, as observed for parthenogenetic eggs (Fig 3A, Fig 3B, S3 Movie and S4 Movie)
Our results revealed that vitelline envelope (VE) formation took some time in the parthenogenetic eggs of Daphnia magna
Summary
Crustaceans of the genus Daphnia are used as model organisms in the fields of toxicology, ecology, and evolutionary biology [1]. Daphnia species exhibit an alternative reproductive mode. They usually undergo parthenogenesis to generate diploid female offspring without a fertilization event [2]. Parthenogenesis is maintained when surrounding conditions are favorable. When Daphnia are exposed to unfavorable conditions, male offspring are parthenogenetically produced, and sexual reproduction proceeds [2,3,4]. Various environmental factors can affect this process, such as temperature, photoperiod, food availability, and population density [2, 3, 5]
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