The polyembryonic wasp Copidosoma floridanum produces two castes by differentially parceling the germ line to daughter embryos during embryo proliferation

Abstract

Eggs of the polyembryonic wasp Copidosoma floridanum undergo a clonal phase of proliferation, which results in the formation of thousands of embryos called secondary morulae and two castes called reproductive and soldier larvae. C. floridanum establishes the germ line early in development, and prior studies indicate that embryos with primordial germ cells (PGCs) develop into reproductive larvae while embryos without PGCs develop into soldiers. However, it is unclear how embryos lacking PGCs form and whether all or only some morulae contribute to the proliferation process. Here, we report that most embryos lacking PGCs form by division of a secondary morula into one daughter embryo that inherits the germ line and another that does not. C. floridanum embryos also incorporate 5-bromo-2′-deoxyuridine (BrdU), which allows PGCs and other cell types to be labeled during the S phase of the cell cycle. Continuous BrdU labeling indicated that all secondary morulae cycle during the proliferation phase of embryogenesis. Double labeling with BrdU and the mitosis marker anti-phospho-histone H3 indicated that the median length of the G2 phase of the cell cycle was 18 h with a minimum duration of 4 h. Mitosis of PGCs and presumptive somatic stem cells in secondary morulae was asynchronous, but cells of the inner membrane exhibited synchronous mitosis. Overall, our results suggest that all secondary morulae contribute to the formation of new embryos during the proliferation phase of embryogenesis and that PGCs are involved in regulating both proliferation and caste formation.