Stem cells in the reproductive strategy of colonial rhizocephalan crustaceans (Crustacea: Cirripedia: Rhizocephala)

Abstract

Summary In vivo, histological, histochemical, immunochemical and ultrastructural investigations were performed of the colonial internae of Peltogasterella gracilis, Polyascus (Sacculina) polygenea, and Thylacoplethus isaevae (all infesting decapods). It was shown that asexual reproduction in these species occurs through the budding of stolon-like structures. Undifferentiated stem cells were found inside the stolons. They take part in the morphogenesis of the earliest buds, and later migrate to the developing ovary as primary germ cells. The stem cells in P. polygenea, P. gracilis and Th. isaevae selectively express alkaline phosphatase activity, a well known histochemical marker for mammalian embryonic stem cells in vivo and in vitro. It is shown in P. gracilis that stem cells also selectively express proliferating cell nuclear antigen (PCNA), a cellular marker for cell reproduction. The reproductive strategy in colonial rhizocephalans includes a three-level cascade: asexual reproduction by budding in interna; repeated development of multiple externae; and, in some cases, repeated cycles of sexual reproduction in each externa, resulting in large numbers of hatching larvae and allowing the parasite to infest a large proportion of the host population. Thus, sexual and asexual generations alternate in the life-cycle of the colonial species; stem cells evidently contribute the source of cells in asexual as well as sexual reproduction.