Development of polarized female germline cysts in the polychaete, Ophryotrocha labronica.

Polarized oogenic cysts are clonal syncytia in which cellular compartments (cystocytes) differentiate as either oocytes or nurse cells-supportive germ cells that transport material to the oocyte through cytoplasmic bridges. Although polarized cysts have evolved independently in several animal phyla, most research on this phenomenon has focused on insects, particularly Drosophila melanogaster. To provide a foundation for comparative studies of cyst polarization, we have examined the coelomic development of polarized cysts in an annelid worm, Ophryotrocha labronica, using a variety of microscopic techniques. This species has elegantly simple cysts, each consisting of two interconnected cells (one oocyte and one nurse cell) free of somatic follicular tissue. These cysts arise from segmental ovaries, but are released to the coelom early in oogenesis, where they carry out most of their development. Although the oocyte remains transcriptionally active through oogenesis, nurse cells in this species are robust biosynthetic factories, with highly polyploid nuclei, enormous nucleoli, numerous mitochondria, and abundant rough endoplasmic reticulum. Ultimately, the nurse cell dumps its cytoplasm into oocyte, detaches, and undergoes programmed cell death. Although in insects, and possibly some polychaetes, the overall polarity of the cyst is linked to the specification of future embryonic axes in the oocyte, in Ophryotrocha this appears not to be the case. Our findings provide a detailed analysis of cyst development, addressing earlier misconceptions in the literature and adding new insights. This foundation shows the potential of Ophryotrocha labronica as a model system for further experimental analysis of nurse cell-oocyte differentiation.