LE3 .A278 1999

1999

Gibson, Glenys

Acadia University

Master of Science

Masters

Biology

Biology

Nurse eggs are commonly produced by spionid polychaetes and gastropod molluscs and serve as sources of extra-embryonic nutrition for developing young. Although the influence of nurse eggs on offspring development has been well documented, the origin of nurse eggs is not understood. I examined nurse egg production in the spionid Boccardia proboscidea using brightfield and fluorescence microscopy. Nurse eggs in this species appear to arise as do viable oocytes. After spawning, nurse eggs produce fertilization envelopes indicating that development has been activated. Nurse eggs are also capable of producing polar bodies which indicates completion of meiosis in those eggs. In addition, I examined ultrastructural changes that accompany nurse egg formation using fluorescence and electron microscopy. Nurse eggs in B.proboscidea maintain the fertilization envelope with surface granules; however, microvilli are lost, and the plasma membrane invaginates and compartmentalizes the cytoplasm into vesicles that are ingested by developing siblings. Vesicle formation in nurse eggs involves the energy producing and synthetic organelles, mitochondria and endoplasmic reticulum. Additionally, nuclear DNA in nurse eggs is cleaved into fragments that stain with the apoptosis-specific fluorescent probe BODIPY-dUTP, which are packaged into the cytoplasmic vesicles. These morphological changes in B.proboscidea nurse eggs are indicative of apoptosis, which suggests that this process is involved in nurse egg formation in this species. Thus, nurse egg production in B.proboscidea is an active developmental process and not simply an artifact of sperm limitation.

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