Email: bazinetc@stjohns.edu
Cell and Evolutionary Biology of
Gametogenesis: Where Did Sex Come From?
Germ cells, the family of "sex cells" that give rise to eggs and
sperm in higher animals, are particularly interesting to cell and
evolutionary biologists for a number of reasons. First, unlike the
somatic cells that make up the majority of our bodies, they are
potentially immortal. Fusion of germ cells at fertilization
constitutes the first cell of the next generation. Second, the
pathways from undifferentiated germline stem cell to highly
specialized sperm or egg involve extraordinary changes in cell
structure, entailing extensive, coordinated changes in many
organelle systems. This affords the cell biologist with the
opportunity to probe inter-organelle communications. Since all
contemporary biological pathways and life cycles are presumably
evolved from simple, more primitive biological activities, we
expect that a good understanding of the structural basis of
gametogenesis will also yield insights into how germ cells have
evolved to behave the way they do. We are using a combination of
genetic, molecular biological, and structural approaches to
understand the extensive reorganization of the cysts that give rise
to sperm in the fruit fly, Drosophila melanogaster. In all
organisms, cytokinesis following each of the mitotic and meiotic
nuclear divisions in the cell lineages leading to sperm is
incomplete. The result is that sperm morphogenesis takes place in a
large communal cytoplasm. Only very late in sperm formation are the
individual cells separated from each other and encased in their own
membranes. We are studying the specialized cellular machinery of
sperm individualization in Drosophila. Surprisingly, mitochondria
appear to play an active and central role in this process, and in a
number of other specialized pathways of germ cells. We are working
on a hypothesis that these otherwise unexplained mitochondrial
behaviors reflect an endosymbiotic origin to gametogenic pathways.
Since mitochondria are descended from microbes that invaded ancient
proto-eukaryotic cells, a simple way of rationalizing these
mitochondrial behaviors in the germ cell life cycle is that they
are evolved from the life cycle of the ancient endosymbiont from
which mitochondria are descended. In recent years, much work has
been done by cell biologists to understand the mechanisms by which
several classes of bacteria, having gained access to the cytoplasm
of cells they are parasitizing, "hijack" the actin based
cytoskeleton and use it to move about within the host cell.
Detailed studies have shown that these bacteria promote the
polymerization of actin fibers into a comet-tail like structure
that propels them through the cell. We are exploring the hypothesis
that sperm mitochondria, during Drosophila sperm individualization,
recapitulate the "comet-tail" motility of their endosymbiotic
ancestor.
Recent Publications
Fabrizio, J.J., Hime, G., Lemmon, S.K. and C. Bazinet (1998)
Genetic dissection of sperm individualization in Drosophila
melanogaster. Development 125:1833-1843.
Vasyukevich, K. and C. Bazinet (1999) A Drosophila clathrin
light chain gene: sequence, mapping, and absence of neuronal
specialization. DNA and Cell Biology 18:235-241.