Cell division is one of the most fundamental processes of life, and the core molecular mechanisms that guide cells through the cellular events of growth, DNA replication, and division into two daughter cells are evolutionarily conserved from yeast to humans. However while we now understand the basics of this core program, we know much less about the ways that the core cycle is modified and regulated by cell specific factors. In the Shakes' lab we are investigating two aspects of this larger question:
How is the meiotic cell cycle that gives rise to sperm and oocytes coordinately regulated and/or modified by the distinct developmental programs that give rise to either large, nutrient rich oocytes or small, motile sperm?
How have modifications to the meiotic cell cycle and cell division programs enabled evolutionarily related species to adapt distinct reproductive modes?
By design, sperm cells are small, motile, and transcriptionally inactive. Thus a key step in the developmental program of making a sperm involves prepackaging components that need to be synthesized before the turnoff of transcription and stored for later use. Nematode spermatogenesis provides an extreme example of this process; bulk transcription ceases prior to the meiotic divisions and translation ceases immediately after anaphase II. In particular, spermatocytes must synthesize and prepackage a small filament forming protein called the major sperm protein (MSP). Within mature nematode sperm, MSP serves both as the protein that drives sperm motility and as a signaling protein that triggers oocyte maturation and the physical process of ovulation. Our lab has recently identified several factors involved in both the assembly and sequestration of MSP into large, macromolecular structures called fibrous bodies. Furthermore, in a link to our cell division interests, mutants with defects in MSP sequestration exhibit additional defects in meiotic chromosome segregation. These studies promise to reveal both nematode-specific fertility factors that could be targets for nematode control as well as novel insights into the process of cytoskeletal remodeling.
Evo/Cell: Cellular insights from comparative studies of other nematode species
In a second line of investigation, we are examining the program of spermatogenesis in other nematode species. The goal of this comparative evolutionary approach is to both identify core elements that have been conserved across wide evolutionary distances as well as the many "variations on a theme" that expand our notions of what is possible with a similar "genetic toolbox". In a recent study, we discovered how a modification to the program of spermatogenesis enables males of one particular species to sire almost exclusively daughters. We are now studying the details of spermatogenesis in other closely related species to better understand how this modification evolved.
In our favorite lab organism, the nematode worm Caenorhabditis elegans, we can view gametogenesis either through the transparent body wall of the adult worms or in immunocytological preparations of isolated gonads.
We approach these questions from a variety of angles. First and foremost, we use the powerful genetics of C. elegans to identify and isolate mutants that disrupt the cell cycle either generally or specifically within spermatocytes. In addition, we employ a wide variety of molecular biology tools (e.g. the sequenced genome, bioinformatics database, gene knock down techniques, and various methods to investigate protein:protein interactions) to identify and analyze the function of specific candidate genes. In all cases, we routinely use immunocytological techniques to analyze the resulting cellular changes.