Mature sperm are specialized cells that are both motile and capable of fertilizing oocytes. To reach this stage, sperm undergo an intricate process of differentiation, and there are many interesting questions regarding the molecular mechanisms guiding their development. We are using C. elegans sperm to investigate some of these questions, and have recently identified the t-SNARE syntaxin 7 (syx-7) as a new member of the spe (spermatogenesis defective) class of genes, as it plays an important role in sperm development.
syx-7 is an ortholog of mammalian STX12, an endosomal t-SNARE. Using CRISPR/Cas9, we generated several deletion alleles of syx-7, and found that loss of the gene significantly reduces fertility of both male and hermaphrodite worms. This reduction in fertility is not due to embryonic lethality or female germline defects, but is caused by defective sperm. In animals lacking syx-7, the sperm arrest as large, abnormal cells, often containing four nuclei within a common cytoplasm. Preliminary data suggests syx-7 mutant sperm have defects in their ability to bud off from residual bodies and form haploid spermatids. To further characterize the defects in syx-7 mutants, we are using time-lapse microscopy to generate movies of in vitro sperm development. So far, our data suggest syx-7 may function in the membrane dynamics that take place during cytokinesis, possibly through delivery of new membrane components or maintenance of a boundary between the budding spermatid and the residual body. We are working to test these models. We have demonstrated syx-7 functions in sperm to promote fertility, and we are using strains with a CRISPR-generated gfp::syx-7 to visualize the protein’s localization. So far, we have shown that GFP::SYX-7 localizes to the sperm as well as other tissues, and we are currently determining the subcellular localization of the protein within sperm during spermatogenesis, particularly during the meiotic cell divisions.