In the final step of sperm maturation, round spermatids receive an activation signal which initiates a dramatic morphogenesis: spermatids assemble an MSP cytoskeleton, extend a motile pseudopod and begin to crawl. Spermatids from hermaphrodites and males are activated differently. Hermaphrodite spermatids activate and differentiate in response to an endogenous activator soon after budding from residual bodies in the final meiotic division. Male spermatids can accumulate, then rapidly differentiate into spermatozoa when ejaculated with seminal fluid upon mating, crawling up to the spermathecae lest they be expelled out the vulva as eggs are laid. Four genes have been identified that may aid in the understanding of this specialized signaling event. Each has a striking mutant phenotype: spermatids in virgin hermaphrodites fail to activate, yet mated hermaphrodites can produce both outcross and self progeny. Males are fertile. We have recently cloned two of the genes with this strange spermatid activation defect.
spe-12 encodes a novel transmembrane protein. Identification of the molecular lesions suggests that all three
spe-12 alleles are likely nulls. Surface proteolysis of spermatids followed by Western analysis with SPE-12 antibodies reveals that SPE-12 is localized to the spermatid plasma membrane.
spe-29 also encodes a small predicted transmembrane protein.
spe-27, a previously-cloned gene in this class, interacts genetically with both
spe-12 and
spe-29, suggesting that all three gene products may function in a plasma membrane complex to receive or transduce the activation signal. SPE-12 is known to be expressed on male spermatids, yet mutants are still capable of activating. Perhaps there are two distinct pathways for activating spermatids, one that responds to the hermaphrodite activator and requires SPE-12, and another independent pathway that is utilized to transduce the male signal. Since this model predicts that male
spe-12 sperm would not be defective, we assessed the fertility of individual
spe-12 males. When mated to a hermaphrodite that lays very few oocytes,
spe-12 males were nearly as fertile as wild type. However, when mated to hermaphrodites that lay many oocytes, most matings produced few progeny. This demonstrates that
spe-12 male sperm do have a defect. The defect might be due to increased activation time, since this would result in spermatids which would be lost from the worm as oocytes are laid. Based on these results, we propose that the observed difference in fertility between sterile unmated hermaphrodites and somewhat fertile males results from a property of the activators. Perhaps distinct activators are used in males and hermaphrodites, and SPE-12 function is partially bypassed only when spermatids contact male activator. Alternatively, higher male activator concentration or longer length of exposure to the male activator could explain the partial resuce of
spe-12 spermatids after mating.