Fertilization

Abstract

Fertilization-the fusion of gametes to produce a new organism-is the culmination of a multitude of intricately regulated cellular processes. In Caenorhabditis elegans, fertilization is highly efficient. Sperm become fertilization competent after undergoing a maturation process during which they become motile, and the plasma membrane protein composition is reorganized in preparation for interaction with the oocyte. The highly specialized gametes begin their interactions by signaling to one another to ensure that fertilization occurs when they meet. The oocyte releases prostaglandin signals to help guide the sperm to the site of fertilization, and sperm secrete a protein called major sperm protein (MSP) to trigger oocyte maturation and ovulation. Upon meeting one another in the spermatheca, the sperm and oocyte fuse in a specific and tightly regulated process. Recent studies are providing new insights into the molecular basis of this fusion process. After fertilization, the oocyte must quickly transition from the relative quiescence of oogenesis to a phase of rapid development during the cleavage divisions of early embryogenesis. In addition, the fertilized oocyte must prevent other sperm from fusing with it as well as produce an eggshell for protection during external development. This chapter will review the nature and regulation of the various cellular processes of fertilization, including the development of fertilization competence, gamete signaling, sperm-oocyte fusion, the oocyte to embryo transition, and production of an eggshell to protect the developing embryo.

TheC. elegansVAPB homolog VPR-1 is a permissive signal for gonad development

“…A stem cell population at the distal gonad tip gives rise to sperm and oocyte precursors (Kimble and Crittenden, 2005). Sperm develop first at the L4 stage, followed by oocytes during early adulthood (L'Hernault, 2006; Marcello et al, 2013). Germ cell development and meiosis proceed from the distal to proximal ends (Fig.…”

The regulation of spermatogenesis and sperm function in nematodes

“…In nematodes, the sperm proteins that carry out these functions are members of the spe-9 class, which is named after its founding member [reviewed by 140]. Mutations in this group of spe genes share a specific set of phenotypes — the spermatids appear normal, they signal to oocytes and activate at the appropriate time, the mature sperm move normally and compete for positions in the spermatheca, but the sperm cannot fertilize oocytes.…”

Gamete interactions require transmembranous immunoglobulin-like proteins with conserved roles during evolution

“…5,6 For example, a C. elegans oocyte lacks accessory cells and a thick egg coat, unlike mammalian oocytes ( Fig. 1, right), so that the fusogenic reaction in C. elegans can be examined more simply than those in mammals.…”

“…The sperm becomes acrosome-reacted prior to contacting with the ZP, 34 and gamete fusion occurs on the equatorial segment of the acrosome-reacted sperm 1 (shown by a red square). (Right) Since a C. elegans oocyte lacks any types of accessory cells and egg coats such as the CL and ZP, respectively, a sperm directly binds to and fuses with the oocyte plasma membrane through the pseudopod 5,6 (red square). ejected into the uterus.…”