Identification of a new gene family expressed during the onset of sexual reproduction in the centric diatom Thalassiosira weissflogii

An intriguing feature of the diatom life cycle is that sexual reproduction and the generation of genetic diversity are coupled to the control of cell size. A PCR-based cDNA subtraction technique was used to identify genes tha t are expressed as small cells of the centric diatom Thalassiosira weissflo gii initiate gametogenesis. Ten genes that are up-regulated during the earl y stages of sexual reproduction have been identified thus far. Three of the sexually induced genes, Sig1, Sig2, and Sig3, were sequenced to completion and are members of a novel gene family. The three polypeptides encoded by these genes possess different molecular masses and charges but display many features in common: they share five highly conserved domains; they each co ntain three or more cysteine-rich epithelial growth fatter (EGF)-like repea ts; and they each display homology to the EGF-like region of the vertebrate extracellular matrix glycoprotein tenascin X. Interestingly, the five cons erved domains appear in the same order in each polypeptide but are separate d by variable numbers of nonconserved amino acids. SIG1 and SIG2 display pu tative regulatory domains within the nonconserved regions. A calcium-bindin g, EF-hand motif is found in SIG1, and an ATP/GTP binding motif is present in SIG2. The striking similarity between the SIG polypeptides and extracell ular matrix components commonly involved in cell-cell interactions suggests that the SIG polypeptides may play a role in sperm-egg recognition. The SI G polypeptides are thus important molecular targets for determining when an d where sexual reproduction occurs in the field.