Gonadal growth and maturation of sea urchins

characterized by intra-gonadal nutrient storage and its use for gametogenesis (Walker et al., 2001). Sea urchin gonads contain two main types of cells: germ cells and nutritive phagocytes, somatic cells common in both sexes that store nutrients necessary for gametogenesis (Walker, 1982). Before the initiation of gametogenesis, the gonads increase in size by accumulating nutrients such as proteins, lipids and carbohydrates into the nutritive phagocytes that fill the gonadal lumina in both sexes. After gametogenesis begins, nutritive phagocytes gradually decrease in size, supplying nutrients to the developing germ cells, and finally the lumina are filled with ova and sperm (Walker et al., 2001). The major protein contained in immature gonads is a glycoprotein of molecular mass of 170 kDa (Unuma et al., 1998), which was originally identified as the most abundant component of yolk granules in sea urchin eggs and termed the major yolk protein or major yolk glycoprotein (MYP; Harrington and Easton, 1982; Kari and Rottmann, 1985; Yokota and Kato, 1988; Scott and Lennarz, 1989). Unlike other oviparous animals, where the yolk protein is femalespecific, both male and female sea urchins synthesize MYP (Shyu et al., 1986; Unuma et al., 2001) and store it in nutritive phagocytes before gametogenesis (Ozaki et al., 1986; Unuma et al., 1998). As oogenesis proceeds, MYP is transferred from nutritive phagocytes to the ripe ova to form yolk granules (Ozaki et al., 1986). As spermatogenesis proceeds, most of the MYP disappears from the testis (Unuma et al., 1998). Considering the consensus among researchers that nutritive phagocytes are the main storage site of nutrients for spermatogenesis as well as for oogenesis (Walker, 1982), it has been postulated that male MYP serves as a nutrient source for spermatogenesis (Unuma et al., 1998). The nutritional role of MYP in sea urchin gametogenesis is, however, still not evident because MYP and other biochemical components in the gonads have so far not been quantitatively analyzed. In the present study, we examined the changes in MYP content and the biochemical composition of the gonads in the course of gametogenesis in Pseudocentrotus depressus, in order to clarify the definite role of MYP.