ENDOCRINE REGULATION OF GAMETOGENESIS IN FISH

Bull. Inst. Zool., Academia Sinica, Monograph 16: 139-172 (1991)

ENDOCRINE REGULATION OF GAMETOGENESIS IN FISH

lvi. YOSHIKUNI and Y. NAGAHAMA Laboratory of Reproductive Biology, National Institute for Basic Biology,

Okazaki 444, Japan

It is well established that in

teleosts, as in other vertebrates, the

pituitary-gonadal axis plays an

important role in regulating gameto­

genesis. In most cases, gonadotropins

act through the biosynthesis of

gonadal steroid hormones which In

turn mediate various stages of

gametogenesis including oocyte

growth, oocyte maturation, spermato­

genesis and spermiation. Thus, the

determination of steroid hormones in­

volved in each process is a significant

research goal for scientists interested

in the fish reproduction. This article

reviews some of recent findings on

the identification of steroidal regu­

lators in gametogenesis and the sites

and mechanisms of these regulators.

GONADOTROPIN

The question of whether fish

possess one or two gonadotropins in

the pituitary gland has been con­

troversial (Burza wa -Gerard, 1982;

139

Van Oordt and Peute, 1983; Idler and

Ng, 1983). There is now, however,

biochemical evidence that teleosts,

similar to other vertebrates, possess

two gonadotropins. Idler and his

colleagues (Idler and N g, 1983) have

isolated two gonadotropins, carbo­

hydrate-rich ((maturational" gonado­

tropin and carbohydrate-poor Hvitello­

genic" gonadotropin. More recently,

two distinct carbohydrate gonado­

tropins, designated as GTH I and

GTH II, have been isolated and

characterized from pituitaries of

spawning chum salmon, Oncorhynchus

keta (Suzuki et al., 1988a, 1988b; Itoh

et al., 1988; Kawauchi -et al., 1989).

Each of these gonadotropins consists

of a and f3 subunits, the f3 subunits

ha ving amino acid sequence identities

of 31%. GTH I and GTH II have

also been purified in coho salmon,

Oncorhynchus kisutch, (Dickhoff and

Swanson, 1990) and they are chemi­

cally similar to the chum salmon

gonadotropins. Both GTH I and GTH