9.2 Cellular source of steroid hormones
9.3 Androgenic steroids
9.4 Sex accessory glands
9.5 Sex recognition and pheromones
The testes of teleost fishes show greater morphological variation than in other vertebrates (Lofts, 1968; Dodd, 1972; de Vlaming, 1974; Callard et al., 1978). Inmost cases, testes are a pair of elongated structures composed of branching seminiferous tubules embedded in the stroma. The testis consists of thin-walled tubules or lobules that contain germ cells - the spermatogonia - which are endodermal in origin. Germ cells divide in clusters enclosed by a cyst. Primary spermatogonia - the stem cells - which are present throughout the year, divide mitotically to give rise to secondary spermatogonia which get transformed into primary spermatocytes. They divide by meiosis and give rise to spermatids from which spermatozoa are formed. The seminiferous tubules are packed with spermatozoa in the pro-spawning and spawning periods,
Lofts and Bern (1972) and Guraya (1976b, 1979) have summarized the. literature on cellular sources of testicular steroids in teleost fishes. Leydig cells or interstitial cells, the large polygonal cells usually located within the interlobular spaces, produce androgen (Guraya, 1976b; Hoar and Nagahama, 1978). Lobule boundary eel Is which are homologous with Sertoli cells, are not convincingly steroidogenic in Cichlosoma nigrofasciatus (Nicholls and Graham, 1972), in salmon and goldfish (Hoar and Nagahama, 19787 and Mystus vittatus (Saidapur and Nadkarni, 1976). van den Hurk et al. (1978), who have made an ultrastructural and enzyme cytochemical study of the testis in rainbow trout, reported that the Leydig cells are the main. source of steroids and the steroidogenic activity is at a peak when the testes are mature and new spermatogonia are being formed; Sertoli. cells also have enzymes involved in steroidogenesis when males are in spermiation. Further, stromal cells around the vas dererens epithelium are also steroidogenic when spermatozoa are stored in the. lumen of vas deferens (see Guraya, 1976b).
Bilateral castration followed by replacement therapy experiments have clearly shown that in teleost males, androgenic steroids are. produced by the testes and regulate secondary sexual characters, sex accessories and sexual behaviour (Dodd, 1960; de Vlaming, 1974; Callard et al., 1978).
Testosterone, androstenedione and dehydroepiandrosterone have been extracted and isolated from the testes of a wide variety of fishes (see Ozon, 1972a) and the biosynthetic pathways involved in the formation of androgens have been reviewed by Ozon (1972a), Colombo and Colombo Belvedere (1977) and Tamaoki (1980). Testosterone and androstenedione have been. obtained following incubation or various steroid precursors with testicular pieces or homogenates from different species of teleost fishes (Arai, Shikita and. Tamaoki, 1964; Chan and Phillips, 1969; Colombo, Lupo and Binder, 1970; Colombo, del Conte and Clemenze, 1972; Colombo, Pesavento and Johnson, 1.972; Lupo, Matterazzi and Chieffi, 1970).
A unique steroid, 11-ketotestosterone, was first isolated and identified in the plasma of sockeye salmon, Oncorhynchus nerka (Idler, Schmidt and Ronald, 1960). Incubation of testicular homogenates of rainbow trout and the Atlantic salmon with androstenedione and testosterone also yields 11-ketotestosterone and 11ß-hydroxytestosterone suggesting the testicular origin of the 11-oxygenated steroids found in the plasma (Arai and Tamaoki, 1967a, b; Idler and MacNab, 1967; Idler, Truscott and Stewart, 1968; Kime, 1979). The androgenic activity of 11-ketotestosterone is 10 (Arai, 1967) to 17 (Tamaoki, 1980) times that of testosterone and it is considered to be the true androgenic hormone in fishes. 11-oxygenated androgens have been isolated from in vitro incubations of appropriate precursors with testicular tissue of 12 species of teleost fishes belonging to six different orders (see Colombo and Colombo Belvedere, 1975; Kime, 1979). The conversion of androstenedione to 11-ketotestosterone via testosterone and 11(3-hydroxytestosterone has been postulated as the major biosynthetic pathway in fish testes (Idler, Truscott and Stewart, 1968; Tamaoki, 1980). Formation of 11-ketotestosterone from tritiated testosterone by testes is optimum between 6° and 21°C in rainbow trout and 11° and 36°C in goldfish thus corresponding broadly to favoured breed ins temperatures, whereas testosterone glucuronide formation is optimal from 21°-37°C in rainbow trout and 21°-41°C in goldfish (Kime, 1979). Glucuronidation may play a role in limiting androgen production to the biologically preferred environmental temperature for the species.
Androgens have been estimated from plasma of many teleost fishes (see Ozon, 1972a; Schreck, Flickinger and Hopwood, 1972; Schreck, Lackey and Hopwood, 1972; Schreck and Hopwood, 1974; Campbell, Walsh and Idler, 1976; Wingfield and Grimm, 1977; Simpson and Wright, 1977; Billard et al., 1978; Sangalang, Freeman and Flemming, 1978; Sundararaj, Lamba and Goswami, 1980).
Seminal vesicles, the sex accessory glandular structures, attached to the testes have been reported in various species of catfishes and gobiid fishes (Sundararaj, 1958; Dodd, 1.960). They show seasonal variations in secretory activity correlated with those of the testes (Nayyar and Sundararaj, 1970a). A synergism among androgen, prolactin and growth hormone has been demonstrated in the regulation of secretory activity in the seminal vesicles of catfish, Heteropneustes fossills (Sundararaj and Nayyar, 1969a, b; Nayyar and Sundararaj, 1969, 1970b) and the gobiid fish, Gillichthys mirabilis (de Vlaming and Sundararaj, 1972). The secretion contains mucoproteins and acid mucopolysacharides. The presence of secretory seminal vesicles enhances the fertilizing capacity of males (Sundararaj and Nayyar, 1969c; Nayyar and Sundararaj, 1970a).
In many teleost' fishes, social communication is based on the exchange of pheromonal messages which serve to regulate territoriality, schooling, sexual and parental behaviour (see Pfeiffer, 1974). Very little information is available on fish pheromones and many workers have suggested that gonadal secretions may provide the signals for sexual recognition in many fishes (Liley, 1969). It has been proposed that the mesorchial glandular mass of the testis of the black goby, Gobius jozo (Colombo, Colombo Belvedere and Pilati, 1977) and Gobius paganellus (Stanley, Chieffi and Botte, 1965) may be a source of pheromonal steroids. In both species, the impressive steroidogenic apparatus seems to satisfy the necessity for producing huge amounts of steroid conjugates as glucuronides (Colombo, Colombo Belvedere and Marconato, 1979). In Gobius jozo, etiocholanolone glucuronide attracts the ripe. female and triggers the release of eggs (Colombo, Colombo Belvedere and Marconato, 1979). Formation of water-soluble conjugates possibly facilitates secretion into the surrounding medium.