THE ROLE OF THE GONADS
In addition to the central role of gametogenesis the gonads possess com-
plex endocrine properties which, under pituitary control, contribute to the
regulation of reproductive cycles by direct action on gamete differentiation,
by controlling the activity of different organs and tissues involved in repro-
duction (such as the liver, fat and bone mineral stores) and also by control-
ling the development of secondary sexual characteristics. Finally the endo-
crine secretions of the gonads participate in the regulation of pituitary
activity (a feed-back mechanism) and also act on the central nervous system,
allowing different kinds of behavioral patterns to occur during the successive
periods of the sexual cycles.
Sexual steroids are the main hormones produced by the gonads, but other
compounds, the occurrence of which have not been investigated in cichlids,
must certainly be secreted (Breton et al. 1980). Steroids are generally
considered to be produced by specialized cells presenting histochemical and
morphological features common to all vertebrates (Hoar and Nagahama
1978).
In males these cells form a typical interstitial tissue which has been
characterized as steroidogenic by demonstration of 3p-hydroxy-steroid-
ogenase (Yaron 1966) and &om ultrastructural evidence (Nicholls and
Graham 1972). They exhibit increased activity (number, size, lipid concen-
tration) during spermatogenetic progression, with maximum activity during
spermiation, when there is also rapid development of sexual coloration, nest
building activity and territoriality (Hyder 1970b), and when the testis
contains high levels of testosterone (Hyder and Kirschner 1969). The fact
that interstitial tissue activity is under pituitary control can be demonstrated
by several methods. Treatment with human chorionic gonadotropin (HCG),
which exhibits gonadotropic activity in some fishes,induces both stimulation
of interstitial tissue and an increase in the testosterone content of the testes
(Hyder et al. 1970). Conversely, methallibure inhibition of pituitary gonad-
otropin secretion lowers interstitial cell activity, an effect which can be over-
come by administration of HCG or Sarotherodon pituitary extracts (Hyder
1972; Hyder et al. 1974). But testosterone is probably neither the only nor
the main steroid mediator produced in the gonad, as testosterone propionate
administered to methallibure-treated Sarotherodon failed to restore sperm-
atogenesis, producing only some spermiation at high doses (Hyder et al.
1974). As a range of androgenic steroids has already been found in fish,
like 11-ketotestosterone in female S. aureus (Eckstein 1970), further studies
would be of great interest in attempting to identify the major active steroids
in Tilapia and Sarotherodon, and to understand their action on the different
steps of spermatogenesis, sexual behavior and secondary sexual character-
istics: for example, the genital tassel of male S. macrochir which seems to be
an important signal in the spawning behavior of that species (Wickler 1965,
1966b).