x S. leucostictus (Pruginin 1965) and S. niloticus x S. spilurus niger (Pruginin
1965).
Several theories have been proposed to explain these ratios. Hickling
(1960) and Chen (1969) suggested that the sexdetermining mechanisms
XX female-XY male and WZ female-ZZ male are both found in tilapias.
However, not all the sex ratios of tilapia hybrid progeny can be explained
by these. Hammerman and Avtalion (1979) have presented a model which
takes into account the possible sexdetermining effects of autosomes as well
as sex chromosomes.
Culture of tilapia hybrids has resulted in high yields of harvestable-
size fish (Hickling 1960; Pruginin 1967; Lovshin et al. 1977; Avault and
Shell 1967). Although hybrid vigor has been reported by Hickling (1960)
and Avault and Shell (1967), the findings of Pruginin (1967) and Lovshin
et al. (1977) showed otherwise. One other advantage of using crosses which
give monosex hybrid progeny is that the wastage of females from manual
sexing is eliminated.
The main disadvantages of such crosses are: difficulty in maintaining pure
parental stocks that consistently produce 100% male offspring (Pruginin et
al. 1975), poor spawning success (Lee 1979) and incompatibility of breeders
resulting in low fertility (Lovshin et al. 1977).The use of steroid hormones for production of monosex broods has
proved effective in many tilapias. Artificial sex-reversal of genotypic females
of five species has been achieved using methyltestosterone and ethynyl-
testosterone (Table 1) and feminization of genotypic males has been induced
with estrogens: ethynylestradiol, estrone and diethylstilbestrol (Table 2)., Treatment is per os for periods ranging from^15 to^60 days in tanks or
aquaria. Survival of fry under hormone treatment does not differ significantly
from controls indicatingno differential mortality (Guerrero 1974). Effective-
ness of the sex reversal treatment depends on the kind and dosage of steroid
used, the method of administration, time and duration of treatment, and on
the species (Yamamoto 1969). Shelton et al. (1978) recommend oral treat-
ment of 9 to 11 mm S. aureus fry at a density of 2,600/m2 or less with
ethynyltestosterone dosage of 60 mg/kg of feed for 6 weeks at 25 to 29°C
for production of all-male broods.
Sex-reversal using androgens on a commercial scale has been shown to
be feasible by Guerrero (1979b) and Koplin et al. (1977). The disadvantages
of the method are the need for holding facilities for treatment of large num-
bers of fry (Mires 1977) and the sophisticated skill needed for its application.
Hopkins (1979) produced all-male progeny of S. aureus with sex-reversed
females (genotypic males) treated with ethynylestradiol. He also produced
all-female fry with sex-reversed males of S. niloticus treated with androgens.
The low percentage of spawns with monosex fish, however, points out that
further research is needed to establish the practicality of the method.