with a homogametic female (XX) and heterogametic male (XY) and in S.
macrochir with a heterogametic female (WZ) and homogametic male (ZZ)
did not explain the results obtained in some crosses. Jalabert et al. (1971)
hypothesized that autosomes were influencing sex determination in some
hybrid crosses. Avtalion and Hammerman (1978) theorized that a pair of
autosomes compliments the sexdetermining chromosomes. Using the
sex ratios obtained in Chen's crosses with S. mossambicus and S. hornorum,
they were able to postulate the relative influence of the different chromo-
somes and the sex of all the genotypes. The sex of each genotype as well as
the resulting theoretical sex ratios for all possible hybrid crosses were pre-
dicted. It remains to be seen if the autosomal theory of Avtalion and Ham-
merman holds true when the actual crosses are performed.
Commercial production of all-male hybrids has been difficult to maintain
over a long period of time due to contamination of pure broodstock lines
resulting in the appearance of varying proportions of females. At this time,
there is no way to determine the genotype of tilapia broodstock routinely on
a commercial scale. This problem has been especially troublesome in Israel
with S. niloticus x S. aureus hybrids.
Avtalion et al. (1975) suggested electrophoretic comparisons of the
bldod proteins of broodstock to select pure genetic strains that will produce
a high percentage of male offspring. However, this method has not been
sufficiently developed to permit determination of the genotype and cannot
be claimed as a complete method for ensuring all-male broods at this time.
Hulata et al. (in press) have propos etermining geneti-
cally pure strains of S. aureus and S t all-male hybrids
can be consistently produced. The system involves a series of aquarium
spawnings of female S. niloticus and male S. aureus and their reciprocal
cross. Those females and males of each species producing the correct male to
female hybrid offspring ratios, 100% males for the S. niloticus x S. aureus
cross and 3 to 1 males to females in the S. aureus x S. niloticus cross, will be
mated with the selected individual of the opposite sex of the same species.
The pure S. aureus male and S. niloticus female offspring from the selected
matings will then be hybridized. This process will be continued until all-male
offspring are consistently produced. The pure line broodstock of S. aureus
and S. niloticus will then be isolated so that contamination will not occur
again.
Fingerling Production: Installations and SystemsThe maintenance of pure genetic lines to produce consistently all-male
tilapia hybrids is difficult. Control of tilapia spawning and fry raising is often
best accomplished in artificial environments: aquaria, concrete tanks, plastic
pools, etc. Rothbard and Pruginin (1975) described a technique for induced
spawning of S. niloticus and S. aureus to produce hybrids between the two
species. The authors used aquaria that were 200 cm long, 50 cm wide, and
40 cm high in which one male S. aureus and 7 to 10 female S. niloticus were