norum, S. aureus, S. macrochir and S. jipe dd. Sex ratios (9:d) of 1: 3 were
consistently obtained in reciprocal crosses (Hickling 1960, 1963; Fishelson
1966a, 196613; Chen 1969; Jalabert et al. 1971; Pruginin et al. 1975; Lovshin
and Da Silva 1975; Haller pers. comm.). On the other hand, the back cross of
the F1 male hybrids and their female parents gives rise to equal numbers of
males and females. In general, male hybrids give rise to progeny with a low per-
centage of males when crossed with their parents or grandparents. In practice,
the infiltration of broodstocks by hybrids seems to be the reason for the
decrease in the percentage of males in group spawning. Actually, for many
reasons (e.g., human error, transfer of fry from one pond to another by
predatory birds, occurrence of fry in the water used to fill the pond prior to
stocking) it is quite impossible to avoid contamination of the parental
species in large-scale commercial operations.
The main purpose of our work was to establish criteria, based on genetic,
biochemical, electrophoretic and immunological markers, to identify parental
species and their hybrids, in order to control broodstocks and to eliminate
individuals presenting xenogeneic markers, and to carry out a comprehensive
study on blood markers in tilapia.
Sampling for Serum MarkersFor the commercial production of F1-male hybrids in group spawning
in Israel, S. aureus males are currently crossed with females of S. nilo-
ticus of different origins: S. niloticus (G) (originating from Ghana) and S.
niloticus (V) (otherwise called S. vulcani, originating from Kenya). It is a
matter of opinion whether to designate these as separate subspecies. In
practice, it is difficult, if not impossible, to differentiate between the F1
hybrid progeny of these crosses and their parents on the basis of external
morphometric criteria. For this reason, a systematic check on the brood-
stocks is carried out using several blood protein markers. Once or twice a
year, in various farms which produce F1-male hybrids for commercial
purposes, we test all the selected broodstocks (which are normally kept in
aquaria) and take samples from S. aureus dd and S. niloticus 90 destined for
the mass production of F1 male hybrids in ponds.
The test consists of withdrawing a small quantity of blood (0.1 to 0.4 ml)
from the hemal arch of tagged fish, and testing their sera by polyacrylamide
gel electrophoresis. Since the tested fish are destined for reproduction, the
blood sampling is performed carefully to minimize injury. The mortality
following sampling is normally very low, or zero, and even small fish of 8 to
10 g can be sampled.
The serum markers which were found significant for this purpose, and
which are routiqely tested using 6 and 7% polyacrylamide gels, are the
transferrins, male sex-protein (MSP) and esterase isoenzymes. These markers,
which exhibit sex and/or species-specific polymorphism, enable an easy
identification of S. aureus and S. niloticus parents and their hybrids (Avtalion
et al. 1975,1976).