nants seem to be present in higher quantities in mature than in immature
males. The S. aureus MSP was found to display cross antigenicity with S.
galilaeus MSP. Some males were found to possess especially high quantities
of this antigen, and we wonder whether these MSP-rich males are more
capable than other males with less MSP, in their ability to give rise to a
higher percentage of male offspring. This would be in agreement with our
autosomal theory on sex determination in Sarotherodon (Hammerman and
Avtalion 1979) which is summarized below.
Sex DeterminationThe autosomal theory suggests that the sex determination is by female
and male gene products, which interact quantitatively (Avtalion and Harn-
merman 1978; Hammerman and Avtalion 1979). The crossing of males,
which produce more male-sex regulating factors (SRF) with females, which
produce less female-SRF, would provide a high percentage of male hybrids
and vice versa. This balance theory is called "autosomal" because it is based
on the assumption that the regulatory genes are located on autosomes
(A,a) as well as on gonosomes (X, Y, W). This theory explains in a highly
satisfactory manner the unusual sex ratios obtained by Chen (1969) who
performed a comprehensive study in which hybrids of S. mossambicus and S.
hornorum were extensively crossed between themselves, and were back-
crossed with parents for up to four generations. This theory also explains
most of the sex-ratio results obtained by Jalabert et al. (1971). It is note-
worthy that a four-chromosome theory was originally suggested by Bellamy
(1936) (XX-XY and WZ-ZZ) and modified by Gordon (1946, 1947) (3 sex
chromosome theory, X, Y, 2) on the basis of their results on platyfish. This
theory was reviewed by different authors (Kosswig and Oktay 1955; Anders
and Anders 1963; and Kallman 1965) and was applied to Sarotherodon
species by Hickling (1960), Chen (1969) and Jalabert et al. (1971) in order
to explain the sex-ratio results they obtained in intercrossing these species.
Since some of these sex-ratio results could not be explained on the basis of
this theory, they all came to the conclusion that an autosomal influence on
the sex determination process must be taken into account.
Assuming that autosomes indeed exert an influence on sex determination,
the simplest system of sex-influencing chromosomes would consist of three
gonosomes (X, W, Y) appearing as a complement of two, in any one of the
possible combinations (XX, XY, WX, WW, WY or YY) and a combination of
a pair of autosomes (AA, Aa or aa), all involved in primary sex determina-
tion. Within each pure species the pair of autosomes would be identical AA
or aa. Thus, the complete set of chromosomes influencing sex determination
in the pure species was suggested to be AAXX Q and AAXY d in S. niloticus
and S. mossambicus (homogametic female and heterogametic male), and
aaWY Q and aaYY d in S. aureus and S. hornorum (heterogametic female and
homogametic male). The number of genotypes resulting from the combina-
tion of autosomes and gonosomes is 18. The sex of all the genotypes was
determined on the basis of analysis of the sex ratio results obtained in Chen's
crosses (Chen 1969).