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the presence of sex chromosomes; for instance, the ratio of MLH1 loci per
chromosome in males versus females is 1 : 1.55 indicating the presence of
sex chromosome in zebrafi sh (Kochakpour and Moens, 2008), in which no
heteromorphic sex chromosomes could earlier be recognized (e.g., Wallace
and Wallace, 2003). Of the 13,000 marine fi sh species, fewer than 2% have so
far received cytogenetic attention (Brum, 1996). Of the 400 and odd species
displaying one or another form of hermaphroditism, sex chromosomes have
been hardly recognized in a half dozen species. The Neotropical fi shes are
of special interest, as about 8,000 freshwater fi shes, representing more than
25% of all fi sh species are considered to be present there (Schaefer, 1998).
Of the 4,475 species thus far recognized, the presence of sex chromosomes
has been claimed only for 5.9% species (Oliveira et al., 2009). Using more
sensitive chromosome painting and other techniques, the presence of sex
chromosomes in another half a dozen Neotropical
Characidium
has been
recently reported (Machado et al., 2011). Nevertheless, we seem to have just
touched the tip of the iceberg and our basic understanding of cytogenetics
is limited to a few thousand species and much is awaited especially from
the coral-reef inhabiting hermaphroditic and Neotropical fi shes.
A heterogametic individual produces genetically different gametes (X, Y
or Z, W), one of which may determine the sex of its progeny generated by its
fusion with the gamete of the opposite sex. On the basis of reliable evidences
from heteromorphic sex chromosomes, 264 fi sh species belonging to about
160 families are recognized as heterogametic; of them, 67% are recognized
as male heterogametic (Pandian, 2011). Male and female heterogametics
also exist within a genus (e.g.,
Gambusia, Xiphophorus
) and within a species
(
Gambusia affi nis
) too. More intriguingly, 9% fi sh species are known to
have multiple sex chromosome-systems. There are frequent reports on
the existence of multiple sex chromosomes, especially from the Neotropic
freshwater fishes and endemic species from the Antarctic (Pandian,
2011). The presence of male and female heterogametic sex determination
systems as well as simple and multiple sex chromosomes clearly indicates
the absence of a common, universal sex chromosome system in fi shes, as
mammals have it. It is not known whether these diversities in gamety and
simple/multiple sex chromosome systems are refl ected in sex differentiation
or not. For instance, high androgen ratios in male heterogametic vertebrates
may be the consequence of the Y-linked gene, whose product is an inhibitor
of
cyp19
, whereas the W chromosome may carry a gene inducer of
cyp19
in
female heterogamatics (Bogart, 1987; see also Piferrer and Guiguen, 2008)
The family Cichlidae, one of the most species-rich families of
vertebrates, is composed of 3,000 species. More than 1,500 species inhabiting
the Great Lakes of eastern Africa are endemic. Because of their impressive
