Biology Reference
In-Depth Information
2
Differentiation in Gonochores
Sex is a luxury and costs time and energy but ensures recombination to
generate genetic diversity (Carvalho, 2003). As benefi ts accruing from
genetic recombination outweigh the costs of time and energy, sex is
successful and has evolved as early as 1.6-2.0 billion years ago (Butlin,
2002) and has been successfully manifested in a wide range of microbes,
plants and animals. Of the 30, 000 fi sh species, more than 98% are bisexuals
and the others are non-bisexuals, i.e., hermaphrodites or unisexuals. Even
among the hermaphrodites, hardly two species are self-fertilizing; all other
hermaphrodites function as male or female at a given point of time/age
(Pandian, 2011). Importantly, the sex changing sequentials provide a distinct
opportunity to study the processes of sex differentiation, de-differentiation
and re-differentiation over a period of time.
2.1 Sex chromosomes
Sexuality in fi shes ranges from gonochorism to unisexualism and self-
fertilizing hermaphroditism to sequential and serial hermaphroditism,
suggesting the presence and operation of diverse sex determination
mechanisms. Cytological mechanisms of hereditary transmission by
chromosomes as carriers of genes have received much attention. Yet
our present knowledge is based on cytogenetic characterization of only
about 1,700 species, of which 188 species belonging to 86 families, i.e.,
6-11% of fi shes alone are reported to have cytogenetically differentiated
heteromorphic sex chromosomes (Devlin and Nagahama, 2002). The use of
sensitive techniques like the anti-body staining of Human MutL Homologue
1 (MLH1) protein to mark the sites of putative chiasmata, the physical
basis for the pattern of recombination, opens a new avenue to recognize
