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and a cross between it and
X. helleri
Sarabia male produces homogametic
YY male progenies. Hence
X. maculatus
Jp 163A strain is regarded as an
ideal model to understand the sex determining/differentiation processes.
Employing the 454-FLX massively parallel DNA sequencing platform,
Zhang et al. (2011) have obtained 742,771 and 721,543 reads from whole
adult female and male, respectively. Among the identifi ed genes 54 are
differentially expressing sex genes. Further, the Poisson-based enriched
testing has identifi ed 2,250 male-enriched and 2,304 female-enriched
contigs with various degrees of difference. Among these contigs, 22 and
23 are female-and-male predominant transcripts, respectively. Within these
contigs, eight female predominant and nine male predominant transcripts
are recognized as differentially expressing sex genes.
Even among these 6-11% fi sh species, in which the existence of sex
chromosomes has been recognized, many claims are adduced from genetic
rather than cytological evidence. Emphasizing the need for molecular
cytogenetic approach, Ross and Peichel (2008) showed the presence of sex
chromosomes in the stickleback
Gasterosteus aculeatus
, in which the sex
chromosomes are morphologically homomorphic. A BAC (Bacterial Artifi cial
Chromosome) containing
cyp19b
, located at 16.7 Mbp in the X chromosome
assembly, hybridizes only to the X but not to the Y nor to any other location
in the genome, suggesting that a part of the Y chromosome has been deleted
(Fig. 5). The conclusion is supported by the presence of
cyp19b
on the X but
not on Y, even when detected with other probes around
cyp19b
. This large
(6 Mbp) deletion on the Y is equivalent to 30% of the sequence content of X
chromosome. However, X and Y chromosomes are morphologically similar
in size at metaphase. This is due to a pericentric inversion resulting in
difference in the centromere positions of X and Y chromosomes. Comparing
a few hundred thousand base pairs of sequence from the non-recombination
region of the Y chromosome and homologous region of the X chromosome,
Peichel et al. (2004) have shown the accumulation of many sequence
characteristics in the sex chromosome, including an elevated transposable
element content and a small intra-chromosomal duplication and inversions
(
Wtla
). Briefl y, the sex determining region of the Y or Z chromosome is a
'hot spot' for deletion, inversion, duplication, amplication and transposition
and other kinds of rearrangements (see Pandian, 2011). For instance, Y
choromosome-autosome fusions have occurred in at least 25 fi sh species,
including twice independently in stickebacks (Ross et al., 2009). Hence there
is a need for molecular cytogenetic approach to recognize sex chromosomes
(see also Volff et al., 2007).
A further complication in sex differentiation is caused by autosomal
genes inducing departures from expected sex ratio. Examples for such
unexpected departures in sex ratio are plentiful in relevant literature (e.g.,
Herpin et al., 2007). In the context of gynogenesis and androgenesis, some of
