Biology Reference
In-Depth Information
containing a high mobility group (HMG) box domain but the
DMY
is a
transcription factor similar to
the double-sex and male-3 related transcription
factor
(
Dmrt1
). Another candidate gene, the anti-Mullerian hormone
amh
has
been mapped but within an autosomal sex determining Quantitative Trait
Loci (QTL, Shirak et al., 2006). Using a plethora of traditional molecular
approaches such as AFLPs, RAPDs and subtractive hybridization, the search
for sturgeon sex determining gene(s) has remained unsuccessful (e.g.,
McCornick et al., 2008), as the sturgeon genome is polyploid and most loci
are inherited tetrasomically or octosomically (Blacklidge and Bidwell, 1993).
However, Hale et al. (2010) claim the discovery of candidate sex-determining
genes
Dmrt1
and
Tra-1 Dmrt1
in
Acipenser fulvescens.
Incidentally,
Dmrt1
is
the master sex determining gene in birds and medaka, whereas
Tra-1
helps
direct sexual differentiation in nematodes. Hale et al. have employed more
advanced methods like 454 pyrosequencing, expressed sequential tags,
ontologies and so forth, used to characterize non-model species bearing
homomorphic chromosomes. They have also attempted to adduce evidence
with
Trichomonas
BLAST hits limited to the female gonad. However, they
have not located the claimed sex determinant genes to a specifi c sex
chromosome or autosome. The claims by Shirak et al. (2006) and Hale et al.
(2010) need to be confi rmed in other cichlid and acipenserid species.
Employing a novel procedure to study genome-wide linkage, Bradley
et al. (2011) have also claimed the discovery of sex determination genes in
zebrafi sh, in which the identifi cation of sex chromosomes and sex linked
genes has remained elusive (e.g., Wallace and Wallace, 2003). Genotyping,
genome-wide linkage analysis and recombination estimates of an accurately
developed SNP (Single Nucleotide Polymorphism) genetic map of zebrafi sh
have shown that sex determination is indeed a complex trait in zebrafi sh
and does not employ sex chromosomes in the conventional homogametic/
heterogametic mode. Bradley et al. have estimated the LOD scores for
linkage to sex determination, which have identifi ed two loci, that exceed
genome-wide signifi cance for linkage to sex determination. One locus
resides on chromosome 5 at 53.5 cM with an LOD score of 7.9 and the other
on chromosome 16 at 32.0 cM with an LOD score of 9.3. Further analysis
of mutants has suggested that the
Dmrt1
and
cyp21a2
are the candidate
genes harbored in chromosomes 5 and 16, respectively. Mutagenetic data,
that defi ned the highly conserved 3΄ UTR protein binding motif shown to
be responsible for transcription stability and translation effi ciency, predict
that the female associated allele 5΄ CUGCUACAGAU-3΄ would yield to
lower
Dmrt1
expression relative to male allele 5΄ CUGCUGCAGAU-3΄
in the developing gonadal primordium.
cyp21a2
encodes 21-hydroxylase
biosynthesizing corticosteroids. Reduced 21-hydroxylase activity results
in shunt of precursors toward steroid biosynthesis. Corticosteroid as well
