Abstract
In the culture of tilapia (Oreochromis niloticus), monosex male tilapia are
preferred because it not only can prevent uncontrolled reproduction but also
can increase production as male tilapia grow faster than females. Development
of genetic markers and linkage maps have offered opportunities to understand
the complex mode of sex determination which in turn can play significant role
in monosex production. As a first step towards producing genetically male
tilapia, neofemales were produced by applying the synthetic estrogen
Diethylstilbestrol (DES) in three doses- 100 mg, 200 mg and 300 mg DES/kg
feed. DES treatments resulted in 95% female while the rate of female in the nontreated group was only 55%. Among six DES treated females used for progeny
test experiment, three families - HRT23, HRT26 and HTZF824 were found to
follow a 3:1 (male: female) sex-ratio (p>0.05) which are indicative to families of
'XY' females. Fifteen microsatellite DNA markers putatively linked to the sex
determining QTL were analyzed for molecular identification of male tilapia.
Three markers UNH995, UNH985 and ARO172 showed some variation among
different genotypes of male and female fish. Among the three markers ARO172
was found to be the most informative in differentiating male and female
genotypes. Four offsprings of family HRT23 (‘XY' sex-reversed female)
identified as YY male on the basis of marker analysis were verified by progeny
testing. Upon crossing with normal females (XX), one of them produced 100%
male offspring and the other three produced 88.40%, 96.84% and 86.36% male
offspring, respectively. Thus we have produced and identified YY supermales
that can give 100% or nearly 100% male progeny. We also have identified one
molecular marker located in LG23 that can at least correctly differentiate YY
males from normal male and female fish. This YY male can be used for large
scale production of genetically all male tilapia. Further research however is
needed to explain the variations in sex ratios in progeny of YY supermales. This
would allow greater understanding of the apparently complex sex determining
system of tilapia and stronger control over large scale production of genetically
male tilapia for aquaculture.