Sex-Determination System In Fishes
Kiran Rasal; Makwana Nayan; Amar Gaikwad;
Central Institute of Fisheries Education,
Mumbai 400 061.
Correponding Author- firstname.lastname@example.org
sex-determination system is a biological system that determines the development
of sexual characteristics in an organism. Most sexual organisms have two sexes.
In many cases, sex determination is genetic: males and females have different
alleles or even different genes that specify their sexual morphology. In animals,
this is often accompanied by chromosomal differences. In other cases, sex is
determined by environmental variables (such as temperature) or social variables
(the size of an organism relative to other members of its population). The
details of some sex-determination systems are not yet fully understood
XX/XY sex-determination system is the most familiar sex-determination systems.
In XY/XO system, females have two copies of the sex chromosome (XX) but males
have only one (X0). The ZW sex-determination system is found in birds and some
insects and other organisms many other sex-determination systems exist. In some
species of reptiles, including alligators, some turtles, and the tuatara, sex
is determined by the temperature at which the egg is incubated. Some species,
such as some snails, practice sex change: adults start out male, and then
become female. In tropical clown fish, the dominant individual in a group
becomes female while the other ones are male, and blue wrasse fish are the
reverse. In Bonellia viridis, larvae
become males if they make physical contact with the female and females if they
end up on soil.
Some species, however, have no sex-determination system.
Hermaphrodites include the common earthworm and certain species of snails. A
few species of fish, reptiles, and insects reproduce by parthenogenesis and are
In some arthropods, sex is
determined by infection, as when Bacteria of the genus Wolbachia alter their sexuality; some species consist entirely
of ZZ individuals, with sex determined by the presence of Wolbachia.
Temperature-dependent sex determination
Temperature-dependent sex determination (TSD) is a system
in which the temperature the eggs experience determines the sexes of the
organisms that hatch. It is most prevalent and common among amniote vertebrates
that are classified under the reptile class, but is also used among some birds,
such as the Australian Brush-turkey. It also happens in some fishes such as
salmon, gold fish, tilapia etc. It differs from the chromosomal
sex-determination systems common among vertebrates. It is a type of
environmental sex determination (ESD); in other ESD systems, some factors such
as population determine the sex of organisms (see Polyphenism).
The eggs are affected by the temperature at which they are
incubated during the middle one-third of embryonic development. This critical
period of incubation is known as the thermosensitive period (TSP). The specific
time of sex-commitment is known due to several authors resolving histological
chronology of sex differentiation in the gonads of turtles with TSD
A variety of sex determination systems exists in reptiles
and fishes, including temperature-dependent sex determination (TSD). While the
precise physiological basis of TSD is unknown, numerous past studies have
provided a general characterization of this phenomenon. The period of
thermosensitivity encompasses the middle one third to one half of embryonic
development. It has been shown that in several species of fishes this period
starts prior to (histologically defined) gonadal differentiation and extends to
a time when sex specific changes are becoming evident in the gonads
on the mechanism of sex determination of the commercially important tilapia, Oreochromis
niloticus, have demonstrated that this
species exhibits a predominantly monofactorial genotypic system with male
heterogamety and female homogamety. The effects of environmental conditions
during the period of sex differentiation on the sex ratio of the Nile tilapia, Oreochromis
niloticus. (Jose S. Abucay et al
1999). Different sex genotypes
were exposed to varying temperatures putative all-female, all-male and all-YY
males.and salinities putative all-female progeny only. For a minimum period of
21 days after first feeding and were on grown prior to sexing by gonad squash.
The majority of the putative all-female progeny exposed to high temperature
36.54¼C.produced significantly higher percentages of males compared to controls
reared at ambient temperature 27.87 ¼C
Similarly, at high temperature, some of the all-male and YY male
progenies had significantly lower percentage of males compared to controls. Sex
differentiation in YY males appears to be more labile than in normal XY males
although this could possibly be attributable to different levels of inbreeding.
Low temperature 25.78¼C.and varying levels of salinity 11.30 to 26.65 ppt. did
not significantly affect sex ratios. The apparent sensitivity of sex
differentiation to some environmental factors is considered in the context of a
predominantly monofactorial genetic sex determining
of the effect of environment on sex ratio has been observed in a number of fish
species. A number of studies have reported on the varying degrees of
dependent sex determination
and its adaptive significance in the Atlantic Silverside,
Menidia menidia (Conover and Kynard, 1981; Conover, 1984.) Other
extremes such as low pH
have also been observed to affect sex ratio in Cichlids and
Poecilids Rubin, 1985.
number of studies have also demonstrated effects of temperature on sex ratio in
tilapia. In an early study, Mair et al.1990.observed that at a cold temperature
19¼C.the sex ratio of O. mossambicus in
one experiment had a significantexcess of males 89%. compared to the control reared at 28¼C mechanism.
effect of temperature on sex differentiation is variable among species. Fish
species that do not possess a clear distinct chromosomal sex, show strong
temperature dependent sex determination/differentiation (TSD) (Piferrer et
al.,2005; StrŸssmann et al., 1996a; Conover and Kynard) 1981)
gold fish sex differentiation is affected during a specific period of larval
development, and that the timing of the sensitive period is itself dependent on
temperature. Sex differentiation depended on the initial temperature until Day
17 in the group reared at 30 ¡C and Day 22 in the group reared at 23 ¡C. The
sex ratio was greatly affected by a high temperature regime (0Ð7.7% females at
30 ¡C). At temperatures
between 17 and 30 ¡C, the
percentage of males increased with temperature.( Rie G. k. et al ;2006) In
conclusion, water temperature can play an important role in the process of sex
differentiation in goldfish.
temperature sensitive period (TSP) has also been demonstrated in several
species of fish (Koumoundourus et al., 2002) they show that Temperature is
responsible for sex determination in the European sea bass, Dicentrarchus
in phenotypic expression of sex was examined in hime salmon, a land-locked type
of sockeye salmon Oncorhynchus nerka (Teruo A. et al., 2004), thus produce
all-female populations in practice to develop effective salmon aquaculture in a
safe, simple, environmental friendly. Craig et al.(1996) observed female-biased
sex ratios in fish reared at high temperature during embryonic development in
hatcheries, suggesting that incubation temperature is probably the factor
responsible for a shift in sex ratio in sockeye salmon, O. nerka. Although
Craig et al. suggested further studies were necessary, there were no rigorous
tests on thermosensitivity of gonadal sex determination in salmoids thereafter.
experiments showed that effects of male producing temperature on sex
differentiation were greater than the effect of temperature that allowed the
expression of the genotypic sex. Although we have seen consistent effects of
temperature on sex differentiation of goldfish in laboratory experiments, such
effects may not be seen in the fish from the wild. Even though there is great
temperature fluctuation at goldfish spawning grounds, sex may be determined by
genetic factors, provided that the basal temperature is below male-producing
temperature. In conclusion, water temperature can play an important role in the
process of sex differentiation in goldfish (Rie G.K. et al 2006).
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