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Sex-Determination System In Fishes

Kiran Rasal; Makwana Nayan; Amar Gaikwad; Rashmi Ambulkar

Central Institute of Fisheries Education, Mumbai 400 061.

Correponding Author-



A 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


The 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 female altogether.

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

Studies 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 36.54C.produced significantly higher percentages of males compared to controls reared at ambient temperature 27.87 C  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 25.78C.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


Evidence 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 temperature

dependent sex determination and its adaptive significance in the Atlantic Silverside,

Menidia menidia (Conover and Kynard, 1981; Conover, 1984.) Other environmental

extremes such as low pH have also been observed to affect sex ratio in Cichlids and

Poecilids Rubin, 1985.

A 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 19C.the sex ratio of O. mossambicus in one experiment had a significantexcess of males  89%. compared to the control reared at 28C mechanism.

            The 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; Strssmann et al., 1996a; Conover and Kynard) 1981)

In 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 (07.7% 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.

            A 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 labrax

Thermolability 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.



Temperature fluctuation 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).




Baroiller, J.F., D'Cotta, H., 2001. Environment and sex determination in farmed fish. Comp. Biochem. Physiol. 130, 399409


Baroiller, J.F., Chourrout, D., Fostier, A., Jalabert, B., 1995. Temperature and sex chromosomes govern sex ratios of the mouthbrooding Cichlid fish Oreochromis niloticus. The Journal of Experimental Zoology 272, 213223.


Baroiller, J.F., Chourrout, D., Fostier, A., Jalabert, B., 1995b.Temperature and sex chromosomes govern sex ratios of the mouthbrooding cichlid fish Oreochromis niloticus. J. Exp. Zool. 273, 216223


Baroiller, J.F., Nakayama, I., Foresti, F., Chourrout, D., 1996. Sex determination studies in two species of teleost fish, Oreochromis niloticus and Leporinus elongatus. Zool. Stud. 35, 279285.


Conover, D.O., 1984. Adaptive significance of temperature dependant sex determination in a fish. Am. Nat.

123, 297313


Conover, D.O., Kynard, B.O., 1981. Environmental sex determination: interaction of temperature and genotype in a fish. Science 213


Craig, J.K., Foote, C.J., Wood, C.C., 1996. Evidence for temperature-dependent sex determination in sockeye salmon (Oncorhynchus nerka). Can. J. Fish. Aquat. Sci. 53, 141147.

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