Sex determination in Fish

 

Published By, Degonto Islam

Department of Fisheries and Marine Science, 

Noakhali Science and Technology University.

Introduction

In many species, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual morphology. Genotype in case of fish is not exact same like other animal. In animals this is often accompanied by chromosomal differences, generally through combinations of XY, ZW, XO, ZO chromosomes which is  triggered by a main gene (a "sex locus").

Sex determination

Sex determination is generally considered to give rise to the genetic sex of the individual and take place at the moment of fertilization. A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most organisms that create their offspring using sexual reproduction have two sexes. In higher vertebrates, sex determination in different species is considered as highly correlated processes. The means by which the development of the differences between the sex is initiated called sex determination. The actual development of primary and secondary sexual characteristics is dependent upon hormonal cues, particularly these from the gonad themselves.

Method of sex determination are known for relatively few species of fish, but there are nine known systems in fish, and sex is controlled by sex chromosomes in eight of these systems. The sex chromosome are some species are morphologically distinct and can be identified. But those in other species are not morphologically district and are inferred from sex-reversal studies, hybridization experiments, chromosomal manipulation or analysis of sex-linked phenotypes.

Expression of the male and female phenotypes depends on the genotypes of the fish. The germinal cells start colonizing the gonadal ridges, increasing in number by successive mitotic division. The bipotential germ cells is entirely dependent on the genotype of the individuals. This is associated with the presence or absence of SRY (SEX  DETERMINATING REGION Y), a gene located on the Y chromosome coding for a testis determining factor. Fish exhibit a higher degree of plasticity in terms of sex determination than mammals.

 

Table : Nine sex determination system

Species	System
Channel catfish	XY           XY
Rainbow trout
Lake trout
Coho salmon
Chinook salmon
Sockeye salmon
Tilapia nilotica
Tilapis mossambica
Common carp
Silver carp
Bighead carp
Grass carp
Gold fish
Guppy
Medka
Tilapia aurea	WZ
Tilapia hornorum
Mosquito fish
Japanese eel
Congor eel
Platy fish
Doller hatch fish	WXY
Dwarft gourami	XO
Catarine pupfish	ZO
File fish	X1X1X2X2/X1X2Y
Freshwater gobi
Virolito
Hoplians sp. from Aripuna river	ZZ/ZW1W2
Brazil	XY1Y2/XX
Swordtail	Autosomal
Blue Poecilia	Autosomal

 

Sex Determination: Origin and Evolution

Hierarchical  development controlled process would facilitate the control of sex ratio by a single-gene mechanism, but at the same time it would open the opportunity for changing the SD factor in response to new evolutionary scenarios. Theories on the evolution and genetic architecture of SD in animals have been largely influenced by studies on Drosophila, mammals, and birds, all of them showing convergent patterns, with a heteromorphic sex chromosome pair and, as a consequence, a particular sex-linked inheritance model .

A generalized theory on the origin and evolution of SD systems emerged from these data, which assumed a sexual conflict between antagonistic alleles at specific loci favorable to one sex but detrimental to the other . To maintain the beneficial association between the antagonistic allele and the SD locus, recombination would be restricted, giving rise to the permanent heterozygous state of that portion of the sexual pair . That circumstance would promote the accumulation of repetitive elements and deleterious variants in the SDg-bearing chromosome, contributing to its progressive degeneration and the typical heteromorphic shape of the sexual pair . Mathematical models based on this theory suggested that only one gene should underlie the SD system, and if more than one gene were segregating, this should represent an unstable equilibrium towards a new SD mechanism.

XY sex determination system:

Sex chromosomes in female are identical and these is male are mismatched pair. Female are XX and male are XY. So female are called homogametic and male are heterogametic. Heterogametic always determine the sex of the progeny. Regardlessof sex-determining system, the heterogametic sex is the sex tha tis usually determines sex of the offspring.in the xy sex determinating system, the Y chromosome is the chromosome that determines sex. Eg; Nile tilapia ( Oreochromis NIloticus)

   Male                                               Female

Because Y chromosome is the chromosome that determines sex, the mael is the parent that determines sex in species that have the XY sex determining system.offspring that receive his Y chromosome become males , while those that receive his X chromosome become female.

WZ sex determination system:

Sex chromosome in male are identical and female are mismatched pair. Females are WZ and males are ZZ. Heterogamatic female determines the sex. Eg; Red tilapia ( Oreochromis aureus).

                                      Female                                       Male

This system is the mirror image of the XY sex determining system. In this system, the sex chromosome in the male from an identical pair, while those in the female form a mismatched pair. Male are the homogametic sex , while females are the heterogametic sex.

In the WZ sex determining system, the W chromosome is the chromosome that determines sex. Offspring that receive her W chromosome become female, While those receive her Z chromosome become males.

Multiple sex chromosomes:

This sex determination system have  multiple chromosomes. All these are variations on either the XY or the WZ sex determinating systems.

 

Multiple X chromosome System:

In this sex determination system, Female are X₁X₁X₂X₂ amd male are X₁X₂ Y are homogametic and males are heterogametic. Heterogametic determine the sex of the offspring. Consequently the male determine the sex of his offspring. E.g.; File fish.

Male                          Female

Multiple W chromosome system:

Female are W₁W₂Z and males are ZZ. Female are heterogametic. Female determine the sex of the offspring .males are the homogametic sex, and female are the heterogametic sex. As a result, the female determines the sex of her offspring. Those that receive her W₁W₂ chromosomes become females, while those that received her Z chromosome become male.Eg; Virolito.

Multiple Y chromosome system:

Males are XY₁Y₂ and females are XX. Heterogametic male determine the sex of the offspring  .

Male are heterogametic sex, and female are homogametic sex. Thus a male determines the sex of the offspring. Those that receive his Y₁Y₂ chromosomes become males, while those that receive his X chromosome become female. Eg; Hoplias sp.

 

WXY  chromosome system:

This is a variant on the XY sex determination system. The W chromosome is a modified X chromosome. It can block the male determining ability of the Y chromosome. So XY and YY fishes are male while XX,WX,WY fishes are female. Both male and female can either homogametic heterogametic. E.g.; platy fish.

XO system:

There is only one sex chromosome. The XO is a variant of the XY system. Female are XX and males XO ( O is the symbol for no chromosome) heterogametic male determine the sex of the Progeny, e.g.; those received X chromosome become female, while those receive no sex chromosome become male. e.g.; Doller hatchet fish.

ZO system:

It is a variant of the WZ system. Here female are ZO, and males are ZZ. Heterogametic females determine the sex of the offspring. Here females are heterogametic sex, while males are the homogametic sex. In the ZO sex-determining system, a female determines the sex of her offspring. Those that receive her Z chromosome become males, while those that receive no sex chromosome become females. e.g.; Dwarf gourami.

Autosomal sex determination system:

Here there is no sex chromosome and sex determined autosomally. Sex is determined by the number of male or female genes that are located on the autosomes. Eg; Sword tail Male swordtail has a long codal fin. But in case of female the codal fin was short.

Figure : Swordtail Fish

Figure: Blue Poecilia

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 such in some fishes (salmon, gold fish, tilapia etc)

A variety of sex determination systems exists in reptiles and fishes, including temperature-dependent sex determination (TSD).. 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.

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

 

Application of Sex determination system

·         Hybridization experiment

·         Production of monosex population

·         Chromosomal manipulation

Understanding the way sex is determined is not trivial. It is used in production of monosex population through hormonal sex reversal and chromosomal manipulation. The major goal of tilapia breeding projects is to produce monosex populatons to prevent reproduction during growout.

Hybridization is one technique that has been used to try and produce all male populations. The use of this technique in producing all male populations is based on a simple system of sex determining, one controlled only by sex chromosome.

 

Conclusion

The XX/XY sex-determination system is the most familiar sex-determination systems. Sex determining in fish is not simple method. If species have X1X1X2X2/X1X2Y, Z W1W2/ZZ, XY1Y2/XX, XO, ZO sex determining system , number of chromosomes is not constant within species. Sex determination dependent also varying for the degrees of temperature. Sex determination technique has the most advantage in the hatcheries in producing selective offspring.

 

Reference

Douglas, T., (1986), “Genetics for Fish Hatchery Managers”, AVI Publishing Company.

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