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reprofish

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Sexual differentiation & sex control

Technical factsheet describing the sexual differentiation process in fish


© Marie,D.Inra
The development of a fish into a male or a female may be dependent on many factors.

What we know about sexual differentiation in teleost fish
In numerous species of fish, sex is determined principally genetically. Different sex determination mechanisms co-exist, such as the "mammalian" XX/XY, or the "avian" ZZ/ZW, sometimes even within the same fish gender. Only a few species possess morphologically differentiated chromosomes. For most fish species the differentation of the gonad into a testis or an ovary can be strongly influenced by hormonal, environmental or social factors, whose modes of action are on the whole poorly understood.Among the environmental factors influencing fish sex differentiation, temperature is the best characterised for an increasing number of fish species. However, sensitivity to pH, density and population sex ratio has also been observed. Hormonal influence on sex differentiation in fish is well documented, in particular the key role of oestrogen, a female sex steroid hormone, which is crucial for ovarian differentiation.
The stage in development at which sexual differentiation occurs differs widely between species. It may be defined before hatching, as in the medaka, or after hatching, as for the trout or the tilapia. However, in some species sexual differentiation does not occur until several years into life; for example the European sea bass does not mature until 1-2 years of age, and the European eel may take up to 6 years to mature. Some fish species possess both male and female gonads, either simultaneously (simultaneous hermaphrodites), or successively. Some are even capable of differentiating into both males and females, and in a reversible manner.
Why controlling sex in fish is so valuable to the fish farming industry
Many, if not all, commercially important species display significant differences in size between the sexes. In some species, one sex enters maturity earlier than the other (e.g. male salmonids); in others reproduction may start very early in life and occur all year round in captivity (e.g. tilapia). Reproductive status greatly affects individual growth rates as less energy is committed to somatic growth.
The ability of a fish farmer to control the ratio of male to female fish, or to produce single-sex stocks, has significant economic implications. By controlling the ratio of males to females, the fish farmer can reduce the quantity of broodstock necessary to obtain a given egg take, or increase egg take by rearing mainly females. The sex showing the greatest growth potential can also be selected for; for example, the female turbot or sea bass can grow to a much larger size than the male. The target may also be to produce a product of great commercial value that only one of the sexes can produce, as with caviar production in the sturgeon. The selection for sex can also help when there are problems associated with precocious puberty in males, as is the case with salmonid fish, such as trout and salmon, and also the sea bass (see the puberty and biotechnology factsheets).
In addition, some species of hermaphrodite fish, such as the Atlantic dusky grouper (Ephinephelus marginatus) are only present as males in the wild at a certain size; it is therefore difficult to maintain a sex-balanced broodstock in captivity without considering sex-ratio control.
How is fish sex currently controlled?
Fish sex may be manipulated by the direct application of hormones. The administration of oestrogens can produce females, anti- or androgen administration may produce males. However, direct hormonal methods are not permitted in many countries, indeed within the European Union only the use of certain androgens (methyltestosterone) is permitted in fish farming and then only under certain derogations (see legislation section for further details). Methyltestosterone treatment produces fish that are genetically female, but phenotypically male, or "neo-males", which are then crossed with normal females in order to obtain monosex female progeny (i.e. the treated fish are not consumed directly and must be discarded according to strict regulations). This method is currently used on rainbow trout in order to obtain large female individuals for the market, and to avoid the problem of precocious male puberty. Further details on monosex populations can be found in the biotechnology factsheet.
Hormonal treatments have certain limitations and associated problems. One major limitation with steroid treatments is that one protocol is not effective for all species (dosage, timing of treatment, method of administration etc.), even for species that are genetically very close (e.g. within the salmonid family). Hormonal treatments are very efficient, but they could pose environmental problems in the future, due to uncontrolled discharge of the resulting waste water (i.e. steroids and / or metabolites are not removed from the water before discharge). The potential for localised pollution downstream of the discharge has not yet been investigated.
Production of all female trout populations

What are the hopes for future developments?
Among the alternatives to hormonal treatments, methods based on the environmental sex determination sensitivity of some fish have great potential. However, despite significant advances in the understanding of environmental sensitivity, especially for temperature, these methods are not yet practicable by the industry.
There is therefore a need to improve our general understanding of the hormonal and environmental factors determining sexual differentiation, in order to improve sex control methods in captivity and to ensure that they are environmentally sustainable. This global understanding may be achieved by both comparative and integrative biological methods. The development of functional genomic approaches, and the use of different fish models, will also booster knowledge on the role genes play in the sex differentiation process and how their action is affected by sex hormones and environmental factors.
The outputs of this research should help to provide sex control methods that are more efficient and environmentally sustainable (moving away from hormone use). Research into the delineation of the sensitive period for hormonal treatments will aid towards finding a more direct application for such treatments in emerging aquaculture species.

This factsheet was prepared with the expert assistance of scientist Yann Guiguen.

See also

Laboratories working on sexual differentiation & control
A selection of useful references to consult:

Piferrer, F. et al. 2005. Genetic, endocrine and environmental components of sex determination and differentiation in the European sea bass (Dicentrarchus labrax). GCE 142: 102-110.
Baroiller, J.F., Guiguen, Y. 2001. Endocrine and environmental aspects of sex differentiation in gonochoristic fish. EXS (91): 177-201.
Piferrer, F. 2001. Endocrine sex control strategies for the feminisation of teleost fish. Aquaculture 197: 229-281.
Baroiller, J.F., D'Cotta, H. 2001. Environment and sex determination in farmed fish. Comp Biochem Physiol C Toxicol Pharmacol. 130(4): 399-409.
Sadovy, Y., Shapiro, D.Y. 1987. Criteria for the diagnosis of hermaphroditism in finfish. Copeia 1: 136-156.