Conservation genetics

9.5.4 Conservation genetics

The desirability of retaining or conserving genetic diversity is now widely accepted. Once lost, diver- sity cannot be restored. Unfortunately, we often have little knowledge of the extent of diversity

within a species or how this diversity is parti- tioned among populations. Without this informa- tion, it is not possible to assess the effectiveness or appropriateness of conservation measures. Some quantification of the extent of genetic population structuring of threatened or vulnerable species is therefore desirable. However, if this structuring is extensive, as is frequently the case for freshwater and anadromous species but less so for marine species (Ward et al. 1994), it may not be feasible to enact conservation measures to protect all compo- nents. Pacific salmonids, for example, are sepa- rated into numerous genetically distinct populations through their strong homing behav- iour. Allendorf et al. (1997) consider that more than 300 native stocks of these salmonids are at risk in the Pacific northwest and suggest how they may be prioritized for conservation actions using ranks based on the genetic and ecological conse- quences of extinction. Such actions might include habitat protection, reduction in harvesting rate, stock enhancement using strains from the stock to

be protected or, if that is not possible, the most closely related population, or translocation to a new habitat. Some genetic effects associated with the last two issues have already been considered. Another concern is the need to maintain genetic diversity levels as high as possible in any translo- cated population. The minimum effective popula- tion size of a hatchery or transplanted stock should

be, whenever possible, around 50–100 animals. A high population size and diversity permits some measure of adaptive evolution and reduces the deleterious effects of inbreeding.

Ryman et al. (1995) review the many threats to genetic diversity in fish and suggest three respons- es to these threats: (i) to ensure that those involved in operations such as wild harvesting, aquaculture and ocean ranching are aware of possible incom- patibilities with conservation objectives; (ii) to require a risk assessment before any potentially threatening activity commences; and (iii) to accept that whoever proposes a threatening activity is responsible for providing the burden of proof of the likely impact of that action. Further aspects of conservation are discussed by Reynolds et al. (Chapter 15, Volume 2).

Genetics

217

Chapter 9

et al. 1998) and inferred in Acipenser and Rhodeus

9.6 GENETICS OF SEX

by ploidy and gynogenesis manipulations

DETERMINATION IN FISH

(Kawamura 1998; Van Eenennaam et al. 1999). The lake trout (Salvelinus namaycush) is one of the few

Reproductive mechanisms in fish are highly salmonids with morphologically differentiated variable (reviewed by Devlin and Nagahama 2002). sex chromosomes, and the sex-determining region While most reproduce sexually, some are asexual. appears to lie on the short arm of the Y chromo- Some of the asexual species consist of all-female some (Reed et al. 1995). Given the difficulty of dis- gynogens, whereby matings with a male of a tinguishing sex chromosomes in most salmonids, related species are necessary to initiate embryoge- and the importance of sex manipulations in nesis but the male does not contribute genetic ma- aquaculture production, there have been consider- terial. Examples are the gynogenetic Poecilia able efforts put into isolating salmonid Y-specific formosa (sailfin molly), whose embryogenesis is probes. These are now available for five different initiated by sperm from Poecilia latipinna in Oncorhynchus species (Devlin et al. 1994; mixed-species shoals (Schlupp and Ryan 1996), Donaldson and Devlin 1996; Nakayama et al. and gynogenetic minnows of the Phoxinus eos- 1999), although not yet for Atlantic salmon. neogaeus complex. In the latter, examinations of

Environmental sex determination, principally DNA variability have revealed only one clone or through temperature, has been suggested for a genotype in more than 400 gynogenetic fish sam- range of fish from lampreys (Beamish 1993) to pled (Elder and Schlosser 1995). Another type of eels (Krueger and Oliveira 1999) and atherinids asexual reproduction is hybridogenesis. In Poecil- (Strussmann et al. 1997). In the cichlid genus iopsis , the all-female hybridogenetic P. monacha- Apistogramma , pH as well as temperature is a lucida biotype arose as an interspecific hybrid of P. significant determinant of sex in some species monacha and P. lucida. Subsequent maintenance (Romer and Beisenherz 1996). There is evidence of this biotype relies on matings with males of P. that environmental factors often overlay or modify lucida , but the lucida genome is discarded during the outcomes of a genetic switch (e.g. Conover oogenesis (Schultz 1969; Vrijenhoek 1994).

et al. 1992; Strussmann et al. 1997; Abucay et al.

Among sexual fish species, there is a wide 1999). Finally, some fish are protandrous hermaph- range of sex-determining systems, including both rodites, with sex changing from male to female genetic and environmental mechanisms. Genetic with age (e.g. sea bass or barramundi, Lates calcar- mechanisms in fish are varied, with Tave (1993) ifer ; Guiguen et al. 1993), while some are protogy- listing nine types. Fifteen fish species had the nous, with sex changing from female to male XX/XY chromosome type with homogametic (e.g. bluehead wrasse, Thalassoma bifasciatum; females and heterogametic males; five had the Kramer and Imbriano 1997). ZZ/ZW type with homogametic males and het- erogametic females; eight had other sex chro- mosome systems; and two had autosomal sex

9.7 CONCLUSIONS

determination. The XX/XY system is found in many aquacultured species, including channel It should be clear from the discussion in this chap- catfish, salmonids and carp. Often the sex chromo- ter that genetic analyses have a great deal to offer in somes are not morphologically differentiated and the study of fish populations. The range of tools is cannot be identified by chromosome staining tech- ever-expanding as new ways to harness the power niques; in such cases the system has to be inferred of PCR analysis are brought on-line. These dif- from sex-reversal or other genome manipula- ferent tools are suited to different problems, and I tions. For example, a ZZ/ZW mechanism has been have tried to outline both their benefits and limita- shown in Leporinus and Clarias by direct chromo- tions. The focus has been on the uses of genetics in some staining (Pandey and Lakra 1997; Molina fish identification and population structure analy-

Genetics

sis, but molecular genetic studies have also made numerous contributions to other areas such as sys-

ACKNOWLEDGEMENTS

tematics and aquaculture. Genetic identification of unknown or disputed

I thank Sharon Appleyard, Nick Elliott and Dan samples is usually unambiguous and simple; McGoldrick for comments on this chapter. protein or mtDNA-based tests are very suitable.

The analysis of population structure is far more demanding. Interpretation of frequently large and

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