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ABSTRACT1. Grouper are of considerable economic value in tropical and subtropical regions and most particularly in south-east Asia. Moreover, it is in Asia, where this fish is highly prized, that the grouper farming began in the 1980s.2. The organoleptic qualities of the groupers are not the only characteristics to be appreciated; in culture, their robustness in heavily populated conditions, as well as their rapid growth at elevated temperatures makes them a good species for aquaculture. However, it is, above all, market demand that is outstripping the supply of fish, which is motivating the expansion of grouper aquaculture.3. At the present time, aquaculture is unable to satisfy demand. The chronic shortage of broodstock means that the alevins, destined for aquaculture, are taken mainly from natural habitats with the aid of fish traps, nets and cyanide, thus contributing to the destruction of south-east Asian reefs. Nevertheless, research is progressing and the number of species whose reproduction is totally controlled is increasing. It is in Taiwan that grouper aquaculture is at its most advanced with a total of five species being raised. 4. Aquaculture is divided into four stages, carried out by different farmers, allowing a more rapid turnover for each farmer and therefore a better financial return. On the other hand, in the Mediterranean at present time, there are no completely controlled grouper captive-breeding programmes. The first attempts at controlled breeding have been carried out since 1995 in Italy, Croatia and Greece. However, farming groupers in captivity, and in particular the control of their reproduction, poses numerous problems.

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Abstract.-Differential introgression of mitochondrial genomes has been used to explain the occurrence in some species of individuals bearing mtDNA from a related species. This situation has been observed for Drosophila mauritiana (endemic to Mauritius) where a high proportion of individuals (88%) carries an mtDNA also found in D. simulans populations from Madagascar and Reunion. Using these two species, experiments were carried out to test for differential mtDNA introgression. A single virgin female from one species (initial frequency 0.03) was introduced into a population of the other. D. simulans mtDNA can, within three generations, almost entirely displace (frequency up to 0.80) D. mauritiana mtDNA. Hybrid male sterility probably curtails to a large degree parallel introgression of nuclear genes. The progress of cytoplasmic introgression is dependent on the degree of inbreeding of the recipient D. mauritiana strains. In reciprocal experiments, introgression was much less likely: few D. mauritiana migrant females are inseminated and their mtDNA frequency always remains very low. The results of these experiments support the hypothesis that a selective advantage of hybrids (probably at the nuclear level) has promoted mtDNA transfer from D. simulans Madagascar or Reunion populations into D. mauritiana through introgressive hybridization.