MITOCHONDRIAL-DNA DIVERSITY IN FISHES AND ITS IMPLICATIONS FOR INTRODUCTIONS

Patterns of mitochondrial DNA (mtDNA) variation have now been examined in more than 40 fish species. This work has established that mitochondrial genomes of fish show constrained size variation, but that genomes of closely related species often vary in size, suggesting that counterbalancing deletions and insertions act to stabilize genome size. Gene order and composition are also stable, but nucleotide sequences show rapid divergence. Populations of most fish species contain a few common mtDNA haplotypes and a spectrum of their rare mutational derivatives. The extent of genetic divergence among dominant haplotypes seems linked to environmental conditions with greater divergence in freshwater than marine species, especially those occupying unglaciated localities. Haplotype frequencies ordinarily show considerable local divergence, largely as a result of stochastic shifts associated with population founding events. Such divergence provides a basis for both the discrimination of local stocks and for the recognition of major genetic discontinuities reflecting past episodes of gene pool fragmentation. Analysis of hatchery stocks has often revealed their mtDNA divergence, but in most cases stocks are fixed for haplotypes which are also common in wild populations. There is, however, the opportunity to develop broodstocks marked with rare haplotypes, permitting their recognition in nature over many generations.