Genetic analysis of population structure in marine teleosts around Japan

Genetic differentiation and genetic variability were estimated from data on electrophoretically detectable isozymic loci in 12 species of marine teleosts belonging to 10 families of six orders, this study permitted conclusions concerning population structures within the species of marine teleosts. As a result the 12 species could be grouped into two classes, one of which showed higher genetic diversity and included the species inhabiting coastal areas; the other showed lower genetic diversity and included the species inhabiting offshore areas and spawning free pelagic eggs. Class I and Class II could be further divided into two groups (I-1 and I-2, and II-1 and II-2) on the basis of the level of average heterozygosity. Based on the above classification and ecological characteristics, the degree of genetic differentiation may be defined mainly by the level of mixing of eggs and by fry migration, and only to a small extent by adult migration between localities. The degree of average heterozygosity should be a measure of effective population size of the breeding units within the species but not of the whole species. Putting together the above facts, six types of population structure have been proposed for marine teleosts as follows: Species in type I-1 have formed some large populations with large breeding units in offshore areas; Type I-2-1, species have formed a large number of breeding populations with relatively large effective sizes of breeding units in the coastal areas or rivers where they were hatched, even though the species may have migrated over a wide pelagic area during their juvenile to adult stages. The species in type I-2-2 would have formed a number of local subpopulations with large to intermediate breeding units in the coastal areas they inhabit. Type I-2-3 species have formed two or more completely isolated local races (or subspecies) with relatively large breeding units, but with a low level of genetic differentiation within each local race. The species in Type II-1 would have formed a single large population consisting of a few breeding units with large effective sizes in offshore areas around Japan. Type II-2 species would have formed one large population consisting of a large number of breeding units with small effective sizes in wide offshore areas around Japan.