Mitochondrial‐dna Restriction‐site Polymorphisms in the Teleost Fundulus Heteroclitus Support Secondary Intergradation

Fundulus heteroclitus is a highly polymorphic fish distributed along the Atlantic coast of North America. Several loci show directional changes in gene frequency with latitude (i.e., clines). Such directional changes have classically been described by two general models: primary and secondary intergradation. Previously, it has not been possible to distinguish between these models for Fundulus heteroclitus on the basis of allelic isozymes or morphological data. However, recent analysis of mitochondrial-DNA (mtDNA) restriction electromorphs helps resolve this issue. Mitochondrial-DNA samples from 48 individuals representing four populations were digested with 17 restriction endonucleases. After electrophoresis, the sizes of the mtDNA fragments were used to analyze the phylogenetic relatedness of fish collected over most of the species range. The analysis clearly identified two major races within the species: a northern and a southern form. The distribution of the mtDNA electromorphs, combined with zoogeographical changes in allelic isozymes and in eggs and adult morphologies (published elsewhere), makes the secondary-intergradation hypothesis most compelling.