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The spatial variation in gene frequencies of four unlinked polymorphic loci was studied in the teleost Fundulus heteroclitus. Three loci (Idh-A, Idh-B, and Est-S) exhibit significant north-south clinal variation in allelic frequencies along the Atlantic Coast of North America, while a fourth locus (6-Pgdh-A) shows a modest clinal variation. These data, together with our previous data for Ldh-B, Mdh-A, Gpi-B, and Pgm-A, reveal a pattern of low gene diversity in the colder northern extremes of the species range and high gene diversity in warmer southern latitudes.

We have isolated, cloned, and sequenced the rainbow trout (Salmo gairdneri) c-myc gene. The presumptive coding region of the trout c-myc gene shows extensive homology to the c-myc genes of chicken, mouse, and human. Comparison of nucleotide sequences reveals that human, mouse, chicken, and trout c-myc genes contain at least two coding exons, interrupted by introns of decreasing size of 1.38 kilobases (kb), 1.2 kb, 0.97 kb, and 0.33 kb, respectively. The exons are clearly delineated by donor-acceptor splice signals. The degree of nucleotide homology between trout, chicken, and human exon II is less than that observed for exon III. However, the greatest homology among these three genes is localized to two specific regions within exon II (myc boxes A and B). At the predicted amino acid level, fish c-myc shows considerable homology to vertebrate c-myc gene products. Trout c-myc is expressed in normal trout cells as a single 2.3-kb mRNA species, similar in size to other vertebrate transcripts.

Starch gel electrophoresis has shown that natural populations of Fundulus heteroclitus have variants at four enzyme-coding loci: Idh-A, Idh-B, 6-Pgdh-A, and Est-S. Analysis of the phenotypic distribution of the F1 generation suggests that each of the variants segregates as autosomally inherited codominant alleles. Tissue specificity and intracellular localization were also determined for the IDH and 6PGDH isozymes.