Characterization of Mitochondrial DNA Variability in a Hybrid Swarm Between Subspecies of Bluegill Sunfish (Lepomis macrochirus)

We begin a characterization of the evolutionary dynamics of mitochondrial DNA (mtDNA) in fishes by examining restriction site variability in 189 bluegill sunfish (Lepomis macrochirus). A total of 15 endonucleases was employed to map 37 restriction sites in mtDNA from selected individuals. Genome size was approximately 16.2 kilobases. All differences between genotypes could be accounted for by gains or losses of individual sites, without additions, deletions, or rearrangements affecting more than about 50-250 base-pairs. Two highly distinct mtDNA genomes, differing by 20 assayed mutation steps and an estimated 8.5% sequence divergence, were discovered. Both genomes were observed in high frequency in a sample of 151 bluegill from a north-Georgia population, which on the basis of allozyme genotype appears to represent a freely-interbreeding hybrid swarm between two bluegill subspecies. Within the hybrid population, mtDNA and allozyme genotypes were associated approximately at random. The distinct genetic markers provided by the mtDNA genomes provided an improved test of the possibility of within-individual mtDNA polymorphism because true heteroplasmicity could readily be distinguished from results of incomplete digests. Nonetheless, in somatic or germ cells, no individual bluegill exhibited mtDNA heteroplasmicity (at 5% or greater level) that could be explained by paternal mtDNA transmission. Additional samples from Louisiana to Florida tentatively confirm that the geographic distributions of the two distinct mtDNA genomes are highly concordant with the previously described ranges of L. m. macrochirus and L. m. purpurescens defined by morphology and allozymes. Overall, the results on mtDNA variability in bluegill conform to and strengthen some of the more straightforward expectations about the pattern of evolution of uniparentally-transmitted genomes in sexually reproducing populations.