Microsatellite DNA variation was examined at 11 loci in five populations of Florida largemouth bass Micropterus salmoides floridanus (N ¼ 175) and eight populations of northern largemouth bass M. s. salmoides (N ¼ 249). Distinct allele frequencies with 33 private alleles between subspecies (threshold frequency, !0.05) and 19 private alleles among three geographic regions distinguished between Florida largemouth bass and northern largemouth bass as well as between northern largemouth bass from northern and southern latitudes in North America. Variation at microsatellite loci also provided sufficient resolution to discriminate among some populations within regions. Allele frequencies indicated that of 37 trophy bass (defined here as fish weighing 5.90 kg or more) donated to the Texas Parks and Wildlife Department (TPWD) between 2004 and 2005, all had more than 50% Florida largemouth bass influence with ancestry genetically similar to that of populations sampled in western Florida. Some fish (N ¼ 24) were direct descendents of Florida largemouth bass (either remnants or direct descendants from introductions), whereas others (N ¼ 13) were admixed with northern largemouth bass. Of the 13 admixed fish, 11 had ancestry in lineages of southernlatitude northern largemouth bass. Genetic variation within northern largemouth bass populations was depressed at northern latitudes (mean heterozygosity, 0.37; SD, 0.26; mean number of alleles per locus, 2.91; SD, 1.51) relative to southern latitudes (mean heterozygosity, 0.52; SD, 0.25; mean number of alleles per locus, 4.57; SD, 2.88); Florida largemouth bass exhibited intermediate heterozygosity (mean, 0.41; SD, 0.32) and an allelic richness (mean, 4.51; SD, 4.58) similar to that of southern-latitude northern largemouth bass. Overall, the variation observed at these loci is greater than that at other codominant markers explored in this species, providing additional power to detect admixture in populations and individuals.
Florida bass Micropterus floridanus are endemic to peninsular Florida and co‐occur with largemouth bass M. salmoides in a natural intergrade zone in the northern portions of the state. In this study, we resolved the genetic population structure among populations of largemouth bass, Florida bass, and their interspecific hybrids from 48 lakes and streams across Florida, and we updated and refined the southernmost boundary of introgression. Nuclear (allozyme and microsatellite) and mitochondrial (restriction fragment length polymorphism) markers produced somewhat different results. Microsatellite genotypes alone and the combination of all nuclear genotypes resolved genetic structure among four regional groups within Florida: (1) northwest, (2) north‐central, (3) central‐west, and (4) south, central‐east, and northeast. Allozyme genotypes alone did not resolve well‐defined groups. The distribution of mitochondrial haplotypes suggested that populations from the St. Johns River drainage should be considered their own distinct group (rather than be included in the south—central‐east—northeast Florida group). Although mitochondrial markers failed to detect introgression throughout much of the intergrade zone, nuclear markers revealed an expected decline in introgression from west to east as well as from north to south. Largemouth bass alleles, however, were detected at low frequencies in 12 populations below the southern boundary of the accepted intergrade zone, some of which are the result of stocking. There was no evidence of nonrandom mating or selection against hybrids in the distribution of nuclear genotypes; however, the rarity of largemouth bass mitochondrial haplotypes suggests either assortative mating or reduced fitness in the progeny of female largemouth bass × male Florida bass crosses.
Introductions of nonnative smallmouth bass Micropterus dolomieu into central Texas streams resulted in introgressive hybridization with an endemic allopatric congener, the Guadalupe bass M. treculii. The objectives of this study were to use the variation at 14 microsatellite DNA loci to examine the frequencies of Guadalupe bass, smallmouth bass, and their hybrids 10 years after supplemental stockings of Guadalupe bass in the Blanco River. Genetic analysis identified 40% of individuals as smallmouth bass, 51% as smallmouth bass 3 Guadalupe bass hybrids, and 9% as other Micropterus hybrids. Pure Guadalupe bass were not collected. Despite supplemental stocking, the frequency of pure Guadalupe bass appears to have declined in the Blanco River in the past 10 years. These results emphasize the need for a current survey of the genetic status of the species throughout its native range.
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