Admixture Analysis of Florida Largemouth Bass and Northern Largemouth Bass using Microsatellite Loci

—Microsatellite DNA variation was examined at 11 loci in five populations of Florida largemouth bass Micropterus salmoides floridanus (N 1⁄4 175) and eight populations of northern largemouth bass M. s. salmoides (N 1⁄4 249). 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 1⁄4 24) were direct descendents of Florida largemouth bass (either remnants or direct descendants from introductions), whereas others (N 1⁄4 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. The divergence of lineages within largemouth bass Micropterus salmoides (Bailey and Hubbs 1949) was largely influenced by Pliocene sea level fluctuations (Near et al. 2003) and Pleistocene glaciation events (Nedbal and Philipp 1994). The native range of the Florida largemouth bass M. s. floridanus consisted of peninsular Florida, while the northern largemouth bass M. s. salmoides extended throughout northeastern Mexico, southeastern Canada, and the U.S. corridor in between (MacCrimmon and Robbins 1975). Furthermore, genetic distinctions between northern largemouth bass at northern and southern latitudes have been identified (Philipp et al. 1983; Nedbal and Philipp 1994). The genetic distinctions among these groups, however, are continually being diluted by introgression (Philipp et al. 1983). The resulting ambiguity over the genetic composition of populations poses problems for researchers and resource agencies working with largemouth bass, particularly when it is of interest to quantify stocking events, identify gene flow within or from targeted systems, or resolve the identity or origins