The Genetic Mosaic ofIrisSeriesHexagonaein Florida: Inferences on the Holocene History of the Louisiana Irises and Anthropogenic Effects on Their Distribution

Abstract

Iris series Hexagonae is a small, monophyletic complex of five species and associated hybrid populations, popularly known as the “Louisiana irises.” The Hexagonae alliance of Iris has been recognized as a textbook case of introgressive hybridization on the basis of numerous studies in Louisiana. In light of field observations of phenotypic heterogeneity among Florida populations, we investigated genetic variation within and among 37 populations of Iris series Hexagonae from Florida, seven from Louisiana, and one from Texas with 19 microsatellite loci. We wished to test the hypothesis that the Lake Wales Ridge and Polk Uplands of Florida, which remained above sea level maxima during the early Pleistocene, might be the sites of origin for all extant iris populations in Florida and possibly the series Hexagonae in its entirety. We also hypothesized that phenotypic heterogeneity observed in Florida iris would be reflected in patterns of allelic diversity. Finally, we wished to seek evidence of Native American influence on the present distribution of iris in Florida. Most populations display clonality, including three populations that are predominantly one or two clones. Levels of gene diversity are high and the populations are highly differentiated, although 19 exhibit significant homozygote excess. The majority of genetic variation occurs within populations. Genetic distance resolves three large clusters. One unites the southernmost populations with those from north-central Gulf coastal Florida. A second encompasses southwest Gulf coastal populations. If three compound repeat loci are dropped from the data, these two clusters are united. The third group joins populations derived from the Central Highlands of peninsular Florida, which are genetically isolated from all other peninsular Florida populations, with populations of Iris fulva, Iris brevicaulis, Iris giganticaerulea, and Iris hexagona s.s., supporting recognition of the latter two as distinct species. Model-based Bayesian clustering supports high population differentiation () and isolation of Central Highlands populations, and it resolves the terminal clusters of the genetic distance topology. Isolation by distance is significant yet weak because of genetic distance relationships that contradict biogeographic expectations. We propose that the Lake Wales Ridge and Polk Uplands, which constituted the Wicomico shoreline during an early Pleistocene interglacial inundation, functioned as refugia for series Hexagonae. Moreover, we suggest that Florida iris populations occupying high, dry habitats close to the Central Highlands ridges represent relicts of once-larger populations that adapted to the more xeric condition during the last glacial maximum. These populations have a distinctive floral phenotype and are related to species of Hexagonae iris outside of peninsular Florida. Excessively clonal populations may have been deliberately cultivated by Native Americans, which may also have influenced the connection between southern and north-central Florida populations, but there is insufficient evidence to validate this hypothesis. Many populations test positively for recent bottlenecks, which we attribute primarily to founder effects, given the low migration rates of the species and the high degree of population differentiation as well as the Holocene geological history of the Florida peninsula. We present evidence of peripatric divergence in series Hexagonae iris and suggest that this may function as a significant generator of species diversity in the group.