The genetic mosiac of iris series Hexagonae in Florida: inferences on the Holocene history of the Louisiana irises and anthropogenic effects on their distribution.

Authors: Meerow, Alan; GIDEON, MICHAEL - Gideon'S Iris; Kuhn, David; MOPPER, SUSAN - University Of Louisiana; Nakamura, Kyoko

Submitted to: International Journal of Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/31/2011
Publication Date: 10/4/2011
Citation: Meerow, A.W., Gideon, M., Kuhn, D.N., Mopper, S., Nakamura, K. 2011. The genetic mosiac of iris series Hexagonae in Florida: inferences on the Holocene history of the Louisiana irises and anthropogenic effects on their distribution. International Journal of Plant Science. 172(8):1026-1052.

Interpretive Summary: The Louisiana irises are a small complex of 3-5 species and associated hybrid populations found in deep south north to the lower Ohio valley. The group has long been a textbook example plant hybridization in nature based on numerous studies in Louisiana, which resulted in as many as 80 species being recognized in the group at one time. Only a single species has been considered to occur in Florida. Field observations revealed a greater degree of variation in Florida populations than expected. We investigated genetic variation within and among 37 populations of the group from Florida, seven from Louisiana and one from Texas with 19 DNA repeat markers. 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 Louisiana iris populations in Florida. We also wished to seek evidence of Native American influence on present distribution of iris in Florida. Levels of gene diversity are high, and the populations are highly differentiated. Nineteen populations are significantly inbred. Genetic distance resolves three large clusters. One unites southernmost populations with those from north-central Gulf coastal Florida. A second encompasses southwest Gulf coastal populations. If three loci containing more than one repeat region 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 I. fulva, I. brevicaulis, I. giganticaerulea and I. hexagona s.s., supporting recognition of the latter two as distinct species. Isolation by distance is significant yet weak due to genetic distance relationships that contradict geographic expectations. We propose that the Lake Wales Ridge and Polk Uplands, which constituted the Wicomico shoreline during an early Pleistocene interglacial sea level rise, functioned as a refugea Lousiana iris. Moreover, we suggest that Florida iris populations occupying high, dry habitats close to the Central Highland ridges represent relicts of once larger populations that adapted to the dry conditions of the Last Glacial Maximum. These populations have a distinctive floral phenotype, and are related to species of Louisiana 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. We present evidence of a model of divergence in the Louisiana iris.

Technical Abstract: Iris series Hexagonae is a small, monophyletic complex of 3-5 species and associated hybrid populations, popularly known as the “Louisiana irises.” The Hexagonae alliance of Iris have been recognized as a textbook case of introgressive hybridization based on numerous studies in Lousiana. On the basis 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 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 is within populations. Genetic distance resolves three large clusters. One unites 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 I. fulva, I. brevicaulis, I. giganticaerulea and I. hexagona s.s., supporting recognition of the latter two as distinct species. Model-based Bayesian clustering supports a high population differentiation (K = 22), isolation of Highlands populations, and resolves the terminal clusters of the genetic distance topology. Isolation by distance is significant yet weak due to 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 Highland 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. 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 this may function as a significant generator of species diversity in the group.