Hybridization between ecotypes in a phenotypically and ecologically heterogeneous population of Iris savannarum (Iridaceae) in Florida.

Authors: Meerow, Alan; GIDEON, MICHAEL - Gideon'S Iris; Nakamura, Kyoko

Submitted to: Plant Species Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/31/2016
Publication Date: 1/6/2017
Citation: Meerow, A. W., Gideon, M. and Nakamura, K. 2017. Hybridization between ecotypes in a phenotypically and ecologically heterogeneous population of Iris savannarum (Iridaceae) in Florida. Plant Species Biol. doi:10.1111/1442-1984.12158

Interpretive Summary: Abstract. Iris series Hexagonae is a small complex of 5 species and associated hybrid populations, popularly known as the “Louisiana irises” that are popular ornamental plants. The Hexagonae alliance of Iris have been recognized as a model of hybridization between numerous studies in Louisiana. We previously explored patterns of genetic structure and diversity in Florida populations of the complex, all of which had been assigned to either I. hexagona or I. savannarum. Populations derived from the Central Highlands of peninsular Florida, which are genetically isolated from all other peninsular Florida populations, occupy high, dry habitats and are more closely related to species of Hexagonae iris outside of peninsular Florida. These populations have a distinctive floral “highlands” form. Aquatic populations of I. savannarum, found towards the Gulf coast and low-lying inland areas, also have a consistent floral form (“coastal”). Jacks Branch slough in Glades County harbors the largest population of Hexagonae iris sampled in our previous studies. It is an ecologically heterogenous site, punctuated by sandy uplands that remain above high water and harbor iris with the highlands floral form as well as coastal types in the wetter areas of the slough. We hypothesize that the two genetic groups have come into secondary contact only in the last 5000 years as sea levels rose to their present extent. We further hypothesized that intermediate sites in the slough would host hybrids of the two ecological groups, and tested these hypotheses with 19 microsatellite DNA markers. All data analyses support our hypothesis of hybridization between the upland and aquatic subpopulations. The aquatic subpopulation shows genetic characteristics of recent expansion. Hybridization was asymmetric with introgression from the “highland” parent. We suggest that the processes described interspecifically for Hexagonae iris commence early at the population level.

Technical Abstract: Iris series Hexagonae is a small, monophyletic complex of 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 Louisiana. We previously explored patterns of genetic structure and diversity in Florida populations of the complex, all of which had been assigned to either I. hexagona or I. savannarum. Populations derived from the Central Highlands of peninsular Florida, which are genetically isolated from all other peninsular Florida populations, occupy high, dry habitats and are more closely related to species of Hexagonae iris outside of peninsular Florida. These populations have a distinctive floral “highlands” phenotype. Hydrophilic populations of I. savannarum, found towards the Gulf coast and low-lying inland areas, also have a consistent floral phenotype (“coastal”). Jacks Branch slough in Glades County harbors the largest population of Hexagonae iris sampled in our previous studies. It is an ecologically heterogenous site, punctuated by sandy uplands that remain above high water and harbor iris with the highlands floral phenotype as well as coastal types in the wetter areas of the slough. We hypothesize that the two genetic groups have come into secondary contact only in the last 5000 years as sea levels rose to their present extent. We further hypothesized that mesic, but non-inundated sites in the slough would host hybrids of the two phenotypic groups, and tested these hypotheses with 19 microsatellite loci. All data analyses support our hypothesis of hybridization between the upland and hydric subpopulations. The hydric subpopulation shows the least admixture, no linkage disequilibrium, highest effective population size, and highest number of private alleles. While two methods of introgression analysis hybrid identify some of the individuals from the admixture zone as first generation hybrids, subsequent admixture was asymmetric with introgression from the “highland” parent. We suggest that the processes described interspecifically for Hexagonae iris commence early at the population level.