ECOLOGICAL AND GENETIC ASSOCIATIONS IN AN IRIS HYBRID ZONE

Chloroplast DNA (cpDNA) markers and 12 nuclear (random amplified polym orphic DNA, or RAPD) markers were used to examine the distribution of genetic variation among individuals and the genetic and ecological ass ociations in a hybrid iris population. Plants in the population occurr ed at various distances from the edge of a bayou in a relatively undis turbed mixed hardwood forest and in an adjacent pasture dominated by h erbaceous perennials with interspersed oak and cypress trees. The majo rity of plants sampled possessed combinations of markers from the diff erent Iris species. Genetic markers diagnostic for Iris fulva and I. b revicaulis occurred at high frequencies, whereas markers diagnostic fo r I. hexagona were infrequent. For the majority of the nuclear markers , significant levels of cytonuclear disequilibria existed because of i ntraspecific associations among the markers in both the pasture and th e forest. The distribution of nuclear markers among individuals was bi modal; intermediate genotypes were absent and the majority of RAPD mar kers were associated with their intraspecific cpDNA haplotypes. Strong intraspecific associations existed among RAPD markers in the forest, but associations tended to be weaker in the pasture area. Ecological c orrelations were detected for all but one of the I. fulva and I. brevi caulis RAPD markers. The ecological associations of hybrids similar to I. brevicaulis resembled associations of I. brevicaulis parental geno types, suggesting that these hybrid genotypes may be relatively fit in the same habitats. The hybrids similar to I. fulva, however, were dis tributed in habitats that were unique relative to the parental species . The patterns of genetic and environmental associations along with ot her available data suggest that (1) only advanced generation hybrids w ere present in the population; (2) formation of F-1 hybrids among Loui siana irises is rare, leading to sporadic formation of hybrid populati ons; and (3) selection and assortative mating have contributed to the formation of hybrid genotypes that tend to be similar to parental geno types. The patterns of ecological and genetic associations detected in this population suggest that assortative mating and environmental and viability selection are important in the structuring and maintenance of this hybrid zone.