Allozyme variation in Louisiana irises: a test for introgression and hybrid speciation

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Genetic variation present in 599 individual Iris samples was examined at 50 presumptive loci (isozymes). The samples included allopatric populations of I. fulva, I. hexagona, I. brevicaulis and the putative hybrid species, I. nelsonii. In addition, we examined two populations from areas of overlap involving I. fulva with I. hexagona, and I. fulva with I. brevicaulis and I. hexagona. Allozyme data for both of these hybrid populations suggest the presence of genes from I. fulva, I. brevicaulis and I. hexagona. Bidirectional introgressive hybridization is indicated between I. fulva and I. hexagona. Individuals from a parapatric association between these two species possess various combinations of I. fulva and I. hexagona diagnostic markers. The pattern of genetic variation in this sample indicates the presence of advanced hybrid generations and localized introgression; this finding is in accord with a previous molecular genetic analysis (Arnold et al., 1990). Furthermore, currently allopatric populations of I. fulva and I. hexagona were found to have low frequencies of marker alleles diagnostic for the alternate species. These populations also contain a low frequency of alternate, diagnostic ribosomal DNA repeat length variants (Arnold et al., 1990). The allozyme (and molecular) data suggest that these allopatric samples represent previously introgressed populations. Findings from the allozyme survey indicate that I. nelsonii contains an array of markers characteristic for I. fulva, I. hexagona and I. brevicaulis. Furthermore, some of these markers are found in a present-day hybrid population involving I. fulva, I. hexagona and I. brevicaulis. These findings suggest that I. nelsonii is of hybrid origin, deriving from hybridization between I. fulva, I. hexagona and I. brevicaulis. The genetic analysis along with previous chromosomal, distributional and demographic studies of the Louisiana Iris species suggests that a number of mechanisms may have been involved in the stabilization of this hybrid derivative.