Document: JIL-3-37-13

Intimate links between environmental gradients and distribution of genotypes in a Louisiana Iris hybrid swarm.

JOHNSTON, J.A.*, R.A.WESSELINGH, A.C.BOUCK, L.A.DONOVAN and M.L.ARNOLD

University of Georgia, Athens, GA 30602 USA 1

Abstract:
Fine-scale ecological measurements were made on 201 Louisiana Iris individuals within an I. fulva X I. brevicaulis hybrid swarm to determine the importance of environmental factors in structuring the spatial distribution of genotypes. Canonical discriminant analysis was used to determine which factors act as the strongest sieve for sorting genotypes in a natural population containing both Iris species and hybrid combinations. Chloroplast DNA and seven species-specific RAPD markers were used to assign Iris genotypes as one species or a hybrid individual. Percent cover by canopy vegetation, elevation from the high water mark, soil pH, and soil organic matter were sampled at each individual plant chosen for DNA analysis. Slight differences in elevation were overwhelmingly important in determining which genotypes were found where. I. brevicaulis demonstrated a more generalist strategy, occurring in a broad range of habitats, while I. fulva had more specific preferences. Hybrids were grouped into two categories based on the parent contributing the majority of the markers to the genotype. I. fulva-like hybrids as a group showed significant separation from their closest parent, but I. brevicaulis-like hybrids did not. Individual RAPD markers showed different magnitude of response to environmental variables, and one marker showed no association with measured environmental variables at all. The statistical distribution of both groups of hybrids extended beyond at least one parental species on at least one axis, indicating the potential to inhabit a different range of habitats. This study indicates that at a very fine scale environmental selection is operating to structure this hybrid zone, and that novel hybrid genotypes may be able to live in unique habitat types, unoccupied by either parental species.

Keywords: Louisiana Iris, hybrid zone, wetland plants, plant deomography, ecological association, hybrid novelty

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