Introgression between Elymus canadensis L. and E. virginicus L. (Triticeae, Poaceae) in south central United States

Abstract

Enzyme electrophoresis and multivariate morphometries were used to analyze hybridization between two species of wildrye, Elymus canadensis L. and E. virqinicus L. (Poaceae). A morphological cline correlated with longitude is found in both species. The clines are not under direct environmental control because transplant studies show that the morphological characters analyzed are not highly plastic Environmental selection of genetically-controlled morphological traits cannot be ruled out because the clines are significantly correlated with precipitation and evaporation rate. An alternative explanation for the parallel clines is that introgression is occurring between the two species in their broad area of contact in the south central United States. Introgression is the most likely explanation for the high correlation between the two species within the river systems. The river systems cross all environmental parameters in the study area, are of Pleistocene origin, and represent natural avenues of past and current propagule dispersal. The data show support for current theories on the geomorphic evolution of the river systems and for the existence of the species in the study area at least since late Pleistocene times. Reductions in character correlations in mixed-species populations compared to single-species populations suggest that recombination via hybridization is taking place. Poor germination, influenced by a prolonged post-harvest dormancy and rapid loss of seed viability, resulted in a small size for the enzyme electrophoretic studies. As a consequence the results from the electrophoretic studies are preliminary. Thirty enzyme systems were investigated, and the genetics of some were determined. Several enzymes appear to be useful in studying introgression between the species or varietal development within a species. Up to 80 percent of the enzyme systems were variable in E. virqinicus and over 50 percent were variable in E. canadensis. The amount of enzyme variability was much higher than that found in previous studies of the species in other regions of the United States. It is argued that high variability is expected in a region where secondary diversification based on repeated hybridizations occurred in Pleistocene refugia and is still occurring today in man-made, disturbed habitats...