Morphometric analysis of the North American creosote bush (Larrea tridentata, Zygophyllaceae) and the microspatial distribution of its chromosome races

Polyploidy is a major mechanism of chromosome evolution and speciation in flowering plants. Delineation of polyploid populations as species or subspecies is complicated because of the uncertainties of distinguishing closely related diploids and polyploids in field conditions. Here we evaluate the practical identification of polyploids-using geographic distributions and morphological features-in the North American creosote bush (Larrea tridentata, Zygophyllaceae). Regarded as a classical autopolyploid complex, L. tridentata comprises diploids, tetraploids, and hexaploids distributed throughout the Chihuahuan, Sonoran, and Mojave Deserts. Transect sampling on cytotype range boundaries revealed strong spatial structuring of the chromosome races, and infrequent sympatry, over small spatial scales (< 50 km). Inter-cytotype hybrids were rare, with putative triploids and pentaploids comprising < 1 % of the sampled plants (N = 1,197). In morphometric analyses of stem, flower, and leaf characteristics (N = 14 traits), we found significant ploidy effects in several cytotype comparisons; reproductive structures and foliage characteristics were particularly discriminatory. Significant transect and ploidy x transect effects for most traits suggest, however, that environmental and genic factors influence plant architecture and morphology. Nonetheless, discriminant function analysis with a combined morphometric data set correctly assigned 68.9 % of plants to ploidal level. Pollen diameters increased significantly with ploidal level, providing another potentially informative trait for comparisons of reproductive plants. Taken together, the spatial distribution and morphometric data presented here suggest that the majority of L. tridentata plants could, in principle, be assigned to cytotype in the field. However, because of potential misclassifications, we suggest recognition of the L. tridentata cytotypes as subspecies.