Polyploidy in Dalea formosa (Fabaceae) on the Chihuahuan Desert

Abstract

Dalea formosa consists of diploids (n = 7), tetraploids (n = 14), and hexaploids (n = 21), the polyploids restricted to the Chihuahuan Desert region or its immediate borders. There is very little morphological differentiation between the three chromosome races and, therefore, the polyploids are assumed to be primarily autoploid. Tetraploids discovered were few and were very similar to hexaploids; the two ploidy levels were combined as “polyploids” for analyses of geographically and cytologically correlated morphological variation. Pollen length generally was found to be greater in known polyploids than in known diploids. Through the use of pollen length and geographic origin, chromosomally unknown specimens were estimated as to ploidy level. This produced four groups, known diploids and polyploids, and putative diploids and polyploids, which were then subjected to stepwise discriminant analysis (SDA) to search for other morphological characters that might indicate ploidy level, to evaluate the assignments to putative ploidy level in unknown plants, and to assess correlation of these plants of putative ploidy level to geographic regions. SDA also indicated that pollen length, among ten morphological features, is the primary discriminator between ploidy levels, and that putative polyploids are confined primarily to the Chihuahuan Desert. Chromosomally unknown specimens that were originally assigned to one ploidy level, but were classified by SDA as another, are viewed as indicative of areas where further cytological sampling is particularly needed. These areas are southeastern Arizona, where pollen among known diploids is comparatively large, northeastern New Mexico, where polyploids might occur off the Chihuahuan Desert, east edge of the Chihuahuan Desert in Texas, a cytologically poorly sampled contact zone between diploids and polyploids, and central Coahuila, where no cytological sampling has been done. Canonical variate analysis is used to aid in the visualization of the general morphological relationship between diploids and polyploids.