Dispersal Patterns and the Evolution of Distyly in Oxalis alpina

Abstract

Individuals from distylous and ti-istylous populations of Oxalis clpiina (Rose) Knuth (section Ionoxalis) from southeastern Arizona were crossed and the fitness of the resultant F1 hybrids analyzed to provide estimates of genetic divergence among populations. Crossability was highest when tristylous individuals were used as seed parents. Ci-osses among distylous populations resulted in the lowest seed set. Proportions of flowering F1 progeny were highest for crosses with ti-istylous individuals as seed parents, and lowest for distylous x distylous crosses. Pollen fer-tility of F1 progeny was higher in the tristylous x tristylous crosses than in distylous x distylous crosses. Results consistently indicated a higher degree of genetic relationship among tristylous populations than among distylous populations. A likely explanation for the present distribution of tristylous and distylous populations in Arizona is the initial migration of a tristylous race into this region, followed by disjunction into isolated populations due to Pleistocene drying. Following isolation, variation in evolutionary rate has apparently resulted in i-etention of tristyly in some populationis, and the independent evolution of distyly in those populations Which have diverged at the greatest r-ates. Crossability data and analysis of F1 fitness thus suggest a likely dispersal pattern for 0. alpina in southeaster-n Arizona as well as the evolutionar-y relationship between tristylous and distylous reproductive systems in this species. In southeastern Arizona, adjacent New Mexico and the Mexican states of Sonora and Chihuahua, populations of Oxalis alpina (Rose) Knuth (section Ionoxalis) occurring at forested higher elevations possess either tristylous or distylous reproductive systems. In distylous populations approximately half the individuals in the population produce flowers with long styles and short stamens, while the remaining individuals produce flowers with short styles and long stamens. Pollinations leading to fertilization and seed set are those occurring between the two forms. Selfpollinations and pollinations among individuals with identical floral form fail to result in seed set. The incompatibility relationships of distylous populations of 0. alpina are similar to those found in many distylous species occurring in other plant families. Three floral forms are found in the tristylous populations. Although the morphological features of the floral forms are similar to those found in many tristylous species, the incompatibility relationships are unusual, resembling those found in distylous populations. On the basis of these unusual incompatibility relationships, I have suggested that distyly has evolved from tristyly in this region (Weller, 1976a). Cytogeographic evidence supports the same general trend in the remaining species of section Ionoxalis (Weller and Denton, 1976). Tristylous and distylous populations of Oxalis alpina in southeastern Biological Sciences, University of Illinois. Chicago IL. 60680.