Populations Of Oxalis Alpina Research Articles

The evolution of distyly from tristyly in populations of Oxalis alpina (Oxalidaceae) in the Sky Islands of the Sonoran Desert

The evolution of distyly from tristyly was investigated in populations of Oxalis alpina at high elevations throughout the Sky Islands of the Sonoran Desert. Incompatibility systems in tristylous populations, where self-incompatible short-, mid-, and long-styled morphs occur in populations, vary from those typical of tristylous species in which each morph is equally capable of fertilizing ovules of the other two morphs, to breeding systems in which incompatibility relationships are asymmetric. In these populations, selection against the allele controlling expression of the mid-styled morph is likely. The degree of modification of incompatibility in the short- and long-styled morphs in 10 populations was strongly associated with fewer mid-styled morphs, supporting models predicting the effect of these modifications of incompatibility on frequency of the mid-styled morph. Self-compatibility of the mid-styled morph may be important for maintaining the frequency of this morph, depending on the level of self-pollination, self-fertilization, and the extent of inbreeding depression. Modifications of incompatibility in tristylous populations and the distribution of distylous populations of O. alpina in the Sky Island region have similar geographic components, indicating the potential importance of historical factors in the evolution of distyly from tristyly.

Pollination biology of heteromorphic populations of Oxalis alpina (Rose) (Oxalidaceae) in south-eastern Arizona

In south-eastern Arizona, heterostylous (often trimorphic) populations of Oxalis alpina (Rose) Knuth are visited most commonly by females of the solitary bee Heterosarus bakeri Cockerell. Bees collect pollen from flowers and are presumed to be the major pollinators of O. alpina in this region. Analysis of pollen loads from the corbiculae of H. bakeri suggests that individual bees may specialize temporally on different floral forms. However, the apparent preferential collection of pollen probably results from spatial segregation of morphs and the localized foraging behaviour of bees rather than preference on the part of individual bees for particular stylar forms. As a group, bees appear to visit floral morphs of O. alpina indiscriminately, even though individual bees may have a preponderance of pollen from one morph type. Despite spatial segregation leading to pollinator flights between members of the same incompatibility group, capsule and seed production in populations of O. alpina is high for all forms. Loss of the mid form in some populations cannot be attributed to pollinator preferences for individual style morphs.

Fecundity in Populations of Oxalis alpina in Southeastern Arizona

Fecundity in Populations of Oxalis alpina in Southeastern Arizona

Variation in Heterostylous Reproductive Systems Among Populations of Oxalis alpina in Southeastern Arizona

Numerous stages in the evolution of distyly from tristyly may be ob- served among populations of Oxalis alpina in southeastern Arizona. At one extreme, the mid form is common in populations and features of floral morphology are similar to those of typical trimorphic species. At the other extreme, the mid form is absent and few of the morphological features of tristyly have been retained. A spectrum of populations with intermediate mid frequencies and floral morphology connects these extremes. Modifications of style form representation and floral mor- phology are consistent with those expected in populations where modified incom- patibility has resulted in selection against the mid form. Disjunction of populations and variation in evolutionary rates have probably resulted in the diversity of inter- mediate reproductive systems occurring in this region.

Dispersal Patterns and the Evolution of Distyly in Oxalis alpina

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.