Breeding systems determine how individuals get together to reproduce, and influence the amount of genetic diversity in populations and the potential for speciation. Plants provide model systems for understanding factors promoting changes in breeding systems because of their variability in reproductive systems. In heterostyly, distinctive floral forms are associated with chemical incompatibilities that usually prevent mating between similar forms and promote crossing between forms (outcrossing). Oxalis alpina appears to be evolving a new breeding system, with loss of one floral morph in some populations. To test previous models indicating that changes in self-incompatibility are responsible for breeding system evolution, crosses will be carried out to determine whether changes in incompatibility are more common in populations where heterostyly is more highly modified. Outcrossing rates will be measured because greater self-compatibility could retard loss of morphs. The relative fitness of offspring from crosses within and between different morphs will be measured to understand the importance of inbreeding depression in this breeding system change. Research will focus on populations of Oxalis alpina occurring in mountain ranges extending through Mexico, Arizona, and New Mexico.
This research provides insights into evolutionary factors that underlie modifications of plant breeding systems, and reproductive systems in general. The research fosters an international collaboration, with participation of faculty, graduate students, and undergraduates from U.S. and Mexican institutions. This project is being supported by the Office of International Science and Engineering.
Heterostylous breeding systems have been useful model systems for addressing questions about breeding system evolution. In heterostyly, two or three floral morphs occur in populations, and because of strong self-incompatibility, each floral morph is normally incapable of producing seed after self fertilization. In tristyly, three floral morphs occur in populations (the short-, mid-, and long-styled morphs), named by the position of the stigmas. Anthers occur at the two remaining levels in each morph. Compatible matings occur between individuals with different style morphs, usually between individuals with anthers and stigmas located at the same level in the flower. In distylous species only two morphs occur in populations and fertilizations capable of leading to seed production are normally those occurring between the two morphs. We have investigating the evolutionary relationship of tristyly and distyly in Oxalis alpina (section Ionoxalis, Oxalidaceae), a perennial herbaceous species found largely in Mexico, but occurring in parts of adjacent southwest United States. Populations of O. alpina in these areas possess both distyly and tristyly, as well as intermediate breeding systems. Incompatibility systems in many of these tristylous populations vary from those of typical 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, because the short- and long-styled style morphs have incompatibility relationships allowing all reciprocal pollinations between the short and long morphs to produce seeds. Pollen from the mid-styled morph is likely to be outcompeted by pollen from short and long style morphs in populations with incompatibility modifications, leading to gradual loss of the mid morph. Our goal has been to test models of the evolution of distyly from tristyly including (1) field studies of frequencies of reproductive morphs; (2) crosses to determine incompatibility relationships in populations with different morph frequencies; (3) estimation of outcrossing rates, and (4) measurements of inbreeding depression in some of these populations. The extent of incompatibility modification was strongly associated with the frequency of the mid morph in the ten populations that were surveyed for frequency of style morphs and that also had controlled crosses completed to determine incompatibility relationships. Self-compatibility of the mid-styled morph was pronounced in populations with modified incompatibility, and may be important in maintaining the mid morph in populations, depending on the extent of self pollination and self-fertilization, and the degree of inbreeding depression. Levels of inbreeding depression varied among the three populations studied to date, although some mid-styled morphs have reduced inbreeding depression, which could increase their fitness. Outcrossing is generally high for all three morphs, suggesting that even partially self-compatible style morphs may not self-fertilize very much under field conditions. High outcrossing rates may explain the conversion of breeding systems from tristyly to distyly, and the general lack of stability of three-morph breeding systems.
