BREEDING SYSTEM AND GENETIC VARIATION IN LEAVENWORTHIA.

Abstract

Plants show a great deal of variation in their mode of fertilization from obligate outbreeding with the two sexes in separate individuals to complete inbreeding with the two sexes of a hermaphroditic individual arranged in such a way that self-fertilization is unavoidable. Any population relies for its immediate source of variation largely on recombination. Autogamy will restrict recombination. Consequently it is of interest to investigate the effect that autogamy has on the genetic variation of a population. The object of this paper is to present the results of an investigation on the genetic variation in a group of related plants with common ecological requirements but different breeding systems. The genus Leavenworthia.-This small group of winter annuals in the family Cruciferae, the mustard family, grows primarily in northern Alabama and central Tennessee, with a few populations in Arkansas, Kentucky, Missouri, Ohio, Oklahoma, Texas and Georgia. The genus has been monographed by Rollins (1963), and the population biology of two out of seven species investigated in detail by Lloyd (1965, 1969). Ecological work on some of the species has been performed by Quarterman and her collaborators at Vanderbilt University. All the species have similar ecological requirements and life cycles. The seeds germinate in the fall in wet depressions that form in the cedar glades. The cedar glades are special edaphic communities characterized by horizontally bedded limestone outcrops. The plants overwinter as rosettes, and bloom in the spring before the summer droughts make the glades quite dry and barren. All seven species are very similar in their morphology, being almost indistinguishable in the vegetative state. Some of the differences that are found in their flowers and fruits have been shown by Rollins (1963) to be related to a change in the breeding system that has taken place. In effect, three of the seven species are normally self-incompatible and consequently obligatory outbreeders, while the other four are self-compatible, and based on their flower structure and field observations, they are presumed to have various degrees of habitual self-pollination. This situation, that of a small group of ecologically and morphologically very similar species with different degrees of selfpollination, makes Leavenworthia an ideal material to investigate the effect of changes in the breeding system on the underlying genetic variation, using the technique of isozyme variation. The questions posed are the following: