THE EVOLUTION OF AUTOGAMY IN SPECIES OF THE MUSTARD GENUS LEAVENWORTHIA.

In about 80% of all species of hermaphroditic flowering plants the pollen is incapable of germinating on the styles of the flowers that produce it, a condition known as selfincompatibility (Fryxell, 1960). The genetic control is usually simple, one-locus, and multiallelic (Lewis, 1954) although more complex two-locus systems are known (Lundquist, 1975). In a number of species the incompatibility mechanism has been shut off by a dominant mutation. These species are capable of self-fertilization. However, self-compatibility does not lead inevitably to self-pollination, and various degrees of selfing and outcrossing can take place (Stebbins, 1957). The general consensus (Lewis, 1954; Fryxell, 1960; Stebbins, 1957) is that self-incompatibility is the primitive condition in Angiosperms and that self-compatibility is secondary and derived. Since autogamy has evolved repeatedly, the evolutionary and genetic consequences of autogamy are of general interest. Autogamy tends to increase inbreeding, and in most normally cross-fertilized plants, inbreeding is known to decrease vigor and seed production (Darwin, 1877; Stebbins, 1957). Consequently it is valid to ask why some plants do become autogamous in spite of the apparently deleterious effects resulting from inbreeding. It is suspected that the losses due to inbreeding must be compensated for in some other way. To test the existence of such tradeoffs, a group of closely related taxa, growing in similar environments is required. One such group is the small genus Leavenworthia (Rollins, 1963). A single species is polyploid; the remaining six diploid species in the genus can be divided into two groups on the basis of chromosome number. One group with 11 pairs of chromosomes is formed by L. alabamica and L. crassa, which have both self-incompatible and selfcompatible populations (Rollins, 1963; Lloyd, 1965) and a derived self-compatible, largely selfed species, L. exigua. The other group of species with 15 pairs of chromosomes is composed of one self-incompatible species, L. stylosa, and two derived selfcompatible species, L. torulosa and L. uniflora. The seventh species in the genus is the polyploid L. aurea (n = 24), restricted in its distribution to local populations in eastern Texas and southeastern Oklahoma (Fig. 1). All species grow on calcareous outcrops, known locally as cedar glades. Leavenworthia alabamica and L. crassa are restricted to glades in northern Alabama; L. stylosa and L. torulosa are found largely in Central Tennessee, while the two selfcompatible and highly autogamous species have a wider distribution. Leavenworthia exigua is found in north central Alabama, northwestern Georgia, central Tennessee, and northern Kentucky; L. uniflora grows from northern Alabama to southern Ohio and Indiana, and in Missouri and Arkansas. Species of Leavenworthia are found on limestone outcrops and occasionally in adjacent agricultural fields. The kinds of tradeoffs that are expected to occur as a result of changes in the breeding system involve the genetic structure of the population, the pollination mechanism, seed set efficiency, growth pattern, and size and distribution of populations. Consequently, the present study focused on all of these biological aspects of the species, and for each aspect we contrasted the behavior of outcrossing and selfing populations. Since such an in-depth study is very