The shift from outcrossing to selfing is one of the most common evolutionary trends in plants, and there is intense interest in why this is so. The genus Leavenworthia has been the focus of research on this question for half of a century, with particular attention paid to the evolution of self-compatibility from self-incompatibility. In this review, we discuss the last 50 years of research concerning this evolutionary transition in Leavenworthia. Selfing appears to have evolved independently at minimum three times within this genus of eight species. Work on the ecological basis of mating system evolution in Leavenworthia has clarified that selection among individuals is likely a major force behind the recurrent evolution of selfing. Although inadequate pollination is appreciated as a factor favoring selfing, definitive ecological mechanisms that act to favor selfing are still not known and future work on the efficacy of pollinating bees and the effects of climate change is needed. Recent research has likely identified the SRK ortholog at the S-locus controlling selfincompatibility in Leavenworthia alabamica. Analyses of S-locus variation have revealed substantial S-allele diversity in outcrossing populations, with the recurrent fixation of mutations at the S-locus permitting the parallel evolution of selfing in this species. Although we appreciate some of the factors that may explain the evolution of selfing in this group, there is less known about the mechanisms underlying the widespread maintenance of outcrossing at the population and species levels. Studies in Leavenworthia have revealed that genetic diversity is lost over the longterm within selfing populations and leads to elevated population subdivision, but work is needed to determine why these genetic consequences of selfing cause lineages to become evolutionary dead ends.

The present study aimed to develop a robust, extensively sampled phylogenetic hypothesis across all species in Leavenworthia and Selenia using chloroplast (psbj-petA) and nuclear encoded (nrDNA ITS and LUMINIDEPENDENS) DNA sequence data. Parsimony and maximum likelihood methodologies were employed. Strong maximum likelihood bootstrap and parsimony jackknife support were recovered for the monophyly of the sister pair Leavenworthia plus Selenia and for the respective monophyly of each genus. Leavenworthia and Selenia are hypothesized to have diverged in the Ozark Mountains and results from an analysis of branch lengths leading to Leavemvorthia and Selenia clades are consistent with Leavenworthia having diversified more recently than Selenia. Polyploidy and extensive aneuploidy complicate the interpretation of chromosomal evolution within and between these genera. Comparative results from phylogenetic analyses indicate that L. aurea and L. texana are hybrids with a shared set of putative parents. Lastly, patterns of diversification within Leavenworthia appear to have included a minimum of three transformations from self-incompatible to self-compatible systems.