Insights Gained From 50 Years of Studying the Evolution of Self-Compatibility in Leavenworthia (Brassicaceae)

Abstract

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 Genetics of Self-Compatibility and Associated Floral Characters inTolpis(Asteraceae) in the Canary Islands

“…The loss of SI has been documented between sister or progenitor-derivative species (Ornduff 1966;Gottlieb 1973;Beck et al 2006;Busch and Urban 2011;Slotte et al 2012), between sister subspecies (Pettengill and Moeller 2011), among populations of a species (Busch 2005;Koelling et al 2011), and among individuals within populations (Brauner and Gottlieb 1987;Tsukamoto et al 1999;Mable et al 2005). The breakdown of SI may be associated with one or more floral features collectively known as the selfing syndrome (Ornduff 1969), so that closely related SI-PSC and SC species differ by several of these floral characters (Ornduff 1966;Foxe et al 2009;Slotte et al 2012).…”

Phylogeny of <I>Leavenworthia</I> and <I>Selenia</I> (Brassicaceae)

“…Further, self-incompatibility is thought to be the ancestral condition in Leavenworthia (e.g. Rollins 1963;Lloyd 1965;Beck et al 2006;Busch and Urban 2011). To understand the frequency that self-compatibility has evolved within Leavenworthia, Beck et al (2006) presented a phylogeny derived from three chloroplast markers (tmT-tmL, tmL-trnF, and psbA-trnH) and mapped the evolution of breeding systems on to the tree.…”

“…brachxjstyla). The putative paternal parent of the hybrids, L. torulosa, is classified as SC (Rollins 1963;Busch and Urban 2011); however, additional study of S-allele diversity in L. torulosa is warranted because insect pollinators were observed on L. torulosa flowers in the field, the species is relatively late flowering, and select populations have sweetiy scented flowers (Rollins 1963; Urban personal observation), traits indicative of self-incompatibility (Rollins 1963). It is also possible that at the time of the hypothesized hybridization event between L. torulosa and L. alabamica, L. torulosa was SI, having yet to transition to a SC mating strategy.…”

“…However, a reexamination of the morphology of Selenia (Urban et al in prep) found that S. mexicana possessed markedly smaller flowers as compared to its putative sister species S. dissecta, suggesting that S. mexicana may be SC. Studies on the evolution of SC in Leavenworthia have contributed to our understandings of the genetic consequences and mechanisms of selfing (Busch and Urban 2011) and this work should be expanded to include members of Selenia, with particular attention paid to the putative SC taxon S. mexicana.…”

“…Leavenworthia has been a focus of study on the evolution of sporophytic self-incompatibility in plants with a tradition of work established by Rollins (1963) and Lloyd (1965). The genus includes self-compatible (SC) and self-incompatible (SI) species, with L. crassa and L. alabamica possessing discrete SI and SC populations (Rollins 1963;Lloyd 1965;Busch and Urban 2011).…”