A Sense of Self: Exploring the Selfing Syndrome in Capsella

Abstract

Over the course of angiosperm evolution, inbreeding or selfing lineages have evolved independently numerous times from selfincompatible outbreeding ancestors. One of the driving forces of the evolution of selfing is thought to be limited pollinator and/or mate availability under low population densities (Lloyd, 1992). The breakdown in self-incompatibility that occurs during the transition to selfing typically is accompanied by specific changes in flower morphology and function, known as the selfing syndrome. Compared with their outbreeding relatives, selfing species tend to have smaller flowers that open less, develop a shorter distance between stigma and anthers, and produce less pollen, nectar, and scent (reviewed in Sicard and Lenhard, 2011). New work by Sicard et al. (2011) explores the genetic and molecular basis of these selfing syndrome traits in the genus Capsella, a close relative of Arabidopsis, through genetic mapping of a recombinant inbred line population from a cross between the selfing species Capsella rubella and outbreeding Capsella grandiflora. C. rubella exhibits many of the typical selfing syndrome traits, such as smaller size and opening angle of petals, shorter distance between stigma and anthers, and reduced pollen-to-ovule ratio (see figure). Analysis of the recombinant inbred line population led to the identification of multiple independent quantitative trait loci influencing these traits, suggesting independent evolution of different traits. The authors concluded that the selfing syndrome in Capsella has a complex genetic basis resulting fromthe accumulated effects of mutations at multiple genetic loci. An assessment of a variety of flower, leaf, and growth traits indicated that the main difference between the two species is in their flower morphology, and the floral differences are independent from differences in vegetative growth and development. The morphological traits associated with the selfing syndrome have been proposed to increase the efficiency of self-pollination relative to the outbreeder. The authors tested this hypothesis using an elegant approach by introgressing the S-locus of C. rubella into C. grandiflora. This resulted in introgression lines that were fully selfcompatible but with floral morphology very similar to the C. grandiflora outbreeder. Experiments with these lines suggested that flower morphology in C. rubella, and in particular the reduction in flower opening, is more conducive to efficient self-pollination than that of C. grandiflora. Thus, selfing syndrome flower traits may be selected for directly because of their positive effect on efficient self-pollination. Analysis of different C. rubella accessions showed that the small flower size and opening angle of petals are fixed traits within the species, consistent with a single origin for these traits. This is of interest as C. rubella is geographically far more widespread than C. grandiflora. Further studies, including the identification of genes underlying quantitative trait loci associated with the selfing syndrome in Capsella species, are necessary to determine the timing of the breakdown in self-incompatibility versus the evolution of morphological traits and the primary forces driving evolution.