Search

Self-incompatibility systems based on self-recognition evolved in hermaphroditic plants to maintain genetic variation of offspring and mitigate inbreeding depression. Despite these benefits in diploid plants, for polyploids who often face a scarcity of mating partners, self-incompatibility can thwart reproduction. In contrast, self-compatibility provides an immediate advantage: a route to reproductive viability. Thus, diploid selfing lineages may facilitate the formation of new allopolyploid species. Here, we describe the mechanism of establishment of at least four allopolyploid species in Brassicaceae (Arabidopsis suecica, Arabidopsis kamchatica, Capsella bursa-pastoris, and Brassica napus), in a manner dependent on the prior loss of the self-incompatibility mechanism in one of the ancestors. In each case, the degraded S-locus from one parental lineage was dominant over the functional S-locus of the outcrossing parental lineage. Such dominant loss-of-function mutations promote an immediate transition to selfing in allopolyploids and may facilitate their establishment., (© 2022. The Author(s).)

Crucihimalaya himalaica , a close relative of Arabidopsis and Capsella , grows on the Qinghai-Tibet Plateau (QTP) about 4,000 m above sea level and represents an attractive model system for studying speciation and ecological adaptation in extreme environments. We assembled a draft genome sequence of 234.72 Mb encoding 27,019 genes and investigated its origin and adaptive evolutionary mechanisms. Phylogenomic analyses based on 4,586 single-copy genes revealed that C. himalaica is most closely related to Capsella (estimated divergence 8.8 to 12.2 Mya), whereas both species form a sister clade to Arabidopsis thaliana and Arabidopsis lyrata , from which they diverged between 12.7 and 17.2 Mya. LTR retrotransposons in C. himalaica proliferated shortly after the dramatic uplift and climatic change of the Himalayas from the Late Pliocene to Pleistocene. Compared with closely related species, C. himalaica showed significant contraction and pseudogenization in gene families associated with disease resistance and also significant expansion in gene families associated with ubiquitin-mediated proteolysis and DNA repair. We identified hundreds of genes involved in DNA repair, ubiquitin-mediated proteolysis, and reproductive processes with signs of positive selection. Gene families showing dramatic changes in size and genes showing signs of positive selection are likely candidates for C. himalaica 's adaptation to intense radiation, low temperature, and pathogen-depauperate environments in the QTP. Loss of function at the S-locus, the reason for the transition to self-fertilization of C. himalaica , might have enabled its QTP occupation. Overall, the genome sequence of C. himalaica provides insights into the mechanisms of plant adaptation to extreme environments., Competing Interests: The authors declare no conflict of interest., (Copyright © 2019 the Author(s). Published by PNAS.)

The evolution of self-compatibility (SC) is the first step in the evolutionary transition in plants from outcrossing enforced by self-incompatibility (SI) to self-fertilization. In the Brassicaceae, SI is controlled by alleles of two tightly linked genes at the S-locus. Despite permitting inbreeding, mutations at the S-locus leading to SC may be selected if they provide reproductive assurance and/or gain a transmission advantage in a population when SC plants self- and outcross. Positive selection can leave a genomic signature in the regions physically linked to the focus of selection when selection has occurred recently. From an SC population of Leavenworthia alabamica with a known nonfunctional mutation at the S-locus, we collected sequence data from a ∼690 Kb region surrounding the S-locus, as well as from regions not linked to the S-locus. To test for recent positive selection acting at the S-locus, we examined polymorphism and the site-frequency spectra. Using forward simulations, we demonstrate that recent selection of the strength expected for SC at a locus formerly under balancing selection can generate patterns similar to those seen in our empirical data., (© 2016 The Author(s). Evolution © 2016 The Society for the Study of Evolution.)

Evolutionary transitions between mating systems have occurred repetitively and independently in flowering plants. One of the most spectacular advances of the recent empirical literature in the field was the discovery of the underlying genetic machinery, which provides the opportunity to retrospectively document the scenario of the outcrossing to selfing transitions in a phylogenetic perspective. In this review, we explore the literature describing patterns of polymorphism and molecular evolution of the locus controlling self-incompatibility (S-locus) in selfing species of the Brassicaceae family in order to document the transition from outcrossing to selfing, a retrospective approach that we describe as the 'mating system genes approach'. The data point to strikingly contrasted scenarios of transition from outcrossing to selfing. We also perform original analyses of the fully sequenced genomes of four species showing self-compatibility, to compare the orthologous S-locus region with that of functional S-locus haplotypes. Phylogenetic analyses suggest that all species we investigated evolved independently towards loss of self-incompatibility, and in most cases almost intact sequences of either of the two S-locus genes suggest that these transitions occurred relatively recently. The S-locus region in Aethionema arabicum, representing the most basal lineage of Brassicaceae, showed unusual patterns so that our analysis could not determine whether self-incompatibility was lost secondarily, or evolved in the core Brassicaceae after the split with this basal lineage. Although the approach we detail can only be used when mating system genes have been identified in a clade, we suggest that its integration with phylogenetic and population genetic approaches should help determine the main routes of this predominant mating system shift in plants., (© 2014 The Authors. Journal of Evolutionary Biology © 2014 European Society For Evolutionary Biology.)

Summary: A crucial step in the transition from outcrossing to self‐fertilization is the loss of genetic self‐incompatibility (SI). In the Brassicaceae, SI involves the interaction of female and male specificity components, encoded by the genes SRK and SCR at the self‐incompatibility locus (S‐locus). Theory predicts that S‐linked mutations, and especially dominant mutations in SCR, are likely to contribute to loss of SI. However, few studies have investigated the contribution of dominant mutations to loss of SI in wild plant species.Here, we investigate the genetic basis of loss of SI in the self‐fertilizing crucifer species Capsella orientalis, by combining genetic mapping, long‐read sequencing of complete S‐haplotypes, gene expression analyses and controlled crosses.We show that loss of SI in C. orientalis occurred < 2.6 Mya and maps as a dominant trait to the S‐locus. We identify a fixed frameshift deletion in the male specificity gene SCR and confirm loss of male SI specificity. We further identify an S‐linked small RNA that is predicted to cause dominance of self‐compatibility.Our results agree with predictions on the contribution of dominant S‐linked mutations to loss of SI, and thus provide new insights into the molecular basis of mating system transitions. [ABSTRACT FROM AUTHOR]

Self-incompatibility (SI) is a genetically controlled system adopted by many flowering plants to avoid inbreeding and thus to maintain species diversity. Generally, self-pollen rejection occurs through active pollen and pistil recognition and subsequent signaling responses. So far, three different molecular controls of pollen and pistil recognition have been characterized and are exemplified by three families: the Solanaceae, the Papaveraceae, and the Brassicaceae. With more components involved in these SI systems coming to light, recent studies have provided intriguing insights into the downstream reactions that follow the initial SI signal perception. The process of pollen rejection is closely associated with rapid and effective proteolytic events, including the ubiquitin-proteasome pathway and the vacuolar sorting pathway. Here, we review our current understanding of the roles of proteolysis in SI responses of flowering plants. [ABSTRACT FROM AUTHOR]

Frequency-dependent selection is a major force determining the evolutionary dynamics of alleles at the self-incompatibility locus (S-locus) in flowering plants. We introduce a general method using numerical simulations to test several alternative models of frequency-dependent selection on S-locus data from sporophytic systems, taking into account both genetic drift and observed patterns of dominance interactions among S-locus haplotypes (S-haplotypes). Using a molecular typing method, we estimated S-haplotype frequencies in a sample of 322 adult plants and of 245 offspring obtained from seeds sampled on 22 maternal plants, collected in a single population of Arabidopsis halleri (Brassicaceae). We found eight different S-haplotypes and characterized their dominance interactions by controlled pollinations. We then compared the likelihood of different models of frequency-dependent selection: we found that the observed haplotype frequencies and observed frequency changes in one generation best fitted a model with (1) the observed dominance interactions and (2) no pollen limitation. Overall, our population genetic models of frequency-dependent selection, including patterns of dominance interactions among S-haplotypes and genetic drift, can reliably predict polymorphism at the S-locus. We discuss how these approaches allow detecting additional processes influencing the evolutionary dynamics of the S-locus, such as purifying selection on linked loci. [ABSTRACT FROM AUTHOR]

The self‐incompatibility (SI) response in Papaver rhoeas depends upon the cognate interaction between a pollen‐expressed receptor and a stigmatically expressed ligand. The genes encoding these components are situated within the S‐locus. In order for SI to be maintained, the genes encoded by the S‐locus must be co‐inherited with no recombination between them. Several hypotheses, including sequence heterogeneity and chromosomal position, have been put forward to explain the maintenance of the S‐locus in the SI systems of the Brassicaceae and the Solanaceae. A region of the Papaver rhoeas genome encompassing part of the self‐incompatibility S1 locus has been cloned and sequenced. The clone contains the gene encoding the stigmatic component of the response, but does not contain a putative pollen S‐gene. The sequence surrounding the S1 gene contains several diverse repetitive DNA elements. As such, the P. rhoeas S‐locus bears similarities to the S‐loci of other SI systems. An attempt to localize the P. rhoeas S‐locus using fluorescence in situ hybridization (FISH) has also been made. The potential relevance of the findings to mechanisms of recombination suppression is discussed. [ABSTRACT FROM PUBLISHER]