The selfing syndrome and beyond: diverse evolutionary consequences of mating system transitions in plants.

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.

Floral scent is an important way for plants to communicate with insects, but scent emission has been lost or strongly reduced during the transition from pollinator-mediated outbreeding to selfing. The shift from outcrossing to selfing is not only accompanied by scent loss, but also by a reduction in other pollinator-attracting traits like petal size and can be observed multiple times among angiosperms. These changes are summarized by the term selfing syndrome and represent one of the most prominent examples of convergent evolution within the plant kingdom. In this work the genus Capsella was used as a model to study convergent evolution in two closely related selfers with separate transitions to self-fertilization. Compared to their outbreeding ancestor C. grandiflora, the emission of benzaldehyde as main compound of floral scent is lacking or strongly reduced in the selfing species C. rubella and C. orientalis. In C. rubella the loss of benzaldehyde was caused by mutations to cinnamate:CoA ligase CNL1, but the biochemical basis and evolutionary history of this loss remained unknown, together with the genetic basis of scent loss in C. orientalis. Here, a combination of plant transformations, in vitro enzyme assays, population genetics and quantitative genetics has been used to address these questions. The results indicate that CNL1 has been inactivated twice independently by point mutations in C. rubella, leading to a loss of benzaldehyde emission. Both inactivated haplotypes can be found around the Mediterranean Sea, indicating that they arose before the species´ geographical spread. This study confirmed CNL1 as a hotspot for mutations to eliminate benzaldehyde emission, as it has been suggested by previous studies. In contrast to these findings, CNL1 in C. orientalis remains active. To test whether similar mechanisms underlie the convergent evolution of scent loss in C. orientalis a QTL mapping approach was used and the results suggest that this closely related species followed a different evolutionary route to reduce floral scent, possibly reflecting that the convergent evolution of floral scent is driven by ecological rather than genetic factors. In parallel with studying the genetic basis of repeated scent loss a method for testing the adaptive value of individual selfing syndrome traits was established. The established method allows estimating outcrossing rates with a high throughput of samples and detects successfully insect-mediated outcrossing events, providing major advantages regarding time and effort compared to other approaches. It can be applied to correlate outcrossing rates with differences in individual traits by using quasi-isogenic lines as demonstrated here or with environmental or morphological parameters. Convergent evolution can not only be observed for scent loss in Capsella but also for the morphological evolution of petal size. Previous studies detected several QTLs underlying the petal size reduction in C. orientalis and C. rubella, some of them shared among both species. One shared QTL is PAQTL1 which might map to NUBBIN, a growth factor. To better understand the morphological evolution and genetic basis of petal size reduction, this QTL was studied. Mapping this QTL to a gene might identify another example for a hotspot gene, in this case for the convergent evolution of petal size.

The change from outbreeding to selfing is one of the most frequent evolutionary transitions in flowering plants. It is often accompanied by characteristic morphological and functional changes to the flowers (the selfing syndrome), including reduced flower size and opening. Little is known about the developmental and genetic basis of the selfing syndrome, as well as its adaptive significance. Here, we address these issues using the two closely related species Capsella grandiflora (the ancestral outbreeder) and red shepherd's purse (Capsella rubella, the derived selfer). In C. rubella, petal size has been decreased by shortening the period of proliferative growth. Using interspecific recombinant inbred lines, we show that differences in petal size and flower opening between the two species each have a complex genetic basis involving allelic differences at multiple loci. An intraspecific cross within C. rubella suggests that flower size and opening have been decreased in the C. rubella lineage before its extensive geographical spread. Lastly, by generating plants that likely resemble the earliest ancestors of the C. rubella lineage, we provide evidence that evolution of the selfing syndrome was at least partly driven by selection for efficient self-pollination. Thus, our studies pave the way for a molecular dissection of selfing-syndrome evolution.

In angiosperm evolution, autogamously selfing lineages have been derived from outbreeding ancestors multiple times, and this transition is regarded as one of the most common evolutionary tendencies in flowering plants. In most cases, it is accompanied by a characteristic set of morphological and functional changes to the flowers, together termed the selfing syndrome. Two major areas that have changed during evolution of the selfing syndrome are sex allocation to male vs. female function and flower morphology, in particular flower (mainly petal) size and the distance between anthers and stigma. A rich body of theoretical, taxonomic, ecological and genetic studies have addressed the evolutionary modification of these two trait complexes during or after the transition to selfing. Here, we review our current knowledge about the genetics and evolution of the selfing syndrome. We argue that because of its frequent parallel evolution, the selfing syndrome represents an ideal model for addressing basic questions about morphological evolution and adaptation in flowering plants, but that realizing this potential will require the molecular identification of more of the causal genes underlying relevant trait variation.

Pollen:ovule ratios are often lower in species and populations with higher selfing rates. This may be due either to higher pollination efficiency through selfing, or to lower male competition when less allo-pollen is available. Changes in pollination can also impact pollen traits, such as the number of apertures. Viola arvensis has experienced a rapid recent increase in selfing rates, and a rapid floral trait evolution towards the selfing syndrome. This study tests the hypothesis that V. arvensis is also undergoing a rapid evolution in its pollen:ovule ratio and pollen heteromorphism. Using the resurrection ecology methodology, we compared four ancestral populations (from ca. 30 years ago) to their descendants (from 2021). We counted ovules and pollen and measured the different pollen aperture morphs in 50 individuals per population. We also developed a model to better understand the links between the number of apertures and the mating system. We found no temporal change in pollen or ovule production. However, populations with the lowest pollen:ovule ratios were also those with the highest ancestral selfing rates, suggesting that the pollen:ovule ratio could have evolved on a similar timescale to population differentiation. Our model predicts a positive correlation between number of apertures and selfing rates, if pollination parameters remain constant. However, this positive correlation was not found in our results, neither across populations nor through time. Unlike floral morphology, pollen and ovule production did not evolve rapidly with increased selfing rates, suggesting a delayed change of the pollen:ovule ratio compared to other traits of the selfing syndrome. The absence of correlation between pollen heteromorphism and selfing rate can be explained by multiple (non-mutually exclusive) factors: a decrease in allo-pollen deposition correlated with the evolution of the selfing syndrome, pollinator declines or the absence of selection in this trait.

The evolutionary transition from outcross- to self-fertilization is one of the most common in angiosperms and is often associated with a parallel shift in floral morphological and developmental traits, such as reduced flower size and pollen to ovule ratios, known as the “selfing syndrome.” How these convergent phenotypes arise, the extent to which they are shaped by selection, and the nature of their underlying genetic basis are unsettled questions in evolutionary biology. The genus Collinsia (Plantaginaceae) includes seven independent transitions from outcrossing or mixed mating to high selfing rates accompanied by selfing syndrome traits. Accordingly, Collinsia represents an ideal system for investigating this parallelism, but requires genomic resource development. We present a high quality de novo genome assembly for the highly selfing species Collinsia rattanii. To begin addressing the basis of selfing syndrome developmental shifts, we evaluate and contrast patterns of gene expression from floral transcriptomes across three stages of bud development for C. rattanii and its outcrossing sister species Collinsia linearis. Relative to C. linearis, total gene expression is less variable among individuals and bud stages in C. rattanii. In addition, there is a common pattern among differentially expressed genes: lower expression levels that are more constant across bud development in C. rattanii relative to C. linearis. Transcriptional regulation of enzymes involved in pollen formation specifically in early bud development may influence floral traits that distinguish selfing and outcrossing Collinsia species through pleiotropic functions. Future work will include additional Collinsia outcrossing-selfing species pairs to identify genomic signatures of parallel evolution.

The evolution of predominant self-fertilisation frequently coincides with the evolution of a collection of phenotypes that comprise the 'selfing syndrome', in both plants and animals. Genomic features also display a selfing syndrome. Selfing syndrome traits often involve changes to male and female reproductive characters that were subject to sexual selection and sexual conflict in the obligatorily outcrossing ancestor, including the gametic phase for both plants and animals. Rapid evolution of reproductive traits, due to both relaxed selection and directional selection under the new status of predominant selfing, lays the genetic groundwork for reproductive isolation. Consequently, shifts in sexual selection pressures coupled to transitions to selfing provide a powerful paradigm for investigating the speciation process. Plant and animal studies, however, emphasise distinct selective forces influencing reproductive-mode transitions: genetic transmission advantage to selfing or reproductive assurance outweighing the costs of inbreeding depression vs the costs of males and meiosis. Here, I synthesise links between sexual selection, evolution of selfing and speciation, with particular focus on identifying commonalities and differences between plant and animal systems and pointing to areas warranting further synergy.

Plants that are highly selfing typically exhibit a suite of morphological traits termed a “selfing syndrome,” including reduced corollas and reproductive structures, loss of corolla pigmentation, little anther-stigma separation, and a lower pollen/ovule (P/O) ratio. While it is typically assumed that these changes are adaptive, few attempts have been made to determine whether they result from the operation of natural selection or genetic drift. In the southeastern United States, Ipomoea lacunosa has evolved a typical selfing syndrome compared to its close relative, Ipomoea cordatotriloba. Microsatellite markers confirmed that selfing rates are substantially higher in I. lacunosa. Furthermore, using a standard QST – FST comparison, we evaluated the relative importance of selection and drift in the evolution of selfing syndrome traits in I. lacunosa. The analysis demonstrated that natural selection is responsible for the evolution of reduced corolla size, anther-stigma distance, and style length in this species. By contrast, leaf characteristics unrelated to selfing were found to have diverged largely by genetic drift. Our study provides one of the first confirmations that natural selection drives the evolution of selfing-syndrome traits.

Evolutionary transitions from outcrossing to selfing in flowering plants have convergent morphological and genomic signatures and can involve parallel evolution within related lineages. Adaptive evolution of morphological traits is often assumed to evolve faster than nonadaptive features of the genomic selfing syndrome. We investigated phenotypic and genomic changes associated with transitions from distyly to homostyly in the Primula oreodoxa complex. We determined whether the transition to selfing occurred more than once and investigated stages in the evolution of morphological and genomic selfing syndromes using 22 floral traits and both nuclear and plastid genomic data from 25 populations. Two independent transitions were detected representing an earlier and a more recently derived selfing lineage. The older lineage exhibited classic features of the morphological and genomic selfing syndrome. Although features of both selfing syndromes were less developed in the younger selfing lineage, they exhibited parallel development with the older selfing lineage. This finding contrasts with the prediction that some genomic changes should lag behind adaptive changes to morphological traits. Our findings highlight the value of comparative studies on the timing and extent of transitions from outcrossing to selfing between related lineages for investigating the tempo of morphological and molecular evolution.

Reinforcement can contribute to speciation by increasing the strength of prezygotic isolating mechanisms. Theoretical analyses over the past two decades have demonstrated that conditions for reinforcement are not unduly restrictive, and empirical investigations have documented over a dozen likely cases, indicating that it may be a reasonably common phenomenon in nature. Largely uncharacterized, however, is the diversity of biological scenarios that can create the reduced hybrid fitness that drives reinforcement. Here I examine one such scenario-the evolution of the "selfing syndrome" (a suite of characters including reductions in flower size and in nectar, pollen, and scent production) in highly selfing plant species. Using a four-locus model, where the loci are (1) a discrimination locus, (2) a target-of-discimination locus, (3) a pollen-production locus, and (4) a selfing-rate locus, I determine the conditions under which this syndrome can favor reinforcement, an increase in discrimination through change at locus 1, in an outcrossing species that experiences gene flow from a highly selfing species. In the absence of both linkage disequilibrium between loci and pollen discounting, reinforcement can occur, but only in a very small fraction of the parameter combinations examined. Moderate linkage ([Formula: see text]) between one pair of loci increases this fraction moderately, depending on which two loci are linked. Pollen discounting (a reduction in pollen exported to other plants due to increased selfing), by contrast, can increase the fraction of parameter combinations that result in reinforcement substantially. The evolution of reduced pollen production in highly selfing species thus facilitates reinforcement, especially if substantial pollen discounting is associated with selfing.

The transition from outcrossing to predominant self-fertilization is one of the most common evolutionary transitions in flowering plants. This shift is often accompanied by a suite of changes in floral and reproductive characters termed the selfing syndrome. Here, we characterize the genetic architecture and evolutionary forces underlying evolution of the selfing syndrome in Capsella rubella following its recent divergence from the outcrossing ancestor C. grandiflora. We conduct genotyping by multiplexed shotgun sequencing and map floral and reproductive traits in a large (N= 550) F2 population. Our results suggest that in contrast to previous studies of the selfing syndrome, changes at a few loci, some with major effects, have shaped the evolution of the selfing syndrome in Capsella. The directionality of QTL effects, as well as population genetic patterns of polymorphism and divergence at 318 loci, is consistent with a history of directional selection on the selfing syndrome. Our study is an important step toward characterizing the genetic basis and evolutionary forces underlying the evolution of the selfing syndrome in a genetically accessible model system.

How and why plants evolve to become selfing is a long-standing evolutionary puzzle. The transition from outcrossing to highly selfing is less well understood in self-compatible (SC) mixed-mating (MM) species where potentially subtle interactions between floral phenotypes and the environment are at play. We examined floral morphological and developmental traits across an entire SC MM genus, Collinsia, to determine which, if any, predict potential autonomous selfing ability when pollinators are absent (AS) and actual selfing rates in the wild, s(m), and to best define the selfing syndrome for this clade. Using polymorphic microsatellite markers, we obtained 30 population-level estimates of s(m) across 19 Collinsia taxa. Species grand means for the timing of herkogamy (stigma-anther contact) and dichogamy (stigmatic receptivity, SR), AS, floral size, longevity and their genetic correlations were quantified for 22 taxa. Species fell into discrete selfing and outcrossing groups based on floral traits. Loss of dichogamy defines Collinsia's selfing syndrome. Floral size, longevity and herkogamy also differ significantly between these groups. Most taxa have high AS rates (>80 %), but AS is uncorrelated with any measured trait. In contrast, s(m) is significantly correlated only with SR. High variance in s(m) was observed in the two groups. Collinsia species exhibit clear morphological and developmental traits diagnostic of 'selfing' or 'outcrossing' groups. However, many species in both the 'selfing' and the 'outcrossing' groups were MM, pointing to the critical influence of the pollination environment, the timing of AS and outcross pollen prepotency on s(m). Flower size is a poor predictor of Collinsia species' field selfing rates and this result may apply to many SC species. Assessment of the variation in the pollination environment, which can increase selfing rates in more 'outcrossing' species but can also decrease selfing rates in more 'selfing' species, is critical to understanding mating system evolution of SC MM taxa.

Many angiosperms prevent inbreeding through a self-incompatibility (SI) system, but the loss of SI has been frequent in their evolutionary history. The loss of SI may often lead to an increase in the selfing rate, with the purging of inbreeding depression and the ultimate evolution of a selfing syndrome, where plants have smaller flowers with reduced pollen and nectar production. In this study, we used approximate Bayesian computation (ABC) to estimate the timing of divergence between populations of the plant Linaria cavanillesii that differ in SI status and in which SI is associated with low inbreeding depression but not with a transition to full selfing or a selfing syndrome. Our analysis suggests that the mixed-mating self-compatible (SC) population may have begun to diverge from the SI populations around 2810 generation ago, a period perhaps too short for the evolution of a selfing syndrome. We conjecture that the SC population of L.cavanillesii is at an intermediate stage of transition between outcrossing and selfing.

As part of global change, climate warming and pollinator decline are expected to affect plant phenology and plant-pollinator interactions. This paper aims at characterizing rapid evolution of life history traits and floral traits over two decades in the wild pansy (Viola arvensis), a common weed in agrosystems. We used a resurrection ecology approach with genotypes sampled in 1991 and 2012 from a population in Burgundy (France). The species has a mixed mating system (hereafter: mixed selfer) and presents a floral polymorphism. To correct for maternal effects, we measured plant traits in the second generation in a common garden (after a refreshing generation) to characterize plant evolution during the two decades. In addition, historical population selfing rates in 1991 and 2012 were inferred from microsatellites markers through heterozygote deficiency and identity disequilibrium. Phenotypic data revealed a significant advance in flowering date, reduced flower sizes and a higher propensity of plants to set seed by autonomous selfing. Moreover, we detected a change in color morph frequency with an increase of the pale morph frequency. In accordance with phenotypic data, the neutral genetic data revealed an increase in historical selfing rates from 0.68 in 1991 to 0.86 in 2012. Taken together, such data suggest that the wild pansy, a mixed selfer, is evolving a selfing syndrome that may be the consequence of reduced pollinator activity in agrosystems.

The selfing syndrome constitutes a suite of floral and reproductive trait changes that have evolved repeatedly across many evolutionary lineages in response to the shift to selfing. Convergent evolution of the selfing syndrome suggests that these changes are adaptive, yet our understanding of the detailed molecular genetic basis of the selfing syndrome remains limited. Here, we investigate the role of cis-regulatory changes during the recent evolution of the selfing syndrome in Capsella rubella, which split from the outcrosser Capsella grandiflora less than 200 ka. We assess allele-specific expression (ASE) in leaves and flower buds at a total of 18,452 genes in three interspecific F1 C. grandiflora x C. rubella hybrids. Using a hierarchical Bayesian approach that accounts for technical variation using genomic reads, we find evidence for extensive cis-regulatory changes. On average, 44% of the assayed genes show evidence of ASE; however, only 6% show strong allelic expression biases. Flower buds, but not leaves, show an enrichment of cis-regulatory changes in genomic regions responsible for floral and reproductive trait divergence between C. rubella and C. grandiflora. We further detected an excess of heterozygous transposable element (TE) insertions near genes with ASE, and TE insertions targeted by uniquely mapping 24-nt small RNAs were associated with reduced expression of nearby genes. Our results suggest that cis-regulatory changes have been important during the recent adaptive floral evolution in Capsella and that differences in TE dynamics between selfing and outcrossing species could be important for rapid regulatory divergence in association with mating system shifts.