Diverse mating consequences of the evolutionary breakdown of the sexual polymorphism heterostyly

Converging on long and short: The genetics, molecular biology and evolution of heterostyly

Heterostyly is a fascinating floral polymorphism that enhances outcrossing. In heterostylous species the flowers of the two or three morphs differ in multiple traits, including reciprocal reproductive-organ placement and self-incompatibility. These traits are controlled by individual genes within an S-locus supergene, whose suppressed recombination ensures the coordinated inheritance of the morph phenotypes. Recent breakthroughs about the genetic and molecular basis of heterostyly have resulted from studies on many independently evolved instances and include the following: The S-locus is a hemizygous region comprising several individual genes in multiple heterostylous taxa. In many systems, a single gene within the S-locus plays dual roles in regulating both female traits of style length and self-incompatibility type, often involving brassinosteroid signalling. The S-loci have evolved through stepwise or segmental duplication in different lineages. The frequent breakdown of heterostyly generally results from individual mutations at the S-locus and leads to a genomic selfing syndrome. These discoveries suggest convergent and genetically constrained evolution of heterostyly at the molecular level.

Quantitative nectar spur length governs nonrandom mating in a bee-pollinated Aquilegia species

Mating patterns in angiosperms are typically nonrandom, yet the mechanisms driving nonrandom mating remain unclear, especially regarding the effects of quantitative floral traits on plant mating success across male and female functions. In this study, we investigated how variation in spur length and flower number per plant influences mating patterns in Aquilegia rockii within a natural population. Using marker-based paternity analyses and manipulative experiments, we assessed the role of these traits in mating success across both sexual functions. We found significant variation in the mate composition between male and female function, with spur-length frequency positively associated with female outcrossing rate and mate number, but not with male outcrossing or mate number. Most mating events occurred within 10 m, and spur-length frequency positively correlated with mating distance. Regardless of selfing, there was evidence for assortative mating for spur length. Although spur length did not correlate with pollinator visitation, plants with mid-length spurs had higher seed set than those with shorter or longer spurs when autonomous selfing was excluded. Flowers number per plant was only associated with mating distance and female outcrossing rate. Our results suggest that spur length plays a key role in nonrandom mating by frequency-dependent mating, with implications for stabilizing selection and maintenance of genetic diversity. This study advances our understanding of floral diversity by dissecting the role of quantitative floral traits in plant mating through both female and male functions.

Does the occurence of homostyly necessarily accompany the breakdown of heteromorphic incompatibility system?

Introduction

Heterostyly is a genetic polymorphism that facilitates precise pollen transfer through reciprocal herkogamy. The loss or variation of reciprocal herkogamy is usually accompanied by the breakdown of heteromorphic incompatibility system. Homostyly, which is characterized by self-compatibility and same stigma-anther height is a common floral morph in the variation and evolution of heterostyly. Limonium aureum is a distylous species distributed in the desert of northwest China, in which a floral morph with the same stigma-anther height (H-morph) widely distributed in the natural populations, resembling classical homostyly. The aim of this study was to clarify whether the occurrence of H-morph is also accompanied by the breakdown of heteromorphic incompatibility system, and the relationship between the H-morph and long-styled-/shortstyled-morph (L-/S-morph).

Methods

The floral morphs composition and frequency, heterostylous syndrome, pollinators and visiting efficiency were investigated in five natural populations of L. aureum based on field observation, artificial control pollination experiment and so on.

Results and conclusion

All populations were composed of L-, S- and H-morphs, except for ATS population with only H-morph, and there were significant differences in flower size parameter, fruit set, and degree of pollination limitation, while no differences among morphs within population. However, each population demonstrated dimorphic pollen-stigma morphology and a strict heteromorphic incompatibility system, especially ATS population, in which they were compatible between morphs with heteromorphic pollen-stigma morphology, regardless of the reciprocal herkogamy, and vice versa. It is speculated that the H-morph in different populations may be at different stages of heterostylous evolution. The ATS population may be a dimorphic population without reciprocal herkogamy which is in the stage before distyly formation, while the other 4 populations may be dimorphic populations with significant variation in reciprocal herkogamy which is in the stage after distyly formation. The H-morph may be caused by stigma-anther separation shortening of L- and S-morph in other 4 populations. These phenomenons that the variation of floral morph is independent of physiological incompatibility breakdown, as well as the coexistence of populations from different origins and evolutionary stages within the same species have been reported for the first time in the Plumbaginaceae.

Pollination ecotypes and the origin of plant species

Ecological niche shifts are a key driver of phenotypic divergence and contribute to isolating barriers among lineages. For many groups of organisms, the history of these shifts and associated trait-environment correlations are well-documented at the macroevolutionary level. However, the processes that generate these patterns are initiated below the species level, often by the formation of ecotypes in contrasting environments. Here, I review the evidence in plants for 'pollination ecotypes' as microevolutionary responses to environmental gradients in pollinator availability. Pollinators are critical for population establishment and persistence in most species, thereby forming part of their fundamental niche. Novel floral trait combinations allow species to exploit particular pollination opportunities in local habitats and evolve primarily through sexual selection due to their effects on mating success. I examine selected case studies on the evolution of pollination ecotypes, including self-pollinating forms, and use these to illustrate challenging practical and conceptual issues. These issues include the paucity of reliable natural history data, the problem of implementing and interpreting reciprocal translocation experiments, and establishing criteria for when allopatric ecotypes should be considered species.

Evolution from mixed to fixed handedness in mirror-image flowers: insights from adaptive dynamics

Mirror-image flowers (enantiostyly) involve a form of sexual asymmetry in which a flower's style is deflected either to the left or right side, with a pollinating anther orientated in the opposite direction. This curious floral polymorphism, which was known but not studied by Charles Darwin, occurs in at least 11 unrelated angiosperm families and represents a striking example of adaptive convergence in form and function associated with cross-pollination by insects. In several lineages, dimorphic enantiostyly (one stylar orientation per plant, both forms occurring within populations) has evolved from monomorphic enantiostyly, in which all plants can produce both style orientations. We use a modelling approach to investigate the emergence of dimorphic enantiostyly from monomorphic enantiostyly under gradual evolution. We show using adaptive dynamics that depending on the balance between inbreeding depression following geitonogamy, pollination efficiency, and plant density, dimorphism can evolve from an ancestral monomorphic population. In general, the newly emergent dimorphic population is stable against invasion of a monomorphic mutant. However, our model predicts that under certain ecological conditions, for example, a decline of pollinators, dimorphic enantiostyly may revert to a monomorphic state. We demonstrate using population genetics simulations that the observed evolutionary transitions are possible, assuming a plausible genetic architecture.

Parallel evolution of morphological and genomic selfing syndromes accompany the breakdown of heterostyly

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.

Two floral forms in the same species-distyly

MAIN CONCLUSION

This paper reviews the progress of research on the morphology, physiology and molecular biology of distyly in plants. It will help to elucidate the mysteries of distyly in plants. Distyly is a unique representative type of heterostyly in plants, primarily characterized by the presence of long style and short style within the flowers of the same species. This interesting trait has always fascinated researchers. With the rapid development of molecular biology, the molecular mechanism for the production of dimorphic styles in plants is also gaining ground. Researchers have been studying plant dimorphic styles from various perspectives. The researchers are gradually unravelling the mechanisms by which plants produce distyly traits. This paper reviews advances in the study of plant dimorphic style characteristics, mainly in terms of the morphology, physiology and molecular biology of plants with dimorphic styles. The aim is to provide a theoretical basis for the study of the mechanism of distyly formation in plants.