DEMOGRAPHY, POLLINATION, AND BAKER'S LAW

Perspectives on mating-system evolution: comparing concepts in plants and animals

The study of mating systems, defined as the distribution of who mates with whom and how often in a sexually reproducing population, forms a core pillar of evolution research due to their effects on many evolutionary phenomena. Historically, the "mating system" has either been used to refer to the rate of self-fertilization or to the formation of mating pairs between individuals of distinct sexes. Consequently, these two types of mating systems have tended to be studied separately rather than jointly. This separation often means that mating systems are not necessarily researched in a coherent manner that might apply to different types of organisms (e.g., plants versus animals, or hermaphrodites versus dioecious species), even if similar mechanisms may drive the evolution of self-fertilization and mating pair formation. Here, we review the evolution of both plant and animal mating systems, highlighting where similar concepts underlie both these fields and also where differing mechanisms are at play. We particularly focus on the effects of inbreeding, but also discuss the influence of spatial dynamics on mating-system evolution. We end with a synthesis of these different ideas and propose ideas for which concepts can be considered together to move towards a more cohesive approach to studying mating-system evolution.

Breakdown of gametophytic self-incompatibility in subdivided populations

In many hermaphroditic flowering plants, self-fertilization is prevented by self-incompatibility (SI), often controlled by a single locus, the S-locus. In single isolated populations, the maintenance of SI depends chiefly on inbreeding depression and the number of SI alleles at the S-locus. In subdivided populations, however, population subdivision has complicated effects on both the number of SI alleles and the level of inbreeding depression, rendering the maintenance of SI difficult to predict. Here, we explore the conditions for the invasion of a self-compatible mutant in a structured population. We find that the maintenance of SI is strongly compromised when a population becomes subdivided. We show that this effect is mainly caused by the decrease in the local diversity of SI alleles rather than by a change in the dynamics of inbreeding depression. Strikingly, we also find that the diversity of SI alleles at the whole population level is a poor predictor of the maintenance of SI. We discuss the implications of our results for the interpretation of empirical data on the loss of SI in natural populations.

Colonization, Baker's law, and the evolution of gynodioecy in Hawaii: implications from a study of Lycium carolinianum

PREMISE

As Baker's law suggests, the successful colonization of oceanic islands is often associated with uniparental reproduction (self-fertility), but the high incidence of dimorphism (dioecy, gynodioecy) on islands complicates this idea. Lycium carolinianum is widespread, occurring on the North American mainland and the Hawaiian Islands. We examined Baker's ideas for mainland and island populations of L. carolinianum and examined inbreeding depression as a possible contributor to the evolution of gynodioecy on Maui.

METHODS

Controlled crosses were conducted in two mainland populations and two populations in Hawaii. Treatments included self and cross pollination, unmanipulated controls, and autogamy/agamospermy. Alleles from the self-incompatibility S-RNase gene were isolated and compared between mainland and island populations. Given self-compatibility in Hawaii, we germinated seeds from self- and cross- treatments and estimated inbreeding depression using seven traits and a measure of cumulative fitness.

RESULTS

Mainland populations of Lycium carolinianum are predominately self-incompatible with some polymorphism for self-fertility, whereas Hawaiian populations are self-compatible. Concordantly, S-RNase allelic diversity is reduced in Hawaii compared to the mainland. Hawaiian populations also exhibit significant inbreeding depression.

CONCLUSIONS

Self-compatibility in Hawaii and individual variation in self-fertility in mainland populations suggests that a colonization filter promoting uniparental reproduction may be acting in this system. Comparison of S-RNase variation suggests a collapse of allelic diversity and heterozygosity at the S-RNase locus in Hawaii, which likely contributed to mate limitation upon arrival to the Pacific. Inbreeding depression coupled with autonomous self-fertilization may have led to the evolution of gynodioecy on Maui.

Heterogeneity in local density allows a positive evolutionary relationship between self-fertilisation and dispersal

Despite empirical evidence for a positive relationship between dispersal and self-fertilization (selfing), theoretical work predicts that these traits should always be negatively correlated, and the Good Coloniser Syndrome of high dispersal and selfing (Cf. Baker's Law) should not evolve. Critically, previous work assumes that adult density is spatiotemporally homogeneous, so selfing results in identical offspring production for all patches, eliminating the benefit of dispersal for escaping from local resource competition. We investigate the joint evolution of dispersal and selfing in a demographically structured metapopulation model where local density is spatiotemporally heterogeneous due to extinction-recolonization dynamics. Selfing alleviates outcrossing failure due to low local density (an Allee effect) while dispersal alleviates competition through dispersal of propagules from high- to low-density patches. Because local density is spatiotemporally heterogeneous in our model, selfing does not eliminate heterogeneity in competition, so dispersal remains beneficial even under full selfing. Hence the Good Coloniser Syndrome is evolutionarily stable under a broad range of conditions, and both negative and positive relationships between dispersal and selfing are possible, depending on the environment. Our model thus accommodates positive empirical relationships between dispersal and selfing not predicted by previous theoretical work and provides additional explanations for negative relationships.

The scope of Baker's law

Baker's law refers to the tendency for species that establish on islands by long-distance dispersal to show an increased capacity for self-fertilization because of the advantage of self-compatibility when colonizing new habitat. Despite its intuitive appeal and broad empirical support, it has received substantial criticism over the years since it was proclaimed in the 1950s, not least because it seemed to be contradicted by the high frequency of dioecy on islands. Recent theoretical work has again questioned the generality and scope of Baker's law. Here, we attempt to discern where the idea is useful to apply and where it is not. We conclude that several of the perceived problems with Baker's law fall away when a narrower perspective is adopted on how it should be circumscribed. We emphasize that Baker's law should be read in terms of an enrichment of a capacity for uniparental reproduction in colonizing situations, rather than of high selfing rates. We suggest that Baker's law might be tested in four different contexts, which set the breadth of its scope: the colonization of oceanic islands, metapopulation dynamics with recurrent colonization, range expansions with recurrent colonization, and colonization through species invasions.

Mating system in Mexican populations of the annual herb Solanum rostratum Dunal (Solanaceae)

Traditionally, annual colonising species are expected to have high rates of self-fertilisation, although recent theoretical and empirical studies have shown that cross-fertilisation can be selected for under heterogeneous pollination environments. Solanum rostratum is a self-compatible annual herb that colonises disturbed habitats. Despite the lack of physiological mechanisms to prevent self-fertilisation, pollen transfer between individuals is expected to be favoured because of its complex floral morphology. In previous studies of S. rostratum it has been shown that anther dimorphism within flowers results in precise pollen placement on the pollinator's body, and the presence of mirror-image floral morphs within plants promotes outcrossing in experimental arrays. However, the mating system of natural populations of S. rostratum has never been assessed, and thus whether it is predominantly selfing or outcrossing remains unknown. We hypothesise that floral and inflorescence morphology of S. rostratum should facilitate cross-fertilisation, making it a predominantly outcrossing despite its lack of a self-incompatibility system. To test this hypothesis, we estimated outcrossing rates by genotyping 700 individuals at 13 microsatellite loci, sampled from four populations across a 690-km transect in the species' native range. We found that populations had mean outcrossing rates of 0.70 ± 0.03, with multiple sires contributing to paternity of each progeny array (average effective number of sires = 8.97 ± 0.57). This indicates that natural populations S. rostratum have relatively high levels of outcrossing, probably facilitated by its floral and inflorescence morphology. We speculate that partial selfing in this species may be an unavoidable consequence of displaying multiple flowers at the same time (geitonogamy), as well as the result of self-pollen transfer by illegitimate visitors.

Evolution of uni- and bifactorial sexual compatibility systems in fungi

Mating systems, that is, whether organisms give rise to progeny by selfing, inbreeding or outcrossing, strongly affect important ecological and evolutionary processes. Large variations in mating systems exist in fungi, allowing the study of their origin and consequences. In fungi, sexual incompatibility is determined by molecular recognition mechanisms, controlled by a single mating-type locus in most unifactorial fungi. In Basidiomycete fungi, however, which include rusts, smuts and mushrooms, a system has evolved in which incompatibility is controlled by two unlinked loci. This bifactorial system probably evolved from a unifactorial system. Multiple independent transitions back to a unifactorial system occurred. It is still unclear what force drove evolution and maintenance of these contrasting inheritance patterns that determine mating compatibility. Here, we give an overview of the evolutionary factors that might have driven the evolution of bifactoriality from a unifactorial system and the transitions back to unifactoriality. Bifactoriality most likely evolved for selfing avoidance. Subsequently, multiallelism at mating-type loci evolved through negative frequency-dependent selection by increasing the chance to find a compatible mate. Unifactoriality then evolved back in some species, possibly because either selfing was favoured or for increasing the chance to find a compatible mate in species with few alleles. Owing to the existence of closely related unifactorial and bifactorial species and the increasing knowledge of the genetic systems of the different mechanisms, Basidiomycetes provide an excellent model for studying the different forces that shape breeding systems.

Sex, outcrossing and mating types: unsolved questions in fungi and beyond

Variability in the way organisms reproduce raises numerous, and still unsolved, questions in evolutionary biology. In this study, we emphasize that fungi deserve a much greater emphasis in efforts to address these questions because of their multiple advantages as model eukaryotes. A tremendous diversity of reproductive modes and mating systems can be found in fungi, with many evolutionary transitions among closely related species. In addition, fungi show some peculiarities in their mating systems that have received little attention so far, despite the potential for providing insights into important evolutionary questions. In particular, selfing can occur at the haploid stage in addition to the diploid stage in many fungi, which is generally not possible in animals and plants but has a dramatic influence upon the structure of genetic systems. Fungi also present several advantages that make them tractable models for studies in experimental evolution. Here, we briefly review the unsolved questions and extant hypotheses about the evolution and maintenance of asexual vs. sexual reproduction and of selfing vs. outcrossing, focusing on fungal life cycles. We then propose how fungi can be used to address these long-standing questions and advance our understanding of sexual reproduction and mating systems across all eukaryotes.

Clarifying Baker's Law

BACKGROUND

Baker's Law states that colonization by self-compatible organisms is more likely to be successful than colonization by self-incompatible organisms because of the ability for self-compatible organisms to produce offspring without pollination agents. This simple model has proved very successful in plant ecology and has been applied to various contexts, including colonizing or ruderal species, islands colonizers, invasive species or mating system variation across distribution ranges. Moreover, it is one of the only models in population biology linking two traits of major importance in ecology, namely dispersal and mating system. Although Baker's Law has stimulated a large number of empirical studies reporting the association of self-fertilization and colonizing ability in various contexts, the data have not established a general pattern for the association of traits.

SCOPE

In this paper, a critical position is adopted to discuss and clarify Baker's Law. From the literature referring to Baker's Law, an analysis made regarding how mating success is considered in such studies and discrepancies with population genetics theory of mating systems are highlighted. The data reporting the association of self-fertilization and colonizing ability are also briefly reviewed and the potential bias in interpretation is discussed. Lastly, a recent theoretical model analysing the link between colonizing ability and self-fertilization is considered.

CONCLUSIONS

Evolutionary predictions are actually more complex than Baker's intuitive arguments. It appears that Baker's Law encompasses a variety of ecological scenarios, which cannot be considered a priori as equivalent. Questioning what has been considered as self-evident for more than 50 years seems a reasonable objective to analyse in-depth dispersal and mating system traits.