Self-incompatibility systems in angiosperms are exemplars of extreme allelic poly-2 morphism maintained by long-term balancing selection. Pollen that shares an allele with the pollen recipient at the self-incompatibility locus is rejected, and this rejec-4 tion favors rare alleles as well as preventing self-fertilization. Advances in molecular genetics reveal that an ancient, deeply conserved, and well-studied incompatibility 6 system functions through multiple tightly linked genes encoding separate pollenexpressed F-box proteins and pistil-expressed ribonucleases. We show that certain 8 recombinant haplotypes at the incompatibility locus can drive collapse in the number of incompatibility types. We use a modified evolutionary rescue model to calculate 10 the relative probabilities of increase and collapse in number of incompatibility types given the initial collection of incompatibility haplotypes and the population rate of 12 gene conversion. We find that expansion in haplotype number is possible when population size or the rate of gene conversion is large, but large contractions are likely 14 otherwise. By iterating a Markov chain model derived from these expansion and collapse probabilities, we find that a stable haplotype number distribution in the realistic 16 range of 10-40 is possible under plausible parameters. However, small or moderatesized populations should be susceptible to substantial additional loss of haplotypes 18 1 beyond those lost by chance during bottlenecks. The same processes that can generate many incompatibility haplotypes in large populations may therefore be crushing 20 haplotype diversity in smaller populations. 28 (Stein et al. 1991), and Asteraceae (Hiscock et al. 2003), and some form of SI is thought to be present in nearly 40% of plant species across 100 families (Igić et al. 2008). Rejection occurs 30when the pollen specificity matches the pistil specificity, which is likewise encoded by the Slocus. Pollen with a rare specificity has an advantage because it is less likely to encounter a 32 pistil with a matching specificity and is thus less likely to be rejected. This advantage of rarity results in balancing selection, which maintains polymorphism at the S-locus by protecting S-locus 34 alleles (S-alleles) from loss through drift (Wright 1939). While it is "fairly obvious that selection would tend to increase the frequency of any additional alleles that may appear" (Wright 1939), 36 it is not obvious how novel S-alleles do appear. Molecular genetic understanding of SI has advanced rapidly, and modern theory to explain the diversification of S-alleles must account for 38 what is now known about the structure of the S-locus. We develop a population genetic model of the expansion, collapse, and long-term evolution of S-allele number under the widespread 40 solanaceous SI system.The SI system originally discovered in Nicotiana (East and Mangelsdorf 1925) is particularly 42 well-studied. Counts of 10-28 alleles have been directly observed in several other species in Solanaceae, ...
Isolation allows populations to diverge and to fix different alleles. Deleterious alleles that reach locally high frequencies contribute to genetic load, especially in inbred or selfing populations, in which selection is relaxed. In the event of secondary contact, the recessive portion of the genetic load is masked in the hybrid offspring, producing heterosis. This advantage, only attainable through outcrossing, should favour evolution of greater outcrossing even if inbreeding depression has been purged from the contributing populations. Why, then, are selfing‐to‐outcrossing transitions not more common? To evaluate the evolutionary response of mating system to heterosis, we model two monomorphic populations of entirely selfing individuals, introduce a modifier allele that increases the rate of outcrossing and investigate whether the heterosis among populations is sufficient for the modifier to invade and fix. We find that the outcrossing mutation invades for many parameter choices, but it rarely fixes unless populations harbour extremely large unique fixed genetic loads. Reversions to outcrossing become more likely as the load becomes more polygenic, or when the modifier appears on a rare background, such as by dispersal of an outcrossing genotype into a selfing population. More often, the outcrossing mutation instead rises to moderate frequency, which allows recombination in hybrids to produce superior haplotypes that can spread without the mutation's further assistance. The transience of heterosis can therefore explain why secondary contact does not commonly yield selfing‐to‐outcrossing transitions.
Self-incompatibility alleles (S-alleles), which prevent self-fertilisation in plants, have historically been expected to benefit from negative frequency-dependent selection and invade when introduced to a new population through gene flow. However, the most taxonomically widespread form of self-incompatibility, the ribonuclease-based system ancestral to the core eudicots, functions through collaborative non-self recognition, which can affect both shortterm patterns of gene flow and the long-term process of S-allele diversification.We analysed a model of S-allele evolution in two populations connected by migration, focussing on comparisons among the fates of S-alleles initially unique to each population and those shared among populations.We found that both shared and unique S-alleles from the population with more unique Salleles were usually fitter compared with S-alleles from the population with fewer S-alleles. Resident S-alleles often became extinct and were replaced by migrant S-alleles, although this outcome could be averted by pollen limitation or biased migration.Collaborative non-self recognition will usually either result in the whole-sale replacement of S-alleles from one population with those from another or else disfavour introgression of S-alleles altogether.
Self-incompatibility systems in angiosperms are exemplars of extreme allelic polymorphism maintained by long-term balancing selection. Pollen that shares an allele with the pollen recipient at the self-incompatibility locus is rejected, and this rejection favors rare alleles as well as preventing self-fertilization. Advances in molecular genetics reveal that an ancient, deeply conserved, and well-studied incompatibility system functions through multiple tightly linked genes encoding separate pollenexpressed F-box proteins and pistil-expressed ribonucleases. We show that certain recombinant haplotypes at the incompatibility locus can drive collapse in the number of incompatibility types. We use a modified evolutionary rescue model to calculate the relative probabilities of increase and collapse in number of incompatibility types given the initial collection of incompatibility haplotypes and the population rate of gene conversion. We find that expansion in haplotype number is possible when population size or the rate of gene conversion is large, but large contractions are likely otherwise. By iterating a Markov chain model derived from these expansion and collapse probabilities, we find that a stable haplotype number distribution in the realistic range of 10-40 is possible under plausible parameters. However, small or moderatesized populations should be susceptible to substantial additional loss of haplotypes 1 beyond those lost by chance during bottlenecks. The same processes that can generate many incompatibility haplotypes in large populations may therefore be crushing haplotype diversity in smaller populations.
In angiosperm self-incompatibility systems, pollen with an allele matching the 2 pollen recipient at the self-incompatibility locus is rejected. Extreme allelic polymorphism is maintained by frequency-dependent selection favoring rare alleles. However, 4 two challenges limit the spread of a new allele (a tightly linked haplotype in this case) under the widespread "collaborative non-self recognition" mechanism. First, there is 6 no obvious selective benefit for pollen compatible with non-existent stylar incompatibilities, which themselves cannot spread if no pollen can fertilize them. However, a 8 pistil-function mutation complementary to a previously neutral pollen mutation may spread if it restores self-incompatibility to a self-compatible intermediate. Second, we 10 show that novel haplotypes can drive elimination of existing ones with fewer siring opportunities. We calculate relative probabilities of increase and collapse in haplotype 12 number given the initial collection of incompatibility haplotypes and the population gene conversion rate. Expansion in haplotype number is possible when population 14 gene conversion rate is large, but large contractions are likely otherwise. A Markov chain model derived from these expansion and collapse probabilities generates a sta-16 ble haplotype number distribution in the realistic range of 10-40 under plausible parameters. However, smaller populations might lose many haplotypes beyond those 18 lost by chance during bottlenecks. Introduction 20Self-incompatibility (SI), a common strategy by which plants ensure outcrossing, is a classic example of extreme allelic polymorphism maintained by long-term balancing selection. An SI plant 22 rejects self pollen, which is identified by a specificity phenotype encoded by a highly polymorphic self-incompatibility locus (S-locus). SI is widespread in plants: distinct non-homologous SI 24 systems have been discovered in the Poaceae (Li et al. 1997), Papaveraceae (Foote et al. 1994), Solanaceae (McClure et al. 1989), Brassicaceae (Stein et al. 1991), and Asteraceae (Hiscock et al. 26 2003), and some form of SI is thought to be present in nearly 40% of plant species across 100 families (Igić et al. 2008). Rejection occurs when the pollen specificity matches the pistil specificity, 28which is likewise encoded by the S-locus. Pollen with a rare specificity has an advantage because it is less likely to encounter a pistil with a matching specificity and is thus less likely to be re-30 jected. This advantage of rarity results in balancing selection, which maintains polymorphism at the S-locus by protecting S-locus alleles (S-alleles) from loss through drift (Wright 1939). While it 32 is "fairly obvious that selection would tend to increase the frequency of any additional alleles that may appear" (Wright 1939), it is not obvious how any additional alleles may appear. The origin 34 of novel S-alleles is most mysterious in the "collaborative non-self recognition" SI system, whose molecular mechanism has been recently unraveled. Collaborat...
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