Biology subjects

Siljestam, M.

Publications and source records attributed to Siljestam, M..

6 recordsLinked to original sources

bioRxiv2026

Sex-specific dominance maintains multilocus polymorphism and can resolve sexual conflict

Sexually antagonistic selection arises when females and males favour different phenotypes but share their genetic basis. Beneficial sex-specific dominance can alleviate this conflict by allowing each allele to be more dominant in the sex in which it is beneficial. Whether the coevolution of dominance and allelic diversity can generate and maintain polymorphism across a polygenic trait remains unclear. We investigate a quantitative trait encoded by multiple diploid loci, with allelic effects shared between the sexes and evolving sex-specific promoter affinities determining dominance. We show that sex-specific dominance can generate polymorphism across many loci, including under weak or asymmetric selection. Whereas additive allelic effects lead to the concentration of polymorphism at a single major locus, evolving dominance allows polymorphism to become distributed across loci. This distributed polygenic architecture generates sexual dimorphism while reducing within-sex phenotypic variance: increasing the number of polymorphic loci, as well as allelic diversity within loci, allows female and male phenotypes to approach their respective optima with lower segregation load. Sufficiently strong selection can instead cause polymorphism to collapse toward a single major locus, but the selection strength required for this transition increases sharply with the number of contributing loci. Thus, evolving sex-specific dominance can make widespread genetic polymorphism part of the resolution of sexual conflict.

evolutionary biology↗

bioRxiv2024

Sex-specific dominance and its effects on allelic diversity in sexually antagonistic loci

Sexually antagonistic (SA) selection can promote genetic diversity. Current theory predicts biallelic polymorphism in SA loci primarily under strong selection or dominance reversal between the sexes. Yet, selection is often weak, several candidate SA loci harbour more than two alleles, and the prevalence of dominance reversal remains unclear. We present a model in which allelic effects at a quantitative-trait locus coevolve with sex-specific promoter affinities that determine allelic dominance, under distinct female and male phenotypic optima. We show that gradual coevolution between allelic values and promoter affinities can generate a positive feedback between beneficial sex-specific dominance and allelic divergence. Importantly, this feedback does not require an established balanced polymorphism, but can be initiated during selective sweeps, during evolutionarily transient protected polymorphism, or from variation maintained by mutation-selection-drift balance. Under weak or asymmetric selection, repeated diversification can generate polyallelic polymorphism, with several alleles forming dominance hierarchies that reduce segregation load. These results hold for both autosomal and X-linked loci and do not require complete dominance reversal. To assess these findings, we analyse segregating genetic variation in three insect populations and find stronger signals of polyallelic polymorphism in candidate SA loci and those showing sex-specific dominance, while genes with the strongest signals are enriched for functions associated with known SA phenotypes.

evolutionary biology↗

bioRxiv2021

Body size mediates latitudinal population differences in response to Bd infection in two amphibian species.

Populations of the same species may differ in their sensitivity to pathogens but the factors behind this variation are poorly understood. Moreover, infections may cause sub-lethal fitness effects even in species resistant or tolerant to disease. The chytrid fungus Batrachochytrium dendrobatidis (Bd), is a generalist pathogen which has caused amphibian population declines worldwide. In many species, Bd infection causes the disease chytridiomycosis, often leading to high mortality. We investigated how geographical origin affects tolerance to Bd by exposing newly metamorphosed individuals of two North European amphibians (moor frog Rana arvalis, common toad Bufo bufo) from two latitudinal regions to two different BdGPL strains. Bd exposure strongly lowered survival in B. bufo, and in both species survival was lower in the northern region, this difference being much stronger in B. bufo. Northern individuals were smaller in both species, and the survival difference between the regions was size-mediated with smaller individuals being more sensitive to Bd. In both species, Bd exposure led to sub-lethal effects in terms of reduced growth suggesting that even individuals surviving the infection may have reduced fitness mediated by smaller body size. Bd strain affected size-dependent mortality differently in the two regions. We discuss the possible mechanisms how body size and geographical origin can contribute to the present results.

ecology↗

bioRxiv2020

The evolution of anisogamy does not always lead to male competition

Anisogamy has evolved in a large proportion of sexually reproducing multicellular organisms allowing the definition of the female and male sexes, producing large and small gametes, respectively. Anisogamy is the initial sexual dimorphism: it has lead the sexes to experience selection differently, which makes it a good starting point to understand the evolution of further sexual dimorphisms. For instance, it is generally accepted that anisogamy sets the stage for more intense intrasexual competition in the male sex than in the female sex. However, we argue that this idea may rely on assumptions on the conditions under which anisogamy has evolved in the first place. We consider here two widely accepted scenarios for the evolution of anisogamy: gamete competition or gamete limitation. We present a mechanistic mathematical model in which both gamete size and an intrasexual competition trait for fertilisation can coevolve in a population starting without dimorphism between its two mating types. Two different intrasexual competition traits are investigated, gamete motility and the ability of gametes to capture gametes of the opposite mating type. We show that gamete competition and gamete limitation can lead to greatly different outcomes in terms of which sex competes most for fertilisation. Our results suggest that gamete competition is most likely to lead to stronger competition in males. On the other hand, under gamete limitation, competition in form of motility can evolve in either sex while gamete capture mainly evolves in females. This study suggests that anisogamy does not per se lead to more intense male competition. The conditions under which anisogamy evolves matter, as well as the competition trait considered.

evolutionary biology↗

bioRxiv2019

Latitudinal MHC variation and haplotype associated differential survival in response to experimental infection of two strains of Batrachochytrium dendrobatitis (Bd-GPL) in common toads

While both innate and adaptive immune system mechanisms have been implicated in resistance against the chytrid fungus Batrachochytrium dendrobatitis, studies on the role of specific MHC haplotypes on Bd infection are rare. Here, we studied latitudinal variation in MHC Class IIB loci along a latitudinal gradient from southern to northern Sweden in common toads, Bufo bufo. Swedish toad populations had fewer MHC Class IIB haplotypes compared to a previous study of populations in Britain. Furthermore, we found MHC diversity to decline from south to the north within Sweden. The low diversity may compromise the ability of northern populations to fight emerging disease, such as the chytrid fungus Bd. In a laboratory experiment, we infected newly metamorphosed toads with two strains of the Global Pandemic Lineage of the fungus (Bd-GPL) and compared survival with sham controls. We found Bd-infected toads had lower survival compared to controls. Survival was dependent on Bd-strain and whether experimental toads where collected in the south or the north of Sweden with lower survival in northern individuals. MHC diversity was lower in toads of northern origin, all northern animals being monomorphic for a single MHC haplotype, whereas we found seven different haplotypes in southern animals. Survival of infected animals was dependent on both Bd-strain and MHC haplotype suggesting differential infection dynamics depending on both Bd-strain and host MHC characteristics.

evolutionary biology↗

bioRxiv2018

The evolution and maintenance of extraordinary allelic diversity

AbstractThe majority of highly polymorphic genes are related to immune functions and with over 100 alleles within a population, genes of the major histocompatibility complex (MHC) are the most polymorphic loci in vertebrates. How such extraordinary polymorphism arose and is maintained is controversial. One possibility is heterozygote advantage (HA), which can in principle maintain any number of alleles, but biologically explicit models based on this mechanism have so far failed to reliably predict the coexistence of significantly more than ten alleles. We here present an eco-evolutionary model showing that evolution can result in the emergence and maintenance of more than 100 alleles under HA if the following two assumptions are fulfilled: first, pathogens are lethal in the absence of an appropriate immune defence; second, the effect of pathogens depends on host condition, with hosts in poorer condition being affected more strongly. Thus, our results show that HA can be a more potent force in explaining the extraordinary polymorphism found at MHC loci than currently recognized.

evolutionary biology↗