| 1. |
- Meurling, Sara, et al.
(författare)
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Body size mediates latitudinal population differences in the response to chytrid fungus infection in two amphibians
- 2024
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Ingår i: Oecologia. - 0029-8549 .- 1432-1939. ; 204:1, s. 71-81
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Tidskriftsartikel (refereegranskat)abstract
- Factors behind intraspecific variation in sensitivity to pathogens remain poorly understood. We investigated how geographical origin in two North European amphibians affects tolerance to infection by the chytrid fungus Batrachochytrium dendrobatidis (Bd), a generalist pathogen which has caused amphibian population declines worldwide. We exposed newly metamorphosed individuals of moor frog Rana arvalis and common toad Bufo bufo from two latitudinal regions to two different BdGPL strains. We measured survival and growth as infections may cause sub-lethal effects in fitness components even in the absence of mortality. Infection loads were higher in B. bufo than in R. arvalis, and smaller individuals had generally higher infection loads. B. bufo had high mortality in response to Bd infection, whereas there was little mortality in R. arvalis. Bd-mediated mortality was size-dependent and high-latitude individuals were smaller leading to high mortality in the northern B. bufo. Bd exposure led to sub-lethal effects in terms of reduced growth suggesting that individuals surviving the infection may have reduced fitness mediated by smaller body size. In both host species, the Swedish Bd strain caused stronger sublethal effects than the British strain. We suggest that high-latitude populations can be more vulnerable to chytrids than those from lower latitudes and discuss the possible mechanisms how body size and host geographical origin contribute to the present results.
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| 2. |
- Siljestam, Mattias, 1989-, et al.
(författare)
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Anisogamy Does Not Always Promote the Evolution of Mating Competition Traits in Males
- 2024
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Ingår i: American Naturalist. - : University of Chicago Press. - 0003-0147 .- 1537-5323. ; 203:2, s. 230-253
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Tidskriftsartikel (refereegranskat)abstract
- Anisogamy has evolved in most sexually reproducing multicellular organisms allowing the definition of the male and female sexes, producing small and large gametes. Anisogamy, as the initial sexual dimorphism, is 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 mating competition in males than in females. We argue that this idea stems from a restrictive assumption on the conditions under which anisogamy evolved in the first place: the absence of sperm limitation (assuming that all female gametes are fertilized). Here, we relax this assumption and present a model that considers the coevolution of gamete size with a mating competition trait, starting in a population without dimorphism. We vary gamete density to produce different scenarios of gamete limitation. We show that, while at high gamete density the evolution of anisogamy always results in male investment in competition, gamete limitation at intermediate gamete densities allows for either females or males to invest more into mating competition. Our results thus suggest that anisogamy does not always promote mating competition among males. The conditions under which anisogamy evolves matter, as well as the competition trait.
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| 3. |
- Siljestam, Mattias, 1989-
(författare)
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Mathematical Solutions to Divergent Evolution
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Diversity is ubiquitous in nature and manifests through various forms of divergent evolution. Using mathematical models that consider the interplay between ecology and evolution, I explore mechanisms driving two types of such divergence: the emergence of genetic diversity in diploid organisms, and the initial sexual dimorphism of anisogamy.Genetic diversity is typically studied as a consequence of competition or local adaptation. However, diploidy introduces an alternative mechanism: heterozygote advantage (HA), where alleles provide complementary functionalities. A classical example is the immune genes of the Major Histocompatibility Complex (MHC), where alleles can protect against complementary sets of pathogens. HA can emerge if individuals encounter multiple pathogens. When pathogens are distributed over habitats, divergence can be driven by local adaptation, or an emerging HA if migration is high. Alternatively, if MHC alleles provide full defence as a single copy (adaptive context-specific dominance), HA can also emerge under low migration. I challenge the view that HA alone cannot explain the high polymorphism observed at MHC loci by demonstrating that over 100 alleles can be maintained based on two critical assumptions: pathogens can be lethal if not properly countered by an immune response, and the combined effect of multiple pathogens can exceed the sum of their individual impacts.For loci under sexually antagonistic selection, divergent evolution can facilitate the coexistence of alleles, each producing a homozygote genotype with an optimal phenotype in one sex while the heterozygote exhibits an intermediate maladapted phenotype. However, I show that sex-specific dominance is expected to evolve, resulting in a marginal HA across the sexes: a heterozygote carrying alleles optimal for each sex exhibits an optimal phenotype in both sexes, whereas the corresponding homozygotes are maladapted in one sex. This leads to further divergence and the coexistence of many alleles, for wide parameter ranges.Additionally, I challenge the traditional view that male-biased competition for mating is an inevitable consequence of anisogamy---the evolutionary differentiation in gamete size between the sexes. I present the first theoretical description of the coevolution of anisogamy and mating competition, demonstrating that anisogamy does not inherently favour male competition. Instead, the specific evolutionary conditions and the nature of the competition trait significantly influence which sex invests more in mating competition.This thesis not only enhances our understanding of the underlying drivers of genetic and phenotypic diversity but also challenges longstanding evolutionary paradigms, shedding light on the complex dynamics that shape life’s vast diversity.
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