THE ROLE OF DELETERIOUS MUTATIONS IN ALLOPATRIC SPECIATION

ABSTRACTSex-specific lifespans are ubiquitous across the tree of life and exhibit broad taxonomic patterns that remain a puzzle, such as males living longer than females in birds and vice versa in mammals. The prevailing “unguarded-X” hypothesis (UXh) explains this by differential expression of recessive mutations in the X/Z chromosome of the heterogametic sex (e.g., females in birds and males in mammals), but has only received indirect support to date. An alternative hypothesis is that the accumulation of deleterious mutations and repetitive elements on the Y/W chromosome might lower the survival of the heterogametic sex (“toxic Y” hypothesis). Here, we report lower survival of the heterogametic relative to the homogametic sex across 138 species of birds, mammals, reptiles and amphibians, as expected if sex chromosomes shape sex-specific lifespans. We then analysed bird and mammal karyotypes and found that the relative sizes of the X and Z chromosomes are not associated with sex-specific lifespans, contrary to UXh predictions. In contrast, we found that Y size correlates negatively with male survival in mammals, where toxic Y effects are expected to be particularly strong. This suggests that small Y chromosomes benefit male lifespans. Our results confirm the role of sex chromosomes in explaining sex differences in lifespan, but indicate that, at least in mammals, this is better explained by “toxic Y” rather than UXh effects.

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Transmission of Functional, Wild-Type Mitochondria and the Fittest mtDNA to the Next Generation: Bottleneck Phenomenon, Balbiani Body, and Mitophagy

Genes ◽

10.3390/genes11010104 ◽

2020 ◽

Vol 11 (1) ◽

pp. 104 ◽

Cited By ~ 1

Author(s):

Waclaw Tworzydlo ◽

Malgorzata Sekula ◽

Szczepan M. Bilinski

Keyword(s):

Membrane Potential ◽

Respiratory Activity ◽

Metabolic Energy ◽

Oxygen Species ◽

Deleterious Mutations ◽

Next Generation ◽

Wild Type ◽

Female Germline ◽

Final Effect

The most important role of mitochondria is to supply cells with metabolic energy in the form of adenosine triphosphate (ATP). As synthesis of ATP molecules is accompanied by the generation of reactive oxygen species (ROS), mitochondrial DNA (mtDNA) is highly vulnerable to impairment and, consequently, accumulation of deleterious mutations. In most animals, mitochondria are transmitted to the next generation maternally, i.e., exclusively from female germline cells (oocytes and eggs). It has been suggested, in this context, that a specialized mechanism must operate in the developing oocytes enabling escape from the impairment and subsequent transmission of accurate (devoid of mutations) mtDNA from one generation to the next. Literature survey suggest that two distinct and irreplaceable pathways of mitochondria transmission may be operational in various animal lineages. In some taxa, the mitochondria are apparently selected: functional mitochondria with high inner membrane potential are transferred to the cells of the embryo, whereas those with low membrane potential (overloaded with mutations in mtDNA) are eliminated by mitophagy. In other species, the respiratory activity of germline mitochondria is suppressed and ROS production alleviated leading to the same final effect, i.e., transmission of undamaged mitochondria to offspring, via an entirely different route.

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The role of deleterious mutations in the adaptation to a novel environment

Artificial Life 13 ◽

10.7551/978-0-262-31050-5-ch004 ◽

2012 ◽

Cited By ~ 1

Author(s):

Arthur W. Covert III ◽

Jared Carlson-Stevermer

Keyword(s):

Deleterious Mutations ◽

Novel Environment

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Expansion load: recessive mutations and the role of standing genetic variation

10.1101/011593 ◽

2014 ◽

Cited By ~ 2

Author(s):

Stephan Peischl ◽

Laurent Excoffier

Keyword(s):

Genetic Variation ◽

Species Range ◽

Human Populations ◽

Deleterious Mutations ◽

Recessive Mutations ◽

Mutation Burden ◽

Natural Range ◽

Mean Fitness ◽

Expansion Load

Expanding populations incur a mutation burden – the so-called expansion load. Previous studies of expansion load have focused on co-dominant mutations. An important consequence of this assumption is that expansion load stems exclusively from the accumulation of new mutations occurring in individuals living at the wave front. Using individual-based simulations we study here the dynamics of standing genetic variation at the front of expansions, and its consequences on mean fitness if mutations are recessive. We find that deleterious genetic diversity is quickly lost at the front of the expansion, but the loss of deleterious mutations at some loci is compensated by an increase of their frequencies at other loci. The frequency of deleterious homozygotes therefore increases along the expansion axis whereas the average number of deleterious mutations per individual remains nearly constant across the species range. This reveals two important differences to co-dominant models: (i) mean fitness at the front of the expansion drops much faster if mutations are recessive, and (ii) mutation load can increase during the expansion even if the total number of deleterious mutations per individual remains constant. We use our model to make predictions about the shape of the site frequency spectrum at the front of range expansion, and about correlations between heterozygosity and fitness in different parts of the species range. Importantly, these predictions provide opportunities to empirically validate our theoretical results. We discuss our findings in the light of recent results on the distribution of deleterious genetic variation across human populations, and link them to empirical results on the correlation of heterozygosity and fitness found in many natural range expansions.

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Population-level consequences of inheritable somatic mutations and the evolution of mutation rates in plants

10.1101/2020.09.29.318402 ◽

2020 ◽

Author(s):

Thomas Lesaffre

Keyword(s):

Life Expectancy ◽

Inbreeding Depression ◽

Life Histories ◽

Somatic Mutations ◽

Population Level ◽

Mutation Rates ◽

Deleterious Mutations

ABSTRACTInbreeding depression, that is the decrease in fitness of inbred relative to outbred individuals, was shown to increase strongly as life expectancy increases in plants. Because plants are thought to not have a separated germline, it was proposed that this pattern could be generated by somatic mutations accumulating during growth, since larger and more long-lived species have more opportunities for mutations to accumulate. A key determinant of the role of somatic mutations is the rate at which they occur, which likely differs between species because mutation rates may evolve differently in species with constrasting life-histories. In this paper, we study the evolution of the mutation rates in plants, and consider the population-level consequences of inheritable somatic mutations given this evolution. We show that despite substantially lower per year mutation rates, more long-lived species still tend to accumulate larger amounts of deleterious mutations because of higher per generation, leading to higher levels of inbreeding depression in these species. However, the magnitude of this increase depends strongly on how mutagenic meiosis is relative to growth.

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Deleterious mutation accumulation and the long-term fate of chromosomal inversions

PLoS Genetics ◽

10.1371/journal.pgen.1009411 ◽

2021 ◽

Vol 17 (3) ◽

pp. e1009411

Author(s):

Emma L. Berdan ◽

Alexandre Blanckaert ◽

Roger K. Butlin ◽

Claudia Bank

Keyword(s):

Adaptive Evolution ◽

Simulation Study ◽

Feedback Loop ◽

Deleterious Mutation ◽

Adaptive Divergence ◽

Deleterious Mutations ◽

Dynamic Features ◽

Chromosomal Inversions

Chromosomal inversions contribute widely to adaptation and speciation, yet they present a unique evolutionary puzzle as both their allelic content and frequency evolve in a feedback loop. In this simulation study, we quantified the role of the allelic content in determining the long-term fate of the inversion. Recessive deleterious mutations accumulated on both arrangements with most of them being private to a given arrangement. This led to increasing overdominance, allowing for the maintenance of the inversion polymorphism and generating strong non-adaptive divergence between arrangements. The accumulation of mutations was mitigated by gene conversion but nevertheless led to the fitness decline of at least one homokaryotype under all considered conditions. Surprisingly, this fitness degradation could be permanently halted by the branching of an arrangement into multiple highly divergent haplotypes. Our results highlight the dynamic features of inversions by showing how the non-adaptive evolution of allelic content can play a major role in the fate of the inversion.

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