Genetic load has potential in large populations but is realized in small inbred populations

Mathur, Samarth; DeWoody, J. Andrew

doi:10.1111/eva.13216

Cited by 84 publications

(79 citation statements)

References 149 publications

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“…Alpine ibex, which were once reduced to 100 individuals, had fewer highly deleterious alleles but more mildly deleterious alleles compared to Iberian ibex (bottleneck size 1,000 individuals). Empirical genetic data suggest small populations have higher drift load (41)(42)(43) which has resulted in lower population growth in populations with lower genetic variation (2,3). In agreement with theoretical expectations outlined above, these data suggest that purging is insufficient to maintain high fitness in the face of strong genetic drift and inbreeding.…”

Section: Is Genetic Variation Predictive Of Inbreeding and Inbreeding Depression?mentioning

confidence: 99%

The crucial role of genome-wide genetic variation in conservation

Kardos

Armstrong

Fitzpatrick

et al. 2021

Preprint

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The unprecedented rate of extinction calls for efficient use of genetics to help conserve biodiversity. Several recent genomic and simulation-based studies have argued that the field of conservation biology has placed too much focus on the conservation of genome-wide genetic variation, and that this approach should be replaced with another that focuses instead on managing the subset of functional genetic variation that is thought to affect fitness. Here, we critically evaluate the feasibility and likely benefits of this approach in conservation. We find that population genetics theory and empirical results show that the conserving genome-wide genetic variation is generally the best approach to prevent inbreeding depression and loss of adaptive potential from driving populations towards extinction. Focusing conservation efforts on presumably functional genetic variation will only be feasible occasionally, often misleading, and counterproductive when prioritized over genome-wide genetic variation. Given the increasing rate of habitat loss and other environmental changes, failure to recognize the detrimental effects of lost genome-wide variation on long-term population viability will only worsen the biodiversity crisis.

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Section: Is Genetic Variation Predictive Of Inbreeding and Inbreeding Depression?mentioning

confidence: 99%

The crucial role of genome-wide genetic variation in conservation

Kardos

Armstrong

Fitzpatrick

et al. 2021

Preprint

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“…In addition, on a per site basis, the frequency of deleterious mutations is between 40 -68% less in S. tergeminus (data not shown). Together these patterns argue that although S. tergeminus has higher "Potential" genetic load in the form of absolute numbers of deleterious mutations, the extent to which the negative effects of these mutations ("Realized" load) is blunted by the fact that proportionately more occur in presumably unexpressed heterozygous genotypes and are also found at lower frequencies on a per site basis (Mathur & DeWoody, 2021).…”

Section: Genetic Loadmentioning

confidence: 97%

“…Under the assumption that most deleterious mutations that arise in a population are recessive (Agrawal & Whitlock, 2011) the genotypic configuration of mutations can have a substantial impact on their realized negative impacts on individual fitness (Mathur & DeWoody, 2021). In this respect, S. catenatus has a significantly greater proportion of homozygotes relative to S. tergeminus , demonstrating greater levels of realized genetic load on a per-individual basis for two of three classes of mutations (Figure 6c).…”

Section: Genetic Loadmentioning

confidence: 99%

“…Thus, the type and level of genetic load present in a given population reflects a balance of the relative magnitudes of these distinct processes (inbreeding versus reduced selection due to drift) with opposite effects on mutation frequencies. As a result, assessing levels of inbreeding, effective population size, and levels of genetic load are an important goal of empirical studies that seek to assess the genetic risks impacting long term viability of threatened species (Mathur & DeWoody, 2021) and for assessing the appropriateness of specific management activities such as genetic rescue for mitigating these risks (Kyriazis, Wayne, & Lohmueller, 2020; but see Ralls, Sunnucks, Lacy, & Frankham, 2020).…”

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Genomic signatures of inbreeding and genetic load in a threatened rattlesnake

Ochoa

Gibbs

2021

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Theory predicts that threatened species living in small populations will experience high levels of inbreeding that will increase their negative genetic load but recent work suggests that the impact of load may be minimized by purging resulting from long term population bottlenecks. Empirical studies that examine this idea using genome-wide estimates of inbreeding and genetic load in threatened species are limited. Here we use genome resequencing data to compare levels of inbreeding, levels of genetic load and population history in threatened Eastern massasauga rattlesnakes (Sistrurus catenatus) which exist in small isolated populations and closely-related yet outbred Western massasauga rattlesnakes (S. tergeminus). In terms of inbreeding, S. catenatus genomes had a greater number of ROHs of varying sizes indicating sustained inbreeding through repeated bottlenecks when compared to S. tergeminus. At the species level, outbred S. tergeminus had higher genome-wide levels of genetic load in the form of greater numbers of derived deleterious mutations compared to S. catenatus presumably due to long-term purging of deleterious mutations in S. catenatus. In contrast, mutations that escaped the “drift sieve” and were polymorphic within S. catenatus populations were more abundant and more often found in homozygote genotypes than in S. tergeminus suggesting a reduced efficiency of purifying selection in smaller S. catenatus populations. Our results support an emerging idea that the historical demography of a threatened species has a significant impact on the type of genetic load present which impacts implementation of conservation actions such as genetic rescue.

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“…At the individual level, deleterious mutations that are recessive reduce fitness only when they are homozygous (Robinson et al, 2019) as their phenotypic impacts are masked in heterozygotes (Crnokrak & Roff, 1999;García-Dorado, 2012). Thus, the potential genetic load (Load P ) harbored by populations due to the number and frequency of deleterious mutations is functionally and mechanistically different from the realized genetic load (Load R ) that reduces individual fitness due to homozygosity of deleterious mutations (Mathur & DeWoody, 2021).…”

Section: Introductionmentioning

confidence: 99%

An evolutionary perspective on contemporary genetic load in threatened species to inform future conservation efforts

Mathur

Tomeček

Tarango-Arámbula

et al. 2021

Preprint

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In theory, genomic erosion can be reduced in fragile “recipient” populations by translocating individuals from genetically diverse “donor” populations. However, recent simulation studies have argued that such translocations can, in principle, serve as a conduit for new deleterious mutations to enter recipient populations. A reduction in evolutionary fitness is associated with a higher load of deleterious mutations and thus, a better understanding of evolutionary processes driving the empirical distribution of deleterious mutations is crucial. Here, we show that genetic load is evolutionarily dynamic in nature and that demographic history greatly influences the distribution of deleterious mutations over time. Our analyses, based on both demographically explicit simulations as well as whole genome sequences of potential donor-recipient pairs of Montezuma Quail (Cyrtonyx montezumae) populations, indicate that all populations tend to lose deleterious mutations during bottlenecks, but that genetic purging is pronounced in smaller populations with stronger bottlenecks. Despite carrying relatively fewer deleterious mutations, we demonstrate how small, isolated populations are more likely to suffer inbreeding depression as deleterious mutations that escape purging are homogenized due to drift, inbreeding, and ineffective purifying selection. We apply a population genomics framework to showcase how the phylogeography and historical demography of a given species can enlighten genetic rescue efforts. Our data suggest that small, inbred populations should benefit the most when assisted gene flow stems from genetically diverse donor populations that have the lowest proportion of deleterious mutations.

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Genetic load has potential in large populations but is realized in small inbred populations

Cited by 84 publications

References 149 publications

The crucial role of genome-wide genetic variation in conservation

The crucial role of genome-wide genetic variation in conservation

Genomic signatures of inbreeding and genetic load in a threatened rattlesnake

An evolutionary perspective on contemporary genetic load in threatened species to inform future conservation efforts

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