4.3 Conservation Implications
We see implications from our results for managing genetic erosion inS. catenatus due to range-wide declines in population numbers. First, at the species level, reduced genome-wide genetic load due to long term purging suggests that future population declines in less impacted regions such as the northern parts of the species range will have fewer negative genetic impacts than comparable declines in threatened snakes with historically larger effective sizes. S. catenatus may be more resistant to the process of mutational meltdown that is proposed to occur in small populations of threatened species (Frankham et al., 2017) leading to a focus on the greater importance of ecological factors to long term population viability (Lande, 1988).
Second, consideration of the potential effects of long-term purging on mutational load have led to suggestions that conventional guidelines for conducting genetic rescue of small inbred populations of threatened species need to be reconsidered (Kyriazis et al., 2020; van der Valk et al., 2019; but see Ralls et al., 2020). Specifically, Kyriazis et al. (2020) argue that the typical approach for managing these populations is to maintain high genetic diversity through the transfer of individuals from large genetically diverse populations but that this carries a risk of introducing large numbers of deleterious mutations that can be exposed by inbreeding. Our empirical results show that, in fact, there is an inverse relationship between levels of mutational load and effective size among S. catenatus populations or that large potential donor populations have fewer not more potential deleterious mutations present. This observation, combined with the fact that donor individuals from larger populations will have greater numbers of potentially adaptive variants (Ochoa et al., 2020) broadly supports the idea that choosing donor individuals from populations with large effective sizes is a sound strategy for genetic rescue in this species. This claim comes with two important caveats. It assumes a high degree of sharing of deleterious mutations between populations such that the inter-population transfer of individuals will not introduce large numbers of new distinct negative mutations. Second, it also assumes that adaptive variation that evolves through local adaptation will have similar positive effects on individual fitness in populations receiving translocated individuals.
More broadly, our study provides an example of how whole genome sequence from threatened and endangered species can provide new approaches to assess patterns of functional variation that impact genetic erosion in these species (Leroy et al., 2018). When combined with the results of Ochoa et al. (2020) it provides a rare comprehensive assessment of two of the key aspects of genetic erosion in a single endangered species namely the magnitude and evolutionary mechanisms shaping negative genetic load and adaptive genetic variation. As such it represents an important realization of the application of genomic tools and analyses for addressing genetic issues related to conservation of biodiversity at the species and population levels (Funk et al., 2019; Hohenlohe, Funk, & Rajora, 2021).