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).