Genetic variation in the two E-mink captive breeding programs
Our results show low genetic polymorphism in both captive E-mink
populations. Microsatellite markers for both populations exhibited low
allelic richness and heterozygosity indices, with the western population
having the lowest values, in line with previously published results
(Michaux et al., 2005; Cabria et al., 2007; Cabria et al. 2015).
However, the eastern E-mink sampled in this study might not reflect the
full genetic variation within the entire population, as collected E-mink
originated from a subset of the EEP since its start 25 years ago (Becker
et al., 2009). Conversely, western E-mink sampled came from wild-born
and captive-born individuals from a recent breeding program. Our
Bayesian clustering analysis suggests the existence of at least two main
genetic units of E-mink defined by their origin with the captive
programs, validating our use of the two E-mink groups for studying their
genetic and gut microbial variation.
Nonetheless, the two MHC genes investigated revealed differential
variation between the two E-mink populations, the MHC-I gene being more
divergent in eastern E-mink and the MHC-II gene exhibiting more richness
and divergence in western E-mink. Interestingly, the adaptive genetic
diversity followed the neutral markers trend only for one gene and not
the other, making the assessment of genetic diversity in captive
breeding complex. The maintenance of genetic variation in neutral
markers through non-selective evolutionary forces (genetic drift,
inbreeding) depend on the number of founders in a population, as well as
the breeding system of the species. However, balancing selection is
believed to counteract those non-selective evolutionary forces in
functional genes (Hedrick, 1999), resulting in an excess of
heterozygotes in small, isolated populations for MHC-II loci. This
pattern has been observed in several isolated populations (Aguilar et
al., 2004; Jarvi et al., 2004; Schad et al., 2004), but all species
investigated were free ranging, implying less restrictions in the mating
system compared to CBPs and therefore stronger sexual selection.
In line with previous evidence of the role of sexual selection for MHC
pattern distribution in vertebrates (Edwards & Hedrick, 1998), we
observed that sex had an influence on MHC-I gene composition. It has
been shown that MHC class I genes may be involved in pheromone
recognition, and that mate preferences can be reflected in dissimilarity
of MHC patterns (Penn, 2002). In the case of the E-mink, captive-bred
males are less successful breeders compared to wild-born males (Kiik et
al., 2013). Therefore, mate pairing based only on pedigree might not
provide enough information and might be hindered by MHC-I similarities
between potential mates. It is particularly striking knowing that the
only successful breeding of an eastern pair in 2019 at the French
facility was composed of mates having dissimilar MHC-I gene composition
(Zazu-Rosin, Figure S5; L. Berthomieu pers. comm., 2020). Variation at
neutral markers may thus not accurately reflect variation at potentially
relevant genes, particularly those under selection like the MHC (Ujvari
& Belov, 2011), and a global genetic assessment should be taken in
consideration in conservation genetics for management decisions (Mardsen
et al., 2013).