Major histocompatibility complex variation is similar in little brown bats before and after white-nose syndrome outbreak

Abstract

White-nose syndrome (WNS), caused by the fungal pathogen Pseudogymnoascus destructans (Pd), has driven alarming declines in North American hibernating bats, such as little brown bat (Myotis lucifugus). During hibernation, infected little brown bats are able to initiate anti-Pd immune responses, indicating pathogen-mediated selection on the major histocompatibility complex (MHC) genes. However, such immune responses may not be protective as they interrupt torpor, elevate energy costs, and potentially lead to higher mortality rates. To assess whether WNS drives selection on MHC genes, we compared the MHC DRB gene in little brown bats pre- (Wisconsin) and post- (Michigan, New York, Vermont, and Pennsylvania) WNS (detection spanning 2014-2015). We genotyped 131 individuals and found 45 nucleotide alleles (27 amino acid alleles) indicating a maximum of 3 loci (1-5 alleles per individual). We observed high allelic admixture and a lack of genetic differentiation both among sampling sites and between pre- and post-WNS populations, indicating no signal of selection on MHC genes. However, post-WNS populations exhibited decreased allelic richness, reflecting effects from bottleneck and drift following rapid population declines. We propose that mechanisms other than adaptive immunity are more likely driving current persistence of little brown bats in affected regions.

Keywords: Myotis lucifugus; North American bat; Pseudogymnoascus destructans; fungal disease; immunity; major histocompatibility complex.

FIGURE 1

Distribution of sampling sites and the amino acid alleles. The 10 sampling locations are marked with symbols that indicate WNS infection status and labeled with site names and sample sizes (in parentheses, # successfully genotyped samples/ # collected samples). Pie charts indicating percent amino acid allele frequencies were placed around their corresponding sites. Site PA was a combination of three sampling locations with their average coordinates (mapped) used as the population coordinates. A bar chart (bottom left) shows the percent frequency of amino acid alleles between pre‐ (WI‐a, WI‐b, WI‐c) and post‐WNS (NY‐a, NY‐b, VT) populations

FIGURE 2

Alignment of amino acid alleles and identification of antigen‐binding sites in MHC DRB exon 2. The 27 amino acid alleles of little brown bat identified in this study were aligned with published sequences of DRB exon 2 in several other bat species (GenBank accession numbers given in the sequence names). Two sets of antigen‐binding sites were identified, the * codons (Richman et al., 2010) and the shaded codons (Salmier et al., 2016), based on different previous studies

FIGURE 3

Relationship between sample size and (a) number of alleles or (b) allelic richness (Theta K). The best‐fit linear line shows significant relationship between sample size and the number of nucleotide alleles. Error bars of Theta K show 95% confidence intervals and line types of error bars indicate WNS infection status of the corresponding sites. Theta K against sample size for the grouped pre‐ and post‐WNS populations was also included in (b)

FIGURE 4

The test of isolation by distance. No significant relationships were detected by mantel tests (p > .05)

FIGURE 5

DAPC analysis across (a) all sampling sites and (b) the pre‐ and post‐WNS populations. The size of the inertia ellipse was set 2.5 in R package adegenet to encompass approximately 95% of the alleles. The sampling sites and populations were highly admixed and showed no spatial differentiation