MHC variation reflects the bottleneck histories of New Zealand passerines

Abstract

Most empirical evidence suggests that balancing selection does not counter the effects of genetic drift in shaping postbottleneck major histocompatibility complex (MHC) genetic diversity when population declines are severe or prolonged. However, few studies have been able to include data from historical specimens, or to compare populations/species with different bottleneck histories. In this study, we examined MHC class II B and microsatellite diversity in four New Zealand passerine (songbird) species that experienced moderate to very severe declines. We compared diversity from historical samples (collected c. 1884-1938) to present-day populations. Using a Bayesian framework, we found that the change in genetic diversity from historical to contemporary samples was affected by three main factors: (i) whether the data were based on MHC or microsatellite markers, (ii) species (as a surrogate for bottleneck severity) and (iii) whether the comparison between historical and contemporary samples was made using historical samples originating from the mainland, or using historical samples originating from islands. The greatest losses in genetic diversity occurred for the most severely bottlenecked species, particularly between historical mainland and contemporary samples. Additionally, where loss of diversity occurred, the change was greater for MHC genes compared to microsatellite loci.

Balancing selection and genetic drift create unusual patterns of MHCIIβ variation in Galápagos mockingbirds

“…; Sutton et al . , ). This difference might be caused by the fact that GM represent different‐sized populations in a mutation–drift equilibrium (for neutral loci), rather than heavily bottlenecked populations where the loss of MHC allelic diversity might be reinforced by the interaction between drift and balancing selection (Ejsmond & Radwan ).…”

“…In populations with small effective population size (Ne), genetic drift has been shown to reduce MHC allelic diversity (Sutton et al . , ), thus counteracting the effect of PMBS (Robertson ; Kimura ; Miller & Lambert ). In several very small populations, MHC diversity was found to be even more reduced than neutral diversity (Sutton et al .…”

Differential patterns of diversity at microsatellite, MHC, and TLR loci in bottlenecked South Island saddleback populations

“…We amplified 18 microsatellite loci (see Table S1 in Supplementary Material) according to Sutton et al (2015). (Tallmon et al 2008).…”

“…It has been shown that factors such as location of the population (insular or mainlandoriginating), bottleneck severity (duration and intensity) and prior history of bottlenecks, are all strong determinants of remaining genetic diversity in threatened species (e.g. Taylor & Jamieson 2008;Sutton et al 2015). However, it is currently unknown whether population bottleneck circumstance (e.g.…”

“…Despite contemporary populations of South Island saddlebacks also having reduced MHC diversity compared to historical populations (Sutton et al 2015), it is currently unclear whether diversity at functional genes has been sufficiently depleted to make the genetic effects of contemporary bottlenecks negligible. However, given the highly polymorphic state of the MHC and, to some extent, TLR loci in wild populations, we predict that (1) MHC and TLR loci will continue to decrease in diversity from source to serially translocated populations, and (2) neutral microsatellite diversity may vary slightly from 2002 to 2012 on Motuara Island due to the effects of genetic drift, while MHC and TLR diversity will show changes from 2002 to 2012 that reflect selective responses to the diseaserelated population bottleneck in 2004.…”