COS 150-2 - Putative polymorphic inversion present in multiple populations of White-breasted Nuthatches (Sitta carolinensis)

Background/Question/Methods

In evolutionary ecology, there has been a strong interest in the role genomic inversions play in speciation and local adaptation. Inversions are rearrangements in the genome where a section is inverted relative to its former colinear state. Inversions are compelling rearrangements to study because they effectively reduce recombination in the inversion area linking together loci. Because of this suppressed recombination, various models have been proposed regarding how inversions can influence local adaptation and speciation. As a part of a project investigating genomic differentiation in White-breasted Nuthatches, we sequenced hundreds of samples across North America using GBS (Genotyping-by-Sequencing). We found that White-breasted Nuthatches are comprised of four highly differentiated populations, Rocky Mountain South, Rocky Mountain North, Pacific, and Eastern, which may be different species.

Results/Conclusions

In principal components analyses, we observed on one chromosome what appears to be a polymorphic inversion which manifests as three clusters (homozygotes of either inversion type and heterozygotes). This inversion is large, approximately 10 Mb, and appears in three populations: Rocky Mountain North, Rocky Mountain South, and Pacific. The inversion frequency is similar in the two Rocky Mountain populations but much lower in the Pacific. Detailed analyses of F_ST (a measure of genetic differentiation) and nucleotide diversity of the two haplotypes reveal that nucleotide diversity is very low in one of the inversion types. We interpret this to mean that the inversion type with low diversity formed recently and/or has been the subject of strong selection and introgression between populations. However, this conclusion is preliminary and will be tested further by ongoing analyses. Whether or not the inversion has formed recently, we postulate that balancing selection explains its shared polymorphism in multiple populations. To our knowledge, this kind of observation in birds is not common. It demonstrates that inversions may be important sources of within-population diversity in birds and can be maintained over speciation events.