J.L. Bollmer’s research while affiliated with University of Wisconsin - Milwaukee and other places

The major histocompatibility complex (MHC) plays a central role in innate and adaptive immunity, but relatively little is known about the evolution of the number and arrangement of MHC genes in birds. Insights into the evolution of the MHC in birds can be gained by comparing the genetic architecture of the MHC between closely related species. We used a fosmid DNA library to sequence a 60.9-kb region of the MHC of the greater prairie chicken (Tympanuchus cupido), one of five species of Galliformes with a physically mapped MHC. Greater prairie chickens have the smallest core MHC yet observed in any bird species, and major changes are observed in the number and arrangement of MHC loci. In particular, the greater prairie chicken differs from other Galliformes in the deletion of an important class I antigen binding gene. Analysis of the remaining class IA gene in a population of greater prairie chickens in Wisconsin, USA revealed little evidence for selection at the region responsible for antigen binding.

Females are thought to gain better-quality genes for their offspring by mating with particular males. Genes of the major histocompatibility complex (MHC) play a critical role in adaptive immunity, and several studies have examined female mate choice in relation to MHC variation. In common yellowthroats, females prefer males that have larger black facial masks, an ornament associated with MHC variation, immune function and condition. Here we also tested whether mating patterns are directly correlated with MHC diversity or similarity. Using pyrosequencing, we found that the presence of extra-pair young in the brood was not related to male MHC diversity or similarity between the female and her within-pair mate. Furthermore, extra-pair sires did not differ in overall diversity from males they cuckolded, or in their similarity to the female. MHC diversity is extremely high in this species, and it may limit the ability of females to assess MHC variation in males. Thus, mating may be based on ornaments, such as mask size, which are better indicators of overall male health and genetic quality.

Previous studies of immunity in wild populations have focused primarily on genes of the major histocompatibility complex (MHC); however, studies of model species have identified additional immune-related genes that also affect fitness. In this study, we sequenced five non-MHC immune genes in six greater prairie-chicken (Tympanuchus cupido) populations that have experienced varying degrees of genetic drift as a consequence of population bottlenecks and fragmentation. We compared patterns of geographic variation at the immune genes with six neutral microsatellite markers to investigate the relative effects of selection and genetic drift. Global F(ST) outlier tests identified positive selection on just one of five immune genes (IAP-1) in one population. In contrast, at other immune genes, standardized G'(ST) values were lower than those at microsatellites for a majority of pairwise population comparisons, consistent with balancing selection or with species-wide positive or purifying selection resulting in similar haplotype frequencies across populations. The effects of genetic drift were also evident as summary statistics (e.g., Tajima's D) did not differ from neutrality for the majority of cases, and immune gene diversity (number of haplotypes per gene) was correlated positively with population size. In summary, we found that both genetic drift and selection shaped variation at the five immune genes, and the strength and type of selection varied among genes. Our results caution that neutral forces, such as drift, can make it difficult to detect current selection on genes.

Genes at the major histocompatibility complex (MHC) are known for high levels of polymorphism maintained by balancing selection. In small or bottlenecked populations, however, genetic drift may be strong enough to overwhelm the effect of balancing selection, resulting in reduced MHC variability. In this study we investigated MHC evolution in two recently diverged bird species: the endemic Galápagos hawk (Buteo galapagoensis), which occurs in small, isolated island populations, and its widespread mainland relative, the Swainson's hawk (B. swainsoni). We amplified at least two MHC class II B gene copies in each species. We recovered only three different sequences from 32 Galápagos hawks, while we amplified 20 unique sequences in 20 Swainson's hawks. Most of the sequences clustered into two groups in a phylogenetic network, with one group likely representing pseudogenes or nonclassical loci. Neutral genetic diversity at 17 microsatellite loci was also reduced in the Galápagos hawk compared to the Swainson's hawk. The corresponding loss in neutral diversity suggests that the reduced variability present at Galápagos hawk MHC class II B genes compared to the Swainson's hawk is primarily due to a founder event followed by ongoing genetic drift in small populations. However, purifying selection could also explain the low number of MHC alleles present. This lack of variation at genes involved in the adaptive immune response could be cause for concern should novel diseases reach the archipelago.

Population bottlenecks may reduce genetic variation and potentially increase the risk of extinction. Here, we present the first study to use historic samples to analyse loss of variation at the major histocompatibility complex (MHC), which plays a central role in vertebrate disease resistance. Balancing selection acts on the MHC and could moderate the loss of variation expected from drift; however, in a Wisconsin population of greater prairie-chickens (Tympanuchus cupido), the number of MHC class II B alleles per individual declined by 44% following a population bottleneck, compared to a loss of only 8% at microsatellites. Simulations indicate that drift likely reduced MHC variation at the population level, as well as within individuals by reducing the number of gene copies per individual or by fixing the same alleles across multiple loci. These multiple effects of genetic drift on MHC variation could have important implications for immunity and fitness.

The role of disease in regulating populations is controversial, partly owing to the absence of good disease records in historic wildlife populations. We examined birds collected in the Galapagos Islands between 1891 and 1906 that are currently held at the California Academy of Sciences and the Zoologisches Staatssammlung Muenchen, including 3973 specimens representing species from two well-studied families of endemic passerine birds: finches and mockingbirds. Beginning with samples collected in 1899, we observed cutaneous lesions consistent with Avipoxvirus on 226 (6.3%) specimens. Histopathology and viral genotyping of 59 candidate tissue samples from six islands showed that 21 (35.6%) were positive for Avipoxvirus, while alternative diagnoses for some of those testing negative by both methods were feather follicle cysts, non-specific dermatitis, or post mortem fungal colonization. Positive specimens were significantly nonrandomly distributed among islands both for mockingbirds (San Cristobal vs. Espanola, Santa Fe and Santa Cruz) and for finches (San Cristobal and Isabela vs. Santa Cruz and Floreana), and overall highly significantly distributed toward islands that were inhabited by humans (San Cristobal, Isabela, Floreana) vs. uninhabited at the time of collection (Santa Cruz, Santa Fe, Espanola), with only one positive individual on an uninhabited island. Eleven of the positive specimens sequenced successfully were identical at four diagnostic sites to the two canarypox variants previously described in contemporary Galapagos passerines. We conclude that this virus was introduced late in 1890's and was dispersed among islands by a variety of mechanisms, including regular human movements among colonized islands. At present, this disease represents an ongoing threat to the birds on the Galapagos Islands.

Small and isolated island populations provide ideal systems to study the effects of limited population size, genetic drift and gene flow on genetic diversity. We assessed genetic diversity within and differentiation among 19 mockingbird populations on 15 Galápagos islands, covering all four endemic species, using 16 microsatellite loci. We tested for signs of drift and gene flow, and used historic specimens to assess genetic change over the last century and to estimate effective population sizes. Within-population genetic diversity and effective population sizes varied substantially among island populations and correlated strongly with island size, suggesting that island size serves as a good predictor for effective population size. Genetic differentiation among populations was pronounced and increased with geographical distance. A century of genetic drift did not change genetic diversity on an archipelago-wide scale, but genetic drift led to loss of genetic diversity in small populations, especially in one of the two remaining populations of the endangered Floreana mockingbird. Unlike in other Galápagos bird species such as the Darwin's finches, gene flow among mockingbird populations was low. The clear pattern of genetically distinct populations reflects the effects of genetic drift and suggests that Galápagos mockingbirds are evolving in relative isolation.

The major histocompatibility complex (MHC) is characterized by a birth and death model of evolution involving gene duplication, diversification, loss of function, and deletion. As a result, gene number varies across taxa. Birds have between one and 7 confirmed MHC class II B genes, and the greatest diversity appears to occur in passerines. We used multiple primer sets on both genomic DNA (gDNA) and complementary DNA (cDNA) to characterize the range of class II B genes present in a passerine, the common yellowthroat (Geothlypis trichas). We confirmed 39 exon 2 sequences from gDNA in a single individual, indicating the presence of at least 20 class II B loci. From a second individual, we recovered 16 cDNA sequences belonging to at least 8 transcribed loci. Phylogenetic analysis showed that common yellowthroat sequences fell into subgroups consisting of classical loci, as well as at least 4 different clusters of sequences with reduced sequence variability that may represent pseudogenes or nonclassical loci. Data from 2 additional common yellowthroats demonstrated high interindividual variability. Our results reveal that some passerines possess an extraordinary diversity of MHC gene duplications, including both classical and nonclassical loci.

Debido al tamaño más pequeño y al aislamiento de las poblaciones de las islas, éstas tienden a ser más divergentes y menos variables genéticamente que las poblaciones continentales. Recolectamos muestras de ADN de nueve poblaciones isleñas de Buteo galapagoensis, cubriendo el rango total de distribución de la especie. Usamos marcadores neutrales de ADN minisatelital para calcular la diversidad genética dentro de las islas y la diferenciación genética (FST) entre islas. Típicamente, estos marcadores mutan demasiado rápido como para ser relevantes en estos estudios. Sin embargo, en poblaciones aisladas muy pequeñas, las preocupaciones sobre altas tasas de mutación son descartadas por la fuerza relativa de la deriva génica. Los individuos dentro de las islas presentaron los niveles más altos de uniformidad genética que se conocen actualmente para poblaciones naturales de aves, con valores de similitud promedio de bandas compartidas dentro de las poblaciones que fluctuaron entre 0.693 y 0.956. Estos valores se incrementaron al disminuir el tamaño de la isla. B. galapagoensis exhibe poliandría cooperativa de varios niveles en distintas islas; sin embargo, no encontramos una asociación entre el grado de poliandría y la variabilidad genética. Los valores de FST entre las islas fluctuaron entre 0.017 y 0.896, con un valor global para el archipiélago de 0.538. Por lo tanto, la mayoría de las poblaciones fueron genéticamente distintas. Además, documentamos niveles más altos de similitud genética entre poblaciones vecinas. Nuestros resultados indican que el flujo génico entre la mayoría de las poblaciones de B. galapagoensis es despreciable, y que la deriva génica ha jugado un rol importante determinando la estructura en estos loci minisatelitales.

Interspecific variation in sexual size dimorphism has commonly been attributed to variation in social mating system, with dimorphism increasing as intrasexual competition for mates increases. In birds, overall body size has also been found to correlate positively with size dimorphism. In this study, we describe variation in morphology and mating system across six populations of the endemic Galápagos Hawk (Buteo galapagoensis). Galápagos Hawks exhibit cooperative polyandry, a mating system in which long-term social groups contain a single female and multiple males. Comparisons among islands revealed significant differences in overall body size for both adults and immatures. Populations ranged from completely monogamous to completely polyandrous, with varying mean group sizes. Data did not support our prediction that sexual size dimorphism would increase with the degree of polyandry (number of males per group) or with body size; there was no correlation between mating system and sexual dimorphism. We did find a significant negative relationship between degree of polyandry and body size among islands, opposite of the pattern predicted.