Variation at the major histocompatibility complex in Savannah sparrows

Area of Science:

• * Immunogenetics
• * Evolutionary biology
• * Avian genomics

Background:

• * The major histocompatibility complex (Mhc) is crucial for vertebrate adaptive immunity.
• * Avian Mhc gene organization, size, and variation are poorly understood, with initial studies suggesting significant interspecies differences.
• * Previous research in chickens and pheasants indicated distinct Mhc gene clustering compared to mammals.

Purpose of the Study:

• * To investigate the Mhc gene structure and diversity in a wild population of Savannah sparrows (Passerculus sandwichensis).
• * To determine the organization of class II B genes within the Savannah sparrow Mhc.
• * To explore potential correlations between Mhc characteristics and the species' migratory and social behaviors.

Main Methods:

• * Utilized a 157-base-pair probe from the second exon of a class II B gene to analyze Mhc variation.
• * Employed segregation analysis of Mhc bands to infer gene arrangement and copy number.
• * Genotyped 48 adult Savannah sparrows from a breeding population.

Main Results:

• * Savannah sparrow Mhc class II B genes appear organized into two independently assorting clusters, similar to other bird species but distinct from mammals.
• * The Mhc in this population is extensive, containing numerous class II B genes.
• * High Mhc genetic diversity was observed, with 42 unique genotypes identified among 48 individuals.

Conclusions:

• * The findings support hypotheses that long-distance migratory birds exhibit greater Mhc polymorphism and gene numbers compared to sedentary species.
• * The observed Mhc diversity may also be influenced by social mating systems, specifically polygyny and high rates of extra-pair paternity.
• * This study provides critical insights into the evolutionary dynamics shaping the avian Mhc.