Early diversification of sperm size in the evolutionary history of the old world leaf warblers (Phylloscopidae).

Sperm morphological traits are highly variable among species and are commonly thought to evolve by post-copulatory sexual selection. However, little is known about the evolutionary dynamics of sperm morphology, and whether rates of evolutionary change are variable over time and among taxonomic groups. Here, we examine sperm morphology from 21 species of Old World leaf warblers (Phylloscopidae), a group of generally dull, sexually monochromatic birds, which are known to have high levels of extra-pair paternity. We found that sperm length differs markedly across species, spanning about 40% of the range observed across a larger selection of passerine birds. Furthermore, we found strong support for an 'early-burst' model of trait evolution, implying that the majority of divergence in sperm length has occurred early in the evolutionary history of this clade with subsequent evolutionary stasis. This large early divergence matches the early divergence reported in ecological traits (i.e. body size and feeding behaviour). Our findings demonstrate that rates of evolution in sperm morphology can change over time in passerine taxa, and that evolutionary stasis in sperm traits can occur even in species exhibiting characteristics consistent with moderate-to-high levels of sperm competition. It remains a major challenge to identify the selection mechanisms and possible constraints responsible for these variable rates of sperm evolution.

Ancestral polymorphism in exon 2 of bluethroat (Luscinia svecica) MHC class II B genes.

The genes of the major histocompatibility complex (MHC) are important model genes for understanding selective forces in evolution. Here, we document, using a cloning and sequencing approach, high polymorphism at the exon 2 of the MHC class II B (MHCIIB) genes in the bluethroat (Luscinia svecica); a minimum of 61 unique alleles were detected in 20 individuals, and at least 11 functional loci. In addition, several pseudogenes were revealed. The specimens originated from three different bluethroat subspecies (azuricollis, cyanecula and svecica), and we also analysed four specimens of the closely related thrush nightingale (L. luscinia) for comparison. Phylogenetic analyses of the functional alleles revealed 258 equally parsimonious trees with poor statistical support for the majority of nodes. The distribution of the sequences in the trees point to an ancestral origin of the polymorphism in MHC class II B genes, a portion of which predated the phylogenetic split between the bluethroat and the thrush nightingale. Strong signatures of balancing selection were uncovered for the codons coding for the peptide-binding residues of the functional MHCIIB exon 2 alleles. Our results highlight the importance of duplication and recombination events for shaping passerine MHC and give insights in the evolutionary dynamics of MHC variation among closely related taxa.

Cell-mediated immunity and multi-locus heterozygosity in bluethroat nestlings.

Recent evidence suggests that marker-based heterozygosity-fitness correlations may be driven by only one or a few markers, indicating local heterozygosity effects caused by linkage disequilibrium with functional genes. In this study, we investigated the relationship between microsatellite heterozygosity and a measure of cell-mediated immunity (phytohaemagglutinin; PHA) in bluethroat (Luscinia s. svecica) nestlings using a full-sibling design. We found significant positive associations between PHA response and two different indices of microsatellite heterozygosity, i.e. multi-locus heterozygosity and mean d(2). However, model comparisons disclosed that both associations were more likely caused by local effects rather than general effects and that the two local effects appeared to be realized through two different genetic mechanisms. Our results indicate that both the random assortment of parental chromosomes during meiosis as well as inbreeding can drive heterozygosity-fitness correlations.

Female bluethroats enhance offspring immunocompetence through extra-pair copulations.

Female birds frequently copulate with extra-pair males, but the adaptive value of this behaviour is poorly understood. Some studies have suggested that 'good genes' may be involved, where females seek to have their eggs fertilized by high-quality males without receiving any material benefits from them. Nevertheless, it remains to be shown that a genetic benefit is passed on to offspring. Here we report that nestling bluethroats, Luscinia svecica, sired by extra-pair males had a higher T-cell-mediated immune response than their maternal half-siblings raised in the same nest. The difference could not be attributed to nestling body mass, sex or hatching order, but may be an effect of paternal genotype. Extra-pair young were also more immunocompetent than their paternal half-sibs raised in the genetic father's own nest, which indicates an additional effect of maternal genotype. Our results are consistent with the idea that females engage in extra-pair copulations to obtain compatible viability genes, rather than 'good genes' per se.

Genetic relationships in the peregrine falcon (Falco peregrinus) analysed by microsatellite DNA markers.

Microsatellite DNA markers were developed from a peregrine falcon (Falco peregrinus) and genetic relationships among peregrine falcons in southern Norway were analysed using the markers. The genomic DNA library was screened for the presence of dinucleotide microsatellite repeats. Twelve loci revealed polymorphism through the initial analysis of 24 unrelated peregrine falcons, and Mendelian inheritance was confirmed in two peregrine falcon families bred in captivity. The estimated mean probability of identical genotypes in two unrelated individuals was 3 x 10-8, and the combined exclusion probability for parentage testing was 0.99 and 0.94 for one or both parents unknown, respectively. The markers were used to investigate the parentage of peregrine broods from the same nest site from different breeding seasons, and subsequently the nest-site fidelity of the breeding peregrines. High nest-site fidelity was found by studying pairwise comparisons of relatedness (rxy) estimates among chicks at six nest sites from three different breeding seasons. Cross-species amplifications showed that most loci also appeared to amplify polymorphic products in the gyrfalcon (F. rusticolus), merlin (F. columbarius), hobby (F. subbuteo) and kestrel (F. tinnunculus), demonstrating that the loci will provide powerful genetic markers in these falcons too.