Evidence for convergent nucleotide evolution and high allelic turnover rates at the complementary sex determiner (csd) gene of western and Asian honey bees

Our understanding of the impact of recombination, mutation, genetic drift and selection on the evolution of a single gene is still limited. Here we investigate the impact of all of these evolutionary forces at the complementary sex determiner (csd) gene which evolves under a balancing mode of selection. Females are heterozygous at the csd gene and males are hemizygous; diploid males are lethal and occur when csd is homozygous. Rare alleles thus have a selective advantage, are seldom lost by the effect of genetic drift and are maintained over extended periods of time when compared to neutral polymorphisms. Here, we report on the analysis of 17, 19 and 15 csd alleles of Apis cerana, Apis dorsata and Apis mellifera honey bees respectively. We observed great heterogeneity of synonymous (pi S) and nonsynonymous (pi N) polymorphisms across the gene, with a consistent peak in exon 6 and 7. We propose that exons 6 and 7 encode the potential specifying domain (csd-PSD) which has accumulated elevated nucleotide polymorphisms over time by balancing selection. We observed no direct evidence that balancing selection favors the accumulation of nonsynonymous changes at csd-PSD (pi N/pi S ratios are all < 1, ranging from 0.6 to 0.95). We observed an excess of shared nonsynonymous changes, which suggests that strong evolutionary constraints are operating at csd-PSD resulting in the independent accumulation of the same nonsynonymous changes in different alleles across species (convergent evolution). Analysis of a csd-PSD genealogy revealed relatively short average coalescence times (~6 million years), low average synonymous nucleotide diversity (pi S < 0.09) and a lack of trans-specific alleles which substantially contrasts with previously analyzed loci under strong balancing selection. We excluded the possibility of a burst of diversification after population bottlenecking and intragenic recombination as explanatory factors, leaving high turn-over rates as the explanation for this observation. By comparing observed allele richness and average coalescence times with a simplified model of csd-coalescence, we found that small long term population sizes (i.e. Ne <104), but not high mutation rates, can explain short maintenance times, implicating a strong impact of genetic drift on the molecular evolution of highly social honey bees

A simple and distinctive microbiota exclusively associated with honey bees and bumble bees

Abstract: Specialized relationships with bacteria often allow animals to exploit a new diet by providing a novel set of metabolic capabilities. Bees are a monophyletic group of Hymenoptera that transitioned to a completely herbivorous diet from the carnivorous diet of their wasp ancestors. Recent culture-independent studies suggest that a set of distinctive bacterial species inhabits the gut of the honey bee, Apis mellifera. Here we survey the gut microbiotae of diverse bee and wasp species to test whether acquisition of these bacteria was associated with the transition to herbivory in bees generally. We found that most bee species lack phylotypes that are the same or similar to those typical of A. mellifera, rejecting the hypothesis that this dietary transition was symbiont-dependent. The most common bacteria in solitary bee species are a widespread phylotype of Burkholderia and the pervasive insect associate, Wolbachia. In contrast, several social representatives of corbiculate bees do possess distinctive bacterial phylotypes. Samples of A. mellifera harboured the same microbiota as in previous surveys, and closely related bacterial phylotypes were identified in two Asian honey bees (Apis andreniformis and Apis dorsata) and several bumble bee (Bombus) species. Potentially, the sociality of Apis and Bombus species facilitates symbiont transmission and thus is key to the maintenance of a more consistent gut microbiota. Phylogenetic analyses provide a more refined taxonomic placement of the A. mellifera symbionts. apis mellifera | bacterial microbiota | insect symbiosis | microbiology | molecula

Variance in spermatozoa number among Apis dorsata drones and among Apis mellifera drones

Published estimates of the mean spermatozoa numbers for Apis dorsata drones vary from 1.2 × 106 and 2.4 × 106; the number of spermatozoa per individual drone vary from 0.22 × 106 to 2.65 × 106. Counts presented here revealed 1.19 × 106 + 0.25 × 106 spermatozoa in drones sampled near a colony and 1.59 × 106 + 0.18 × 106 in drones sampled at a drone congregation area (DCA) in Sabah, Borneo. The difference between the two sites is significant. Further, the degree of variation in sperm numbers among drones near the colonies was higher than at the DCA. Possible reasons are discussed for spermatozoa number variation between drone samples in A. dorsata and in A. mellifera (published estimates). Furthermore, it is discussed if differences in spermatozoa numbers among fathering males may contribute to differences in patriline proportions within colonies

Using drones for estimating colony number by microsatellite DNA analyses of haploid males in Apis

In social insects the number of colonies rather than the actual number of individuals in the population primarily determines the effective population size. Here we present a method where microsatellite data of haploid males can be used to estimate the number of male producing queens in honeybee populations. A cluster analysis based on the allelic identity by descent (AID) among male genotypes is used to group potential brother males. For each “brother cluster” the corresponding mother queen genotype is determined by Mendelian inference. We show in various simulations that although limited number of screened loci can result in slightly biased estimates, the precision improves considerably with increasing number of loci. Empirical data from microsatellite studies of the Western honeybee Apis mellifera and the giant Asian honeybee Apis dorsata are presented to illustrate the application of the procedure