Effective population size in eusocial Hymenoptera with worker-produced males

Sexual maturation and allometry of reproductive traits in large- and small-sized male honeybees

The body size of male honeybees (Apis mellifera L.) is a cause of skewed reproductive success. Large males are usually produced in colonies and have advantages in competition for mating and fertilisation. However, distinct small-sized males were produced depending on the colony conditions, particularly under queen-less conditions. Understanding the reproductive traits of small-sized males is currently limited, but it may provide insight into the developmental patterns and reproductive strategies that maximise reproductive success depending on body size and colony conditions. This study evaluated the process of sexual maturation in large- and small-sized males and the allometry between reproductive traits and body size. Changes in reproductive traits, including reproductive organs, number of spermatozoa, and sperm density, occurred earlier in small-sized males than in large-sized males after emergence. These results suggest that small males are precocious. The relatively early development of small-sized males would reflect the low developmental cost, which is likely to allow the production of many males and ensure reproductive success under circumstances in which available resources are limited. Furthermore, reproductive traits were positively correlated with body size, but allometry was different for these traits. Hence, the findings suggest that there is a given investment pattern toward reproductive traits with increasing body size, which would be responsible for high mating and fertilisation success in large males.

Eusociality influences the strength of negative selection on insect genomes

While much of the focus of sociobiology concerns identifying genomic changes that influence social behaviour, we know little about the consequences of social behaviour on genome evolution. It has been hypothesized that social evolution can influence the strength of negative selection via two mechanisms. First, division of labour can influence the efficiency of negative selection in a caste-specific manner; indirect negative selection on worker traits is theoretically expected to be weaker than direct selection on queen traits. Second, increasing social complexity is expected to lead to relaxed negative selection because of its influence on effective population size. We tested these two hypotheses by estimating the strength of negative selection in honeybees, bumblebees, paper wasps, fire ants and six other insects that span the range of social complexity. We found no consistent evidence that negative selection was significantly stronger on queen-biased genes relative to worker-biased genes. However, we found strong evidence that increased social complexity reduced the efficiency of negative selection. Our study clearly illustrates how changes in behaviour can influence patterns of genome evolution by modulating the strength of natural selection.

GC-biased gene conversion conceals the prediction of the nearly neutral theory in avian genomes

BACKGROUND

The nearly neutral theory of molecular evolution predicts that the efficacy of natural selection increases with the effective population size. This prediction has been verified by independent observations in diverse taxa, which show that life-history traits are strongly correlated with measures of the efficacy of selection, such as the dN/dS ratio. Surprisingly, avian taxa are an exception to this theory because correlations between life-history traits and dN/dS are apparently absent. Here we explore the role of GC-biased gene conversion on estimates of substitution rates as a potential driver of these unexpected observations.

RESULTS

We analyze the relationship between dN/dS estimated from alignments of 47 avian genomes and several proxies for effective population size. To distinguish the impact of GC-biased gene conversion from selection, we use an approach that accounts for non-stationary base composition and estimate dN/dS separately for changes affected or unaffected by GC-biased gene conversion. This analysis shows that the impact of GC-biased gene conversion on substitution rates can explain the lack of correlations between life-history traits and dN/dS. Strong correlations between life-history traits and dN/dS are recovered after accounting for GC-biased gene conversion. The correlations are robust to variation in base composition and genomic location.

CONCLUSIONS

Our study shows that gene sequence evolution across a wide range of avian lineages meets the prediction of the nearly neutral theory, the efficacy of selection increases with effective population size. Moreover, our study illustrates that accounting for GC-biased gene conversion is important to correctly estimate the strength of selection.

Prediction and estimation of effective population size

Effective population size (Ne) is a key parameter in population genetics. It has important applications in evolutionary biology, conservation genetics and plant and animal breeding, because it measures the rates of genetic drift and inbreeding and affects the efficacy of systematic evolutionary forces, such as mutation, selection and migration. We review the developments in predictive equations and estimation methodologies of effective size. In the prediction part, we focus on the equations for populations with different modes of reproduction, for populations under selection for unlinked or linked loci and for the specific applications to conservation genetics. In the estimation part, we focus on methods developed for estimating the current or recent effective size from molecular marker or sequence data. We discuss some underdeveloped areas in predicting and estimating Ne for future research.

Evolution of recombination and genome structure in eusocial insects

Eusocial Hymenoptera, such as the European honey bee, Apis mellifera, have the highest recombination rates of multicellular animals.(1) Recently, we showed(2) that a side-effect of recombination in the honey bee, GC biased gene conversion (bGC), helps maintain the unusual bimodal GC-content distribution of the bee genome by increasing GC-content in high recombination areas while low recombination areas are losing GC-content because of biased AT mutations and low rates of bGC. Although the very high recombination rate of A. mellifera makes GC-content evolution easier to study, the pattern is consistent with results found in many other species including mammals and yeast.(3) Also consistent across phyla is the association of higher genetic diversity and divergence with high GC and high recombination areas.(4) (,) (5) Finally, we showed that genes overexpressed in the brains of workers cluster in GC-rich genomic areas with the highest rates of recombination and molecular evolution.(2) In this Addendum we present a conceptual model of how eusociality and high recombination rates may co-evolve.