Explore the vast range of titles available.

Invasive populations often experience founder effects: a loss of genetic diversity relative to the source population, due to a small number of founders. Even where these founder effects do not impact colonization success, theory predicts they might affect the rate at which invasive populations expand. This is because secondary founder effects are generated at advancing population edges, further reducing local genetic diversity and elevating genetic load. We show that in an expanding invasive population of the Asian honey bee ( Apis cerana ), genetic diversity is indeed lowest at range edges, including at the complementary sex determiner , csd , a locus that is homozygous-lethal. Consistent with lower local csd diversity, range edge colonies had lower brood viability than colonies in the range centre. Further, simulations of a newly-founded and expanding honey bee population corroborate the spatial patterns in mean colony fitness observed in our empirical data and show that such genetic load at range edges will slow the rate of population expansion. Invasive populations often have low genetic diversity because they originated from a small number of founding individuals. This study shows that in an invasive honey bee, one consequence of low genetic diversity is a reduced rate of population expansion due to serial founder effects at range edges.

Background Polyandrous social insects such as the honey bee are prime candidates for parental manipulation of gene expression in offspring. Although there is good evidence for parent-of-origin effects in honey bees the epigenetic mechanisms that underlie these effects remain a mystery. Small RNA molecules such as miRNAs, piRNAs and siRNAs play important roles in transgenerational epigenetic inheritance and in the regulation of gene expression during development. Results Here we present the first characterisation of small RNAs present in honey bee reproductive tissues: ovaries, spermatheca, semen, fertilised and unfertilised eggs, and testes. We show that semen contains fewer piRNAs relative to eggs and ovaries, and that piRNAs and miRNAs which map antisense to genes involved in DNA regulation and developmental processes are differentially expressed between tissues. tRNA fragments are highly abundant in semen and have a similar profile to those seen in the semen of other animals. Intriguingly we also find abundant piRNAs that target the sex determination locus, suggesting that piRNAs may play a role in honey bee sex determination. Conclusions We conclude that small RNAs may play a fundamental role in honey bee gametogenesis and reproduction and provide a plausible mechanism for parent-of-origin effects on gene expression and reproductive physiology.

Cross-kingdom RNA interference (RNAi) is a biological process allowing plants to transfer small regulatory RNAs to invading pathogens to trigger the silencing of target virulence genes. Transient assays in cereal powdery mildews suggest that silencing of one or two effectors could lead to near loss of virulence, but evidence from stable RNAi lines is lacking. We established transient host-induced gene silencing (HIGS) in wheat, and demonstrate that targeting an essential housekeeping gene in the wheat powdery mildew pathogen ( f. sp. ) results in significant reduction of virulence at an early stage of infection. We generated stable transgenic RNAi wheat lines encoding a HIGS construct simultaneously silencing three effectors including , a virulence factor involved in the suppression of the powdery mildew resistance gene. We show that all targeted effectors are effectively downregulated by HIGS, resulting in reduced fungal virulence on adult wheat plants. Our findings demonstrate that stable HIGS of effector genes can lead to quantitative gain of resistance without major pleiotropic effects in wheat.

Some invasive hymenopteran social insects found new populations with very few reproductive individuals. This is despite the high cost of founder effects for such insects, which generally require heterozygosity at a single locus—the complementary sex determiner, csd—to develop as females. Individuals that are homozygous at csd develop as either infertile or subfertile diploid males or not at all. Furthermore, diploid males replace the female workers that are essential for colony function. Here we document how the Asian honey bee (Apis cerana) overcame the diploid male problem during its invasion of Australia. Natural selection prevented the loss of rare csd alleles due to genetic drift and corrected the skew in allele frequencies caused by founder effects to restore high average heterozygosity. Thus, balancing selection can alleviate the genetic load at csd imposed by severe bottlenecks, and so facilitate invasiveness.Hymenopteran social insects require heterozygosity at the csd locus for female development. Here, it is shown how balancing selection overcomes founder effects to maintain this heterozygosity and allow the Asian honeybee to become invasive.

The haplodiploid system of sex determination of Hymenoptera acts as an exaptation for species to evolve novel forms of asexual reproduction including thelytoky (clonal offspring of the mother). During normal reproduction in Hymenoptera, three of the four products of meiosis that are present in newly-laid eggs are lost as polar bodies, while the remaining pronucleus either develops as a haploid male or fuses with a sperm nucleus to produce a diploid zygote. In contrast, in thelytokous reproduction, which is uncommon but taxonomically widespread, two of the four products of meiosis fuse, as if one acted as a sperm. Queenless workers of Apis mellifera capensis, a subspecies of honey bee from South Africa, routinely reproduce thelytokously. Unmated A. m. capensis queens can also be induced to lay thelytokously by narcosis with carbon dioxide, but mated queens are never thelytokous. We artificially inseminated A. m. capensis queens using CO2 narcosis. Up to 1/3 of offspring workers carried two maternal alleles and an allele of one father whereas no three-allele progeny were seen in control queens of the arrhenotokous (unfertilized eggs result in males) subspecies A. m. scutellata. Flow cytometry of three-allele individuals revealed that they were triploid and arose from the fertilization of a thelytokous fusion nucleus. We then reared six queens from a narcotized A. m. capensis queen and determined the ploidy of the offspring queens based on microsatellites. One of the five daughters was triploid. Following artificial insemination, this queen produced unfertilized thelytokous diploid eggs at high frequency, and unfertilized triploid eggs at much lower frequency. If fertilized, thelytokous diploid eggs were non-viable, even though triploidy in itself does not impede normal development. In contrast, when the rarer triploid eggs were fertilized, a proportion developed into viable tetraploids. Our study highlights the extraordinary developmental flexibility of haplo-diploid systems.

The evolutionary significance of epigenetic inheritance is controversial. While epigenetic marks such as DNA methylation can affect gene function and change in response to environmental conditions, their role as carriers of heritable information is often considered anecdotal. Indeed, near-complete DNA methylation reprogramming, as occurs during mammalian embryogenesis, is a major hindrance for the transmission of nongenetic information between generations. Yet it remains unclear how general DNA methylation reprogramming is across the tree of life. Here we investigate the existence of epigenetic inheritance in the honey bee. We studied whether fathers can transfer epigenetic information to their daughters through DNA methylation. We performed instrumental inseminations of queens, each with four different males, retaining half of each male’s semen for whole genome bisulfite sequencing. We then compared the methylation profile of each father’s somatic tissue and semen with the methylation profile of his daughters. We found that DNA methylation patterns were highly conserved between tissues and generations. There was a much greater similarity of methylomes within patrilines (i.e., father-daughter subfamilies) than between patrilines in each colony. Indeed, the samples’ methylomes consistently clustered by patriline within colony. Samples from the same patriline had twice as many shared methylated sites and four times fewer differentially methylated regions compared to samples from different patrilines. Our findings indicate that there is no DNA methylation reprogramming in bees and, consequently, that DNA methylation marks are stably transferred between generations. This points to a greater evolutionary potential of the epigenome in invertebrates than there is in mammals.

Key messageWe have isolated a novel powdery mildew resistance gene in wheat that was originally introgressed from rye. Further analysis revealed evolutionary divergent history of wheat and rye orthologous resistance genes.Wheat production is under constant threat from a number of fungal pathogens, among them is wheat powdery mildew (Blumeria graminis f. sp. tritici). Deployment of resistance genes is the most economical and sustainable method for mildew control. However, domestication and selective breeding have narrowed genetic diversity of modern wheat germplasm, and breeders have relied on wheat relatives for enriching its gene pool through introgression. Translocations where the 1RS chromosome arm was introgressed from rye to wheat have improved yield and resistance against various pathogens. Here, we isolated the Pm17 mildew resistance gene located on the 1RS introgression in wheat cultivar ‘Amigo’ and found that it is an allele or a close paralog of the Pm8 gene isolated earlier from ‘Petkus’ rye. Functional validation using transient and stable transformation confirmed the identity of Pm17. Analysis of Pm17 and Pm8 coding regions revealed an overall identity of 82.9% at the protein level, with the LRR domains being most divergent. Our analysis also showed that the two rye genes are much more diverse compared to the variants encoded by the Pm3 gene in wheat, which is orthologous to Pm17/Pm8 as concluded from highly conserved upstream sequences in all these genes. Thus, the evolutionary history of these orthologous loci differs in the cereal species rye and wheat and demonstrates that orthologous resistance genes can take different routes towards functionally active genes. These findings suggest that the isolation of Pm3/Pm8/Pm17 orthologs from other grass species, additional alleles from the rye germplasm as well as possibly synthetic variants will result in novel resistance genes useful in wheat breeding.

Honey bees ( Apis spp.) are important pollinators in many natural and agro-ecosystems across the world. Effective means of surveying wild populations are therefore key to their conservation and management. One available survey method infers honey bee colony density from the genotype of drones (males) sampled from sites known as Drone Congregation Areas (DCAs). While this approach has been commonly used for the Western honey bee ( A. mellifera ), its feasibility for other Apis species is unknown. Here, we investigate drone congregation behaviour in the Asian honey bee Apis cerana in north-east Australia and its suitability for inferring colony density. As this A. cerana population is invasive, surveys in this case can aid in monitoring the population’s growth and spread. Over 5 years, we identified 30 DCAs, many of which were stable across time. DCAs were sheltered areas beside tree-lines or openings in the forest canopy. A. cerana drones joined DCAs during 1–2-h afternoon intervals and could be sampled at heights of 2–24 m via adhesive lines attached to helium balloons carrying lures coated in queen pheromone. Drones were more likely to be present at a DCA as temperature increased, though abiotic factors did not predict overall drone abundance. Drones could be sampled in low numbers even where colony density was extremely low. Based on the genotyping and inferred sibship of drones sampled at DCAs between 2016 and 2021, we estimate population density in Australia’s A. cerana to be in the range 1.1–8.1 colonies/km 2 . This extrapolates to a total population size in the range 11,000–83,000 colonies, with more refined estimates requiring better knowledge of drone flight distance and the effect of habitat on colony density. We conclude that population surveys based on drones from DCAs are possible for A. cerana and propose that this approach be part of a toolkit of methods used to monitor Asian honey bee populations in both their native and invasive ranges.

AbstractHoney bees (Apis spp.) are important pollinators in many natural and agro-ecosystems across the world. Effective means of surveying wild populations are therefore key to their conservation and management. One available survey method infers honey bee colony density from the genotype of drones (males) sampled from sites known as Drone Congregation Areas (DCAs). While this approach has been commonly used for the Western honey bee (A. mellifera), its feasibility for other Apis species is unknown. Here, we investigate drone congregation behaviour in the Asian honey bee Apis cerana in north-east Australia and its suitability for inferring colony density. As this A. cerana population is invasive, surveys in this case can aid in monitoring the population’s growth and spread. Over 5 years, we identified 30 DCAs, many of which were stable across time. DCAs were sheltered areas beside tree-lines or openings in the forest canopy. A. cerana drones joined DCAs during 1–2-h afternoon intervals and could be sampled at heights of 2–24 m via adhesive lines attached to helium balloons carrying lures coated in queen pheromone. Drones were more likely to be present at a DCA as temperature increased, though abiotic factors did not predict overall drone abundance. Drones could be sampled in low numbers even where colony density was extremely low. Based on the genotyping and inferred sibship of drones sampled at DCAs between 2016 and 2021, we estimate population density in Australia’s A. cerana to be in the range 1.1–8.1 colonies/km2. This extrapolates to a total population size in the range 11,000–83,000 colonies, with more refined estimates requiring better knowledge of drone flight distance and the effect of habitat on colony density. We conclude that population surveys based on drones from DCAs are possible for A. cerana and propose that this approach be part of a toolkit of methods used to monitor Asian honey bee populations in both their native and invasive ranges.