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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.

Pollen is one of the main food sources for honeybees. The honeybee gut microbiota plays a crucial role in maintaining digestive function and host health during long-term coevolution. While the consumption and utilization of pollen have been extensively studied, there is limited information about the effects of pollen on the gut microbiota of Apis cerana. In this study, we used 16S rRNA amplicon sequencing to evaluate the effects of four natural pollens (oilseed rape pollen, camellia pollen, lotus pollen and buckwheat pollen) and two pollen substitutes (Diet 1 and Diet 2) on the hindgut microbiota of newly emerged A. cerana worker bees, following feeding periods of 5, 10 and 15 days. The results showed that Firmicutes and Proteobacteria are dominant in the gut microbiota of A. cerana. A. cerana workers fed with pollen diets had a higher diversity of gut microbiota than those fed with pollen substitutes. There have been significant differences in the gut microbiota structure and relative abundance of the core microbial community among A. cerana workers supplied with different diets. Our results confirm that gut bacterial communities of A. cerana can be influenced by pollen diets and may play an important role in host adaptation.

In recent years, honey bees have been stressed by multiple factors, with malnutrition posing a significant threat to the healthy development of honey bee colonies. To keep a colony healthy and productive, beekeepers supply their colonies with supplementary pollen or commercial pollen substitutes during periods of pollen dearth or insufficient pollen quantity or quality. In this study, we evaluated the effects of four natural pollen types (oilseed rape pollen, camellia pollen, lotus pollen and buckwheat pollen) and two commercial pollen substitutes (Diet 1 and Diet 2) against a control group (sucrose solution) on Apis cerana through cage experiments. The food consumption, live body weight, longevity, hypopharyngeal gland development and midgut proteolytic enzyme activity of caged workers were measured. The food consumption rates of oilseed rape pollen and buckwheat pollen were greater than the other diets. Oilseed rape pollen and camellia pollen were recognized as excellent-quality diets for hypopharyngeal gland development and midgut proteolytic enzyme activity. Over the entire experimental period, the caged workers fed with lotus pollen had a similar diet consumption and body weight to those fed with pollen substitutes, and these bees had a significantly higher survival rate than those fed with other diets. The results indicated that the commercial pollen substitutes appeared to be less beneficial to caged A. cerana workers than the natural pollen resources.

Polyandry is widespread among eusocial Hymenoptera, and the honey bee is a typical representative of this. It has been widely shown that polyandry can confer benefits to queens and their offspring, including enhanced productivity and fitness, stronger resistance to pathogens, and resilient division of labor, which promotes colony-level homeostasis. A previous study conducted in Australia demonstrated that 33.8% commercial Apis mellifera queens produced in autumn were not adequately mated. Beekeepers of Apis cerana in China also claimed that the queens reared in autumn are inferior to those reared in spring. To confirm whether the quality difference of queens produced in different periods is related to their mating frequency, we estimated the observed mating frequency (k) and the effective mating frequency (me) of A. cerana queens produced at the beginning and end of the queen production season in Liaoning Province. We found that all the queens were suitably mated and there was no significant difference in the mating frequency between early spring queens and late summer queens. In addition, our study indicated that the queens and their offspring workers owned a high level of heterozygosity and their inbreeding coefficients were universally low. Further studies on the queens’ performance and health are required to verify the statement that the queens reared in autumn are not as good as those reared in spring.

Haploid males of hymenopteran species produce gametes through an abortive meiosis I followed by meiosis II that can either be symmetric or asymmetric in different species. Thus, one spermatocyte could give rise to two spermatids with either equal or unequal amounts of cytoplasm. It is currently unknown what molecular features accompany these postmeiotic sperm cells especially in species with asymmetric meiosis II such as bees. Here we present testis single-cell RNA sequencing datasets from the honeybee ( Apis mellifera ) drones of 3 and 14 days after emergence (3d and 14d). We show that, while 3d testes exhibit active, ongoing spermatogenesis, 14d testes only have late-stage spermatids. We identify a postmeiotic bifurcation in the transcriptional roadmap during spermatogenesis, with cells progressing toward the annotated spermatids (SPT) and small spermatids (sSPT), respectively. Despite an overall similarity in their transcriptomic profiles, sSPTs express the fewest genes and the least RNA content among all the sperm cell types. Intriguingly, sSPTs exhibit a relatively high expression level for Hymenoptera-restricted genes and a high mutation load, suggesting that the special meiosis II during spermatogenesis in the honeybee is accompanied by phylogenetically young gene activities.

When selection favours rare alleles over common ones (balancing selection in the form of negative frequency-dependent selection), a locus may maintain a large number of alleles, each at similar frequency. To better understand how allelic richness is generated and maintained at such loci, we assessed 201 sequences of the complementary sex determiner (csd) of the Asian honeybee (Apis cerana), sampled from across its range. Honeybees are haplodiploid; hemizygotes at csd develop as males and heterozygotes as females, while homozygosity is lethal. Thus, csd is under strong negative frequency-dependent selection because rare alleles are less likely to end up in the lethal homozygous form. We find that in A. cerana, as in other Apis, just a few amino acid differences between csd alleles in the hypervariable region are sufficient to trigger female development. We then show that while allelic lineages are spread across geographical regions, allelic differentiation is high between populations, with most csd alleles (86.3%) detected in only one sample location. Furthermore, nucleotide diversity in the hypervariable region indicates an excess of recently arisen alleles, possibly associated with population expansion across Asia since the last glacial maximum. Only the newly invasive populations of the Austral-Pacific share most of their csd alleles. In all, the geographic patterns of csd diversity in A. cerana indicate that high mutation rates and balancing selection act together to produce high rates of allele genesis and turnover at the honeybee sex locus, which in turn leads to its exceptionally high local and global polymorphism.

Pollen is essential to the development of honey bees. The nutrients in bee pollen vary greatly among plant species. Here, we analyzed the differences in the amino acid compositions of pear (Pyrus bretschneideri), rape (Brassica napus), and apricot (Armeniaca sibirica) pollens and investigated the variation in hemolymph metabolites and metabolic pathways through untargeted metabolomics in caged adult bees at days 7 and 14. The results showed that the levels of five essential amino acids (isoleucine, phenylalanine, lysine, methionine, and histidine) were the highest in pear pollen, and the levels of four amino acids (isoleucine: 50.75 ± 1.93 mg/kg, phenylalanine: 87.25 ± 2.66 mg/kg, methionine: 16.00 ± 0.71 mg/kg and histidine: 647.50 ± 24.80 mg/kg) were significantly higher in pear pollen than in the other two kinds of bee pollen (p < 0.05). The number of metabolites in bee hemolymph on day 14 (615) was significantly lower than that on day 7 (1466). The key metabolic pathways of bees, namely, “sphingolipid metabolism (p = 0.0091)”, “tryptophan metabolism (p = 0.0245)”, and “cysteine and methionine metabolism (p = 0.0277)”, were significantly affected on day 7. There was no meaningful pathway enrichment on day 14. In conclusion, pear pollen had higher nutritional value among the three bee pollens in terms of amino acid level, followed by rape and apricot pollen, and the difference in amino acid composition among bee pollens was reflected in the lipid and amino acid metabolism pathways of early adult honey bee hemolymph. This study provides new insights into the physiological and metabolic functions of different bee pollens in bees.

Mitochondrial DNA is inherited maternally in bees, and the mitochondrial DNA characteristics of a single bee can represent those of the entire colony. We determined the complete mitochondrial genomes of the Apis cerana -Diannan and Apis cerana -Yun-Gui Plateau populations by using PacBio HiFi sequencing technology. The A. cerana -Diannan and A. cerana -Yun-Gui Plateau mitochondrial genome lengths were 16,214 and 16,304 bp, respectively. Both A. cerana mitochondrial genomes contained 13 protein-coding genes, 22 transfer RNAs, 2 ribosomal RNAs and an AT-rich region. The length of the AT-rich region of two A. cerana was different. Phylogenetic analyses showed that these two A. cerana populations belong to the same phylogenetic branch, but the genetic distance between DN and YG is further than that between YG and A. cerana -Aba and A. cerana -Central China. Our study shows that different geographical distributions of A. cerana show genetic diversity at the mitochondrial genome level.

With the availability of various plants in bloom simultaneously, honey bees prefer to collect some pollen types over others. To better understand pollen’s role as a reward for workers, we compared the digestibility and nutritional value of two pollen diets, namely, pear (Pyrus bretschneideri Rehd.) and apricot (Armeniaca sibirica L.). We investigated the visits, pollen consumption, and pollen extraction efficiency of caged Apis mellifera workers. Newly emerged workers were reared, and the effects of two pollen diets on their physiological status (the development of hypopharyngeal glands and ovaries) were compared. The choice-test experiments indicated a significant preference of A. mellifera workers for apricot pollen diets over pear pollen diets (number of bees landing, 29.5 ± 8.11 and 9.25 ± 5.10, p < 0.001 and pollen consumption, 0.052 ± 0.026 g/day and 0.033 ± 0.013 g/day, p < 0.05). Both pollen diets had comparable extraction efficiencies (67.63% for pear pollen and 67.73% for apricot pollen). Caged workers fed different pollen diets also exhibited similar ovarian development (p > 0.05). However, workers fed apricot pollen had significantly larger hypopharyngeal glands than those fed pear pollen (p < 0.001). Our results indicated that the benefits conferred to honey bees by different pollen diets may influence their foraging preference.

Pollen contains vegetative and generative cells that influence the seed set. Accurate pollen assessments help understand the breeding biology of oilseed rape. In order to evaluate the number of pollen grains in a large number of samples, an effective method should be designed. A new method was established to evaluate the pollen counts of five oilseed rape cultivars. This method mainly relies on 5% sucrose + 0.1% agar solution to suspend the pollen and uses a standard curve corresponding to the absorbance and the number of pollen grains to estimate the number of pollen grains. The linear fit formula of this standard curve was precision, and the R value between the pollen count and absorbance reached 0.998. Furthermore, the variation in the pollen counts at three flowering stages and on two kinds of stamens was assessed. The pollen count per flower varied significantly among oilseed rape cultivars and flowering stages (GLM: p<0.001). Moreover, the number of pollen grains on long-stamen anthers was significantly higher than that on short-stamen anthers (GLM: p<0.001). In conclusion, we established a rapid, accurate method for quantifying pollen grains based on absorbance determined in a liquid suspension by light spectroscopy, which is a feasible method. In addition, the number of pollen grains under different physiological conditions also provides basic data for oilseed rape breeding.