Uncovering the Changing Gene Expression Profile of Honeybee (Apis mellifera) Worker Larvae Transplanted to Queen Cells

Cellular heterogeneity of the developing worker honey bee (Apis mellifera) pupa: a single cell transcriptomics analysis

It is estimated that animals pollinate 87.5% of flowering plants worldwide and that managed honey bees (Apis mellifera) account for 30-50% of this ecosystem service to agriculture. In addition to their important role as pollinators, honey bees are well-established insect models for studying learning and memory, behavior, caste differentiation, epigenetic mechanisms, olfactory biology, sex determination, and eusociality. Despite their importance to agriculture, knowledge of honey bee biology lags behind many other livestock species. In this study, we have used scRNA-Seq to map cell types to different developmental stages of the worker honey bee (prepupa at day 11 and pupa at day 15) and sought to determine their gene expression signatures. To identify cell-type populations, we examined the cell-to-cell network based on the similarity of the single-cells transcriptomic profiles. Grouping similar cells together we identified 63 different cell clusters of which 17 clusters were identifiable at both stages. To determine genes associated with specific cell populations or with a particular biological process involved in honey bee development, we used gene coexpression analysis. We combined this analysis with literature mining, the honey bee protein atlas, and gene ontology analysis to determine cell cluster identity. Of the cell clusters identified, 17 were related to the nervous system and sensory organs, 7 to the fat body, 19 to the cuticle, 5 to muscle, 4 to compound eye, 2 to midgut, 2 to hemocytes, and 1 to malpighian tubule/pericardial nephrocyte. To our knowledge, this is the first whole single-cell atlas of honey bees at any stage of development and demonstrates the potential for further work to investigate their biology at the cellular level.

Extent and complexity of RNA processing in honey bee queen and worker caste development

The distinct honeybee (Apis mellifera) worker and queen castes have become a model for the study of genomic mechanisms of phenotypic plasticity. Here we performed a nanopore-based direct RNA sequencing with exceptionally long reads to compare the mRNA transcripts between queen and workers at three points during their larval development. We found thousands of significantly differentially expressed transcript isoforms (DEIs) between queen and worker larvae. These DEIs were formatted by a flexible splicing system. We showed that poly(A) tails participated in this caste differentiation by negatively regulating the expression of DEIs. Hundreds of isoforms uniquely expressed in either queens or workers during their larval development, and isoforms were expressed at different points in queen and worker larval development demonstrating a dynamic relationship between isoform expression and developmental mechanisms. These findings show the full complexity of RNA processing and transcript expression in honey bee phenotypic plasticity.

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Honeybee caste differentiation has a complexity of RNA processing

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Isoforms differentially express between queens and workers during larval development

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Isoforms are formatted by a flexible alternative splicing system

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Poly(A) tails are negatively correlated with isoform expression

Transcriptomic, Morphological, and Developmental Comparison of Adult Honey Bee Queens (Apis mellifera) Reared From Eggs or Worker Larvae of Differing Ages

Queens and workers are very distinct phenotypes that develop from the same genome. Larvae from worker cells up to 3.5 d old can be transferred to larger queen cells and will subsequently be reared as queens and develop into functional queens. This has become a very popular queen rearing practice in contemporary apiculture. Here we used RNA-Seq to study the consequences of rearing queens from transplanted worker larvae on the transcriptome of the adult queens. We found that queens reared from transferred older larvae developed slower, weighted less, and had fewer ovarioles than queens reared from transferred eggs, indicating queens were cryptically intercaste. RNA-Seq analysis revealed differentially expressed genes between queens reared from transferred larvae compared with queens reared from transferred eggs: the older the larvae transferred, the greater the number of differentially expressed genes. Many of the differentially expressed genes had functions related to reproduction, longevity, immunity, or metabolism, suggesting that the health and long-term viability of queens was compromised. Our finds verify the previous studies that adult queens reared from older transferred larvae were of lower quality than queens reared from transferred eggs or younger larvae.

Transgenerational accumulation of methylome changes discovered in commercially reared honey bee (Apis mellifera) queens

Whether a female honey bee (Apis mellifera) develops into a worker or a queen depends on her nutrition during development, which changes the epigenome to alter the developmental trajectory. Beekeepers typically exploit this developmental plasticity to produce queen bee by transplanting worker larvae into queen cells to be reared as queens, thus redirecting a worker developmental pathway to a queen developmental pathway. We studied the consequences of this manipulation for the queen phenotype and methylome over four generations. Queens reared from worker larvae consistently had fewer ovarioles than queens reared from eggs. Over four generations the methylomes of lines of queens reared from eggs and worker larvae diverged, accumulating increasing differences in exons of genes related to caste differentiation, growth and immunity. We discuss the consequences of these cryptic changes to the honey bee epigenome for the health and viability of honey bee stocks.

Reproductive pattern in the solanum mealybug, Phenacoccus solani: A new perspective

Background

The reproductive pattern of most scale insects is ovoviviparity. The solanum mealybug, Phenacoccus solani (Hemiptera: Pseudococcidae), is known as a thelytokous parthenogenetic species, but there is still debate about the reproductive strategies of this species.

Methods

Here, we investigated the oviposition characteristics of P. solani and used scanning/transmission electron microscopy and RNA-seq to identify the differences between two types of eggs.

Results

We found that P. solani laid two types of eggs in one batch, with no significant difference in apparent size: one with eyespots that hatched and another without eyespots that failed to hatch. Furthermore, the physiological and molecular differences between the two types of eggs were highly significant. KEGG enrichment analysis revealed significant enrichment for the JAK-STAT, Notch, Hippo, and Wnt signaling pathways and dorsoventral axis formation, wax biosynthesis, cell cycle, insulin secretion, and nitrogen metabolism pathways. The results suggest that the embryo of the egg undergoes development inside the mother and only a short molting period outside the mother.

Discussion

Ovoviviparous species produce eggs and keep them inside the mother's body until they are ready to hatch, and the offspring exits the egg shell during or immediately following oviposition. Therefore, we suggest that the reproductive pattern of P. solani can be described as ovoviviparity.