Transcriptional identification of differentially expressed genes associated with division of labor in Apis cerana cerana

While Apis cerana cerana, like Apis mellifera, undergoes a behavioral transition from in-hive nursing to outdoor foraging duties, nothing is known about the genes underlying this social signal-triggered aged-related transition in this species. Here, we simultaneously sequenced the head transcriptomes of the 7-day-old normal nurses (N7BY), 18- and 22-day-old normal foragers (N18CJ and N22CJ), 7-day-old precocious foragers (Tq7CJ) and 22-day-old overaged or reverted nurses (Tq22BY) of A. cerana cerana by RNA-seq and made a 3-tier comparison (from pairwise to group-wise and between-group) to unravel the genes associated with this transition. Six pairwise comparisons revealed 165-492 differentially expressed genes between nurses vs. foragers. Subsequent 3 group-wise and 1 between-group comparisons narrowed the transition-associated genes down to 18 nurse- and 41 forager-unique genes and 29 (14 and 15 genes upregulated in nurses and foragers, respectively) differentially expressed genes between the 3 types of foragers and 2 types of nurses. The uniquely expressed genes are usually low-abundance long noncoding RNAs, transcription factors, transcription coactivators, RNA-binding proteins, kinases or phosphatases involved in signaling transduction and/or gene expression regulation, whereas the differentially expressed genes are often high-abundance downstream genes that directly perform the tasks of nurses or foragers, such as major royal jelly proteins for nurses and the genes involved in sugar/protein digestion, lipids/fatty acids metabolism, plant allelochemicals detoxification and defense against pathogens and predators for foragers. Mapping of the clean reads to the published A. mellifera genome uncovered that the 3 types of foragers had a greater percentage of reads from annotated exons and intergenic regions, whereas the 2 types of nurses had a greater percentage of reads from introns. Taken together, these results suggest that the reciprocal nurse-forager behavioral transition of the A. cerana cerana is regulated by a social signal-triggered intron-exon/intergenic epigenetic shift and the resulted transcriptional shift of the nurse- and forager-associated genes.

尽管中华蜜蜂（Apis cerana cerana）与西方蜜蜂（Apis mellifera）一样，会经历从巢内哺育到野外采集的行为转变，但目前学界对该物种中由社会信号触发的年龄相关转变背后的分子机制仍一无所知。本研究通过RNA测序（RNA-seq），同时对中华蜜蜂的5类样本头部转录组进行测序：7日龄正常哺育蜂（N7BY）、18日龄与22日龄正常采集蜂（N18CJ、N22CJ）、7日龄早熟采集蜂（Tq7CJ），以及22日龄老龄或反转型哺育蜂（Tq22BY）；并开展三级比较分析（从两两比较到组间比较，再到跨组比较），以解析与该行为转变相关的基因。六项两两比较分析显示，哺育蜂与采集蜂之间存在165至492个差异表达基因（differentially expressed genes）。后续的3项组间比较与1项跨组比较，将与转变相关的基因筛选至：18个哺育蜂特异性基因、41个采集蜂特异性基因，以及29个（其中14个在哺育蜂中上调、15个在采集蜂中上调）在3类采集蜂与2类哺育蜂之间存在差异表达的基因。特异性表达的基因通常为低丰度长链非编码RNA（long noncoding RNAs）、转录因子（transcription factors）、转录辅激活因子（transcription coactivators）、RNA结合蛋白（RNA-binding proteins）、激酶或磷酸酶，参与信号转导和/或基因表达调控；而差异表达基因多为高丰度下游基因，直接执行哺育蜂或采集蜂的生理功能：例如哺育蜂的主蜂王浆蛋白基因，以及采集蜂相关的糖/蛋白质消化、脂质/脂肪酸代谢、植物他感化学物质解毒，以及抵御病原体与天敌的相关基因。将优质读段（clean reads）比对至已发表的西方蜜蜂基因组后发现，3类采集蜂的读段比对到注释外显子与基因间区的比例更高，而2类哺育蜂的读段比对到内含子的比例更高。综上，本研究结果表明，中华蜜蜂的哺育蜂-采集蜂双向行为转变，由社会信号触发的内含子-外显子/基因间区表观遗传转变，以及由此导致的哺育蜂与采集蜂相关基因的转录组转变所调控。