Abundant small RNAs in the reproductive tissues and eggs of the honey bee, Apis mellifera

Abstract

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.

What Are the Functional Roles of Piwi Proteins and piRNAs in Insects?

“…The high abundance of piRNAs mapping to transposons in both ovaries and eggs suggests maternal deposition of piRNAs as protection against transposon activation during embryogenesis. Among the different classes of small RNAs (which also include miRNAs and tRNA fragments), piRNAs showed the greatest divergence between eggs from mated queens and eggs from virgin queens [ 251 ].…”

“…The piRNAs that map to transposons (mainly the DNA transposons Mariner and PiggyBac and the non-LTR retrotransposon R2) show the hallmarks of both the primary pathway (antisense orientation, 1U bias) and the ping-pong mechanism. In reproductive tissue, however, the majority of piRNAs seem to arise by the primary pathway [ 251 ]. A majority of the transposon-associated piRNAs can be mapped to piRNA clusters of which many are differentially active in drones, queens, and workers [ 250 ].…”

“…A majority of the transposon-associated piRNAs can be mapped to piRNA clusters of which many are differentially active in drones, queens, and workers [ 250 ]. The piRNAs derived from piRNA clusters also show a phased pattern indicative of the production of trailer piRNAs, especially in the semen [ 251 ]. Furthermore, the piRNA clusters have a strong tissue-specific expression.…”

First Identification and Investigation of piRNAs in the Larval Gut of the Asian Honeybee, Apis cerana

“…Based on RT-qPCR detection, Wang et al identified piRNA clusters among drone, worker, and queen bees, and found that the expression level of piRNAs in reproductive individuals of A. mellifera was greater than that in sterile workers, suggesting the reproductive bias of the piRNA expression [ 18 ]. Watson et al further surveyed the expression levels of piRNAs in various A. mellifera reproductive tissues such as the ovaries, spermatheca, semen, fertilized eggs, unfertilized eggs, and testes [ 19 ]. Recently, on the basis of transcriptome data and bioinformatics, our team identified 843 piRNAs in the larval gut of Apis mellifera ligustica and uncovered the potential roles of DEpiRNAs during the developmental process of the larval gut [ 20 ].…”

First identification and investigation of piRNAs in the larval guts of Asian honey bee,Apis cerana

“…Compared with Drosophila and Aedes albopictus , study on honey bee piRNAs is still limited. Liao et al identified two PIWI genes, Am-aub and Am-ago3 , for the first time in worker, drone, and queen bees of A. mellifera , and the expression of these two genes showed a significant sex bias, confirming that piRNA are truly expressed in western honey bee, indicating that piRNA may have a potential association with honey bee reproduction under the influence of nutritional factors as well as bee sex determination [14]; Based on RT-qPCR detection, Wang et al identified piRNA clusters content among drone, worker and queen, found that the expression level of piRNAs in reproductive individuals of A. mellifera was greater than that in sterile workers, suggesting the reproductive bias of piRNA expression[15]; Watson et al further surveyed the expression level of piRNAs in various A. mellifera reproductive tissues such as ovaries, spermatheca, semen, fertilised eggs, unfertilised eggs, and testes, and observed that [16]. Recently, on basis of transcriptome data and bioinformatics, our team identified 843 piRNAs in of the larval guts of Apis mellifera ligustica and uncovered the potential roles of DEpiRNAs during the developmental process of larval guts [17].…”