Abstract
Piwi-interacting RNAs (piRNAs) are known to regulate transposon activity in germ cells of several animal models that propagate sexually. However, the role of piRNAs during asexual reproduction remains almost unknown. Aphids that can alternate sexual and asexual reproduction cycles in response to seasonal changes of photoperiod provide a unique opportunity to study piRNAs and the piRNA pathway in both reproductive modes. Taking advantage of the recently sequenced genome of the pea aphid Acyrthosiphon pisum, we found an unusually large lineage-specific expansion of genes encoding the Piwi sub-clade of Argonaute proteins. In situ hybridisation showed differential expressions between the duplicated piwi copies: while Api-piwi2 and Api-piwi6 are “specialised” in germ cells their most closely related copy, respectively Api-piwi5 and Api-piwi3, are expressed in the somatic cells. The differential expression was also identified in duplicated ago3: Api-ago3a in germ cells and Api-ago3b in somatic cells. Moreover, analyses of expression profiles of the expanded piwi and ago3 genes by semi-quantitative RT-PCR showed that expressions varied according to the reproductive types. These specific expression patterns suggest that expanded aphid piwi and ago3 genes have distinct roles in asexual and sexual reproduction.
Highlights
Small non-coding RNAs play key roles in the regulation of gene expression and control of transposon activity in eukaryotes
The main residues involved in the binding of the 59 end of small RNAs and the DDH catalytic triad are conserved in most of the A. pisum proteins, suggesting that these proteins are functional Argonautes
As for genes that were preferentially expressed in some morphs, the following specific expression patterns were noteworthy: (1) The expression profiles of Api-piwi2 and Api-piwi8 discriminated between parthenogenetic females and sexual females, with Api-piwi2 clearly overexpressed in sexual females and in asexuals; (2) Api-piwi5 was preferentially expressed in males while it was only weakly expressed in the female morphs; (3) Api-piwi6 and Api-ago3b were preferentially overexpressed in sexuparae females. These results indicate that the expansion of the Piwi-interacting RNAs (piRNAs)-related piwi and ago3 genes in the pea aphid appears to be linked to its reproductive plasticity
Summary
Small non-coding RNAs play key roles in the regulation of gene expression and control of transposon activity in eukaryotes These RNAs include microRNAs (miRNAs) that regulate gene expression, endogenous small interfering RNAs (endo siRNAs) that regulate gene expression and transposition in somatic cells, and Piwi-interacting RNAs (piRNAs) that silence transposons in germ cells. The MID and PIWI domains, on the other hand, are associated with the 59 ends of the small RNAs. The MID and PIWI domains, on the other hand, are associated with the 59 ends of the small RNAs Among these three domains, PIWI contains the catalytic DDH triad composed of two Aspartic Acid residues (D) and one Histidine (H) residue and responsible for the RNAseH-like endonuclease activity. It can trigger the double-stranded RNA (dsRNA) guided hydrolysis of single-stranded RNA (ssRNA) and it possesses the endonuclease ‘‘slicer’’ activity [2,3]
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