Diabrotica vergifera
Image Credit: “Diabrotica virgifera side” by Siga – Own work. Licensed under CC BY-SA 4.0 via Commons
Khajuria et al (2015) have published a short paper in Insect Biochemistry and Molecular Biology describing their observations supporting the idea that Diabrotica vigifera exhibits parental RNAi following the oral delivery of dsRNA.
Parental RNAi (pRNAi) refers to an inter-generational RNA silencing in which, for example, dsRNA is introduced into an adult female and gene silencing can be seen in the progeny.
The authors reference prior reports of this in insects beginning with Tribolium (Bucher et al 2002) and other examples including Oncopeltus, Gryllus, Orchesella, Athalia, Blattella, Bombyx and Acrythosiphon.
Khajura et al note that these prior examples of pRNAi involved direct injection of dsRNA into the hemocoel of the parental female.
Here the authors present evidence for pRNAi in Diabrotica vigifera following consumption of dsRNA by mated females. The target genes in this case were brahma and hunchback. Both genes play important roles in embryogenesis. hunchback is a ‘gap gene’ while brahma is involved in chromatin remodeling.
brahma (BRM) is part of the SWI/SNF chromatin remodelling complex.
This image is from 10.1016/S0968-0004(00)01689-3
pRNAi assays were fairly simple and consisted of allowing mated females to feed on dsRNA-containing artificial diet followed by an analysis of gene expression and egg hatch rates. Consult the paper for details of the data but they found that treated females laid eggs that did not hatch (no hatching using ds-brahma RNA, 2.4% hatching using ds-hunchback RNA). In terms of gene expression the authors report high levels of knock-down of both targets in adult females and in embryos. In addition, embryos dissected from eggs showed developmental defects.
Protein gradients in early Drosophila embryos including hunchback.
pRNAi is a feature of some insects that allows for functional genomic analyses of embryonic development without relying on technically challenging methodologies such as embryo micro injection. It also presents opportunities for those thinking about using RNA-based gene silencing as a pest control tool.
In Drosophila, most of the mRNA in the egg is maternally provisioned. The developing embryo doesn’t start to transcribe its own mRNA for some time. So, strictly speaking, if maternal feeding on dsRNA disrupts the mother’s provisioning of maternal mRNA into the egg, does this constitute GENE silencing in the embryo? It certainly disrupts the mRNA present in early embryonic development, but it may not disrupt the embryo’s own transcription. Nitpicking I know but Hunchback in particular is known to be maternally provisioned in Drosophila. By constrast, the two genes in Bucher, Scholten, and Klinger’s 2002 paper on Tribolium were known not to be maternally provisioned.