DRUSE

In hybrid dysgenesis (Drosophila melanogaster), the progeny of intercrosses between wild males and laboratory-strain females are sterile because of defects in gamete formation. This has something to do with the mobilization of transposons. Depending on the parent of transposon origin, there exist cytoplasmically inherited determinants of the outcome phenotype. These are often transmitted through the maternal germ line.

Networks of small RNAs act through RNA-interference (RNAi) pathways to restrain the spread of selfish genetic elements, regulate gene expression, and many other functions. Like microRNAs and small interfering RNAs, such small species guide Argonaute proteins to silencing targets. The control of mobile elements in germ cells depends on a system composed of Piwi-family proteins (Piwi, Aubergine, and AGO3) and piRNAs. Piwi and Aubergine (Aub) transfer maternal piRNAs into the germ line.

Do maternally deposited small RNAs affect transposon suppression in a heritable fashion? Are piRNAs the maternal suppressor of hybrid dysgenesis?

Julius Brennecke and colleagues recently reported an epigenetic role for maternally inherited piRNAs in transpososon silencing. They found that small RNAs themselves serve as vectors for epigenetic information. “In both P- and I-element–mediated hybrid dysgenesis models, daughters show a markedly different content of Piwi-interacting RNAs (piRNAs) targeting each element, depending on their parents of origin. Such differences persist from fertilization through adulthood. This indicates that maternally deposited piRNAs are important for mounting an effective silencing response and that a lack of maternal piRNA inheritance underlies hybrid dysgenesis.”

Science 28 November 2008:
Vol. 322. no. 5906, pp. 1387 – 1392
DOI: 10.1126/science.116517

Fig. 2. I-R hybrid dysgenesis correlates with maternal piRNA inheritance. (A) Normalized piRNA counts for Repbase transposons are plotted for w¹¹¹⁸ inducer and w^K reactive ovaries. (B) (Left) Fold differences in piRNA counts comparing w¹¹¹⁸ and w^K mothers are shown (red line indicates a 1:1 ratio). (Right) Transposon piRNA ratios for mothers, embryos, and F1 progeny (SF: RSF ratio) are shown as a heat map. (C) Scatter plots indicating transposon piRNA correlations between w¹¹¹⁸ and w^K mothers (top) and their respective intercross progeny (bottom).