Lack of paternal silencing and ecotype-specific expression in head and body lice hybrids.

Paternal genome elimination (PGE) is a non-Mendelian inheritance system in which males develop from fertilised eggs but their paternally-inherited chromosomes are eliminated before or during spermatogenesis. Therefore, PGE males only transmit their maternally inherited set of chromosomes to their offspring. PGE has been described in numerous arthropod species, many of which are pests or parasites, posing a severe economic burden on crop production and/or with implications for human health. In order to understand how PGE has evolved on the molecular level, to potentially develop novel control strategies, we need to examine species which display basal forms of PGE. The human louse, Pediculus humanus, represents an ideal model system to understand the molecular underpinnings of PGE. In this study we analysed parent-of-origin allele specific expression patterns in male offspring of crosses between head and body lice ecotypes. We have shown that hybrid adult males of P. humanus display biparental gene expression, which constitutes the first known case of a species with PGE in which genetic activity of paternal chromosomes in the soma is not affected by embryonic heterochromatinization or (partial or complete) elimination. We have also identified maternally-biased genes (potentially imprinted genes) which may be involved in the elimination of paternal chromosomes during spermatogenesis. Finally, we have identified genes which show ecotype-specific expression bias. Given the low genetic diversity between ecotypes this is suggestive for a role of epigenetic processes in ecotype differences. These findings have implications for models of pediculicide resistance in human lice and for the development of novel epigenetic-mediated control strategies.