The variations of mos1 in natural populations of D. simulans
The mecanisms that repress mos1 activity
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The aim of my work is to understand the populational variations of mos1 activity in Drosophila simulans, a sister species of D. melanogaster that invaded the world less than one hundred years ago. The mos1 transposable element is present and active in all populations, but displays high variations in activity. However, this measure is based on a phenotypic assay that relies on a genetic cross with a tester strain, and gives access to somatic excision level for the male progeny only since the reporter gene is localized on the X chromosome. A RT-QPCR assay was designed to directly measure the transcriptional variations of mos1 in populations. We first identified a strong sexual dimorphism regarding mos1 expression: transcription in males is always significantly higher than in females, and is restricted to the testes. This differential expression is observed in all natural populations studied. Furthermore, the measured transcription level is variable between populations, and correlated with both the excision rate estimated from somatic tissue and the genomic copy number estimated by QPCR. Interestingly, the copy number and its variance are positively linked to the invasive distance – that is the distance of the geographic area of the population to the ancestral area of the species (West Africa) – suggesting a link between mos1 activity and the invasive history of the populations. A deeper analysis of three invasive populations (far from the origin area) suggests that the mos1 transcriptional variations are related to transcriptional variations of genes (ago3 and aub) involved in the epigenetic piRNA pathway specific to the germline, but not to the somatic siRNA pathway (dicer and ago2). We show that (unlike mos1 expression) the transcription level of ago3 in testes is always low compared to that in ovaries. Moreover, it is negatively correlated with mos1 expression at least in invasive populations. These results suggest a central role of epigenetic pathways in TE regulation in changing environments.
Saint-Leandre, B., I. Clavereau, A. Hua-Van & P. Capy (2017) Transcriptional polymorphism of piRNA regulatory genes underlies the mariner activity in Drosophila simulans testes. Molecular Ecology, 26, 3715-3731. 10.1111/mec.14145.