PUTATIVE M6A-DEPENDENT DOSAGE COMPENSATION IN TRIPLOID OYSTERS SUGGESTS EXTENSION OF THE X CHROMOSOME INACTIVATION PARADIGM OUTSIDE MAMMALS
Résumé
Chromosome dosage compensation is a critical feature across animal phylogeny, with failures leading to sterility or lethality as illustrated by the X-chromosome inactivation, where m6A-RNA regulated chromatin compaction plays a key role. However, such chromosome lockdown paradigm has never been described outside Mammals. Triploid oyster C. gigas populations are mostly sterile (3nbeta), but some individuals escape triploidy-induced sterility (3nalpha). Here, we hypothesized that dosage compensation exists and may imply m6A-RNA mediated heterochromatinization in the oyster. To test this hypothesis, we compared 2n, fertile 3nalpha, sterile 3nbeta and RNA-methyltransferase inhibitor (STM2457) treated 3n alpha oysters. Quantitative histology showed that 3nalpha animals display bigger cells and nuclei than 2n animals, and that 3nalpha display an intermediate chromatin conformation between 2n and 3nbeta oysters. 3nb have less compact chromatin than 3nalpha, and STM treatment disrupts chromatin organization more potently in alpha than beta animals. Nanopore sequencing revealed that 3nalpha have 19797 transcripts associated to the chromosomes (caRNA) with differential methylation profiles across caRNA types. 3nbeta have 13423 caRNAs and STM treatment dramatically reduced their number to 5770, suggesting a m6A-dependent regulation of transcription. We identified two ca. 1kb long non-nascent lnc-caRNAs in alpha triploids that were absent in beta and STM treated animals. These transcripts do not display homology in the present version of the oyster genome and their m6A modification ratio is above the mean level. Altogether, our results indicate that the binding of methylated chromosome associated long non-coding RNAs could be associated to chromatin compaction and transcription regulation in triploid fertile oysters, suggestive of an extension of the X chromosome inactivation lockdown paradigm outside Mammals. This has to be validated by additional work such as proximity-ligation chromatin conformation and functional characterization of the carRNA candidates, presently in progress in our laboratory.
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