PgmNr C37: Both maternal and paternal scnRNAs can target excision of transposon-derived sequences during Paramecium development.

Authors:
E. Meyer; Guillaume Pellerin


Institutes
IBENS, Paris, FR.


Abstract:

Like all ciliates, Paramecium tetraurelia entirely rearranges its genome during development of the somatic macronucleus from the germline micronucleus, in each sexual generation. Rearrangements include the precise excision of IESs (Internal Eliminated Sequences), single-copy intervening sequences likely derived from transposon insertions, from coding and non-coding regions of the genome. Correct genome rearrangements depend on a meiosis-specific class of small RNAs called scnRNAs. Initially produced from the entire maternal germline genome during meiosis, scnRNAs are thought to scan the maternal macronuclear genome which acts as a sponge to remove homologous molecules from the active pool, resulting in the selection of germline-specific scnRNAs that later allow the zygotic macronucleus to reproduce the same deletions. This genome scanning model raises a theoretical problem during conjugation between polymorphic strains : a heterozygous F1 cell should be unable to excise a scnRNA-dependent IES from the incoming (paternal) allele if that IES is absent from the maternal allele, or if its sequence is too divergent. Our results, however, indicate that divergent paternal alleles are correctly rearranged, using scnRNAs produced by the paternal cell. Germline-specific scnRNAs may be exchanged between conjugants along with gametic nuclei, or they may program IES excision at a very early stage, by targeting the deposition of unknown genomic imprints in gametic nuclei. I will present an update on experiments designed to test these hypotheses.