EPIGENETIC REGULATION OF PROGRAMMED GENOMIC REARRANGEMENTS IN PARAMECIUM-AURELIA

In ciliates, development of the polyploid somatic macronucleus after s exual events involves extensive and reproducible rearrangements of the germ-line genome, including chromosome fragmentation and precise exci sion of numerous internal sequence elements. In Paramecium aurelia, al ternative macronuclear Versions of the same germ-line genome can be ma ternally inherited across sexual generations, showing that rearrangeme nt patterns are not strictly determined by the germ-line sequence. Hom ology-dependent maternal effects can be evidenced by transformation of the vegetative macronucleus with cloned macronuclear sequences: new f ragmentation patterns or internal deletions are specifically induced d uring differentiation of a new macronucleus, in sexual progeny of tran sformed clones. Furthermore, transformation of the maternal macronucle us with germ-line sequences containing internal eliminated sequences ( short single-copy elements) can result in a specific inhibition of the excision of the same elements in the zygotic macronucleus. These expe riments show that the processing of many germ-line sequences in the de veloping macronucleus is sensitive to the structure and copy number of homologous sequences in the maternal macronucleus. The generality and sequence specificity of this trans-nuclear, epigenetic regulation of rearrangements suggest that it is mediated by pairing interactions bet ween germ-line sequences and sequences imported from the maternal macr onucleus.