NON-MENDELIAN, HERITABLE BLOCKS TO DNA REARRANGEMENT ARE INDUCED BY LOADING THE SOMATIC NUCLEUS OF TETRAHYMENA-THERMOPHILA WITH GERM LINE-LIMITED DNA

Site-specific DNA deletion occurs at thousands of sites within the gen ome during macronuclear development of Tetrahymena thermophila. These deletion elements are usually not detected in macronuclear chromosomes . We have interfered with the normal deletion of two of these elements , the adjacent M and R elements, by loading vegetative macronuclei wit h these elements prior to sexual conjugation. Transformed cell lines c ontaining the exogenous M or R element, carried on high copy-number ve ctors containing genes encoding rRNA within parental (old) macronuclei , consistently failed to excise chromosomal copies of the M or R eleme nt during formation of new macronuclei. Little or no interference with the deletions of adjacent elements or of unlinked elements was observ ed, The micronucleus (germ line)-limited region of each element was su fficient to inhibit specific DNA deletion. This interference,vith DNA deletion usually is manifested as a cytoplasmic dominant trait: deleti on elements present in the old macronucleus of one partner of a mating pair were sufficient to inhibit deletion occurring in the other partn er. Remarkably, the failure to excise these elements became a non-Mend elian, inheritable trait in the next generation and did not require th e high copy number of exogenously introduced elements. The introductio n of exogenous deletion elements into parental macronuclei provides us with an epigenetic means to establish a heritable pattern of DNA rear rangement.