Inhibition of chromosome breakage and of megachromosomes by intact genomes in Nicotiana.

T H E genome of Nicotiana otophora contains five large blocks of heterochromatm (HB's), whereas N . tabacum possesses no large HB's but only widely scattered, very small segments of heterochromatin (BURNS 1966). Since the chromosomes of N . otophora (n = 12) pair intimately at meiosis with those of one genome of N . tabacum (n = 24), transfer of HB's from the former species to the latter can be achieved by the proper crosses and backcrosses. We have studied in detail the behavior of one HB from N . otophora in the background of N . tabacum (GERSTEL and BURNS 1967) and found that it can break spontaneously in somatic cells. Furthermore, in a small proportion of cells, the chromosome containing the HB undergoes tremendous enlargement to form what we have called a megachromosome (Figure 1 ) . Probably only the heterochromatic portion of the chromosome enlarges and the extent of the enlargement can differ widely from cell to cell. COLLINS, LEGG and ANDERSON (1970) have recently shown that the enlargement is due to an increase in DNA. The particular HB involved in the previous investigations was designated HB". Also studied was HBb which was derived from HBa by translocation from one to another N . tabacum chromosome; it has also undergone some structural modification (BURNS and GERSTEL 1969). Since HBb forms megachromosomes with a higher frequency than HBa, it is better suited for quantitative comparisons and was used exclusively in the present work. When an N . tabacum plant carrying HBb, or HB", is crossed with N . otophora the frequency of breakage of the HB's is much reduced and formation of megachromosomes is nearly completely suppressed in the hybrid. We interpreted this as indicating that N . otophora posse;ses a genetic unit or units which inhibit the proneness of the heterochromatin to behave in aberrant ways and which may preserve the integrity of the genome in that species. During backcrossing of the F, hybrid to N . tabacum the HB's became separated from the inhibitor(s) and the susceptibility to aberrant behavior could be observed (GERSTEL and BURNS 1970). N . otophora is a species of the section Tomentosae. This species, N . tomentosiformis and N . tomentosa are closely related to N . tabacum; probably N . tomentosiformis is closest to the ancestor which contributed one genome to amphi-