Isochromosomes in Tomato.

HROMOSOMES with two similar ends have been recognized since the pioneer work of BELLING and BLAKESLEE (1924) on the secondary trisomes of Datura. More recently, the term, isochromosome, has been applied .by DARLINGTON (1939) to those chromosomes which have two identical arms. When found occurring naturally, the isochromosome has usually been shown to consist of inert, or B-type chromosomal material. Examples of such isochromosomes have been provided in rye by MUNTZING (1944) and MUNTZING and LIMA-DE-FARIA (1950), in sorghum by DARLINGTON and THOMAS (1941), in Anthoxanthum by GSTERGREN (1947), and in the moss Dicranum majus by VAARAMA (1950). On the other hand, in the isochromosomes of Nicandra physaloides studied by DARLINGTON and JANAKI-AMMAL ( 1945), the arms bear nucleolus-organizers and are apparently identical with an arm of the haploid complement. Other examples of isochromosomes formed from members of the basic complement come from experimental cultures. They have usually been additions to the diploid complement, as in the secondary trisomes of Datura (BELLING and BLAKESLEE 19241, of maize (RHOADES 1933, 1940), and of tomato (LESLEY and LESLEY 1941). In cultures of wheat derived from pentaploid hybrids, LOVE (1940) discovered several examples of isochromosomes. In the diploid wheat species, T. monococcum, SMITH (1947) found an interesting situation in which an isochromosome from the long arm and a telocentric fragment from the short arm replaced one of the normal chromosomes. In the genetically best studied example, the attached-X chromosome of D. melanogaster, the duplication originated from exchanges between the X and the two arms of the Y chromosome (NEUHAUS 1936; MORGAN 1938). Complexities in the study of heterochromatin have apparently prevented a detailed analysis of the similarities between the two arms of the attached-X in the region adjacent to the centromere or of the probable homologies between the two arms of the Y chromosome. The present report is concerned with the cytological behavior of two different isochromosomes in the tomato and the transmission of one of them. As isochromosomes, they are unusual because each replaces a normal member of the diploid complement. The ability of the tomato to withstand such genetic unbalance is perhaps not unexpected since LESLEY and LESLEY (1941) have previously demonstrated a half-chromosome deficiency in the diploid complement, On the other hand, the transmission of such a half-chromosome