CYTOGENETIC STUDIES OF DEFICIENT‐DUPLICATE CHROMOSOMES DERIVED FROM INVERSION HETEROZYGOTES IN MAIZE

Abstract

THE PRESENT INVESTIGATION was the outgrowth of studies of the chromosome homology of maize from Central and South America. Strains of maize from Latin America exhibit an extraordinarily wide range of phenotypic appearance owing to genic differences. Since many of these strains presumably have been isolated for centuries, it was thought possible that the genic diversity had been accompanied by changes in chromosomal structure. Cooper and Brink (1937) found no evidence of reciprocal translocations in their study of chromosome homology in 55 strains of maize from Latin America but their method of analysis would not permit the detection of inversions. Inasmuch as inversions in Drosophila populations are of frequent occurrence (Patterson and Stone, 1952) while reciprocal translocations are rarely encountered, it seemed desirable to test exotic races of maize for the presence of inverted segments. F1 seed from crosses of two genetic testers with 90 different races of Latin American origin was obtained through the courtesy of Dr. P. C. Mangelsdorf. Eighty-five of these hybrid combinations involved a Pr-tester and five, a Tu stock. Microsporocytes for cytological examination were collected from a number of plants of each F1 family. Pollen samples were obtained for all of the F1 individuals. Plants heterozygous for a single reciprocal translocation usually have 50 per cent of aborted pollen and ovules while plants heterozygous for inversions produce varying amounts of aborted grains depending on the frequency of crossing over within the inverted segment. The absence of any considerable amount of aborted pollen indicated that all of the 85 progenies involving the Pr-tester possessed chromosomes with no major structural differences. In two of the five families where the Tu line was used, all of the plants had approximately 20 per cent aborted pollen while in two other families one half of the plants had a comparable degree of pollen abortion and the other half had all normal grains. The fifth family consisted only of plants with all normal pollen. When the cytological material from the four families with aborted pollen was examined it was found that all plants with aborted grains were heterozygous for a paracentric inversion in chromosome 3. There was no evidence of reciprocal translocation in any of the 90 exotic races nor was there any indication of inversions. The inversion found was obviously one which came from the Tu stock where it was segregating since some plants were homozygous normal, others homozygous for the inversion and some were heterozygous. It should be recognized