ON SUCCESSIVE DUPLICATE MUTATIONS

Publisher Website
Google Scholar
Abstract
Nilsson-Ehle was the first to show that duplicate and triplicate factors for red are present in certain strains of wheat. He found, moreover, that the same strain may be in one case monomerous and in another case dimerous for this character; and that while, for example, Grenadier wheat possessed three independent units for red, Extra-Squarehead possessed only one. The origin of the original "factor" for red may be accounted for in the wheats as in Œnothera rubricalyx, through the chemical transformation of one chromosome or a pair of homologous chromosomes. The duplicate condition for the character R may have arisen (1) through a chemical mutation in a second pair of chromosomes, (2) through a re-mating of the chromosomes (RR) forming a homozygous pair. The latter method is for various reasons the more probable. Although the original Œnothera rubricalyx was a monohybrid and continued so for at least two generations, yet in subsequent generations involved in crosses with Œ. grandiflora, 15:1 or di-hybrid ratios were derived from the offspring of members of 3:1 families. This can best be accounted for by supposing that in a plant (RR) homozygous for one factor, a re-grouping of the chromosome pairs occurred. This re-grouping involves merely an exchange of mates on the part of the chromosomes RR so that they now belong to different pairs. The formula for the plant may now be written RrR'r', i. e., the plant is heterozygous for two independent units for red and its offspring will give a 15:1 ratio. The second mutation, producing the duplicate condition for R, is thus probably a purely mechanical process, while the original mutation which produced the "factor" R is a chemical change of wholly different nature. It is possible that in some cases the duplicate and triplicate conditions also arise through the chemical transformation of additional chromosomes. When a 15:1 family arises from a 3:1 or 5:1 family, as has happened several times in Œ. rubricalyx hybrids, it is necessary to assume that the regrouping or remating of chromosome pairs which led from the monohybrid to the dihybrid condition, took place at fertilization, or at any rate early in the ontogeny, and is then handed down to the germ cells by mitosis. The chromosomes are known to be paired in the somatic divisions, and it seems probable that the manner of pairing set up in fertilization continues in this case throughout the ontogeny, though this is not true for all organisms. Otherwise it would be necessary to assume that when a plant in a 3:1 family gives rise to a 15:1 family all its germ cells have simultaneously undergone a mismating of the chromosome pairs during meiosis, a highly improbable event. In the F2 and F3 hybrids of Œ. rubricalyx and Œ. grandiflora, in addition to 3:1 and 15:1 ratios, 2:1, 4:1 and 5:1 ratios occur. The 5:1 ratios at least seem to be significant, indicating that R and r gametes are regularly being produced in the ratio 5:3, or that a certain amount of re-grouping of the R chromosomes is regularly occurring during meiosis.

This publication has 0 references indexed in Scilit: