M. M. Rhoades

The cytoplasmic inheritance of male sterility inZea mays

X. Summary1. The analyses of a male-sterile condition inZea mays indicate that the egg cytoplasm plays the chief rôle in the expression of the character.2. There was no transmission of the male-sterility through the pollen of partially sterile plants.3. A replacement of the chromosomes in the male-sterile line with chromosomes known to be free from sterility-producing factors had no apparent effect on the degree of sterility.4. No linkage was found between any of the tested mutant genes in the different linkage groups with male-sterility.5. The nature of the pollen parent had no demonstrable effect upon the expression of male-sterility.6. Attempts to transmit the sterility artificially were unsuccessful.7. The distribution of male-sterile and normal-appearing individuals with respect to the position of the seed on the cob was such as to negative the idea of discrete cytoplasm units, somatically segregated prior to the megasporogenesis, being the cross of sterility.8. Cytological studies show microsporogenesis to be normal. The degeneration of the pollen usually occurs before the first vegetative division. There is a pronounced difference between the cytoplasmic elements of microspores in normal races and those of the male-sterile line.9. There was no indication of the male-sterility being caused by a virus disease.

Duplicate Genes in Maize

Duplicate and triplicate factor ratios for various characters have long been known in tetraploid and hexaploid oats and wheat. Duplicate factors have also been reported in a number of known amphidiploids such as Nicotiana tabacum (Clausen and Cameron, 1950). The occurrence of 15:1 and 63 :1 ratios in polyploid plants is presumptive vidence that some genes are in duplicate or triplicate and that identical or similar loci are present in the different genomes. The recent synthesis by McFadden and Sears (1946) of a vulgare wheat furnishes conclusive evidence that this hexaploid does consist of three different genomic sets of seven chromosomes each. Stadlers (1929) finding of a low induced mutation rate in tetraploid and hexaploid oats and wheat as compared to the much higher mutation rate in the diploid species likewise indicates a considerable duplication of loci in these plants. If 15 :1 and 63 :1 ratios are evidence of duplicated or triplicated loci in polyploids, do similar ratios in essentially diploid plants such as maize also indicate duplicated loci? That duplications played an important role in the evolution of the maize plant, and indeed in all organisms (Metz, 1947), can hardly be questioned, but it is possible that once homologous loci could become so differentiated by mutation that any trace of their former homology would disappear. Although Sprague (1932) has shown that 15 :1 ratios cannot, at least for the inheritance of scutellum color in maize, be taken as critical evidence of duplicate genes, it appears likely that duplicate and triplicate factor inheritance is very suggestive of duplicated loci. At least 14 cases of duplicate factor, two of triplicate and one of quadruplicate factor inheritance have been reported in maize (see Lmerson, Beadle and Fraser, 1935). Unfortunately inkage determinations of duplicate loci are so arduous that nothing is known of the location of many of the sets. In eight of the 14 caves of duplicate factor inheritance the linkage relations of both loci are unknown while in three (xn, xn, zgg Zg2, So1 so2) one of the members of each set has been assigned to a specific chromosome, the other being unplaced. One of the genes of the xnixn2 set is in chromosome 10; in both the zg, zg and so1 so sets one gene of each set has been located in chromosome 9 although the published data do not permit a precise determination of the map position. A rather surprising situation is found in the remaining three sets in that the fr1 fr2 loci are both in chromosome 7 (Jenkins and Pope in Emerson et al., 1935); the w5w6 genes are both in chromosome 6 (Demerec, 1923); and the au1 au2 factors both lie in chromosome 9 (Eyster, 1929 and Emerson et al., 1935). Modified 15 :1 ratios result when the duplicated loci are situated in the same chromosome, the observed ratio being a

Relation of Chromatid Crossing Over to the Upper Limit of Recombination Percentages

The differences between map distances and the percentages of recombination have been accounted for by the occurrence of multiple crossovers (Morgan, 1919). Jennings (1923) has shown that crossing over without interference or with interference extending on the average over a distance of not more than 30 map units should give recombination values not exceeding 50 per cent. These conclusions were arrived at on the basis of the then accepted hypothesis that crossing over takes place between undivided chromosomes. It is important, therefore, to point out the obvious, but to the best of our information unpublished, relation between the occurrence of chromatic crossing over and the fact that the percentage of recombination between two linked genes, irrespective of how great their map distance may be, does not in general exceed 50. It is our contention that random chromatid crossing over, rather than inultiple cross-overs with restricted interference, is the fundamental mechanism which results in recombination percentages approachinz 50 as a limiting value.