Ga 4 and pericarp‑color ratios.

In three previous News Letters (17:8‑10, 1943; 18:7‑8; 1944 and 20:4‑9. 1946), Dr. R. A. Emerson reported aberrant pericarp‑color ratios and a gamete factor, Ga 4, which was shown to be responsible for the deficiency of P alleles in the progenies. It was shown that Ga 4 interferes with the functioning of pollen carrying it. When this gene is linked with P, plants heterozygous for P, when selfed or used as pollen parents in crosses with white, give progenies with an excess of white‑eared plants instead of the respective 3:1 and 1:1 ratios as expected. No disturbance results when these plants are used as pistillate parents.

Before his death in 1947, Dr. Emerson had accumulated two more years' data like that which he previously reported. In addition he left a wealth of material of advanced generations involving crosses of Ga 4 with several translocations in the short arm of chromosome 1. A large amount of this material was planted in 1948 in order to obtain data for a more critical test of linkage relations. However, not enough significant data of this type was obtained to be of use yet. Hence the present report includes that data presented in the 1946 News Letter to which has been added similar data accumulated in the three years' records since.

The ratios of red to white observed in all aberrant cultures of whatever generations are listed below, together with those in which heterozygous reds were used as pistillated parents. It will be noticed that the additional data have only very slightly changed the ratios reported in 1946.

Parent plants		Progenies
Type	Number	Number plants		Ratio	%
		Red	White	R:W	Red
W/R �	117	3816	3079	1.24:1	55.34
W/(W/R)	46	800	3156	1:3.94	20.2
(W/R)/W	54	1895	1919	1:1.01	49.7

In the first two types above, two variables determine the numbers of aberrant and normal cultures resulting in the next generation; namely, the amount of crossing over between Ga 4 and P, and the per cent of functioning Ga 4 pollen. Since these two variables cannot be measured simultaneously here, Emerson used the third type to evaluate the amount of crossing over alone. Since there is here no question of pollen differentials, the per cent of normal cultures, resulting from red ears, should be the per cent crossing over. He found this value to be 17.9. Of 37 additional reds tested in 1947, seven gave normal ratios, an indicated value of 18.9% crossing over. Since Ga 4 is shifted to white‑carrying gametes when lost from red-carrying ones, Emerson made use of cob colors to obtain the number of whites that gave aberrant ratios (deficiency of whites) in the next generation. He thereby found a value of 12.5 for crossing over in 32 progenies tested. The per cent calculated from all of these 97 progenies is 16.5, a slightly higher value than that reported in 1946.

In parallel studies, Emerson used crosses of white with homozygous red, the latter being heterozygous for Ga 4 and used as the pollen parent, to measure the per cent of functioning Ga 4 pollen. This value was calculated to be 30%. Presumably, in order to get a further measure of this value as it is affected by different growing seasons, similar lots of data were obtained in 1946 and 1947. Out of a total of 106 progenies, 40 gave aberrant ratios in F₂, thereby giving a value of 37.7%. Recalculating this value for the total of 136 progenies that have been tested, we find that 36% of the pollen carried Ga 4 as measured by this method.

As in 1946, we see that observed ratios do not correspond at all well with calculated ratios that would result from using the above values for crossing over and functioning of Ga 4 pollen. Therefore, we may conclude that on the basis of the numbers involved and of the seasons in which they were tested, these methods of independent measurements are not satisfactory. Further data involving two genes linked with Ga 4 must be obtained to evaluate the variables simultaneously by the method of Mangelsdorf and Jones and applied to certain 1943 data of this type by Emerson. Evidence on linkage relations of Ga 4 remain inconclusive even with the data accumulated since 1945.

In order to test the hypothesis that Ga 4 acts to reduce the growth rate of the pollen tube, 10 red ears from an aberrant 1947 selfed culture were selected. One hundred kernels were planted separately from the butt and from the tip of the ears in 1948. Of these 10, three ears gave aberrant ratios, However, in none of the three was there a significant difference of aberrant ratios between the tip and the butt of the ear; in fact, one ear gave almost identical numbers of red and white from both areas. Therefore, there seems to be no relation between the distance the pollen tube must grow to effect fertilization and the greater success of the normal pollen.

James E. Wright, Jr.