Ga4 and pericarp-color ratios. In two earlier News Letters (17: 8-10, 1943 and 18: 7-8, 1944), aberrant pericarp-color ratios were reported and a gamete factor, Ga4, was postulated as interfering with the functioning of pollen carrying it. There are now available more data like those previously reported and also a few of more nearly crucial importance. The records here assembled include both the new and most of the previously reported data.
The study involves crosses of lines having red pericarp and cob with lines having colorless pericarp and either white or red cob color. In this account, cob color will be disregarded, except in one section where its designation is essential. In general red and colorless (white) pericarp will be designated, respectively, by R and W. When reference to both pericarp and cob colors is made, the following symbols will be used for the three alleles:
R-R = red pericarp, red cob
W-R = white pericarp, red cob
W-W = white pericarp, white cob
Certain plants with heterozygous red pericarp, when selfed or used as pollen parents in crosses with white, give progenies with an excess of white-eared individuals, instead of the respective 3-1 and 1-1 ratios ordinarily observed. When, however, the same red eared plants are used as pistillate parents in crosses with white, normal ratios result. The ratios of red to white that have been observed to date in all aberrant cultures of whatever generations are given in the tabular statement below, together with first and later generations of crosses in which heterozygous reds were used as pistillate parents.
Not all red eared plants of cultures with an excess of whites, give aberrant ratios in the next generation. Of 42 plants tested from cultures resulting from W/(W/R), line 2 of the above table, 29 gave aberrant and 13 normal ratios in the following generations. Reds of aberrant cultures, which give normal ratios in later generations, are assumed to have lost Ga 4 by crossing over. But, the relative numbers of aberrant and normal progenies resulting is not a measure of the percent of crossing over, because crossover pollen lacking Ga 4 is more likely to function in fertilization than pollen carrying Ga 4.
Of red eared F2 plants lacking Ga 4, two out of three in general are expected to be homozygous. Of 61 such red eared plants of aberrant cultures, only 5 were homozygous, a ratio of 11.2:1 instead of the normal 2-1 ratio. Here again, this ratio is not a measure of percent of crossing over between red and Ga 4 alone or of percent of functioning Ga 4 pollen alone, for both variables are involved together.
Of red eared plants of normal cultures resulting from (W/R)/W, line 3 of the table above (like those of the reciprocal cross W/(W/R), line 2), some have normal and some aberrant progenies in the next generation. Of 28 such reds tested, 23 gave aberrant and 5 normal ratios in the following generation. Since there is here no question of pollen differentials, the percent of normal cultures should measure the percent of crossing over in megasporogenesis. The percent of crossing over indicated is 17.9, but the number of plants tested is far too small to give reliable results.
Of the homozygous red eared plants occurring in aberrant cultures, one was crossed reciprocally with white and two others were used only as pollen parents in crosses with white. The progenies were all red eared, but, of course, segregated in the next generation. The ratios of red to white in the segregating generation indicated that the three homozygous red parents were heterozygous for Ga 4.
The available data are summarized in the following table.
|Type of cross||Number||Red||White|
Of 30 segregating cultures from crosses involving homozygous red as pollen parents, 9 exhibited aberrant and 21 normal ratios. Of 11 segregating cultures from the one cross in which homozygous red was used as pistillate parents 5 gave aberrant and 6 normal ratios. The second of these two categories (homozygous red as pistillate parent) should include equal numbers of aberrantly and normally segregating cultures, since, in homozygous red, crossing over with Ga 4 is not detectable and because Ga 4 was not present in the white pollen parent. The 5-6 ratio is as near equality as is possible with a total of eleven.
The first of the two categories (homozygous red as pollen parent) should, however, afford a direct measure of the percent of functioning Ga 4 pollen. Here crossing over in microsporogenesis cannot be detected and should have no effect on the ratio of aberrant to normal segregating cultures in the succeeding generation. Of the 30 F1 plants tested, 9 gave aberrant and 21 normal segregation ratios. This 9-21 ratio indicates that 30 percent of the functioning pollen carried Ga 4, where 50 percent would be expected if this gene did not work to the disadvantage of the pollen carrying it.
When, in heterozygous red, the Ga 4 gene is lost from red-carrying gametes, it should be picked up in an equal number of instances by gametes carrying white. For this study, a third allele, colorless pericarp with red cob, W-R, may be used. When plants heterozygous for R-R and W-W are crossed with W-R, the red eared plants are W-R/R-R or R-R/W-R and the colorless eared plants are W-R/W-W or W-W/W-R. Data involving the first of these categories have been presented without reference to cob color. In the second category, pericarp is colorless throughout, but it is perhaps less confusing to designate both pericarp and cob color by symbols for the three alleles involved.
When, by crossing over, Ga 4 is shifted from association with R-R to the W-W allele, segregating progenies should show a deficiency of white. In the studies of crosses of R-R with W-W, out-crosses with W-R, as either pollen or pistillate parent, have afforded tests of 137 W-R plants. Their progenies, classified as having normal or aberrant segregation ratios of red to white cob, are summarized as follows.
In these cob-color studies, as in the pericarp-color work reported earlier in this account, when heterozygous red (R-R/W-W or W-W/R-R) is used as the pollen parent in crosses with W-R, there are involved both variables, namely, percent of functioning Ga 4 pollen and percent of crossing over. It is, therefore, impossible to evaluate either one of them. When, however, heterozygous red with heterozygous Ga 4 is used as the pistillate parent and homozygous W-R as the pollen parent, differential fertilization because of Ga 4 is eliminated, and the percent of crossing over in megasporogenesis should be indicated by the relative numbers of normally and aberrantly segregating cultures in the succeeding generation. Data are available for 32 such cultures, as follows.
|Progenies of W-W/W-R|
|R-R Ga 4||4||114||5||22.8:1||4.2|
Here, the ratio of normal to aberrant progenies is 28:4, or 7:1. The percent of aberrant progenies -- equivalent to percent of crossing over -- is 12.5. It will be recalled that the study of segregating red pericarp, reported earlier in this account, involving 23 aberrant to 5 normal progenies, indicated a percent of crossing over of 17.9. The percent calculated from both the pericarp-color and the cob-color lots, 60 progenies in all, is 15.0. It will be recalled also that crosses of white with homozygous red pericarp, the latter as pollen parent, resulted in 21 normal and 9 aberrant cultures. This indicates that 30 percent of the functioning pollen carried Ga4 and 70 percent carried its normal allele.
It remains now to see how nearly aberrant ratios correspond to ratios calculated from the indicated values of the two variables. The answer is easy. They do not fit at all well. It is realized that the number of progenies on which the evaluation of the two variables has been based is wholly inadequate -- 60 for percent of crossing over and 30 for percent of functioning Ga4 pollen.
One further method of evaluating the two variables is available. This method was used by Mangelsdorf and Jones (Genetics 11:423-455. 1926) in their study of the gamete factor in the fourth chromosome. By the use of data involving two genes both linked with Ga, they were able to evaluate the two variables simultaneously. This method can be used with data presented previously. (News Letter 17: 8-10. 1943). These are backcross data, involving pericarp color and ms 17, with a total of 206 plants. The method of Mangelsdorf and Jones applied to these data indicates approximately 13 percent crossing over between Ga4 and pericarp color -- not far from that calculated by the method of eliminating one variable -- but only 5 -- instead of 30 -- percent of the effective pollen carrying Ga4. These percentages, when applied to the data summarized in this account, show a much better fit to observed ratios than do those obtained from evaluation of the two variables independently as presented earlier in this account. A comparison of the two methods is given in the following table.
|13||% crossing over||15|
|Observed||5||% Ga 4 pollen||30|
|B-C -- Red to white||1- 3.7||1 - 5.1||1 - 1.8|
|F2 -- Red to white||1.2 - 1||1.4 - 1||2.1 - 1|
|F2 -- Hetero- to homozygous red||11.2 - 1||6.2 - 1||2.8 - 1|
|F2 -- Red to white||16.0 - 1||11.3 - 1||3.6 - 1|
The data presented in the 1943 News Letter indicate that Ga4 is to the left of ms17. On the assumption of 13 percent crossing over between P and Ga4, the map may be given tentatively as below.
sr <----- Ga4 <----- 10 -----> ms17 <----- 3 ----> P ----- br
A further study, involving Ga4 with sr, ms17, P, and zb4, is underway, but little further evidence can be obtained short of two more years.
R. A. Emerson