Aberrant pericarp-color ratios. In last year's News Letter (17:8-10, 1943), I reported a disturbance of pericarp-color ratios unlike that caused by the recessive zygotic lethal, zl. Selfed red ears gave progenies with approximately equal numbers of red and of white eared plants instead of the expected 3-1 ratio. Such red eared plants, when used as pollen parents in crosses with white gave progenies with about four times as many whites as reds. Only part of the red ears of such cultures gave aberrant progenies. The possibility of this disturbance being transmitted thru the egg had not been determined.
More data of the same kind and a few new data are now available. The new and older data are summarized in the accompanying table.
Normal and aberrant pericarp and cob-color ratios
No. of individuals
|1||-||W-R × R-R||2||26|
The pollen parents of the two F1 cultures shown in line 1 were from the same stocks of chromosome 1 markers, P br f an gs, both homozygous for red pericarp and red cob, R-R. The pistillate parents were from unrelated stocks with colorless pericarp and red cob, W-R. Of 14 F2 cultures, 11 (line 2) showed normal 3:1 segregation and 3 (line 3) gave aberrant ratios approaching 1:1. Other F1 R-R plants were backcrossed as pollen parents to stocks with colorless pericarp and white cobs, W-W. Of 16 such backcross cultures, 9 (line 4) gave normal 1:1 ratios and 7 (line 5) gave aberrant ratios approaching 1:4. Six red eared plants (line 6) and eight red-cob whites (line 7) of the aberrant backcross cultures were again backcrossed this time as pistillate parents; and all gave normal 1:1 ratios. Eight red eared plants (line 8) from these normal second backcross cultures when selfed gave only aberrant cultures. Finally, six red-cob whites from the second backcross cultures (line 9) gave normal ratios on selfing and one (line 10) gave an apparently abnormal ratio.
In summary, it should be noted that red eared plants of aberrant cultures
when selfed or used as pollen parents in backcrosses to white, transmit the disturbance to some but not to all cultures of the next generation. When used as pistillate parents in such backcrosses, no disturbance is shown in the following generation, but both red eared plants and red-cob whites of that normal generation give aberrant results when grown one further generation.
From all this, it is clear that the disturbing factor is carried by a part (presumably one-half) of tho female gametes and by a part (materially less than half) of the functioning male gametes. In its adverse effect on the functioning of male gametes, it is similar to the Ga reported by Rhoades (News Letter 17:7, 1943). I am, therefore, assigning to it tentatively the symbol Ga4.
Since there is evidence (tho slight) of crossing over between Ga4 and the pericarp-color locus and of differential functioning of male gametes, these two variables can be evaluated by use of F2 or backcross ratios only when adequate data are available for a third nearby gene. The percent of crossing over can be determined directly, however, from the ratios of aberrant to normal cultures from (1) F3 from reds of aberrant F2 cultures and (2) from progenies of reds and/or red cob whites of backcross cultures where Ga ga reds are used as the pistillate parents of the backcrosses. In these cases, the ratios of aberrant to normal cultures should be quite independent of the percent of functioning pollen.
Limits can be set for the two variables by use of F2 and backcross ratios of red to white. Thus, the observed 53 percent red eared plants of F2 might be accounted for by various combinations of the two variables with extremes from zero crossing over with 6% functioning Ga pollen to 6% crossing over with zero functioning Ga pollen. But the observed 27 percent red eared plants in backcross cultures indicate very different limits for the two variables, namely, from zero crossing over with 27% functioning Ga pollen to 20% crossing over with 12% functioning Ga pollen. Since the crossover percentage must be the same for the two types of cultures, one of three conclusions must follow, namely, (1) my hypothesis is wrong (2) my calculations are wholly inaccurate, or (3) pollen functioning is affected adversely much more when the pistils to which it is applied are heterozygous for Ga than when they carry only ga. If the latter is true, the gamete factor, Ga4, may be regarded as dominant as is Ga1.
R. A. Emerson