3. Haploid frequencies reported in the News Letter of March, 1938, from untreated and X-rayed pollen involving 150,000 seedling counts indicated that X-raying the pollen materially increased haploid frequencies in maize. Since then additional counts have been made and the numbers at this time are sufficiently large to warrant a comparison not only of frequencies from X-rayed and untreated pollen, but also frequencies in different stocks. These stocks included an inbred line, designated A in the table; a 3-way hybrid involving this same inbred line as one of the 3 inbred parents (B); a commercial strain of Golden Bantam sweet corn (C); a genetic a-tester stock (D); and a group of miscellaneous stocks (E), no one of which was large enough for significant comparison. Haploid frequencies per thousand plants in the several stocks from untreated and from X-rayed pollen (1500 r) are given in the following table:
|Number of plants||Frequency per 1000|
There was a consistent increase in the frequency of haploids among the X-ray progenies, the average increase being 50 per cent. The dosage used (1500 r) decreased the yield of viable seed approximately 50 per cent and also materially increased the difficulty of making classifications. If odds of 40:1 be taken to indicate significance, the least significant difference in frequency of haploids per thousand between untreated and X-rayed pollen in any one stock is 0.18. The least difference observed (stock A) is 0.23 with odds of 66:1 against such a difference being due to errors of random sampling. By the same criterion, the least significant difference for the five stocks together is O.11, which the observed mean difference is 0.32. The odds here are many thousands to one against so consistent a difference being due to chance alone.
A similar comparison of the different stooks shows that stock A is not significantly different from stock E, and B not different from C. Stock C, and possibly stock B, differs significantly from stock A, and stock D differs from all the others. (See also Stadler, this News Letter). It was expected that the haploid frequency in inbred lines and their hybrids would be relatively high, due to the elimination during inbreeding of deleterious genes which might be lethal in the haploid state; but there is no obvious explanation of the extrqmely low frequency noted in the a-tester stock (D). The haploids which did appear in this stock were as vigorous on the average as those in the other stooks with the exception of the inbred line and the 3-way hybrid whose haploids were uniformly more vigorous than those of the other stocks.
The identification of the haploids was made with the aid of recessive endosperm and seedling genes, stomate examination in the seedling stage, and final verification with root tip chromosome counts. The frequencies thus obtained are to be interpreted as minimum frequencies, since it is unlikely that all of the haploids were identified. Only seeds with hybrid (presumably triploid) endosperms were included in the study. All of the haploids. obtained were maternals, although paternal haploids were looked for in some of the crosses which involved easily recognizable recessive seedling characters contributed by the pollen parent.
L. F. Randolph