MONOPLOIDS IN MAIZE 



395 



same proportionate effect of the j)ollen parents. Other data involving other 

 crosses and data taken in other seasons are in agreement with tliose summa- 

 rized here. 



Data summarized in Table 25.2 are presented to show the variation in 

 monoploid frequency dependent on the seed parent. Summaries are given of 

 frequencies in crosses in which a single pollen parent, a brown marker, was 

 used. The differences, expressed in terms of frequencies per 1000 seedlings and 

 also as the frequency per seed parent, are quite striking. The rate of partheno- 

 genesis seems to be roughly proportional to the intensity of agronomic selec- 

 tion to which these various stocks have been previously subjected. 



TABLE 25.2 



FREQUENCIES OF OCCURRENCE OF MONOPLOIDS IN SOME 



DENT STOCKS WHEN CROSSED BY A UNIFORM 



POLLEN PARENT 



* Brown, liguleless stock, a B PI C R" Pr Ig; Randolph 43687-1. 



t Original and first cycle Stiff Stalk Synthetic. 



i 1947 data (Chase 1949), averages of frequencies per thousand. 



Other data, including that from sweet corn varieties, hybrids and inbreds, 

 bear out this relation. A likely explanation, other things being equal, is that 

 the frequency of occurrence of viable monoploids is correlated inversely with 

 the frequency of lethal genes in the source stocks. That the frequency of 

 lethal genes in a stock is not the sole basis of differences between stocks be- 

 comes evident when one compares stocks which have been subject to an 

 equivalent degree of selection. 



It also becomes evident that there is another genetic basis for differences 

 in rates of parthenogenesis when one analyzes the frequency of occurrence of 

 monoploids as a function of the individual seed parent plants. In Table 25.3 

 summaries are given of the numbers of monoploids per seed ])lant in crosses 

 in which the Stiff Stalk Synthetic variety was used as the seed parent. The 

 distribution of none, one, two, and three monoploids per parent is about what 

 one might expect on a chance basis. But the likelihood of getting five, six, and 

 seven monoploids per seed plant by chance in three, three, and two cases 

 respectively in a sample of 1065 parent plants is remote. The likeliest expla- 



