392 SHERRET S. CHASE 



creased naturally by spontaneous doubling of the chromosome complement. 

 About 10 per cent of untreated monoploids have yielded successful self 

 progeny, largely as a result of this spontaneous somatic diploidization. 



Since monoploids are for the most part of maternal origin, these plants 

 should resemble their seed parents. Thus the search for monoploids is greatly 

 facilitated if one looks for them among the progeny of markedly dissimilar 

 parents. If one crosses a purple maize stock as pollen parent onto plants 

 which lack this color and then finds non-purple seedlings in the progeny, one 

 has reason to think these aberrant plants may be monoploids. In practice, the 

 marker phenotype is used to indicate the diploid plants. These are discarded 

 as recognized. Morphological and cytological tests are used for positive recog- 

 nition of the monoploids. 



In brief, the techniques used in isolating monoploids are as follows. The 

 stock from which one wishes to obtain monoploids is pollinated with pollen 

 from a genetic marker stock. The marker may carry the purple plant color 

 genes (.4i A^B PI R) or brown (ai A^ B PI R), purple plumule (.4 Pui Pn-i), 

 or any suitable complex of marker genes not carried by the seed stock. The 

 ears at harvest are checked for kernels resulting from accidental self or cross 

 pollinations. This check is made possible by using marker stocks which carry 

 endosperm marker genes as well as plant marker genes. The markers which 

 have been used, as appropriate, are purple aleurone {AiA^AzCR i Pr) , red 

 aleurone (.4 1 ^4 2 .-1 3 C i? / pr) , starchy endosperm {Su) , and yellow endosperm 

 (F). 



The kernels riot showing the endosperm marker phenotype are discarded 

 (if the pollinations have been carefully made few discards are necessary). 

 Then the kernels saved are germinated and a check made of the embryos or 

 seedlings for the plant marker phenotype. All showing this character are dis- 

 carded. The remainder are transplanted after first taking from each a root tip 

 or two for cytological study. A second screening of? of diploids is carried out 

 after the first seedling leaves of the putative monoploids are fully extended. 

 Those having the first leaf as long as the comparable leaf of the seed parent 

 are almost without exception diploid and are therefore discarded. The true 

 monoploids are then recognized by chromosome number determinations. 

 Errors in classification at each stage result primarily in loss of monoploid 

 plants. Consequently monoploid frequencies as reported are likely to be less 

 than the actual frequencies of occurrence. 



The putative monoploids screened o&. as a result of the genetic check in- 

 clude the actual monoploids and also diploids of the following types: diploid 

 hybrids mutant for marker genes, hybrids carrying strong color suppressor 

 genes, hybrids in which disease (generally fungus infection) has suppressed 

 the development of the color phenotype, and a few maternal diploids. Occa- 

 sionally paternal monoploids also are produced. These may be recognized 



