MONOPLOIDS IN MAIZE 393 



when the hybrid phenolype is unUke that of either the pollen or the seed 

 parent, as is the case in crosses in which the brown marker stocks are used as 

 pollen parents. In such crosses, maternal monoploids of the progeny should 

 resemble the seed parent. Paternal monoploids should be brown (green at 

 early stages) and the hybrids purple. The particular brown stocks used carry 

 recessive markers, liguleless or japonica. These also serve to mark the very 

 rare paternal monoploids. 



When the monoploids reach the reproductive stage the practice has been 

 to self these plants if any self pollen is shed, to cross them by other mono- 

 ploids shedding excess pollen, or to pollinate them by diploids if self pollen is 

 lacking. 



FERTILITY OF MONOPLOIDS 



The estimate of the fertility of monoploids, based on the assumption of 10 

 chromosomes distributed independently at meiosis, is one normal egg in 

 1024. That is, if abundant normal pollen were used in pollination these plants 

 should set one good kernel in 1024 ovules. Actual fertility of the monoploids 

 studied has been much higher than this, in spite of the fact that the amount of 

 pollen used has often been scant. Little is known of the mechanics of meiosis 

 in maize monoploids. Studies of the reactions of unpaired chromosomes at 

 meiosis suggest that monoploid meioses may produce some functional gam- 

 etes with structurally altered chromosomes (Kostoff, 1941). A proportion of 

 the syngamic products in such cases would consequently be structurally 

 heterozygous. If the reproductive tissue of a monoploid becomes diploidized 

 before meiosis is initiated the gametes produced should all be structurally 

 normal and strictly equivalent genetically. Some progenies were checked to 

 determine the extent of chromosome aberration. The percentage of non- 

 viable (actually, non-stainable) pollen produced by the monoploid deriva- 

 tives was used as an indication of chromosome abnormalities. Among the 

 progenies of diploid seed parents by monoploid pollen parents about 1 per 

 cent had 10 per cent or more bad pollen. Among the progenies of monoploid 

 seed parents by diploid pollen parents about 8 per cent had 10 per cent or 

 more bad pollen. Among the progenies of monoploid by monoploid, 17 per 

 cent had 10 per cent or more bad pollen. In the latter two classes, both of 

 those in which monoploids were used as the seed parents, the monoploids 

 thus used were those which had shown no evidence of diploidization in the 

 tassels. 



In a group of 298 monoploids, 282 matured. Of these 139 shed some pollen, 

 68 formed kernels, and 34 yielded successful self progeny. The fertility of this 

 group of plants and of the whole series to date was far in excess of that ex- 

 pected of maize monoploids on theoretical grounds. The difference can be 

 ascribed largely to spontaneous doubling of the chromosome complement in 

 cells giving rise to reproductive tissue (Chase, 1949b). 



