390 SHERRET S. CHASE 



ops into an embryo. Evidence for this is indirect — monoploid embryos are 

 found in kernels having normal (3n) endosperm. It is possible that some or all 

 monoploids arise from reduced cells of the embryo sac other than the egg, 

 from the synergids perhaps. 



As tools for experimental research monoploids offer many possibilities: in 

 the cytological field for studies of the meiotic distributions of unpaired chro- 

 mosomes, non-homologoiis synaptic relations of the chromosomes and me- 

 chanics of chromosome doubling; in the genetic field for direct observation of 

 mutational effects, measurement of mutation rates, studies of cytoplasmic 

 effects, and biochemical investigations; in the agronomic field for the produc- 

 tion of diploid, homozygous stocks directly from the monoploids. The follow- 

 ing discussion is concerned primarily with my own investigations into the 

 latter possibility. 



A monoploid carries in each of its cells, or nuclei, only one chromosome of 

 each type. Thus if the chromosome complement of any cell can be doubled, 

 the affected cell and any derivative of it will consequently be both diploid and 

 homozygous. If such homozygous diploid sectors include the reproductive tis- 

 sues, meiosis should then be normal and the gametes produced functional. 

 Thus such plants can produce diploid progeny — homozygous diploid progeny 

 if the individual is successfully self pollinated — since every gamete of the 

 plant is genetically equivalent to every other gamete. In a monoploid without 

 diploid sectors, since the chromosomes lack synaptic mates, meiosis is highly 

 irregular. Only rarely are functional gametes carrying the full complement of 

 chromosomes produced. If two of these rare functional gametes from a single 

 monoploid do fuse in syngamy, the zygote produced will be diploid, and 

 homozygous, unless the gametic chromosomes were subject to chromosomal 

 aberration during the irregular meiosis. 



Production of homozygous diploid progeny from monoploids results in the 

 fixation of a single gametic complex. In any population, desirable gametes are 

 more frequent than desirable zygotes. Foi example, if one has on hand an 

 individual heterozygous for three pairs of genes and wishes to obtain from it a 

 definite homozygous product by selfing, one individual in sixty-four of the im- 

 mediate self progeny (Si) will, on the average, carry the desired genotype. 

 One gamete in eight, extracted as a monoploid and then converted into a 

 homozygous diploid, will furnish the same genotype (see Fig. 25.1). 



Successful production of homozygous diploids in quantity from mono- 

 ploids depends upon the adequate solution of two main problems. The first 

 of these is the production and recognition in the seedling, or in earlier stages, 

 of large numbers of monoploids. This problem has been solved to the extent 

 that thousands of monoploids can be produced with relatively small expendi- 

 ture of effort. The second problem is that of deriving self, and consequently 

 homozygous, diploid progeny from the monoploids isolated. This problem 

 also has a practical, though partial, solution. 



