SHERRET S. CHASE 



Iowa Sfafe College 



Chapter 25 



Monoploids in Maize 



Haploid sporophytes have been reported in jimson weed (Blakeslee et al., 

 1922), cotton (Harland, 1920), tobacco (Chipman and Goodspeed, 1927), 

 evening primrose (Gates, 1929), maize (Randolph, 1932a, 1932b), wheat 

 (Gaines and Aase, 1926), rice (Ramiah et al., 1933) tomato (Lindstrom, 

 1929), pepper (Christensen and Bamford, 1943), and in many other genera 

 which have been subjects of cytogenetic study. 



A haploid organism, strictly speaking, is one which has only one set of 

 chromosomes, that is, one genome per cell. In the common usage of botanists, 

 geneticists, and others, a sporophy te originating by reduced parthenogenesis 

 or by an equivalent process, and consequently carrying the reduced or gamet- 

 ic complement of chromosomes in each cell instead of the normal zygotic 

 complement, is referred to as a haploid. 



Thus the term, as applied to a sporophyte, has come to carry the connota- 

 tions of both parthenogenetic origin and gametic chromosome number, and 

 the actual genomic condition tends to be ignored. Many so-called haploids are 

 actually diploids or polyploids. Thus the haploids of common wheat are 

 triploids, since the parent species, Triticum vulgare, is a hexaploid. To em- 

 phasize the fact that the haploids of maize carry only one set of chromosomes 

 per cell, that is, only one chromosome of each type instead of the normal pair, 

 the alternate term monoploid is used here to designate these aberrant plants. 



In normal sexual reproduction in maize the pollen tube penetrates the 

 eight nucleate embryo sac. One of the two male gametes released fuses with 

 the egg nucleus to form the zygote, while the other fuses with the two polar 

 nuclei to form the primary endosperm nucleus. In the abnormal type of re- 

 production giving rise to monoploid sporophytes the ])rocesses apparently 

 are the same except that for some reason the first male gamete fails to fuse 

 with the egg nucleus and is lost. The egg nevertheless is activated and devel- 



* Journal Paper No. J-1906 of the Iowa Agricultural Experiment Slalion, Ames, Iowa. 

 Project 1201. 



389 



