392 Intrachromosomal Aberrations 



quently a difference in viability, however, between the megagame- 

 tophyte and the microgametophyte, for deficient chromosomes are 

 sometimes carried along in the female gametophyte even though 

 in the same plant they cause the male gametophyte to die. If it 

 were not for this viability on the female side, heterozygous defi- 

 ciencies would ordinarily be found in plants only if they arose 

 in sporophyte tissue. Because microgametophytes with a defi- 

 ciency rarely survive, homozygous deficiencies are very un- 

 common. 



Stadler has reported an interesting example of deficiency in 

 maize. One X-ray-induced deficiency involved one-sixth of the 

 length of chromosome 10. In the male, microgametophytes bear- 

 ing the deficiency were apparently normal until after the division 

 of the microspore nucleus to form the tube and generative nuclei. 

 After this division, two types of pollen grains were found, large 

 and small. The larger ones divided earlier than the others and 

 were in metaphase when most of the smaller ones were still in 

 early prophase. The generative nucleus of the smaller pollen 

 grains was apparently normal, however, even though its division 

 was delayed; but when the pollen was shed, the smaller ones had 

 accumulated much less starch even in proportion to their size. 

 Normal maize pollen shrivels shortly after it is shed, but the 

 deficient grains shrivel much more rapidly. After 3^4 minutes 

 almost all the small grains, but none of the normal ones, had 

 shriveled. If deficient pollen grains are placed on the silks be- 

 fore they have begun to shrivel, protoplasmic movement and 

 digestion of food reserves occur as in normal grains, but pollen 

 tubes rarely, if ever, emerge from the grains. 



In the female gametophyte the deficiency was injurious but not 

 lethal. Almost half the ovules of the heterozygous deficient plant 

 contained small and subnormal embryo sacs at the time of pol- 

 lination, and only a small proportion was well enough developed 

 to function normally in fertilization and seed development. The 

 proportion of seeds lacking an embryo was higher among defi- 

 cient megagametophytes than among the nondeficient ones on 

 the same ears, and seeds heterozygous for the deficiency were 

 slightly reduced in size. The maize plants heterozygous for 

 the deficiency had somewhat reduced vitality, as shown by a 

 slightly smaller size and a slightly delayed time of flowering, a 



