INDUCED CHROMOSOMAL ALTERATIONS IN MAIZE 1307 



The three deficiencies for yg-i in chromosome 9 reported by Creighton 

 (21) are instructive because of the terminal knob involved. Df 9-3 

 involved no gamete sterility and such a small cytological alteration 

 that its interpretation is open to question. If it is a deficiency, it is 

 not a terminal one. Df 9-2 is a clear case of an internal deficiency which 

 would have been classified as terminal but for the presence of the con- 

 spicuous terminal knob. Df 9-1 lacked the knob entirely as well as 

 about one-fourth of the arm. This is probably the best evidence available 

 for a truly terminal deficiency. But the deficient arm may well end 

 with a short terminal section from another chromosome. Creighton 

 states: "Synapsis of the short arms from the spindle-fiber-insertion 

 regions to the point of deletion was regular in most cases. Sometimes 

 there was a lack of association of the last few chromomeres of the deficient 

 chromosome with their homologues on the standard chromosome." 

 It is also interesting that the plant carrying this deficiency was a mosaic 

 with the greater part normal. 



For tests of the hypothesis that the induced alterations are due to 

 interchange between strands at a double-strand stage, the types not 

 otherwise expected should be watched for in plants from irradiated 

 pollen. Stocks with the largest number of conspicuous knobs and other 

 chromosome markers should be selected for irradiation studies. In 

 deficient plants, the ends of all chromosomes should be observed care- 

 fully. Mosaic plants are of the greatest interest and the nondeficient 

 as well as the deficient sector should be studied. In endosperm studies 

 mosaics where one sector is deficient for one gene and the other sector 

 deficient for a gene from another chromosome should be watched for 

 carefully. 



It is possible, and perhaps even probable, that induced alterations 

 take place with more than random frequency near the extreme ends of 

 chromosomes. If the ends are sticky in the living cell during prophase 

 as they appear to be in fixed preparations, this would tend mechanically 

 to bring the distal portions more frequently in close proximity to other 

 chromosomes. A similar, but perhaps less pronounced increase in 

 alteration frequency might be expected near spindle attachments and 

 knobs. Unfortunately a sufficient number of maize interchanges have 

 not yet been examined cytologically to get a picture of their distribution 

 within the chromosome. 



Little can be said as to the nature of the action of X-rays in inducing 

 chromosome alterations. The obvious suggestion is that for adjacent 

 chromosomes the irradiation somehow brings about conditions resembling 

 the conditions normally present only in or surrounding homologous 

 chromosomes during corresponding phases of meiosis. The recent work 

 of Metz (35) on the role of the chromosome sheath is interesting in this 

 connection. Metz suggests "that irradiation serves to disturb the 



