358 Colchicine 



to this situation, monosomic analysis developed for Nicotiana has 

 proved most useful in many genetic tests, notably in establishing link- 

 age groups; surveying amphidiploids for specific genetic characters. i- 

 The technique applied to Nicotiana suggests that other groups might 

 profit from these methods.--^ There are limitations to this method 

 among such a group as Gossypium, where polyploids are common; 

 yet the use of monosomies is limited. No nullisomics are reported for 

 GossypiiiN}.'^'^ 



Quite another situation exists in hexaploid Triticuni aestiinim L., 

 where nullisomics and monosomies can be applied to genetic prob- 

 lems.''^ As we mentioned for trisomies, the number of different types 

 with one whole chromosome extra should equal the haploid number. 

 For Datura, 12 primary trisomic types exist. In Nicotiana the total 

 monosomies possible is 24. Accordingly, 21 nullisomics would be ex- 

 pected or ecjual to the 21 pairs representing hexaploid wheat. ^^ 



For each pair missing, the 20-chromosome plant has specific 

 characteristics. Nullisomics may be numbered from I to XXI. ^^ None 

 is completely sterile, and certain are fertile in both male and female. 

 Some are female-fertile only, others male-fertile only. Some nulli- 

 somics pollinated by normal plants give more monosomes of a par- 

 ticular type, as well as irisomes. The incidence is more than a random 

 occurrence. For example, nullisomic III produced more monosomic 

 IV and XV than other types of monosomes. 



Particular tetrasomics may cancel the effects of certain nullisomics. 

 Such compensating cases are known for wheat and oats. For example, 

 tetrasomic II compensates for nullisomic XX so that the plant is very 

 nearly normal even as the male gametophyte.^'' There does not seem 

 to be a competitive advantage between pollen-deficient for chromo- 

 some XX and duplicated for II. Common properties in the segments 

 of these chromosomes would appear to be a cause for the compensa- 

 tion. There seems to be no pairing between tetrasome II and nidli- 

 some XX. These are. in very brief sketch, problems related to poly- 

 ploidy. 



Seven chromosomal pairs corresponding to the D genome in hexa- 

 ploid wheat are dwarf nidlisomics and differ from each other accord- 

 ing to the specific pair missing. These nullisomics were derived from 

 among offs]:)ring of Trittcum pojomiciim, genomes AABB, X T. 

 spelta, AABBDD. These 7 nullis(jmics are lettered a, b, c, d, e, f, g, 

 respectively. Twenty-one nidlisomics from a Chinese wheat (T. aesti- 

 vinn L.) should throw light on the D genome by hybridizing the 

 dwarf nidlisomics and those from T. aestiviuit. which had a different 

 origin. -^1 



Success has been achieved in transferring mosaic disease resistance 

 from one species to another in Nicotiana. 1 he commerical tobacco re- 



