gaul: induced mutants in seed-propagated species 213 



Intraspecific Macro-mutations 



Intraspecific macro-mutations are the most common type of 

 all mutations that have been selected. Numerous characters of 

 value for plant breeding have been found, but usually these mutants 

 have a reduced vitality. Reduced general performance may, how- 

 ever, not be unexpected on a priori reasoning. A gross change in 

 the genetic system, though it may mean the induction of a valu- 

 able character, will usually result in a disturbance of the delicate 

 gene balance, which had been built up by the breeder with great 

 efforts over a long time. Perhaps the frequency of productive large 

 mutants might be higher in those plants in which greater breed- 

 ing work has not yet been done (Cf. 25). However, a small portion of 

 these large mutations represent a valuable source for further 

 recombination work as discussed later. 



Micro-mutations 



Small mutations generally deserve more attention from plant 

 breeders than macro-mutations. Like the large mutations, the small 

 ones may affect all morphological and physiological characters. The 

 significance of small mutations in evolution was first recognized 

 and emphasized by Baur (3). Since then it has been repeatedly 

 discussed (119). From the fact that most character differences in 

 race and species hybrids show complex segregation, the majority 

 of research workers (see, e.g., 106) agree today with East (18, page 

 450) that "the deviations forming the fundamental material of 

 evolution are the small variations of Darwin". Though small muta- 

 tions were early described in Antirrhinum (109) and plenty are known 

 in Drosophila (63, 120), it is surprising that relatively little atten- 

 tion has been paid to them. Also, in the extensive work with barley, 

 small deviations of various characters, such as kernel size, leaf size, 

 straw height, protein content, etc., have been known for a long time 

 (43, 57, 73, 79, 93, 95, 96). Yet, the suggestion to put the emphasis in 

 breeding programs on small mutations was not made until recently 

 by Nybom (79) and Gregory (39, 40), and meets perhaps with 

 even stronger emphasis by myself (25, 28). 



Most plant attributes of interest to the breeder are quantitative 

 characters which are controlled by many genes. These are called 

 polymeric genes, multiple genes (or factors), or polygenes. When 



