572 ERNST J. SCHREINER 



BUFFERING BY GENETIC DIVERSIFICATION 



Jensen (1952) cites the practice of growing mixed crops of oats and 

 barley and, occasionally, oats, barley, and spring wheat in some north- 

 eastern states. He suggests that the feature of stability found in crop 

 mixtures has played some part in the persistence of this agricultural 

 practice, often without benefit of scientific approval. Suneson (1960) 

 emphasized the buffering effects of genetic diversification on crop pests, 

 with examples of the nonuniform crop varieties, Coast barley, Red Rust- 

 proof oats, and Kherson oats, which had survived the vicissitudes of 

 diseases and insects for many years with only partial damage. 



Allard and Bradshaw (1964) discussed the implications of genotype- 

 environmental reactions in applied plant breeding. They noted that the 

 stability with which we are concerned does not imply general constancy 

 of phenotype in varying environments; it implies stability of economically 

 important traits. They suggested populational buffering as one approach 

 in that varietal stability can be achieved by constituting a variety with 

 a number of genotypes each adapted to a somewhat different range of 

 environments. Genetically homogeneous populations, such as pure-line 

 varieties or single crosses, obviously depend heavily on individual 

 buffering so that each member of the population is well adapted to a 

 range of environment . Both paths are open to genetically heterogeneous 

 populations. Although there is widespread use of population buffering 

 in corn through the use of genetically heterogeneous double-cross hybrids, 

 Allard and Bradshaw suggested that other types of populations seem 

 feasible, including deliberately compounded mixtures of single crosses, 

 mixtures of double crosses, and synthetic varieties. 



CHROMOSOMAL AND N0NCHR0M0S0MAL INHERITANCE 



Jones (1960) pointed out the necessity for extending genetic termi- 

 nology to include both chromosomal and nonchromosomal hereditary material. 

 He suggested the terms "plasmatype" and "chromotype," respectively, for 

 the two major components of the units of hereditary transmission that 

 make up the genotype. The possible role of the plasmatype must not be 

 overlooked in breeding for forest tree improvement. There should be a 

 conscious effort to bring together the plasmatype and chromotype that 

 will give the most efficient genotype. 



Cytoplasmic inheritance of resistance to a needle blight of European 

 larch has been reported by Langner (1951/19S2) . In corn, Fleming, 

 Kozelnicky, and Browne (1960) obtained significant cytoplasmic effects 

 for silking, ear height, plant height, erect plants, yield, and budworm 

 damage. They found that a significant cytoplasmic difference which occurs 

 one year in a given environment may not occur in another year under a 

 different environment. Hunter and Gamble (1968) reported yield differences 

 as great as 565 kg of shelled corn per hectare between hybrids differing 

 in cytoplasmic source. Nagaich et al. (1968) have published apparently 

 the first report of cytoplasmic control of infection types involving a 

 virus . 



The use of cytoplasmic male sterility and the restoration of 

 fertility is being used more and more extensively in crop breeding, and 

 it undoubtedly will be useful in forest tree breeding. Natural mutations 

 have been the usual source of male-sterile genotypes. However, Erichsen 

 and Ross (1963) found that the colchicine-induced mutants in sorghum with 



