508 MUTATION AND PLANT BREEDING 



mutation breeder than for the conventional breeder. Gregory has 

 shown in irradiated populations of peanuts that the amount of 

 readily visible mutants expressed may be quite unrepresentative 

 of the total (polygenic) variation generated. Moreover, "the 

 remarkable variation in (morphological) mutant expressivity" 

 attributed to polygenic segregation in this material appears to be 

 of a similar order to that described in segregating progenies of inter- 

 specific hybrids in certain other genera, e.g., Lycopersicon and Gos- 

 sypium. There remains the possibility that the amount of poly- 

 genic variation engendered by radiation may be of the same order 

 but differ in kind from that resulting from interspecific hybridiza- 

 tion. If so, it may be easier to recover adapted types from irradiated 

 populations than from segregating populations of interspecific ori- 

 2,in. This remains to be demonstrated. 



The combination of a long generation cycle and complex 

 heterozygosity, sometimes accompanied by aneuploidy and self-in- 

 compatibility, provides a particularly nasty problem for the breed- 

 er of vegetatively propagated crops. The compensating advantages 

 associated with clonal propagation and abundance of meristems 

 cannot be used readily in a conventional (sexual) breeding system 

 and have led to a supplement of the latter by a form of "histolog- 

 ical engineering:". This involves the recognition of somatic mutants 

 and chimeras, and their extraction and isolation by pruning followed 

 by asexual propagation. Under natural conditions the limiting fac- 

 tor is the rate at which somatic mutation occurs. The possibilities of 

 increasing mutation frequency, re-arranging somatic tissues, and 

 breaking self-incompatibility mechanisms through a combination 

 of irradiation and ingenious manipulative techniques have been 

 reviewed by Nybom. In this area, where the opportunities for 

 improving conventional methods seem to be extremely limited, 

 the use of induced "mutation" in a broad sense would seem to 

 offer much promise. 



Mutation induction and conventional breeding systems are 

 not necessarily opposed. Certain specific problems indicate rather 

 clearly the advantages of a combined attack. The breakage of self- 

 incompatibility systems, certainly, and the reconstitution of sexual 

 from apomictic systems, possibly, can be achieved through muta- 

 genic treatment. Success in either would open up new potentialities 



