GENETICS OF SOMATIC CELLS 411 



Particularly interesting was the finding 1275 that the various Minute loci increased 

 the frequency of s.c.o. Minutes are known to occur throughout the chromosomal 

 complement, all producing a similar phenotype, characterized by prolonged larval 

 development, slender bristles, and lowered viability and fertility. Acting as dominants, 

 they are thought to be small deletions, lethal when homozygous. These loci exerted 

 varying degrees of effect on the frequency of s.c.o., the sex-linked Minutes being more 

 effective in causing increases than the autosomal Minutes. The chromosome III 

 Minutes exhibited a peculiar specificity in that they limited s.c.o. to the arm in which 

 they were located. The existence of other genetic factors affecting s.c.o. was indicated 

 by the work of Brown and Welshons 139 who found great variation in the incidence of 

 mosaicism among the several stocks that they studied. Recently, Weaver 1360 

 reported a series of experiments designed to detect and characterize the genetic factors 

 that control s.c.o. These studies led to the conclusion that there are chromosomal 

 factors other than Minutes, not associated with any visible deficiency or aberration, 

 which control the frequency of s.c.o. Such factors, governing the frequency of X- 

 chromosomal s.c.o., occurred on all three major chromosomes. She concluded that 

 s.c.o. is a process under the precise control of genetic factors governing the frequency 

 of its occurrence. The genes responsible might affect either the closeness of somatic 

 pairing, or the physical or chemical processes responsible for the exchange of 

 chromosomal segments, or both. 



Somatic crossing over has received its most important application so far in the 

 work of Pontecorvo and his school, who demonstrated its usefulness as a tool for genetic 

 analysis and emphasized its possible applicability to cells in the tissues of higher organ- 

 isms. The latter possibility has often been dismissed previously as it was assumed that 

 somatic pairing of chromosomes at metaphase is an absolute prerequisite for the 

 occurrence of s.c.o. Whereas somatic pairing is a regular event in Drosophila, butterflies, 

 and moths, and less regular in other lower organisms, it has only been demonstrated 

 exceptionally in higher organisms. 119 However, the validity of this requirement may be 

 seriously questioned. As emphasized by Stern, 1276 we are so ignorant about the mecha- 

 nism of crossing over that negative cytologic evidence regarding the absence of somatic 

 pairing can hardly be taken as a serious objection against the possible occurrence of 

 s.c.o. Or, as Pontecorvo puts it: 1011 "The idea, hard to die, that somatic pairing as 

 cytologically detectable at metaphase in a few organisms has something to do with 

 mitotic crossing over is not very helpful to say the least." In five different species of 

 fungi and in yeast, the occurrence of s.c.o. has been made unquestionable by purely 

 genetic analysis in the complete absence of cytologic observations. Some results 

 of this analysis will now be briefly considered. 



Somatic crossing over has been most extensively studied in Aspergillus nidulans. 1011 

 Since it is a rare event, crossovers had to be selected. This was done by the use of 

 suitable marker characteristics. Five types of selection could be applied: visual (this 

 corresponding to the only method available for the analysis of s.c.o. in Drosophila) ; 

 selection for recessive suppressors of a nutritional requirement; selection for recessive 



