The Influence of Genetic Constitution 



information on the nature of 'constitutional vigour' in relation 

 to growth-rate. The effect is variously explained. Some of the 

 possible complications of heterosis in relation to the criteria of 

 vigour are indicated by the findings of Rutman (1950, 1951), 

 who compared the rates of methionine uptake in liver slices 

 from a fast- and a slow-growing strain of rats. The meth- 

 ionine replacement rate in slices derived from the fast- 

 growing strain was almost double that in the slow, but the 

 growth-rate of the slow strain could be made to approach that 

 of the fast by transposing the litters during suckling, and 

 appeared to be controlled by a milk-borne factor. Interstrain 

 hybrids at first showed a growth pattern like that of the mother, 

 but later followed that of the faster-growing strain. 



Although by a very elegant experiment J. and S. Maynard 

 Smith (1954) have shown that in certain cases at least heterosis 

 appears to result from orthodox heterozygy, the number of 

 instances in which cytoplasmic and environmental factors also 

 appear able to evoke vigour is increasing. This is largely a 

 reflection of the very heterogeneous character of 'vigour'. Some 

 years ago Borisenko (1939, 1941) reported an increase in vigour 

 in the progeny of Drosophila matings where the inbred parents 

 were reared under different environmental conditions. This 

 observation does not appear to have been repeated. The ques- 

 tion of the induction of vigour by non-genic means has since 

 been most actively investigated by avowed anti-Mendelians 

 (Kurbatov, 1951; Hasek, 1953, etc.) but by no means all the 

 positive results come from this school. As far back as 1928, 

 Parkes observed that mice suckled by rats exhibited an extra- 

 ordinary overgrowth, which results simply from excessive nutri- 

 tion. Marshak (1936) found evidence of a maternal cytoplasmic 

 factor influencing the increase of growth-rate due to heterosis 

 in mice. The increased vigour in progeny of pure-line ova 

 transplanted to hybrid mothers (Kurbatov, 1951) is also found 

 in transplanted foetuses (Venge, 1953). Hasek has claimed 

 (1953) that when parabiosis is carried out between Rhode 

 Island and Leghorn embryos in the egg, by an ingenious tech- 

 nique, the pullets occasionally show even greater vigour than 

 the progeny of R.I. R. X Leghorn crosses. Without endorsing the 

 sweeping theoretical claims based by the Russian school upon 



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