INBREEDING DEPRESSION AND THE IIYBRIDITY OPTIMUM 



following segregation in the gametes of the F^ hybrids. In other 

 words the sterility of the backcross is largely genotypic, but it is 

 a consequence of the segregation which caused an even greater 

 sterility in Fi. It is also, we may notice, superimposed on a small 

 amount of segregational sterility in the backcross plants themselves. 



A single mendelian difference, which might be supergenic and 

 distributed over most of one chromosome, would account for the 

 sesreeation seen in this backcross of the Fi from the Lolitim mother. 

 In the second cross, where Festuca was used as the mother of the Fj, 

 segregation also appears, but it is not so clear as in the first. Probably 

 a larger number of differences are being recombined. For this reason, 

 however, the correlation between chiasma and univalent frequency 

 and pollen fertility are all the more clearly manifested (Fig. 56). 



There is also a small overall difference between the two reciprocal 

 crosses and their progenies. The F^ from the Lolium mother has a 

 little good pollen, while that from the Festuca mother has none. In 

 the same way the progeny whose chromosomes show full pairing 

 are more pollen-fertile in the backcross to Lolium of the F^ with 

 Lolium cytoplasm than in the backcross of the F^ with Festuca 

 cytoplasm. This difference may well be due to failure of the other 

 type of adjustment, that between nucleus and cytoplasm. If so, 

 however, it merely serves to emphasize the point that disharmonies 

 at the cellular level generally cause less disturbance than do 

 disharmonies within the nucleus, genie disharmonies such as are 

 revealed by segregation in both these backcrosses. 



Inbreeding Depression and the Hyhridity Optimum 



Hybrid sterility may thus be due, either immediately to geno- 

 typic unbalance of the hybrid, or less immediately to the inability 

 of such wide hybrids to avoid the segregation of unbalanced gametes. 

 Such segregation must, of course, be peculiar to hybrids. But geno- 

 typic sterility, and equally loss of vigour, is not the property of 

 hybrid nuclei alone. If we inbreed maize by self-fertilization, the 

 result is invariably a loss of vigour, or of fertility, or of both, in 

 succeeding generations of the inbred lines. This inbreeding depression, 

 as it is called, is rapid at first but soon begins to slow down (Fig. 57). 

 After five or six generations no great increase o^ the depression is 



235 



