208 University of California Puhlications in Agricultural Sciences [Vol. 2 



INBREEDING AND CROSSBREEDING EXPERIMENTS 



Cultures of Crepis were first grown to study and to isolate certain 

 character variations which had been observed. Forced inbreeding 

 was resorted to as the quickest means of reaching the desired end. 

 After two generations of inbreeding it was noticed that the plants 

 were much smaller and less hardy than at first, notwithstanding the 

 fact that cultural methods had not varied to any marked extent. Ex- 

 periments were then planned and executed with these cultures to 

 demonstrate the effects of continued inbreeding and subsequent cross- 

 ing in a wild species, and the results obtained form the body of this 

 report. 



In Table 1 are given the pedigrees of the cultures in which in- 

 breeding was continued. Cultures 20.113, 20.114, and 20.128 have 

 identical ancestors previous to their parents, which were sibs. In 

 the second generation of inbreeding, sibs of culture e2 were crossed 

 because the strain showed such a high degree of self-sterility that it 

 was feared that not enough viable seed could be secured to maintain 

 the strain. By crossing sibs, which, however, were very similar in all 

 respects, a few viable seeds were secured. Self -fertilized seeds of 

 e32P|5 and of e32Pig were also secured and their cultures were in all 

 measurable respects no less vigorous than the progeny of crossed sibs 

 of culture e32. Plate 41, figure 1, shows culture 113, derived from 

 crossing sibs, and culture 114, derived from selfing one of the sibs 

 used in the cross. Thus for inbreeding purposes the culture e2 had 

 reached an almost homozygous condition in the third generation, since 

 in no case have appreciable changes been noticed in the fourth 

 generation. 



Culture e28 also seemed to reach its maximum reduction of vigor 

 and size in the third generation of inbreeding. No crossbred plants 

 from this inbred strain have yet been grown. In contrast, H-10 

 (pi. 41, fig. 2), resulting from crossing sibs in the two previous gener- 

 ations, shows but little reduction in vigor or size, indicating that it 

 is either still heterozygous genetically or is not affected by inbreeding 

 to the same degree as e28. The latter seems to the writer more prob- 

 able, inasmuch as the entire culture of H-10 was fairly uniform, thus 

 indicating a large degree of homozygosity. 



Cultures 17.192 and Z9 used in the crossbreeding experiments 

 were chosen because they could have no immediate genetic relation 



