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UNIVERSITY OF CALIFORNIA PUBLICATIONS V,f»RARV 



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Vol. 2, No. 10, pp. 297-314, plate 53 March 5, 1925 



IN 



AGRICULTURAL SCIENCES 



CHROMOSOME NUMBER AND INDIVIDUALITY 

 IN THE GENUS CREPIS 



I. A COMPARATIVE STUDY OF THE CHROMOSOME 

 NUMBER AND DIMENSIONS OF NINETEEN SPECIES 



BY 



MARGARET CAMPBELL MANN 



( Contribution from the Division of Genetics, University of California ) 



Because most of the species of the genus Crepis have low chromo- 

 some numbers, it offers obvious advantages for the study of comparative 

 chromosome relations. The chromosome individuality of certain species 

 is very distinct, so much so that it could be used as a diagnostic character 

 in specific determination. These facts lead to an inquiry to discover first, 

 whether upon careful analysis all species would prove to differ in chromo- 

 some individuality, and second, what relations the chromosome groupings 

 of different species bear to one another. This question has been previously 

 touched upon in several papers by Rosenberg (1909, 1918, 1920) and in 

 a recent contribution by Marchal (1920). Rosenberg (1918) called 

 attention to the fact that the genus Crepis possesses a great variety of 

 chromosome numbers. His summary showed species with 3, 4, 5, 8, 

 9, and 20 pairs. In order to determine how such numerical differences 

 had arisen within the genus, he measured the chromosomes of a three 

 and a four-pair species, capillaris (Reuteriana of Rosenberg) and 

 tedorum, respectively, and found, on the basis of measurements of 

 homotypic anaphase chromosomes, that three of the chromosomes of 

 the two species corresponded accurately in size and that the fourth 

 pair of tectoruni averaged slightly shorter than the shortest of capillaris. 

 He noted that the two shortest chromosomes of capillaris often mate 

 later than the other two in p. m. c. and finds associated with this fact 

 a tendency toward lagging and irregular division. From these data he 



