

CHROMOSOME NUMBERS IN THE GENUS BURSA. 



SAMUEL E. HILL, 

 PRINCETON UNIVERSITY.! 



In a recent paper. Shall divides the genus Bursa into two 

 groups on the hasis of genetical studies. The Bursa-pastoris 

 group includes six species and two subspecies, as follows : Bursa 

 bursa-pastoris (L.) Britton, B. bursa-pastoris apetala (Opiz), B. 

 Hecgcri (Solms-Laubach), B. occidentalis Shull ined., B. occi- 

 dcntalis madeira- Shull ined., B. oricntalis Shull ined., B. djur- 

 djurcc Shull ined., and B. penartha: Shull ined. The Rubella 

 group includes four species, as follows: B. grandiflora (Bois.), 

 B. rubella (Reut.), B. Viguicri (Blaringhem), and B. tnscaloosce 

 Shull ined. 



Crosses between members of the same group yield more or less 

 fertile F : hybrids. Crosses between any species of one group and 

 any species of the other group yield sterile hybrids in the F x . 

 Duplication of factors for certain characters is found in the Bursa- 

 pastoris group of species, but not in the Rubella group. 



Rosenberg (1904) had reported the haploid number of chromo- 

 somes of Bursa bursa-pastoris to be 16, and the diploid number to 

 be 32. These counts were confirmed by Laibach (1907). Mar- 

 chal (1920) reported 16 haploid chromosomes for Bursa Heegeri, 

 and 8 haploid chromosomes for Bursa Viguieri. Knowing that 

 two members of the Bursa-pastoris group possessed 16 haploid 

 chromosomes, and that one member of the Rubella group pos- 

 sessed 8 haploid chromosomes, Shull suspected that the cause for 

 the inter-sterility of these two groups might lie in a difference in 

 chromosome number between the two groups. Since rather un- 

 usual opportunities were offered for securing material from Pro- 

 fessor Shull's pedigree cultures of Bursa, I undertook a cytologi- 

 cal examination of the various species of the two groups. 



The cells selected for study were the pollen mother cells, since 

 at the maturation divisions they offer relatively large cells with the 



1 This investigation was carried out principally in the cytological labora- 

 tories of Princeton University. 



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