In last year's MNL (p. 29), we reported the results of a study on the cytological stability of three maize callus lines derived from anther culture in vitro. It was found that among the three lines, one (G7) gradually changed from haploid to diploid chromosome number (2n = 2x = 20), and two (N1 and G9) tended to maintain a haploid level (2n = x - 10) with over 90 percent of the cells having 10 chromosomes, after subculturing for three or more years. As a continued study on two more callus lines was carried out, it was observed, with great interest, that one of these lines evolved toward a polyploid constitution while the other manifested a stable dihaploid condition. In the first year of culturing of these callus lines, their chromosome number was consistently identified to be 10. The increase of chromosome number occurred following the first year of growth. This can be accounted for either by endomitosis or by fusion of adjacent cells. However, why the process stopped for one callus line (Dan-Sun 91) after doubling chromosome number once and continued for another (81-B5) is unclear. For the latter (81-B5), varying chromosome numbers from 30 to more than 100 were identified in different cells.

Y.C. Ting and M.G. Gu
 
 

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