Abstract: 【Objective】 The objective of this study is to analyze B chromosome polymorphisms in maize landrace populations from Southwest China, to evaluate their genetic diversity at cellular level and provide evidence for the introduction path of maize in China. 【Method】 Thirty maize landrace populations from Sichuan Province, Chongqing city, Yunan Province, and Guizhou Province in southwestern China were used. Fifty individuals per landrace population were sampled, and ten cells from a single plant were scored for the presence of Bs under a microscope. A total of 15 000 cells (30 × 50 × 10) in the mitotic metaphase were analyzed. Ten slides, from ten individuals per landrace population, were C-banded using conventional Giemsa staining with modifications.【Result】 Besides 10 pairs of standard chromosomes, the landrace populations contain a metacentric B chromosome (B_M) morphologically similar to the first chromosome of the karyotype, a subtelocentric B (B_ST) identical in size to B_M, and a spot-shape microchromosome (B_S) without centromere. Seven maize landrace populations from Sichuan possessed three forms of Bs. In the populations of maize landraces from Chongqing both B_M and B_ST forms were found, whereas B_ST and B_S were identified in the populations of maize landraces from Yunnan. Nine populations from Guizhou were the only one (B_ST) where B_M and B_S were exclusive. The mean length of B_M, B_ST, and B_S was 2.82 µm, 2.78 µm, and 0.9 µm, respectively. In comparison with the metacentric pair of the standard karyotype, measurements conducted on the cells with Bs estimated a relative length of 34%-52% for the B_M, 32%-58% for B_ST, and 13%-18% for the B_S, and relative lengths for B_M, B_ST, and B_S were on the average 43%, 45%, and 15%, respectively. The mean arm ratios for B_M, B_ST, and B_S were 1.19, 1.98, and 1.00, respectively. Of the eighteen populations with B chromosomes, seven, five, four, and two populations originated geographically from Sichuan, Chongqing, Yunnan, and Guizhou, respectively. Eighteen of the thirty populations sampled exhibited numerical polymorphisms and a total of 487 Bs were identified in 421 individuals. The frequency of B chromosomes per individual varied from 0 to 90% with an average of 29.6%. The populations from Sichuan possess a higher frequency of B chromosomes in both population and individual levels. The highest number of B chromosomes in a cell was 2 with the predominant dose being 0. The mean frequency of B chromosomes for the 0B, 1B, and 2B cell were 97.04%, 2.81%, and 0.15%, respectively. B_ST is the predominant form accounting for 67% of the Bs in the populations, followed in turn by B_M and B_S corresponding to 19% and 14%, respectively. It might be hypothesized that B_ST repres­ented an ancestor B chromosome in maize, whereas the B_ST and B_S were variants of the B_M, implying that the evoluation of B_M and B_S have occurred in a smaller time and space scale. The Giemsa staining exhibited that the B_M and B_ST forms, easily distinguished from A chromosomes, shared some homologous DNA sequences and were both heterochromatic as well as GC-rich. The mean C-band number varied from 11.0 to 20.6 in the 0B cell, 10.8 to 19.4 in the 1B cell, and 10.5 to 18.6 in the 2B cell, indicating a negative correlation between B chromosomes and C-bands in the A chromosome set. 【Conclusion】 Cytological observations revealed that maize landrace populations from southwest China had a higher level of genetic diversity. For the B chromosome in them, there existed morphological and numerical polymorphisms. Sichuan is the geographical centre of maize landraces in this region based on geographical distributions of the B chromosome. This supported the theory on a maize introduction path in China.

Key words: maize, the B chromosome, polymorphisms, southwest China