During the last few years a study on the ploidy stability of three maize callus lines was carried out. These callus lines were obtained through standard anther culture in vitro of different maize varieties and hybrids. It was found that if the majority of cells examined of a callus line were haploid in the beginning, the number of haploid cells became gradually increased during a long-term subculturing, such as callus line N1. After one year of culturing, 90.2 percent of the cells were haploid. Five years later, 98 percent of them reached the same ploidy. On the contrary, for the same callus line, the number of diploid cells changed from 2.1 per cent in the first year to 0.4 per cent five years later. The number of aneuploid cells also decreased from 7.9 percent to 1.6 percent in the same period (Table 1).
For callus line G7, it was found that the majority of cells were diploid in the first five months of growth. A little over three years later, the number of diploid cells in this line rose from 91 percent to 99.2 percent. However, the number of either haploid cells or aneuploids was consistently decreasing during that time. It was further observed after two and a half years of subculturing that about two percent of a total of 275 cells studied belonged to a higher ploidy level, either triploid or tetraploid. One year later cells with larger than diploid chromosome number were no longer present in the same callus line.
Samples of callus line G9 were taken only twice for a term of 42 months of culturing. In the first 15 months, over 50 percent of the cells were identified as haploid with 2n = x = 10. Over two years later, a second sample of the same line was examined. In a total of 307 cells, 97.8 percent were definitely haploid. In contrast, both diploid and aneuploid cells were notably decreasing in number. The details of the change in ploidy level for both lines G7 and G9 are also clearly shown in Table 1.
In view of the above, it appears tenable to conclude that selection was for euploid cells, both haploid and diploid, of those maize callus lines during a long-term of subculturing. Among euploids, a haploid callus line (the majority of the cells are haploid in the beginning) tends to eliminate all the other ploidy cells and eventually becomes an absolute haploid clone. This is also true for a diploid callus line.
Table 1. Ploidy stability during long term subculture of maize callus lines derived from anther culture in vitro.
M. G. Gu and Y. C. Ting
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