Further yearly analyses of spontaneous pink mutant events in the stamen hairs of Tradescantia clone BNL 4430 cultivated in the NSC growth chamber

In order to confirm the results obtained in the previous 1-year-term (Decem ber 12, 1998, through December 10, 1999) scorings and analyses of spontaneo us pink mutant events (PMEs) in the stamen hairs of Tradescantia clone BNL 4430 cultivated in a nutrient solution circulating (NSC) growth chamber, si milar scorings and analyses were continued for another 52-week period from December 11, 1999, through December 8, 2000. The environmental conditions w ere not changed, except for a minor modification in the method of supplying the nutrient solution used. During the scoring period, 732,128 stamen hair s with an average cell number of 24.90 cells were observed, and 2,368 PMEs were detected. The overall spontaneous somatic mutation frequency was 1.35 +/- 0.03 PMEs per 10(4) hair-cell divisions, which was significantly lower than the value of 1.56 +/- 0.03 determined in the previous 52-week period, and the frequencies were lower during April through September than in other months, the period showing lower frequencies lasting 1-month longer than i n the previous year. The present results reconfirmed the occurrence of a cl ear seasonal variation in the spontaneous mutation frequency in the NSC gro wth chamber, and the lower overall frequency, probably related to the minor modification in supplying the nutrient solution, is helpful for conducting mutagenicity tests at low levels, offering a lower background level. The a nalyses of the sectoring patterns of all these PMEs showed that the most of the 203 cases of multiple (two to five) pink sectors observed in the same stamen hairs (scored as 253 PMEs for calculating mutation frequency) were t he results of events involving somatic recombinations occurred in single ce lls or cell lineages, rather than those of two or more independent somatic mutations occurred in different cells, agreeing with our previous study, an d the significance of somatic recombinations in causing single PMEs was als o reconfirmed.