Analyses of transposition frequencies of mGing and mPing in successive subcultures and in different culture conditions. (a) Transposition frequencies of mGing and mPing in five successive subcultures. One callus from a single plant of Jiahua No. 1 (additional file 6a Group I, lane 1) was selected and divided into four pieces (Group I, II, III, and IV). Each callus piece was cultured for five passages (five tubes). In each passage, part of callus piece was used for gDNA extraction and the rest was transferred into a new tube for the next culture passage. (b) The gDNA sample of each lane in TD gels was from one passage of subculture and the lane number indicated the passage of the subculture. Autoradiograph of TD gels of mGing and mPing in different culture conditions. Each group was composed of 6 callus pieces respectively from 6 independent calli. The same lane number in different groups indicated that the callus pieces were originally from the same callus. Group I, calli were inoculated on cell subculture medium and cultured for 28 days. Group II, calli were inoculated on differentiation medium for 28 days. Group III, calli were inoculated on proliferation medium and cultured for 80 days without transferring.