Karyotype Analysis of Autotetraploidy in Brassica Rapa Ssp Chinensis

Summary Brassica rapa ssp. chinensis (non-heading Chinese cabbage) is one of the most important vegetables grown in southern China. Autotetraploidy, induced artificially by colchicine treatment, has increased its nutritional value. For the first time, we report the distribution of repeat DNA sequences in autotetraploid B. rapa ssp. chinensis using fluorescence in situ hybridisation (FISH) with ribosomal DNA (rDNA) probes. Comparisons of inbred lines of a diploid (10P15) and its induced autotetraploid (10P11) of B. rapa ssp. chinensis used genomic in situ hybridisation (GISH) by labelling the total genomic DNA as a probe, and using unlabelled genomic DNA of B. oleracea L. var. italica as the blocking reagent. GISH signals appeared at the centromere and/or adjacent regions of each chromosome; therefore, these could be used as alternative centromere markers. In addition, variations in the number and loci of repeat sequences between the diploid and its autotetraploid were observed, suggesting that some deletions or insertions had occurred in the regions of repeat sequences during formation of the autotetraploid. These findings increase our knowledge of the karyotypes of the mitotic chromosomes in diploid and autotetraploid B. rapa ssp. chinensis, and will facilitate further research in Brassica evolution and marker-assisted breeding.