Abstract
xBrassicoraphanus line BB#5, a new synthetic intergeneric hybrid between Brassica rapa L. ssp. pekinensis and Raphanus sativus L. var. rafiphera induced by N-methyl-N-nitroso-urethane mutagenesis in microspore culture, shows high seed fertility and morphological uniformity. Dual-color fluorescence in situ hybridization (FISH) using 5S and 45S rDNA probes and genomic in situ hybridization (GISH) using B. rapa genomic DNA probe were carried out to analyze the chromosome composition and the meiosis pairing pattern compared to its parental lines. The somatic chromosome complement is 2n = 38, which consists of 17 metacentric and two submetacentric chromosomes with lengths of 2.18 to <TEX>$5.01{\mu}m$</TEX>. FISH karyotype analysis showed five and eight pairs of 5S and 45S rDNA loci. GISH meiosis pairing analysis showed that 19 complete bivalents were most frequent and accounted for 42% of the 100 pollen mother cells examined. Based on chromosome number, size, morphology, rDNA distribution, and meiosis pairing pattern, both parental genomes of B. rapa and R. sativus appear to exist in xBrassicoraphanus line BB#5, demonstrating its genome integrity. Such stable chromosome constitutions and meiotic pairing patterns in somatic and meiotic cells are very rare in natural and synthetic intergeneric hybrids. Chromosomal studies and genetic and phenotypic changes in allopolyploids a re discussed. The results p resented h erein will b e usef ul f or f urther g enomic s tudy o f xBrassicoraphanus lines and their improvement as promising new breeding varieties.
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