Cytogenetical and molecular investigations on somatic hybrids of Sinapis alba and Brassica juncea and their backcross progeny

Somatic hybrids of Sinapis alba+Brassica juncea (Sal Sal AABB) were synthesized by protoplast electrofusion. They were true genomic allopolyploids since they possessed 60 chromosomes, i.e. the sum of S. alba (2n= 24) and B. juncea (2n= 36) chromosomes. Chromosome pairing was predominantly bivalent with the occasional occurrence of multivalents in the pollen mother cells at diakinesis and metaphase I. Hybrids were completely pollen-sterile, but produced seeds on back-crossing with B. juncea and B. campestris. A total of 37 BC₁ plants were raised from two somatic hybrids (JS-1 and JS-2) and 24 of these were analysed cytologically. The 22 plants originating from the pollinations of somatic hybrids with B. juncea showed a chromosome configuration of 18II+12I and had 42–86% pollen fertility. Two plants from the backcrosses of the somatic hybrid with B. campestris formed 10II +20I, and had 0–4% fertile pollen. Total DNA analysis by probing with pTA71 carrying a full-length 18S–25S rDNA fragment of the wheat nuclear genome revealed that the two somatic hybrids possessed all the characteristic bands of both the species, confirming their hybridity. Probing with the mitochondrial coxI and atp9 genes indicated mitochondrial genome recombination in the hybrids. Hybridization with chloroplast-specific psbD indicated that both the somatic hybrids possessed the cp genome of S. alba origin.