Construction of novel Brassica napus genotypes through chromosomal substitution and elimination using interploid species hybridization

A synthetic Brassica napus rapeseed with genome composition of A(r)A(r)C(c)C(c), made by combining A(r) from B. rapa (A(r)A(r)) and Cc from B. carinata ((BBCCc)-B-c-C-c-C-c), is valuable for making new genes available to breeders and gaining heterosis in crosses. An intergenomic hybrid A(n)A(r)C(n)C(c) was made from a hybrid between natural Brassica napus (A(n)A(n)C(n)C(n)) and a synthetic rapeseed. To construct the synthetic Brassica napus, hexaploid plants (2n = 54, A(r)A(r)B(c)B(c)C(c)C(c)) were first obtained through chromosome doubling of trigenomic hybrids (2n = 27, A(r)B(c)C(c)) between Brassica carinata (2n = 34) and B. rapa (2n = 20). Pentaploid hybrids (2n = 46, 46, A(r)A(n)B(c)C(c)C(n)) were then produced by crossing the hexaploid with the pollen of natural B. napus (2n = 38). Chromosomes with dual and single B-c genomes were observed in somatic cells of hexaploid and pentaploid plants. About 80% of pollen mother cells of pentaploid hybrids had 19 or more bivalents, indicating that the bivalents from A(r)/ A(n) and C-c/C-n chromosomes were normally formed. The occurrence of trivalents and quadrivalents at diakinesis suggested that B-c, A(n) and A(r) or B-c, C-n and C-c homologous pairing and exchange might happen. The variable number of laggards, 3 and 4 in most cases, were observed in the majority of PMCs at anaphase. Results from genomic in situ hybridization showed that the laggards belonged mainly to the B-c genome, suggesting that the B-c genome could be eliminated in the gametes of pentaploid hybrids. 16.15% of seeds derived from self-pollinated pentaploids have 38 chromosomes, and 90% of 38-chromosome seeds were completely excluded B-c genome. The cytological results of this experiment suggested that it is possible to obtain new materials with genome composition of A(r)A(r)C(c)C(c) for rapeseed breeding.