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 C(c) from B. carinata (B(c)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, 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.