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

Abstract

A synthetic Brassica napus rapeseed with genome composition of A^rA^rC^cC^c, made by combining A^r from B. rapa (A^rA^r) and C^c from B. carinata (B^cB^cC^cC^c), is valuable for making new genes available to breeders and gaining heterosis in crosses. An intergenomic hybrid AⁿA^rCⁿC^c was made from a hybrid between natural Brassica napus (AⁿAⁿCⁿCⁿ) and a synthetic rapeseed. To construct the synthetic Brassica napus, hexaploid plants (2n=54, A^rA^rB^cB^cC^cC^c) were first obtained through chromosome doubling of trigenomic hybrids (2n=27, A^rB^cC^c) between Brassica carinata (2n=34) and B. rapa (2n=20). Pentaploid hybrids (2n=46, A^rAⁿB^cC^cCⁿ) 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/Aⁿ and C^c/Cⁿ chromosomes were normally formed. The occurrence of trivalents and quadrivalents at diakinesis suggested that B^c, Aⁿ and A^r or B^c, Cⁿ 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 gametesof 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^rA^rC^cC^c for rapeseed breeding.