Phenotypic advantages and improved genomic stability following selection in advanced selfing-generations of Brassica allohexaploids

Allopolyploids often exhibit advantages in vigor and adaptability compared to diploids. A long-term goal in the economically important Brassica genus has been to develop a new allohexaploid crop type (AABBCC) by combining different diploid and allotetraploid crop species. However, early-generation allohexaploids often face challenges like unstable meiosis and low fertility, and the phenotypic performance of these synthetic lines has rarely been assessed. This study analyzes agronomic traits, fertility, and genome stability in A^rA^rB^cB^cC^cC^c lines derived from four crosses between B. carinata and B. rapa after 9-11 selfing generations. Our results demonstrate polyploid advantage in vigor and seed traits, considerable phenotypic variation, and high fertility and genome stability. Meanwhile, parental genotypes significantly influence outcomes in advanced allohexaploids. Structural variants, largely resulting from A-C homoeologous exchanges, contribute to genomic variation and influence hexaploid genome stability, with the A sub-genome showing the highest variability. Both positive and negative impacts of SVs on fertility and seed weight are observed. Pseudo-euploids, frequently appearing, do not significantly affect fertility or other agronomic traits compared to euploids, indicating a potential pathway toward a stable allohexaploid species. These findings provide insights into the challenge and potential for developing an adaptable and stable Brassica hexaploid through selection.