Oral Paper

         Hybrids and Hybridization

Expression response to homoeologous exchange in resynthesized Brassica napus reveals signs of genomic imbalance and relative dosage constraint

Presenting Author
Kevin Bird
Description
The Gene Balance Hypothesis (GBH) proposes that selection acts on gene dosage (copy number) to maintain balanced stoichiometry of interacting proteins (relative gene dosage). Perturbing this stoichiometry results in genomic imbalance and reduced fitness. This selection is further hypothesized to constrain expression responses to dosage changes by making dosage-sensitive genes (those encoding members of interacting proteins) from the same network experience more similar expression changes. In allopolyploids species, where WGD involves hybridization of evolutionarily diverged lineages, there are often also homoeologous exchanges (HEs) that recombine, duplicate, and delete homoeologous regions of the genome and alter total expression of homoeologous gene pairs. Although the GBH makes predictions about the expression response to HEs, they have not been empirically tested. We used paired genomic and transcriptomic data from six independently resynthesized, isogenic Brassica napus lines over ten generations to identify HEs, analyze expression responses, and test for patterns of genomic imbalance and relative dosage constraint. Groups of dosage-sensitive genes (GO terms) had less variable expression responses to HEs than dosage-insensitive genes, a sign of relative dosage constraint. This sign of constraint was absent for homoeologous pairs whose expression was biased toward the BnA subgenome. Finally, the expression response to HEs was more variable than the response to polyploidy, suggesting that HEs produce genomic imbalance. These findings expand our knowledge of the impact of genomic imbalance and relative dosage constraint on genome evolution and potentially connect patterns in polyploid genomes over time; from homoeolog expression bias to duplicate gene retention.