Ereful, N. C.; Alonday, R. S.; Lalusin, A. G.

The Philippines supplies ~85% of the global demand for abaca (Musa textilis Nee) fiber. A backcross (BC2) hybrid, named Bandala, was created by crossing Abuab, an abaca variety with high fiber quality, to Pacol, a wild banana (Musa balbisiana Colla) variety with resistance against Abaca Bunchy Top Virus (ABTV). To assess regulatory difference and expression heterosis, the parents (Abuab and Pacol) and their BC2 were sequenced using RNA-seq. Analysis of expression heterosis showed that a large number of transcripts exhibited non-additive (dominance and transgressive) mode, accounting for ~83.2% of the total heterotic genes. High-parent Abuab dominant genes in the BC2 were identified including genes encoding for cellulose synthase. Results indicated that the combined trans and cis + trans (synergistic interaction) largely explain the combined evolutionary divergence between the parents and repeated backcrossing -- selection procedures. Genes exhibiting compensatory interaction are significantly enriched under the transgressive mode of inheritance, contributing largely to the heterotic effect in the backcross under directional selection. Further statistical analysis suggests that regulatory differences strongly influence expression heterosis. We further fitted fixed line equations describing each mode of expression inheritance in a 3D plot. In our concurrent work, the BC2 demonstrated overdominance over its parents on two economically important phenotypic traits -- fiber length and tensile strength. This study demonstrated that phenotypic and expression heteroses are inherent even in backcrosses owing to the presence of the two alleles in the Bandala genome, providing insights into the genetic mechanisms underlying the heterotic performance of BC2 and offering valuable directions for abaca breeding.