Assessing Loss of Regulatory Divergence, Genome–Transcriptome Incongruence, and Preferential Expression Switching in Abaca/ Banana Backcrosses

The Musa textilis var. Abuab has high fiber quality (FQ) but has low resistance against abaca bunchy top virus (AbBTV); the Musa balbisiana var. Pacol, has low FQ but high resistance against AbBTV. Their backcrosses (BC2 and BC3) possess both desirable traits. Analysis using RNA-seq showed that the regulatory divergence of Abuab and Pacol is largely explained by cis differences with 27.4% and 22.3% if we are to assess it using BC2 and BC3, respectively. Cis differences between the two genotypes are significantly reduced from BC2 to BC3 due to changes in genomic constitution. Trans, on the other hand, is robust to changes in allelic composition. All these are attributed to the loss of heterozygosity in the BC3 relative to BC2. Further analysis showed that both backcrosses exhibited genome-wide preferential expression of Pacol- over Abuab-specific alleles, despite the wider genetic presence of the latter in the hybrids. The ratio of the two genotype-specific expressed transcripts and the ratio of their corresponding genetic make-up are significantly disproportionate, a phenomenon which we refer here as “genome–transcriptome incongruence”. We also observed preferential expression switching in which several genes prefer Abuab- (or Pacol-) specific allele in the BC2 but switched to Pacol- (or Abuab-) specific allele in the BC3 genome.