Biased allelic expression in tissues of F1 hybrids between tropical and temperate lotus (Nelumbo nuicfera)

The genome-wide allele-specific expression in F1 hybrids from the cross of tropical and temperate lotus unveils how cis-regulatory divergences affect genes in key pathways related to ecotypic divergence. Genetic variation, particularly cis-regulatory variation, plays a crucial role in phenotypic variation and adaptive evolution in plants. Temperate and tropical lotus, the two ecotypes of Nelumbo nucifera, show distinction in the degree of rhizome enlargement, which is associated with winter dormancy. To understand the roles of genome-wide cis-regulatory divergences on adaptive evolution of temperate and tropical lotus (Nelumbo nucifera), here we performed allele-specific expression (ASE) analyses on the tissues including flowers, leaves and rhizome from F1 hybrids of tropical and temperate lotus. For all investigated tissues in F1s, about 36% of genes showed ASE and about 3% of genes showed strong consistent ASE. Most of ASEs were biased towards the tropical parent in all surveyed samples, indicating that the tropical genome might be dominant over the temperate genome in gene expression of tissues from their F1 hybrids. We found that promoter sequences with similar allelic expression are more conserved than genes with significant or conditional ASE, suggesting the cis-regulatory sequence divergence underlie the allelic expression bias. We further uncovered biased genes being related to phenotypic differentiation between two lotus ecotypes, especially metabolic and phytohormone-related pathways in the rhizome. Overall, our study provides a global landscape of cis-regulatory variations between two lotus ecotypes and highlights their roles in rhizome growth variation for the climatic adaptation.