Evolution of isoform-level gene expression patterns across tissues during lotus species divergence

Both gene duplication and alternative splicing (AS) drive the functional diversity of gene products in plants, yet the relative contributions of the two key mechanisms to the evolution of gene function are largely unclear. Here, we studied AS in two closely related lotus plants, Nelumbo lutea and Nelumbo nucifera, and the outgroup Arabidopsis thaliana, for both single-copy and duplicated genes. We show that most splicing events evolved rapidly between orthologs and that the origin of lineage-specific splice variants or isoforms contributed to gene functional changes during species divergence within Nelumbo. Single-copy genes contain more isoforms, have more AS events conserved across species, and show more complex tissue-dependent expression patterns than their duplicated counterparts. This suggests that expression divergence through isoforms is a mechanism to extend the expression breadth of genes with low copy numbers. As compared to isoforms of local, small-scale duplicates, isoforms of whole-genome duplicates are less conserved and display a less conserved tissue bias, pointing towards their contribution to subfunctionalization. Through comparative analysis of isoform expression networks, we identified orthologous genes of which the expression of at least some of their isoforms displays a conserved tissue bias across species, indicating a strong selection pressure for maintaining a stable expression pattern of these isoforms. Overall, our study shows that both AS and gene duplication contributed to the diversity of gene function during the evolution of lotus.

Automated summary

This study reveals the relative contributions of alternative splicing and gene duplication to the functional diversity of genes in lotus. Alternative splicing events have rapidly evolved between orthologs and lineage-specific splice variants play a role in gene functional changes during species divergence. Single-copy genes contain more isoforms with a higher conservation of alternative splicing events across species and more complex tissue-dependent expression patterns. In contrast, isoforms of whole-genome duplicates are less conserved and display a less conserved tissue bias, suggesting their contribution to subfunctionalization. Through comparative analysis of isoform expression networks, orthologous genes with conserved tissue bias in isoform expression across species are identified, indicating a strong selection pressure for maintaining a stable expression pattern of these isoforms.