Loss of the IR region in conifer plastomes: Changes in the selection pressure and substitution rate of protein-coding genes.

Plastid genomes (plastomes) have a quadripartite structure, but some species have drastically reduced or lost inverted repeat (IR) regions. IR regions are important for genome stability and the evolution rate. In the evolutionary process of gymnosperms, the typical IRs of conifers were lost, possibly affecting the evolutionary rate and selection pressure of genomic protein-coding genes. In this study, we selected 78 gymnosperm species (51 genera, 13 families) for evolutionary analysis. The selection pressure analysis results showed that negative selection effects were detected in all 50 common genes. Among them, six genes in conifers had higher values than non-conifers, and 12 genes had lower values. The evolutionary rate analysis results showed that 9 of 50 common genes differed between conifers and non-conifers. It is more obvious that in non-conifers, the rates of (, , , , , and ) were 2.6- to 3.1-fold of conifers. In conifers, , , , , and of were 1.2- to 3.6-fold of non-conifers. In addition, the evolution rate of in the IR was significantly reduced. is undergoing dynamic change, with an abnormally high evolution rate as a small portion of it enters the IR region. Although conifers have lost the typical IR regions, we detected no change in the substitution rate or selection pressure of most protein-coding genes due to gene function, plant habitat, or newly acquired IRs.