Chloroplast genome comparative analysis and phylogenetic relationships of 15 Syringa species (Oleaceae)

Syringa is a crucial shrub genus in the family Oleaceae, which has significant ornamental, economic, and medicinal value. However, research on the chloroplast genome (CPG) phylogeny and lineage diversification of this genus remains limited. In this study, all 15 Syringa CPGs exhibited a characteristic quadripartite structure, with genome lengths ranging from 154,019-158,020 bp. These CPGs were highly conserved and moderately differentiated, each containing 130-132 genes. Analysis of inverted repeat (IR) boundaries indicated structural conservation, with six genes: rps19, rpl2, ycf1, trnN, ndhF, and trnH present at the IR/single-copy (SC) junctions. The small single copy (SSC) region displayed greater sequence variability than the IR regions. ycf1, ndhH, trnL-rpl32, ndhF-ycf1, and rbcL-accD were identified as potential molecular markers and rps11, ycf2, and ycf4 may have contributed to the adaptive evolution of Syringa. Phylogenetic reconstruction based on whole CPG data supported the monophyly of the 15 species, which were divided into three distinct subclades. Molecular dating estimated that Syringa diverged from its sister genus approximately 58 million years ago, with most Syringa species diversifying further approximately 47.49 million years ago during the Eocene. Our findings will hopefully stimulate further studies on this genus that may enhance biodiversity knowledge.