Mitochondrial genome evolution of the hemiparasitic plant Taxillus: gene loss, selective constraints, and structural variation.

BACKGROUND: Parasitic and hemiparasitic plants exhibit highly variable mitochondrial genome structures and gene content, but the evolutionary processes underlying this variation remain incompletely understood. This study aimed to characterize the structural features, gene composition, and evolutionary characteristics of the complete mitochondrial genome of Taxillus chinensis. RESULTS: The mitochondrial genome of T. chinensis exhibits a complex multipartite structure composed of multiple linear and circular DNA molecules. Fragmented synteny and heterogeneous genomic organization indicate substantial structural complexity. The genome contains abundant dispersed and palindromic repeats, including long repeat units, contributing to the complex repetitive landscape of the mitogenome. The mitogenome encodes 27 protein-coding genes and three rRNA genes, but only three tRNA genes, indicating substantial reduction of mitochondrial tRNA genes. All rpl genes are absent, indicating a substantial reduction of translation-related components. Homology searches against the nuclear genome and mitochondrial contigs identified only fragmented rpl-related sequences, which likely represent nonfunctional pseudogene remnants rather than intact functional genes. Ka/Ks analyses show that most mitochondrial protein-coding genes remain under strong purifying selection, although evolutionary rate heterogeneity is observed among functional categories. CONCLUSION: The mitochondrial genome of T. chinensis exhibits pronounced structural complexity and extensive loss of translation-related genes, while most retained mitochondrial protein-coding genes remain under strong purifying selection. These findings provide new insights into the structural evolution, functional reduction, and evolutionary constraints of mitochondrial genomes in hemiparasitic plants.