Mitochondrial codon usage bias and novel phylogenetic insights: implications for taxonomic reevaluation of seven Xylotini species (Diptera, Syrphidae, Eristalinae).

BACKGROUNDS: Flies of the tribe Xylotini (Insecta, Diptera, Eristalinae) primarily inhabit dense forests or are frequently observed visiting flowers. Although many genera within this tribe are easily distinguishable and exhibit distinct characteristics, the scarcity of mitochondrial genome resources has limited phylogenetic research. This study addresses this gap by analyzing the mitochondrial genomes of seven newly sequenced species to investigate the tribe's phylogeny and codon usage patterns. RESULTS: All mitochondrial genomes exhibited typical Syrphid structure (37 genes, 15,663 - 16,341 bp) with conserved gene order and strong AT bias (78.56% overall, 93.21% at third codon positions). Codon usage analysis revealed differential evolutionary pressures: Brachypalpoides makiana and Chalcosyrphus curvaria (R² = 0.34-0.56) showed mutation-dominated bias, while other species displayed selection-driven patterns. ENC values (25.6-45.23) and RSCU analysis identified 14 preferred A/U-ending codons, notably UUA-Leu (RSCU = 3.20) and AGA-Arg (RSCU = 2.59). Phylogenetic reconstruction challenged the tribe's monophyly, with Brachypalpoides and Xylota forming a well-supported clade (BS = 100%), while Milesia appeared polyphyletic. Additionally, C. curvaria showed closer affinity to M. ferruginosa than congenerics. CONCLUSIONS: These findings provide novel insights into: (1) the dual influence of mutational pressure and natural selection on codon evolution, and (2) systematic relationships within this ecologically diverse dipteran group. The study establishes molecular foundations for re-evaluating Xylotini taxonomy and understanding mitochondrial genome evolution in hoverflies.