# The first complete mitochondrial genome sequences of an ancient orphan evolutionary lineage of Eumolpinae leaf beetles endemic to the South Pacific

## TL;DR

This study reports the first complete mitochondrial genomes of three leaf beetle species from an ancient lineage in the South Pacific, enhancing our understanding of insect evolution.

## Contribution

The study provides the first complete mitogenomes from an orphan evolutionary lineage of Eumolpinae leaf beetles, enriching phylogenetic data for this poorly represented group.

## Key findings

- The mitogenomes retain ancestral gene order with a synapomorphic switch in trnA and trnR positions.
- Nucleotide composition and codon usage patterns suggest mutation pressure rather than selection as the main driver.
- Phylogenetic analysis supports prior classifications and highlights rapid diversification in Cryptocephalinae.

## Abstract

There are very limited mitogenome data representing most animal groups, particularly among the insects, which are otherwise extremely diverse and fulfill important ecological, sanitary, forest and agroeconomic roles. Increasing taxonomic diversity with new additions to the pool of mitogenomes for unrepresented evolutionary lineages is an opportunity to increase the phylogenetic power of mitogenome data, as well as refining our understanding of mitogenome diversity. Here, we characterize the complete mitogenomes of three species in two subgenera of Taophila leaf beetles, members of the hyperdiverse Eumolpinae, currently very poorly represented by mitogenomes, and the first representing an enigmatic independent evolutionary branch of the Eumolpini tribe, originated in the Cretaceous-Paleogene transition and endemic to the South Pacific.

These mitogenomes, assembled from genomic Illumina libraries, are relatively small (15,484–15,597 bp), retain the ancestral gene order of insect mitogenomes, except for a switch in the positions of trnA and trnR, a synapomorphic trait of the leaf beetle clade including Eumolpinae, Cryptocephalinae and Lamprosomatinae. Nucleotide composition, codon usage, relative synonymous codon usage and initiation and termination codons of protein-coding genes of these mitogenomes are all typical of insect mitogenomes, where nucleotide composition bias may be the result of mutation pressure, rather than natural selection. A mitogenome phylogeny of Eumolpinae, Cryptocephalinae and Lamprosomatinae revealed a strongly supported topology concordant with previous molecular systematic studies of these groups, including the identification of polytomies consistent with rapid early diversification of the Cryptocephalinae tribes and the separation of the main lineages within Eumolpini, one of them represented by Taophila.

New mitogenomes from previously undersampled taxa contribute to an important, collective effort to populate mitogenome databases with an increasingly dense representation of the Tree-of-Life. It is crucial that these efforts are driven and supported by consolidated taxonomic expertise to guarantee the quality of results derived from these data. The availability of South Pacific Eumolpinae in the public mitogenome pool increases the likelihood of finding their missing link, if any, with other Eumolpini.

The online version contains supplementary material available at 10.1186/s12864-025-12170-z.