# Characteristics and Evolutionary Relationships of Two Mitochondrial Genomes of Iguanodectes (Characiformes, Iguanodectidae)

## TL;DR

This study analyzes the mitochondrial genomes of two freshwater fish species to confirm their distinct evolutionary status and provide data for conservation.

## Contribution

The study provides the first complete mitochondrial genomes for Iguanodectes species and confirms their taxonomic distinction using molecular data.

## Key findings

- I. geisleri and I. adujai form a strongly supported monophyletic clade but are distinct evolutionary lineages.
- Genomic structure, base composition, and codon usage differ between the two species despite morphological similarities.
- The study confirms the taxonomic status of I. geisleri and I. adujai using mitochondrial genomic data.

## Abstract

The complete mitochondrial genomes of I. geisleri and I. adujai were obtained using high-throughput sequencing, assembly, and annotation. The mitochondrial genomes of both fish species exhibited typical structural characteristics of vertebrate mitochondria. Concurrently, species-specific differences were observed in genomic structures and codon usage. Further, the ML and BI phylogenetic analyses of a concatenated dataset of 13 PCGs demonstrated that I. geisleri and I. adujai form a strongly supported monophyletic clade, with the two species being sister taxa. However, they were also determined to constitute independent evolutionary lineages, confirming their taxonomic status as distinct species. This study supplements the fundamental mitochondrial genomic data for I. geisleri and I. adujai, provides a reliable basis for the molecular identification of species within the genus Iguanodectes, and offers scientific support for genetic evolutionary research on Characiformes fishes, the conservation of freshwater fish resources in South America, and precise species delineation in the ornamental fish trade.

Iguanodectes geisleri and I. adujai are freshwater fish from South America. Their taxonomic status and phylogenetic relationships are uncertain due to limited molecular data. High-throughput sequencing was applied to obtain and annotate for the first time the complete mitochondrial genomes of I. geisleri and I. adujai to clarify their phylogenetic positions. Mitochondrial genome sequences of 73 Characoidei species were retrieved from GenBank, with Gyrinocheilus aymonieri and Microphysogobio alticorpus designated as outgroups. Phylogenetic trees were constructed using a mitochondrial protein-coding gene dataset and Maximum Likelihood and Bayesian Inference methods. The complete mitochondrial genome measured 16,774 and 16,802 bp, respectively. Both genomes exhibited highly conserved structures. Despite morphological similarities and a close phylogenetic relationship, differences were detected in genomic structure, base composition, codon usage bias, and the control region between the two species. The two species comprise a strongly supported monophyletic clade and are sister species but represent distinct, independent branches. I. geisleri and I. adujai have been recognized as distinct species based on morphological differences, and this study provides molecular confirmation of their separate taxonomic status. The study provides molecular data for the taxonomic identification of fishes of the genus, Iguanodectes, and foundational mitochondrial genomic data for Characiformes. The study advances research on the genetic evolution of this group and resource conservation.