# Integrative Taxonomy Clarifies the Taxonomic Status of the Morphologically Intermediate Form Between Tropidothorax cruciger and T. sinensis (Hemiptera: Lygaeidae)

## TL;DR

This study uses combined data to clarify the taxonomic status of an intermediate form between two insect species, Tropidothorax cruciger and T. sinensis.

## Contribution

The study resolves the taxonomic ambiguity of an intermediate form using integrative taxonomy methods including molecular and ecological data.

## Key findings

- The intermediate form is classified as a morphological variant of T. cruciger.
- A clear genetic barcode gap was found between T. cruciger and T. sinensis.
- Ecological analyses support the intermediate form and T. cruciger as a single species.

## Abstract

The presence of the “intermediate form” between Tropidothorax cruciger and T. sinensis has complicated the identification of these two species. In this study, we performed an integrative taxonomy combining morphological features with molecular and ecological data. The taxonomic status of the “intermediate form” and the species boundaries between T. cruciger and T. sinensis were clarified. The “intermediate form” should be recognized as a morphological variant of T. cruciger. A clear barcode gap was found between interspecific and intraspecific genetic distances of T. cruciger and T. sinensis. Our study provides a robust framework for the accurate delimitation of these two species.

(1) Background: The identification of Tropidothorax cruciger and T. sinensis is often complicated by the presence of the “intermediate form”. Due to the lack of molecular data, the taxonomic status of the “intermediate form” and the species boundaries between T. cruciger and T. sinensis remain uncertain; (2) Methods: In this study, we integrated morphological, molecular, and ecological data to delimit species boundaries of these two species using multiple species delimitation approaches; (3) Results: Most species delimitation analyses based on the cytochrome c oxidase subunit I (COI) fragment suggested that T. cruciger and the “intermediate form” comprised a single species, with T. sinensis representing a separate species. This delimitation result was also supported by the analyses of BFD* and genetic clustering based on genome-wide SNPs. Under this species delimitation scenario, a clear-cut barcode gap was discovered between the interspecific and intraspecific genetic distances. In addition, environmental-related analyses showed highly similar ecological requirements of T. cruciger and the “intermediate form”, supporting their recognition as a single species; (4) Conclusions: This study clarifies the taxonomic status of the “intermediate form” and the species boundaries between T. cruciger and T. sinensis, which is essential for further studies of ecology and evolution of these species.