Evolution of morphological crypsis in the Tetramorium caespitum ant species complex (Hymenoptera: Formicidae)

Herbert C Wagner,Florian M Steiner,Alexander Gamisch,Wolfgang Arthofer,Karl Moder,Birgit C Schlick-Steiner

doi:10.1038/s41598-018-30890-z

Abstract

Cryptic species are morphologically very similar to each other. To what extent stasis or convergence causes crypsis and whether ecology influences the evolution of crypsis has remained unclear. The Tetramorium caespitum complex is one of the most intricate examples of cryptic species in ants. Here, we test three hypotheses concerning the evolution of its crypsis: H1: The complex is monophyletic. H2: Morphology resulted from evolutionary stasis. H3: Ecology and morphology evolved concertedly. We confirmed (H1) monophyly of the complex; (H2) a positive relation between morphological and phylogenetic distances, which indicates a very slow loss of similarity over time and thus stasis; and (H3) a positive relation between only one morphological character and a proxy of the ecological niche, which indicates concerted evolution of these two characters, as well as a negative relation between p-values of correct species identification and altitude, which suggests that species occurring in higher altitudes are more cryptic. Our data suggest that species-specific morphological adaptations to the ecological niche are exceptions in the complex, and we consider the worker morphology in this complex as an adaptive solution for various environments.

Highlights

The family of ants contains many cryptic species[5,22]
Using the same set of species as for those mitochondrial DNA (mtDNA) phylogenies, we reconstruct a SNAPP35 phylogeny based on nuclear data to decide if the nuclear-DNA based tree is in line with the morphological species-complex definition and mtDNA is unsuitable to resolve the species complex’ topology with respect to other species of the T. caespitum group
The relationship of the T. caespitum complex with other species supports its monophyly with respect to the selected species outside the complex, amplified fragment-length polymorphism (AFLP) data are insufficient to reconstruct any topology within the complex, so that not a single sister relationship within the complex is known

Summary

Introduction

The family of ants contains many cryptic species[5,22]. The genus Tetramorium comprises more than 500 species worldwide[23]. Throughout Europe, the species do consistently differ in mitochondrial DNA, nuclear DNA, selected characters of worker morphology, male genitals, and ecology[25]. A positive relation between morphological and phylogenetic distances within the complex would state that morphological similarity is lost over evolutionary time, even if very slowly and even though the species are currently very similar. In contrast to concerted evolution, a negative relation between morphological and ecological distances might indicate character displacement[20,21], which describes the avoidance of competition by utilizing different micro-ecological niches in sympatry, eventually leading to evolution of differences in morphology even if minor. To test H3, possible relations between morphological distance and several ecological parameters, that is, climatic variables, soil variables, altitude, and latitude, are investigated in this study by performing linear regressions and Partial Mantel tests

Methods

Discussion

Conclusion