Convergent evolution in subterranean Coleoptera
Is evolution predictable? The widespread occurrence of evolutionary convergence suggests that this may be the case, and that when confronted with similar circumstances species tend to adopt similar solutions. There is, however, the possibility that this convergence is due to the power of natural selection to drive species to a common optimal design, or to inner development or functional constraints limiting the number of potentially viable phenotypes. Subterranean species are a particularly well suited system to address this question, as many of them show a very similar phenotype that has evolved multiple times independently. This phenotype -what is called "troglomorphy"- includes the reduction or absence of eyes, loss of pigments and of the impermeability of the cuticle, elongated body and appendages and modifications in the sensory organs, among other characters.
We will study evolutionary convergence in two main groups of Coleoptera with subterranean species: 1) tribe Leptodirini (family Leiodidae, ca. 900 species), with a focus on the Pyrenean Speonomus lineage (ca. 200 species+subspecies), and 2) three lineages within tribe Trechini (family Carabidae): the Pyrenean Aphaenops lineage (ca. 120 species+subspecies) and the Trechus fulvus (33 species) and Apoduvalius (ca. 40 species) groups. With the only exception of the Aphaenops lineage, in which all species are subterranean, in all other lineages there are species ranging from epigean environments (usually forest litter), without troglomorphic characters, to the deep subterranean environment, usually highly troglomorphic. For each of the lineages we will build comprehensive molecular phylogenies with 6 mitochondrial and 2 nuclear genes. There are partial phylogenies available, published by members of the project team, and additional material is available in our collections or will be obtained in the field.
We will search for evolutionary convergence in five character systems, each of them in at least two different lineages that have independently colonised the deep subterranean environment, and in at least three species with different degrees of troglomorphy. Character systems will include: 1) body shape and length of antennae and hind legs; 2) eye development; 3) external morphology of the antennal sensory structures (as observed with SEM); 4) antennal chemoreceptors, as sequenced in transcriptomes (obtained with Illumina RNA-Seq); and 5) composition of cuticle hydrocarbons. To study the evolution of antennal chemoreceptors we will obtain transcriptomes from seven species with different degrees of troglomorphy in two lineages of Leptodirini, one transcriptome from the antennae and another from the rest of the body. All data will be analysed in a phylogenetic context using comparative methods.
Our results will allow to document the extent and nature of evolutionary convergence in some lineages of subterranean beetles, determining to what degree natural selection was the driver of the extreme modifications observed in some cave organisms. We also expect that our project may document the existence of evolutionary constraints channelling the evolution of some characters under certain environmental circumstances, ultimately providing evidence for the predictability of evolution