Evolution, diversification and ecology of a clade of strictly subterranean beetles (Troglocharinus, Family Leiodidae)

Abstract

The general aim of this dissertation is to investigate the evolution, the diversification and some ecological traits of the species of the strictly subterranean genus Troglocharinus Reitter, 1908.

The genus Troglocharinus (Tribe Leptodirini, Family Leiodidae, Fig. 4) currently includes 19 species and 19 subspecies, and it is considered to be highly adapted to the subterranean environment (truly troglobiont, sensu Sket 2008), living exclusively in the deepest part of the cave in total darkness and very constant temperature and humidity (Salgado et al., 2008).

Troglocharinus is distributed in two disjunct areas: some mountain systems south of the central area of the Pyrenees, and the coastal karstic formations between Tarragona and Barcelona, separated by extensive areas not suitable for subterranean species (sedimentary soils). Disjunct distributions like that of the genus Troglocharinus are very rare among the strictly subterranean genera, which tend to occupy a well-defined and restricted geographical region, usually within a continuous subterranean environment (Holsinger, 2005).

The aim of this Thesis is to study the evolution and diversification of this genus, and establish a temporal framework for its range expansion. We also aim to test its thermal tolerance with an experimental and an evolutionary approach, to understand its possible role in the current distribution of the species and their biogeographic history.

GENERAL DISCUSSION

This thesis aimed to understand the biogeographic and phylogeographic mechanisms underlying the evolution and the diversification of obligate cave fauna, using as model a clade of strictly subterranean beetles.

With this dissertation we contributed to challenge the traditional orthogenetic view on the origin and evolution of subterranean fauna, where hypogean organisms are classified as the senescence point of phyletic series prone to extinction. These ideas, strongly supported by Racovitza, Jeannel and Vandel during the XXth Centhury, still have a great impact on part of the biospeleological community (Romero 2009).

Recent molecular work has demonstrated that the concept of “phyletic series” has not a phylogenetic mean, and that these series are often poliphyletic groups with still uncertain taxonomy.

Subterranean beetles presents extreme morphological and physiological adaptations, and often their morphologies are very similar even among different groups, likely due to convergent adaptation. It is thus very difficult for morphologists to recognize phylogenetic relationships between species, and to assess their age and phylogenetic origin.

Cryptic lineages can be recognized when applying molecular techniques, and our results support the need for a synergy between morphological and molecular data in the important effort to discover and preserve the fragile and threatened subterranean biodiversity.

In this thesis the first data on thermal tolerance of strictly subterranean fauna is presented. The subterranean environment is a particular natural laboratory for testing evolutionary and ecological traits of their inhabitants, as it lacks many of the confounding factors present in epigean habitats.

It is interesting that, when testing their thermal tolerance limits, and in contrast with the traditional assumption that species fine-tuned their thermal tolerance to the temperatures of their habitats, we found no differences in thermal tolerance between species belonging to clades subjected to different temperature regimes since the Pliocene, suggesting a lack of adjustment to ambient temperature in these organisms. Our results also indicate that models based only on actual conditions and distribution data are insufficient to make predictions on the fate of subterranean species in front of global change scenarios.

In the last part of this dissertation we aimed to demonstrate the influence of the substratum on the dispersal capability and gene flow of cave obligate species, starting form the general principle that the habitat determined actual and historic demographic processes (Southwood 1977, 1988). This is one of the first comprehensive tests of the common view that geology of substratum represents the principal barrier to gene flow underground (e.g. Bellés 1973; Bellés & Martínez 1980).

El origen y la evolución de la fauna cavernícola ha sido tema de estudio y de debate desde los inicios de la biología evolutiva. La adaptación a un ambiente extremo como el subterráneo requiere cambios en múltiples aspectos (morfológico, fisiológico, ecológico...), y su estudio permite plantear cuestiones fundamentales sobre los procesos de adaptación y diversificación de los organismos.

El objetivo general de esta tesis es investigar la evolución, la diversificación y algunos rasgos ecológicos de las especies del género estrictamente subterráneo Troglocharinus Reitter, 1908.

El género Troglocharinus (Tribu Leptodirini, Familia Leiodidae) actualmente incluye 19 especies y 19 subespecies, y se considera altamente adaptado al medio subterráneo (truly troglobiont, sensu Sket 2008), o sea que vive exclusivamente en la parte más profunda de la cueva en la oscuridad total y con una temperatura y humedad muy constante (Salgado et al., 2008).

Análisis previos mostraron que este género está anidado dentro de un ciado de origen Oligocenico que incluye otros géneros subterráneo de Leptodirini de los Pirineos (el grupo Speonomus, Ribera et al. 2010)

El genero Troglocharinus se distribuye en dos áreas disjuntas: algunos sistemas montañosos al sur de la zona central de los Pirineos, y las formaciones kársticas costeras entre Tarragona y Barcelona, separadas por extensas áreas no colonizables por las especies subterráneas (suelos sedimentarios).

Distribuciones disjuntas como la de género Troglocharinus son muy raras entre los géneros estrictamente subterráneos, que tienden a ocupar una región geográfica bien definida y restringida, dentro de un ambiente subterráneo continuo (Holsinger, 2005).

El objetivo de esta tesis es el estudio de la evolución y diversificación de este género, y establecer un marco temporal para su expansión de rango. También tenemos como objetivo poner a prueba su tolerancia térmica con un aproche experimental y un enfoque evolutivo, para entender su posible papel en la distribución actual de la especie y su historia biogeográfica.