Systematic revision of Entomobryidae (Collembola) by integrating molecular and new morphological evidence

The North-Western Mediterranean basin is well known for its high number of relictual endemic taxa, and has been indicated as one of the world's major biodiversity hotspots at the species level. A possible contributing factor may be long-term persistence of populations and their prolonged stability. This study was designed to investigate the phylogeographic structure of three common species of the genus Lepidocyrtus (Hexapoda: Collembola), soil-dwelling arthropods characterized by limited dispersal capabilities and generally associated with forest habitats. We observed a remarkable geographic structure, with numerous deeply divergent genetic lineages occupying islands as well as mainland sites with no apparent gene flow among most sites, even across distances of only tens of kilometres. The reconstructed time frame for the evolution of these lineages suggests divergence between 5 and 15 Ma. This indicates a remarkably ancient origin and long-term persistence of individual lineages over a fine geographic scale despite the occurrence of abrupt sea level and climatic fluctuations in the area. This further suggests that currently recognized morphological species might be a serious underestimation of the true springtail biodiversity within this region.

Emergence from an aquatic environment to the land is one of the major evolutionary transitions within the arthropods. It is often considered that the first hexapods, and in particular the first springtails, were semi-aquatic and this assumption drives evolutionary models towards particular conclusions. To address the question of the ecological origin of the springtails, phylogenetic analyses by optimization alignment were performed on D1 and D2 regions of the 28S rDNA for 55 collembolan exemplars and eight outgroups. Relationships among the orders Symphypleona, Entomobryomorpha and Poduromorpha are inferred. More specifically, a robust hypothesis is provided for the subfamilial relationships within the order Poduromorpha. Contrary to previous statements, the semi-aquatic species Podura aquatica is not basal or 'primitive', but well nested in the Poduromorpha. The analyses performed for the 24 different weighting schemes yielded the same conclusion: semi-aquatic ecology is not ancestral for the springtails. It is a derived condition that evolved independently several times. The adaptation for semi-aquatic life is better interpreted as a step towards independence from land, rather than indication of an aquatic origin.