The most ancient splits in insect phylogeny

To obtain insights into the early events in insect evolution it is crucial to unambiguously reconstruct the most ancient splits within the phylogenetic tree of this group. There are a few groups of extant, primarily wingless insects that are of special relevance for these questions. These so-called Entognatha comprise the three insect orders Diplura, Collembola and Protura, which are considered to be very ancestral for several reasons. For many years, scientists agreed upon a sistergroup relationship between Protura and Collembola. This was challenged by the first mitochondrial genome data, which suggested that the Collembola do not belong to insects but are a separate evolutionary lineage that branched much earlier than the separation of many crustaceans and insects. This unexpected view triggered a number of molecular investigations on several nuclear ribosomal gene sequences. All of these analyses suggested that insects are a monophylum that includes the Collembola. Remarkably, however, those investigations also supported a closer relationship between Protura and Diplura. The name "Nonoculata" was proposed for the resulting clade because both groups are restricted to blind animals living in the soil. Apart from the loss of eyes, however, little evidence is available to support such a sistergroup relationship. Considering this confusing situation, there is a high demand for a comparative, integrative approach in studying these animals, one based on both molecular and morphological data. This project focuses on the "Nonoculata" problem and attempts to clarify whether a well-founded phylogenetic sistergroup relationship is involved or an artificial clade based on a systematic error. The problem of most of the previous analyses is a very restricted number of investigated species, especially of animals that are hard to collect or difficult to determine. In particular, Protura are highly underrepresented or even missing, preventing any clear reconstruction of early character evolution in insects. Our project is designed to fill these gaps. The coordinated analysis of morphological and molecular data on the same set of species will furthermore guarantee an exact comparability of results, which is essential to decide among competing hypotheses. Knowledge about the early splits among these primarily wingless insects is a prerequisite for gaining insight into the evolutionary origin of the ground pattern or bauplan of insects. This, in turn, is the basis for understanding how a radiation unmatched in the history of life could lead to the most species-rich animal group on our planet.

The intensive efforts of the last years to elucidate the major relationships of animals produced a number of conflicting hypotheses. Especially hotly debated are the phylogenetic origin and the early splits of the insects or hexapods. Which are their closest relatives among arthropods and how are the relationships among the basal hexapod lineages traditionally termed the apterygotes? The investigation of these key issues in animal phylogeny necessitates a broad interdisciplinary approach at an analytical level which no longer can be accomplished by a single team. For this reason, the international initiative 1KITE (1K Insect Transcriptome Evolution) was started, which aims to infer robust phylogenetic backbone trees by the study of as many expressed genes as possible. Our research team was strongly involved in the conception and organization of this big research project and cooperates successfully with more than 80 scientists from 12 nations (http://www.1kite.org/). The first goal was the reconstruction of a backbone tree of all major lineages of insects and the resulting publication ranges among the most cited papers in the field. With respect to the origin of hexapods, the new hypothesis that crustaceans and not myriapods are the closest relatives among arthropods, was strongly supported. The elucidation of the early splits among hexapods proved to be a particularly difficult task. Our two phylogenomic analyses revealed remarkably contradictory results. The deeper analysis of these conflicts made our study a prime example to resolve the problems associated with the elucidation of very old splits in animal phylogeny in general. In conclusion, our comprehensive investigations represent a major step forward in a deeper understanding of insect phylogeny and provide crucial insights into the evolution of the most successful animal group on earth. The results are important in all fields of insect research, where interpretations are based on a broader evolutionary background. In the present project this was shown with the analysis of the eyes and light-sensitive proteins in basal hexapods.