Orchesella cincta strain:natural population sampled at Roggebotzand Transcriptome or Gene expression
Collembola (springtails) represent a soil-living lineage of hexapods in between insects and crustaceans, so their genomes may hold key information on the early processes leading to evolution of Hexapoda from a crustacean ancestor. We assembled and annotated transcriptomes of the Collembola Folsomia candida and Orchesella cincta, and compared them to protein sequences of three crustaceans and three insects to identify adaptive signatures associated with the evolution of hexapods within the pancrustacean clade. Assembly of the springtail transcriptomes resulted in 37,730 predicted open reading frames for F. candida and 32,154 for O. cincta, of which 34.2% were functionally annotated for F. candida and 38.4% for O. cincta. Subsequently, we predicted orthologs clusters between eight species and we applied branch-site test to detect episodic positive selection in the Hexapoda and Collembola lineages. A subset of 250 genes showed significant positive selection along the Hexapoda branch and 57 in the Collembola lineage. Gene Ontology analysis revealed that these genes are mostly involved in stress response (i.e. DNA repair, immune response), ion transport, ATP metabolism, regulation and development-related processes (i.e. eye development, neurological development).  We suggest that the identified gene families represent processes that have played a key role in the divergence of hexapods within the pancrustacean clade that eventually evolved into the most species-rich group of all animals, the hexapods. Furthermore, some adaptive signatures in collembolans may provide valuable clues to understand evolution of hexapods on land.