The top-tier journal Nature doesn’t often deal in purely phylogenetic research. So when such a study graces their pages we know it’s big stuff.
Yesterday, Nature published a 62 gene, 75 species analysis of the evolutionary history of the arthropods. Arthropods, as readers of this blog likely know, are animals with a chitinous exoskeleton and jointed legs. They include the insects, arachnids, crustaceans, centipedes, and others. This is a staggeringly diverse group, and one found just about everywhere on the planet. Most animals are arthropods.
This study has been in the works for many years. Jerry Regier’s lab at the University of Maryland has been diligently developing protocols for extracting single-copy nuclear DNA from across the arthropods, and the work has paid off handsomely. They have created the largest and most relevant data set yet assembled for addressing the hard questions in arthropod evolution. This is exciting! Today is like Christmas for arthropod systematists.
There’s a lot to digest here, but below are my first impressions:
- Yes, insects really are crustaceans. This result emerged in every single analysis and upholds earlier molecular findings that the hexapods (insects, collembolans, diplurans) are just a single lineage of crustaceans that left the ocean for a more terrestrial lifestyle.
- The sister-group to hexapods is a surprise: the remipedes Xenocarida. Wow. I’ll let Carl Zimmer explain why this is cool. Note, though, that this relationship was only found in the nucleotide data. Analyses using amino acid sequences were less convincing, so I’d still treat this result with some caution.
- Within hexapods, at least, the morphologists are vindicated. Earlier molecular studies from mitochondrial DNA and ribosomal DNA had found some unconventional relationships- removing springtails from the hexapods, for instance- but this larger and more appropriate data set is basically textbook in its findings. Funny how we spend hundreds of thousands of dollars and ten years of research to confirm that the crusty old taxonomists that none of the cool kids wanted to listen to had been right all along. Hexapods are a clade, bristletails are sister to the remaining insects, and silverfish are sister to the Pterygotes.
- Some of the difficult relationships within the hexapods remain difficult. There’s a hint that Diplura and Collembola are sister, but that doesn’t hold in all analyses. And Paleoptera (Mayflies + Dragonflies) do form a clade, but support is still low.
- The myriapods (centipedes + millipedes) changed position again- they’re now sister to the pancrustacea. Earlier results had them with the chelicerates. I don’t have much to say here. Nor can I say much about the Crustacea, as that’s a group I do not know particularly well.
- Pity the chelicerate systematists. Sixty two genes and the internal relationships of this important group (spiders, scorpions, mites, etc) remain an unresolved mush. It seems the modern lineages arose very quickly in time.
source: Regier et al 2010. Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences. Nature doi:10.1038/nature08742