The phylogeny of arthropods has always been messy. One reason is that studies trying to discern their evolutionary relationships often use too few taxa (this is, after all, the most species-rich of all animal groups), and, especially, too few genes. Conclusions have been based, for example, on only 18S and 28S rRNA and mtDNA (the latter is, of course, effectively one gene). And this has led to conflicting conclusions, some of which contravene morphologically-based systematics.
For example, morphology seems to define a group called the “mandibulata”: all those arthropods that have mandibles. But some molecular work has lumped the myriapods (centipedes and millipedes), which have mandibles, together with the chelicerates, which don’t have mandibles but a nonhomologous biting structure called chelicerae. (Chelicerates include spiders, horseshoe crabs, pycnogonids, and the like; see Fig. 1 below).
Now, a new paper in Nature by Regier et al. has come up with a near-definitive family tree of arthropods that resolves many of the questions that arose from studies using lesser resolution. In their work, Regier et al. used many arthropod species (75, to be exact), and 62 single-copy genes that were orthologous in species from flies to humans. This is a huge genetic sample, allowing for a good, well-supported tree based on 41 kilobases of DNA sequence.
We needn’t go into the messy details, but there are three quite important findings.
1. Insects (“Hexapoda”) are not a sister group of crustaceans, as was indicated by some molecular studies. Nor are they the sister group of myriapods, the traditional arrangement supported by morphology. Instead, insects are nested within crustaceans (see Figure 1). In the same sense that birds are dinosaurs, then, insects are crustaceans.
2. The sister group of insects within crustaceans comprises two rather obscure taxa that were long thought to be primitive: the cephalocarids and the remipedes. Regier et al., however, find these two groups (see below) belong to the monophyletic clade called Xenocarida. Their “primitiveness” is thus deceptive. Now the fact that some crustaceans, like the Xenocarida, are more closely related to insects than to other crustaceans means that the group “Crustacea” is paraphyletic, since, by not including insects, it doesn’t include all descendants of the common ancestor. If we want to be punctilious taxonomists, we’d have to dump the name “Crustacea”, or else reclassify insects as crustaceans.
3. The Mandibulata, mandible-carrying arthropods, is now confirmed as a real monophyletic group, since the myriapods now join all the other mandible-bearing beasts, as they should.
Maybe this seems arcane to those who aren’t interested in arthropods, but it really does seem to settle long-standing questions about where the insects came from. And it sets the standard for the numbers of species and genes that should be included in a good phylogenetic analysis.
Figure 1 (from Regier et al.). The phylogeny of arthropods.
Fig. 2. Speleonectes tulumensis, a marine crustacean, a remipede, and a member of Xenocarida, the sister group of insects
Fig. 3. A cephalocarid, also a member of the Xenocarida
h/t: Phil Ward, Cliff Cunningham
Regier, J. C. J. W. Shultz, A. Zwick, A. Hussey, B. Ball, R. Wetzer, J. W. Martin and C. W. Cunningham. 2010. Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences. Nature 463:1079-1083.