Proposal (135) to South American Classification Committee
Reassign the South American species of "Agelaius" to other genera
Background: Five species (icterocephalus, ruficapillus, cyanopus, xanthophthalmus, and thilius) of blackbirds (Icteridae) long have been assigned to Agelaius. Recent phylogenetic studies, based on analysis of mitochondrial DNA sequences (cytochrome b and ND2), indicate that Agelaius is polyphyletic (Lanyon 1994, Lanyon and Omland 1999, Johnson and Lanyon 1999). Monophyly of Agelaius also was questioned by Freeman and Zink (1995), based on a phylogenetic analysis of restriction enzyme cleavage sites of total mitochondrial DNA. Freeman and Zink included only two species of Agelaius in their study, however, whereas the Lanyon lab has much better taxon sampling (including representatives of all South American species). We currently treat all of the South American species in Chrysomus, by implication restricting Agelaius to North American taxa, but that still leaves a problem of paraphyly within Chrysomus (see below).
New information: Recently Lowther et al. (2004) assessed the taxonomic implications of the DNA sequence data. I'll summarize their arguments briefly; anyone interested in a fuller discussion may contact TSS or JVR for a pdf of the full paper.
Agelaius consists of three clades. North American and Antillean taxa are one clade, and the name Agelaius belongs to this group. The South American species consist of two different clades, one comprising icterocephalus + ruficapillus, and the other xanthophthalmus + cyanopus + thilius.
Analysis: One could combine the five South American species into a single genus (as we currently do, Chrysomus), since the two clades together are part of a monophyletic group; but if the taxonomy were to reflect the proposed phylogeny then Chrysomus would have to be expanded to include Xanthopsar, Pseudoleistes, and Agelaioides. Therefore, to retain familiar generic names (Xanthopsar etc.), Lowther et al. recommended the resurrection of two long-suppressed generic names. Species icterocephalus + ruficapillus would continue to be assigned to Chrysomus, and species xanthophthalmus + cyanopus + thilius would be assigned to Agelasticus.
Recommendation: I vote YES on this proposal. We have known for a "long time" (a decade) that Agelaius was a polyphyletic genus and that some kind of a nomenclatural revision was called for. Finally Peter Lowther worked up the energy to research this and propose a decent solution, part of which Dickinson (2003) already adopted, i.e., recognition of Chrysomus as distinct from Agelaius. That still leaves our current Chrysomus paraphyletic; following Lowther et al. would remedy that.
Freeman, S., and R. M. Zink. 1995. A phylogenetic study of the blackbirds based on variation in mitochondrial DNA restriction sites. Systematic Biology 44: 409-420.
Johnson, K. P. & Lanyon, S. M. 1999. Molecular systematics of the grackles and allies, and the effect of additional sequence (cyt b and ND2). Auk 116: 759-768.
Lanyon, S. M. 1994. Polyphyly of the blackbird genus Agelaius and the importance of assumptions of monophyly in comparative studies. Evolution 48: 679-693.
Lanyon, S. M. & Omland, K. E. 1999. A molecular phylogeny of the blackbirds (Icteridae): Five lineages revealed by cytochrome-b sequence data. Auk 116: 629-639.
Lowther, P. E., R. Fraga, T. S. Schulenberg, and S. M. Lanyon. 2004. Nomenclatural solution for a polyphyletic Agelaius. Bulletin of the British Ornithologists' Club 124: 171-177.
Tom Schulenberg, October 2004
Comments from Remsen: "YES. Data are solid on phylogenetic relationships among these birds, and to maintain monophyletic genera requires a break-up of old Agelaius (which we already do) and now also Chrysomus. The alternative, to move Pseudoleistes and Agelaioides into a single genus to retain a monophyletic Chrysomus, stretches my notion of what a genus 'should be'."
Comments from Robbins: "I vote "YES" on proposal #135 to reassign South American species of the genus Agelaius to other genera. The genetic data clearly support the separation of the South American group as separate from North American and Antillean Agelaius, and Lowther et al. offer a good solution of resurrecting two old names. Their proposal seems more sensible than including such seemingly very different genera as Xanthopsar, Pseudoleistes, and Agelaioides into a catchall Chrysomus."
Comments from Zimmer: "I vote "YES" for reasons already outlined by others."
Comments from Stiles: "YES. Resurrecting Agelasticus to avoid a polyphyletic Chrysomus, as the genetic evidence seems solid."
Comments from Pacheco: "YES. O arranjo sugerido por Lowther e colegas é compatível com as evidências até o momento reunidas."
Comments from Jaramillo: "YES. This is the appropriate route to go, lumping of disparate genera into Chrysomus makes no sense, but dividing Chrysomus is a good course of action. I do recall that thilius is perhaps not that closely related to xanthophthalmus/cyanopus; however splitting it off on its own is not a good solution. Now someone needs to go out and study the relationships of the taxa within xanthophthalmus and cyanopus, I bet that cyanopus is comprised of more than one species level taxon."
Comments from Nores: "YES, aunque no demasiado convencido."