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.
Literature Cited
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
_____________________________________________________________________________________
Voting chart for SACC proposals 100-218
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."