Molecular systematics and biogeography of the Cockatoos (Psittaciformes: Cacatuidae)

Auk, The, Jan 1999 by Brown, David M, Toft, Catherine A

ABSTRACT.-We sequenced a 433-bp region of the mitochondrial 12S ribosomal subunit gene for 15 of the 18 recognized species of cockatoos and also examined previously published data on allozymes. Tests showed that the allozyme and mtDNA data have similar phylogenetic signals. The mtDNA phylogeny placed the Palm Cockatoo (Probosciger aterrimus) as the first extant cockatoo to split, followed by a subclade containing the black-cockatoos (Calyptorhynchus spp.), the Cockatiel (Nymphicus hollandicus), and the Gang-gang Cockatoo (Callocephalon fimbriatum); followed by the Galah (Eolophus roseicapillus) and Major Mitchell's Cockatoo (Cacatua leadbeateri), respectively; and finally followed by two subclades of "white" cockatoos: (1 ) the "corella" clade (Red-vented Cockatoo [Cacatua haematuropygia], Goffin's Cockatoo [C. goffini], Little Corella [C. sanguinea], Ducorps's Cockatoo [C. ducorps)); and (2) the "galerita" clade (Sulphur-crested Cockatoo [C. galerita], Salmon-crested Cockatoo [C. moluccensis], White Cockatoo [C. alba], Blue-eyed Cockatoo [C. ophthalmica], and Lesser Sulphur-crested Cockatoo [C. sulphurea]). If the mtDNA phylogeny accurately represents the evolutionary history of the cockatoos, then several of the phylogenetic problems within the group are resolved, including the positions and relationships of Nymphicus hollandicus, Callocephalon fimbriatum, Eolophus roseicapillus, and Cacatua leadbeateri. The mtDNA phylogeny supports some but not all of the nomenclature recently proposed for the Australian species. Biogeographic analysis of the mtDNA phylogeny supports the hypothesis that the cockatoos originated in Australia and that a combination of vicariant speciation and dispersal may have contributed to the diversification of the genus Cacatua in two separate radiations to the island regions of Indonesia, New Guinea, and the South Pacific. Received 30 July 1997, accepted 18 June 1998.

THE ORDER PSITTACIFORMES is a distinctive group of birds comprising approximately 350 species (Forshaw 1989). Several unique characteristics of the parrots suggest that they compose a monophyletic group, including a distinctive bill and a unique feather pigment (Smith 1975, Dixon 1994). Many classifications of the Psittaciformes have emerged since the classification by Linnaeus in 1758 (Sibley and Ahlquist 1990). Despite this long history, much uncertainty remains about how different groups of parrots are related to one another.

The cockatoos, family Cacatuidae, have long been recognized as a unique group within the Psittaciformes (Adams et al. 1984). Morphologically, cockatoos are distinguished from other parrots by having an erectile crest and lacking dyck texture in their feathers, which produces blue and green in the plumage of other parrots. Cockatoos possess a gall bladder, a non-pericyclic iris, paired patches of powder down in the lumbar region, and several other derived characters that separate them from other parrots (Adams et al. 1984). The monophyly of the cockatoos is also supported by karyological (Van Donzen and DeBoer 1984), allozyme (Adams et al. 1984), and DNA sequence data (Madsen et al. 1992).

The cockatoos have traditionally been organized into two major groups, the predominantly black Calyptorhynchini (Calyptorhynchus and Probosciger) and the predominantly white Cacatuini, including Cacatua (Adams et al. 1984). However, the phylogenetic branching of the various cockatoo groups (and thus their evolutionary relationships) remains unclear. Smith (1975) proposed evolutionary relationships (Fig. 1) for the five cockatoo genera that he recognized (Probosciger, Calyptorhynchus, Cacatua, Nymphicus, Callocephalon), but his hypothesis has not been tested.

A long-standing problem in cockatoo systematics is whether the monotypic Cockatiel (Nymphicus hollandicus) is a cockatoo, and if so, where it belongs within the cockatoo clade. Several workers have gathered morphological and behavioral evidence that the Cockatiel is a diminutive cockatoo (Adams et al. 1984). Others have placed the Cockatiel within other groups of parrots because of its indirect head scratching, auditory meatus, sequence of remigial molt, structure of pineal body, and wing spots in females and immatures. The allozyme evidence of Adams et al. (1984) indicates that the Cockatiel is a cockatoo (Fig. 2). This conclusion is supported by mtDNA (Ovenden et al. 1987), a tandem repeat in parrot nuclear DNA (Madsen et al. 1992, Dixon 1994), and egg shape (Saunders et al. 1984). Although this strong evidence places the Cockatiel with the cockatoos, its exact phylogenetic position within this group remains unresolved (i.e. whether it belongs in the Calyptorhynchinae, the Cacatuinae, or its own subfamily Nymphicinae, following Schodde [1997]). Similarly, there are unresolved questions about the phylogenetic positions (Fig. 2) of other species, including the Galah (Eolophus roseicapillus), the Gang-gang Cockatoo (Callocephalon fimbriatum), and Major Mitchell's Cockatoo (Cacatua leadbeateri; Adams et al. 1984).