Monday, July 21, 2014

A cousin of the dodo

The closely related and extinct Dodo (Raphus cucullatus) and Rodrigues Solitaire (Pezophaps solitaria), both in the subfamily Raphinae, are members of a clade of morphologically very diverse pigeons. Genetic analyses have revealed that the Nicobar Pigeon (Caloenas nicobarica) is the closest living relative of these birds, thereby highlighting their ancestors’ remarkable migration and morphological evolution. The Spotted Green Pigeon (Caloenas maculata) was described in 1783 and showed some similarities to the Nicobar Pigeon. Soon however the taxon fell into obscurity, as it was regarded as simply an abnormal form of the Nicobar Pigeon. The relationship between both taxa has occasionally been questioned, leading some ornithologists to suggest that the two may in fact be different taxa. 

The spotted green pigeon is a historically elusive species. The only known example can be found at the World Museum in Liverpool, and another known specimen is lost. There are no records of the bird in the wild, no one knows where it was found and as described in the paragraph above researchers weren't even really sure if it was its own species - they thought it was just an unusual form of the Nicobar pigeon from around Indonesia.

Researchers from Australia and the UK now used ancient DNA methodologies to investigate the phylogeny and authenticity of the Spotted Green Pigeon. They took DNA from two feathers of the last specimen of the spotted green pigeon. Because of its age, the DNA was highly fragmented, so they focused in on what they called three DNA 'mini barcodes'.  Well, as it turns out those were not DNA Barcodes as they've used some small (~64 bp) 12S fragments. I've often talked about the importance of standards and although this study is really well done and delivers very interesting finds, the generated sequences fail to contribute to any DNA Barcode reference library. If anyone every wanted to compare the results to the some 5 100 species barcodes on BOLD (compared to about 2 200 species with a 12S sequence) they would not be able to do that. Again, a great story but it would be even better if the authors would have abstained from the use of the word 'barcode' as it is clearly misleading in this case.

But let's go back to their findings. They showed that the spotted green pigeon is indeed a separate species, showing a unique mtDNA sequence compared to other pigeons. The pigeon is genetically most closely related to the Nicobar pigeon and the dodo and Rodrigues solitaire. 

Early reports suggested that the spotted green pigeon has a semi-terrestrial island lifestyle and the ability to fly. The closely related Nicobar pigeon shows similar habits and has a preference for travelling between small islands. The researchers conclude that this lifestyle, together with the relationship of both pigeons to the dodo and Rodrigues solitaire supports an evolutionary theory that the ancestors of these birds were 'island hoppers', moving between islands around India and Southeast Asia. The birds that settled on particular islands then evolved into the individual species. The dodo's ancestor managed to hop as far as the island of Mauritius near Madagascar where it then lost the ability to fly.

This study puts an end to any discussions about the species status of the spotted green pigeon. Too bad that the species was never observed again and was consequently added to the list of extinct bird species in 2008. The reasons for its extinction remain unknown.

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