DNA testing challenges traditional species classification

Employing novel techniques to retrieve DNA sequences from thousands of genomic locations, the researchers were able to uncover an unusual case of cryptic speciation in the Streak-eared Bulbul [Pycnonotus blanfordi], a bird widespread throughout South-east Asian countries.

Cryptic speciation produces closely related sister species that are very similar in appearance and often overlooked by scientists until genetic and/or bioacoustic inquiries reveal species-level differences.

Traditionally, the bird identification literature has relied on shape and plumage color to classify bird species. More recently, vocalizations have also been used to uncover cryptic species otherwise similar in body appearance.

In the last two decades, DNA sequence comparison has gradually been added as a tool in the kit for species delimitation, casting light on a number of cryptic species where neither morphological nor bioacoustic differences provided clues for species differentiation.

"Distinguishing different species is a non-trivial task of great importance," said Colin Poole, WCS Regional Director for the Greater Mekong region. "This research helps us better understand the evolution of life and often points to cases in which science has hitherto underestimated the extent of actual species diversity present in any given region. '

After careful examination, two described subspecies of Streak-eared Bulbul [Pycnonotus blanfordi] resident in Myanmar [P. b. blanfordi] and Thailand/Indochina [P. b. conradi] were found to exhibit deep genome-wide differentiation indicating they are two separate species.

Despite negligible nuances in the birds' plumage color, and limited differentiation in their vocalizations, WCS and NUS ornithologists identified a surprising genetic divergence dating back as far as the early Pleistocene.

A closer look also revealed different eye colors between the two forms, which the scientists believe to be an important morphological differentiating trait in mate recognition and reproductive isolation, prompting them to call for an elevation of both forms to species level, and naming the one specific to Myanmar "Ayeyawady Bulbul'"

"Cryptic species have always represented an intriguing challenge for scientists," explains Robert Tizard from WCS Myanmar. "Advancements in DNA methodologies are of tremendous help in understanding the rich biodiversity of Myanmar and tropical Asia."

Through this discovery, scientists at WCS and NUS have demonstrated how novel DNA sequencing technologies that retrieve genome-wide DNA can be put to use in species delimitation, and advocate for more systematic use of genome-wide DNA for the detection of cryptic species.

"Even in birds, which are better-known than most other living beings, the age of new species discoveries is not over," said Dr. Frank Rheindt from NUS. "We hope that future collaborations between academia and conservation NGOs will lead to the discovery of numerous additional cryptic species to help us obtain a more realistic understanding of true levels of species diversity."

Although Streak-eared and Ayeyawady bulbuls are generally common where they occur and neither of the cryptic species involved in this discovery seem to be under threat of extinction, the future discovery of cryptic species may well refer to endangered species that will require immediate conservation action.

"Genome-wide data help identify an avian species-level lineage that is morphologically and vocally cryptic," appears in the journal, Molecular Phylogenetics and Evolution.