More markers, more power? Genomic analyses uncover fine-scale population structure in Philippine Broadbills

Ornithologists reconstruct the evolutionary history of several island populations.

Scientists love a good acronym. A recent study counted more than one million unique acronyms in papers published between 1950 and 2019, but just over 2,000 (0.2%) were used regularly. You can find some clever ones here, including BIGASS (Bright Infrared Galaxy All Sky Survey) and Gandalf (Gas AND Absorption Line Fitting algorithm). Ornithologists working in Asia might be familiar with the abbreviation PAIC, which stands for Pleistocene Aggregate Islands Complexes. This acronym refers to groups of islands that were connected by land bridges during the Pleistocene when sea levels were lower. The avifauna of the Philippine Archipelago seems to adhere to the PAIC model with islands complexes from different PAICs showing clear genetic divergence (see for example this study by Peter Hosner and his colleagues). However, the evolutionary history of bird populations within a single PAIC remains largely unknown. In a recent paper in the Biological Journal of the Linnean Society, researchers addressed this knowledge gap and took a closer look at the Philippine broadbills (genus Sarcophanops) of the Greater Mindanao PAIC.

Genetic Patterns

Luke Campillo and his colleagues collected samples from the Wattled Broadbill (S. steerii) and the Visayan Broadbill (S. samarensis). Genetic analyses – based on thousands of genetic markers – revealed a deep split between both species. A similar pattern emerged in a previous study using mitochondrial DNA. The researchers attributed these findings to the habitat unsuitability of the Leyte Gulf during the Pleistocene, which prevented birds from the northern and southern islands from mixing.

Apart from this deep split, the genetic analyses pointed to fine-scale diversification within the Visayan Broadbills, separating the populations from the islands Samar/Leyte and Bohol. The authors speculate that “rising sea levels at the end of the Pleistocene would have isolated Bohol first, whereas prolonged connectivity between Samar and Leyte could have promoted gene flow, thus obscuring population genetic effects of inter-island diversification.”

The genome-wide markers clearly differentiated between the two species: Wattled Broadbill (in yellow) and Visayan Broadbill (in blue). Within the latter species, further diversification between islands became apparent. Adapted from: Campillo et al. (2020) Biological Journal of the Linnean Society.

Genomic Power

The population structure within the Visayan Broadbill was not apparent in the mitochondrial DNA, highlighting the power of genome-wide markers to detect subtle signatures of population diversification. A few years ago, I covered this recent genomic development in a book chapter with several colleagues, writing that “genomic data has increased the potential for fine-scale resolution of population structure and determination of population boundaries and population membership.” However, this increase in genomic power can complicate analyses because populations tend to fall on a continuum from isolation to panmixia. Delineating populations and drawing species limits with genomic can become a daunting task. It will be interesting to follow how the genetic patterns in this study will impact the taxonomy of Philippine broadbills. Is the fine-scale population structure in the Visayan Broadbill large enough to justify subspecies or not?

References

Campillo, L. C., Manthey, J. D., Thomson, R. C., Hosner, P. A., & Moyle, R. G. (2020). Genomic differentiation in an endemic Philippine genus (Aves: Sarcophanops) owing to geographical isolation on recently disassociated islands. Biological Journal of the Linnean Society, 131(4), 814-821.

Featured image: Wattled Broadbill (Sarcophanops steerii) © Bram Demeulemeester | Flickr