The taxonomic decision has important consequences for breeding programs.
The black-winged myna (Acridotheres melanopterus) is almost extinct in the wild. This species contains three subspecies: one subspecies (melanopterus) is practically extinct in the wild except for a small flock inside a Javan wildlife park, while the other two subspecies (tricolor and tertius) each number less than 200 individuals in the wild. Captive breeding programs have been established the safeguard the future of the black-winged myna. However, a recent taxonomic decision by the IUCN has complicated the conservation efforts of these breeding programs. Based on differences in plumage patterns and biometrics, the IUCN decided to elevate the three subspecies to species level. A decision that is not unanimously supported within the IUCN’s Asian Songbird Trade Specialist Group.
This taxonomic change has important implications for captive breeding strategies. The recognition of three species leads to a three small populations for breeding, increasing the risk of genetic inbreeding. Considering three subspecies opens the opportunity to a bigger breeding population (with the possibility of crossing subspecies), but might result in the loss of evolutionarily unique lineages due to hybridization. A recent study in the journal Scientific Reports tried to solve this dilemma with a genomic perspective on the black-winged myna complex.
Keren Sadanandan and her colleagues sequenced thousands of genome-wide markers from 85 captive individuals across the morphological spectrum of the black-winged myna. The genetic analyses pointed to two population clusters: one cluster with melanopterus individuals from western Java and morphological hybrids (between melanopterus and tricolor), and a second containing tertius individuals from Bali. Moreover, the melanopterus individuals and hybrids were distributed along a gradient with varying levels of shared ancestry with the tertius cluster. These results thus show a smooth genomic cline from melanopterus in western Java to tertius in Bali. This clinal pattern does not support the IUCN’s decision to recognize three species (check out this paper for more on the dangers of clinal variation in taxonomy). The classification into three subspecies is further supported by low differentiation of the mitochondrial gene ND2 (less than 1.5%) which does not exceed the mitochondrial divergence threshold typically used for the species level (2-3%).
Additional analyses suggested ancient introgression between the Javan Myna (A. javanicus) and the tertius subspecies on Bali. It is possible that the genomic regions underlying black plumage introgressed from the dark Javan Myna into the most melanistic tertius subspecies. The researchers could not pinpoint the exact introgressed regions, so this hypothesis remains to be tested. Nonetheless, the findings of this study indicate that melanism (the degree of black plumage) does not reflect the genomic differentiation between the three subspecies and is thus not a reliable character for taxonomic decisions. This leads the researchers to the following conclusion:
Our study showed that the two geographically and morphologically terminal forms of BWM, melanopterus in the west and darker tertius in the east, are characterized by a mtDNA divergence below the species level. Variation in levels of melanism, recently used to separate BWMs into three species, is not reflected by deep genomic differentiation, arguing in favour of the traditional taxonomic arrangement that unites all three forms as subspecies rather than species.
What does this mean for the breeding programs? The researchers recommend “three separate breeding sub-programs under the umbrella of a single species-wide program without a strict separation.” This will hopefully preserve the range of morphological and genetic diversity within the black-winged myna.
Sadanandan, K. R., Low, G. W., Sridharan, S., Gwee, C. Y., Ng, E. Y., Yuda, P., Prawiradilaga, D. M., Lee, J. G. H., Tritto, A. & Rheindt, F. E. (2020). The conservation value of admixed phenotypes in a critically endangered species complex. Scientific reports, 10(1), 1-16.
Featured image: Black-winged Myna (Acridotheres melanopterus) © Doug Jasonjj | Wikimedia Commons
This paper has been added to the Sturnidae page.