The Story of the Black Swan

"The swan genome and transcriptome, it is not all black and white" — published in Genome Biology (2023), with Dr Parwinder Kaur (University of Western Australia, Perth) as a co-author, alongside a large international team including researchers from Baylor College of Medicine, the Broad Institute, and others.

The Australian black swan (Cygnus atratus) has a striking contrast to the white swans native to Europe and North America (like the mute swan, Cygnus olor) — not just in plumage, but in immunology. Because Australia's black swan evolved in relative geographic isolation, it likely developed a limited immune repertoire — having been "largely shielded" from many infectious diseases that circulate elsewhere in the world. The critical finding: unlike mallard ducks and the white/mute swan, the black swan is extremely sensitive to highly pathogenic avian influenza (HPAI) — bird flu hits it far harder.

Understanding why had been impossible because no black swan genome existed — so the team generated the first chromosome-length, annotated genomes for both the black swan and the mute (white) swan, using long-read sequencing pipelines developed for the Vertebrate Genomes Project, and compared the two. The result: comparing the two genomes helped explain the biological basis for the black swan's heightened vulnerability — geographic/evolutionary isolation left it with less immune diversity than its white counterpart, which had presumably faced (and adapted to) a wider range of pathogens over its evolutionary history.

Through their work, they also found that the white swan's SLC45A2 gene had a nucleotide deletion in its reading frame, which caused a frameshift mutation and introduced a premature stop codon. SLC45A2 normally encodes a membrane transport protein that regulates melanin production — so this frameshift breaks the gene, and the researchers concluded this deletion is a strong candidate for why the white swan lost its pigment. This means the white swan is the derived (mutant) form for plumage color, not the black swan — the frameshift knocked out melanin production in the white lineage, while black swans retain the intact, presumably ancestral, functional version of the gene. It's directly analogous to how loss-of-function mutations in the human ortholog of this same gene pathway cause albinism.

The black swan looks different on the outside, but the story is written in the genome. That kind of genetic difference is where precision medicine begins — targeting the genetic drivers of disease in an individual or subgroup, rather than assuming one biology fits all.