The platypus has 52 chromosomes, including 10 sex chromosomes (5 X chromosomes and 5 Y chromosomes) (Grützner et al. 2004). The DNA coding sequence for the platypus’s entire genome (apart from the Y chromosomes) was first described in 2008, based on tissue obtained from a female captured along the Barnard River in New South Wales. This research indicated that the platypus has approximately 18,500 protein-coding genes, most of which (82%) also occur in other vertebrate animals such as mice, dogs, chickens and humans. These genes presumably contribute to basic biological functions that haven’t changed for hundreds of millions of years (Warren et al. 2008). A later, more extensive gene-mapping study increased the estimated number of platypus protein-coding genes to well over 20,000 (Zhou et al. 2021).
The best available genetic evidence suggests that there are four major platypus population units in Australia, respectively inhabiting Tasmania and King Island, New South Wales and Victoria, central Queensland, and northern Queensland (Martin et al. 2018). Genetic differences among the four units are likely due to geographic barriers that have blocked dispersal (and hence genetic exchange) between neighbouring units for possibly hundreds of thousands of years.
At a finer geographic scale, a lesser degree of genetic differentiation is typically evident between platypus populations occupying neighbouring river basins on the Australian mainland. Following on from this, a pattern of “isolation by distance” – populations becoming more dissimilar to each other as the distance between river basins increases – appears to occur both on the mainland and (to a lesser extent) in Tasmania (Kolomyjec et al. 2009; Furlan et al. 2013).
Interestingly, there is no indication that the Great Dividing Range has ever acted as much of a barrier to platypus genetic exchange, implying that it is fairly easy for a dispersing individual to travel between headwater streams located near the top of the Divide, at least in wet years (Furlan et al. 2013). It’s also interesting to note that a surprisingly high number (11%) of 120 platypus tested in the adjoining Hawkesbury-Nepean and Shoalhaven Rivers in New South Wales were deemed to be first-generation migrants (Kolomyjec et al. 2009).
Studies by Furlan et al. (2012) found that genetic diversity in the platypus population on King Island (which has been isolated by the waters of Bass Strait for at least 10,000 years) is among the lowest ever recorded for wild animals, presumably because inbreeding has occurred over a long period. However, no physical abnormalities are evident. Similarly, the highly inbred platypus on Kangaroo Island (descended from 16 founders released in the 1940s, with an effective population size of just 10-11 animals today) have an entirely normal appearance and occur in similar (or slightly higher) abundance as compared to creeks of similar size on the Victorian mainland (Serena and Williams 1997).
Close relatives may sometimes mate with each other even in large platypus populations on the mainland (Martin et al. 2018), likely in part due to the platypus’s long lifespan. This provides a plausible explanation for the King and Kangaroo Island findings, namely that evolution has occurred to reduce the likelihood that physical abnormalities occur in the offspring of close relatives. This process of genetic adaptation to inbreeding, known as “genetic purging”, is believed to contribute to other species (such as albatrosses) thriving in the wild despite their having extremely low genetic diversity (Milot et al. 2007).
Photos courtesy of Mike Sverns (above), D. Illing (below)
LITERATURE CITED
Furlan EM, Griffiths F, Gust N, Handasyde KA, Grant TR, Gruber B and Weeks AR (2013) Dispersal patterns and population structuring among platypuses, Ornithorhynchus anatinus, throughout south-eastern Australia. Conservation Genetics 14, 837-853.
Furlan E, Stoklosa J, Griffiths J, Gust N, Ellis R, Huggins RM and Weeks AR (2012) Small population size and extremely low levels of genetic diversity in island populations of the platypus, Ornithorhynchus anatinus. Ecology and Evolution 2, 844-857.
Grützner F, Rens W, Tsend-Ayush E, El-Mogharbel N, O’Brien PCM, Jones RC, Ferguson-Smith MA and Graves JAM (2004) In the platypus a meiotic chain of ten sex chromosoes share genes with the bird Z and mammal X chromosomes. Nature 432, 913-917.
Kolomyjec SH, Chong JYT, Blair D, Gongora J, Grant TR, Johnson CN and Moran C (2009) Population genetics of the platypus (Ornithorhynchus anatinus): a fine-scale look at adjacent river systems. Australian Journal of Zoology 57, 225-234.
Martin HC, Batty EM, Hussin J, Westall P, Daish T, Kolomyjec S, Piazza P et al. (2018) Insights into platypus population structure and history from whole-genome sequencing. Molecular Biology and Evolution 35, 1238-1252.
Milot E, Weimerskirch H, Duchesne P and Bernatchez L (2007) Surviving with low genetic diversity: the case of albatrosses. Proceedings of the Royal Society B 274, 779-787.
Serena M and Williams GA (1997) Population attributes of platypus (Ornithorhynchus anatinus) in Flinders Chase National Park, Kangaroo Island. South Australian Naturalist 72, 28-33.
Warren WC, Hillier LW, Graves JAM, Birney E, Ponting CP, Grützner F, Belov K, et al. (2008) Genome analysis of the platypus reveals unique signatures of evolution. Nature 453, 175-184.
Zhou Y, Shearwin-Whyatt L, Li J, Song Z, Hayakawa T, Stevens D, Fenelon JC, et al. (2021) Platypus and echidna genomes reveal mammalian biology and evolution. Nature 592, 756-762.