“Extinct” Galápagos Tortoises Reappear in Modern Hybrids
In the world of conservation biology, good news can be all too rare. This week, however, a study published in the 10 January edition of Current Biology reveals a ray of hope for a species long thought to have been another depressing anecdote about human-accelerated extinction.
The animal in question is a subspecies of the Galápagos tortoise. Although some may tend to refer to the Galápagos tortoise as if it were a generic population spread across the archipelago, it is thought that at one time there were up to 15 distinct subspecies (Caccone 2002), some with recognizably different shell morphologies (Chiari et al. 2009). Only 10 subspecies remain today. One of these missing subspecies, Chelonoidis elephantopus,* purportedly disappeared around 150 years ago—shortly after Charles Darwin passed through the archipelago in 1835. Tellingly, one of Charles Darwin’s guides on Floreana was on the island specifically to hunt the towering reptiles, and Darwin " dined exclusively on tortoise meat " during his journey into the island’s highlands.
This subspecies was originally restricted to Floreana Island, but there is new evidence that human explorers and whalers may have transported tortoises between islands, mixing previously isolated populations. While this was by no means an ecologically responsible thing to do, it turns out that it may have prevented C. elephantopus from becoming entirely extinct.
A team of researchers from Yale University, led by Gisella Caccone and Ryan Garrick, elucidated from a previous study that there were mysterious hybrid tortoises on Isabela Island—which is historically home to tortoises in the subspecies C. becki (Poulakakis 2008). They were initially surprised to find that some of the individuals were significantly different from others, and became interested in where these anomalous invidiuals had come from. By comparing the genetic data from their living individuals to a range of Galápagos tortoise specimens stored in museums, they discovered that the outliers looked as though they were C. becki x C. elephantopus hybrids—an incredible revelation, considering that the latter subspecies was thought to have disappeared a century and a half ago.The team proceeded to return to the Galápagos to collect genetic samples from roughly 2,000 tortoises currently living on Isabela Island’s Volcano Wolf.
The results are striking—the genomic analyses showed that 84 of the sampled individuals possess genomes so similar to the—supposedly extinct—Floreana Island subspecies that one of their parents must have been a purebred C. elephantopus. While the remaining purebreds are so rare that none of them happened to be sampled in the study (the total tortoise population of Isabela Island is thought to be around 7,000, and the researchers estimated that 38 of those may be purebred C. elephantopus), they are apparently still there and successfully breeding with the locals.
In general, hybridization is thought to be detrimental for populations of endangered species, and in these cases it is often termed “genetic introgression.” For example, introgression with coyotes ( Canis latrans ) has been a major obstacle to the successful reintroduction of the red wolf ( C. rufus ) in the southeastern United States (Miller et al. 2003).. In the case of the Galápagos tortoises, however, it may be the only way that a devastatingly reduced population was able to persist, allowing C. elephantopus to keep breeding, albeit with other subspecies, rather than dying out due to lack of enough available mates of its own kind.
This indirect evidence that purebred C. elephantopus individuals still exist is exciting news. As the paper asserts, “this is the first rediscovery of a species by way of tracking the genetic footprints left in the genomes of its hybrid offspring.” The broader implications are also intriguing. Efforts can be made to help a rare species proliferate by carefully controlling breedings to produce individuals with the highest possible proportion of their genome stemming from the subspecies of concern.
This case also raises some interesting questions: what should be done with “lighter” hybrids that aren’t eligible for the reconstructive breeding but are still admixing with the Isabela populations? Have any important genes been lost in the course of the subspecies’s decline and hybridization that will have a noticeable effect on the tortoises once they have been line-bred back into purity? Should efforts be made to reintroduce them to their native Floreana Island? It will be a fascinating case to track over the coming years.
- ( Geochelone is sometimes used as a taxonomic synonym for Chelonoidis ).
Caccone, A., G. Gentile, J. P. Gibbs, T. H. Fritts, H. L. Snell, J. Betts, and J. R. Powell. 2002. Phylogeography and history of giant Galápagos tortoises. Evolution 56: 2052-2066.
Chiari, Y., C. Hyseni, T. H. Fritts, S. Glaberman, C. Márquez, J. P. Gibbs, J. Claude, and A. Caccone. Morphometrics parallel genetics in a newly discovered and endangered taxon of Galápagos tortoise. PLoS ONE 4(7): e6272.
Garrick, R., Benavides, E., Russello, M., Gibbs, J., Poulakakis, N., Dion, K., Hyseni, C., Kajdacsi, B., Márquez, L., Bahan, S., Ciofi, C., Tapia, W., & Caccone, A. (2012). Genetic rediscovery of an ‘extinct’ Galápagos giant tortoise species Current Biology, 22 (1) DOI: 10.1016/j.cub.2011.12.004
Grant, K. T. and G. B. Estes. 2009. Darwin in Galapágos: Footsteps to a new world. Princeton University Press. p. 180.
Miller, C. R., J. R. Adams, and L. P. Waits. 2003. Pedigree-based assignment tests for reversing coyote (Canis latrans) introgression into the wild red wolf (Canis rufus) population. Molecular Ecology 12: 3287-3301.
Poulakakis, N., S. Glaberman, M. Russello, L. B. Beheregaray, C. Ciofi, J. R. Powell, and A. Caccone. 2008. Historical DNA analysis reveals living descendants of an extinct Galápagos tortoise. Proceedings of the National Academy of Sciences. 105: 15464-15469.
Galápagos map: discovergalapagos.com
Tortoise: Photographer: Per-Gunnar Ostby of www.pgoimages.com