An Invader In The Galapagos: When History And Genetics Merge

The study of genetics deals with a broad range of topics, from the molecules responsible for heredity to the adaptation and evolution of organisms. Within this spectrum is a niche where genetics can turn into a historical science and can be used to test the historical record of a species, thereby better understanding its story. A particularly interesting example of this is the study of invasive species, which are organisms that have recently occupied ecosystems where they weren’t found before (sometimes posing a threat to the local inhabitants of said environment). Genetics can be used to explore these scenarios, and what better setting than the emblematic Galapagos Islands to study an invasion event and its possible consequences?

A research team from Universidad San Francisco de Quito, Ecuador, recently published a genetic analysis of the common guava (Psidium guajava) from three human-inhabited islands in the Galapagos archipelago¹, where the species is considered invasive and potentially detrimental to the fragile ecosystems of these unique islands. The study combined the power of molecular genetics with the documented history of how this archipelago was inhabited by humans during the 19th century², bringing with them a plethora of plants and animals which had been previously unknown to the endemic ecosystems.

By analyzing the hypervariable microsatellite regions (also known as simple sequence repeats, or SSRs) of individuals from the islands of San Cristobal, Santa Cruz, and Isabela, and analyzing this data under the theoretical framework of population genetics, we constructed a model for the different steps that the common guava took in its journey from mainland Ecuador to colonizing the different islands within the Galapagos (where it is found to this day). The best-supported model appropriately (and somewhat satisfyingly) fits the historical record, suggesting that combining several methods in the population genetics toolbox can provide insight into the story behind ecological invasions. Both genetic and historical records suggest that this invasive guava first reached the island of San Cristobal (the first island to be settled by humans), probably arriving with the influential landowner Manuel J. Cobos³. From here, it was likely taken to Isabela, the largest island in the archipelago, by a group of human settlers. In a final step, human migrations from both San Cristobal and Isabela may have seen multiple introductions of the common guava to the island of Santa Cruz, currently the most populated island in the archipelago.

Figure republished with permission from Bernardo Gutierrez

The story, however, is far from being told in its entirety. The historical record suggests that the guava populations in the Galapagos originated from Central or Southern Ecuador, but the true origin remains unknown. Our current efforts are focused on analyzing the genetic diversity of P. guajava in continental Ecuador in order to understand its evolution in the continent and to identify the number of introductions of the species in the Galapagos and the source populations for the invasion(s).

The applications of this knowledge extend beyond the probing of historical records, and can even lend a hand to the conservation efforts for the endemic inhabitants of the islands. One such species is the guayabillo (Psidium galapagensis), a closely related species whose genetic diversity can be compromised as it competes with the invasive guavas for an ecological niche. The scenario gets even more convoluted, as there is a possibility for both species to form interspecific hybrids^4, resulting in the reduction of the guayabillo’s genetic diversity in a process known as genetic erosion.

Prof Maria de Lourdes Torres, a principal investigator at USFQ’s Plant Biotechnology Laboratory and member of the Galapagos Science Center (GSC) in San Cristobal, Galapagos, is currently working with her research team to determine the genetic diversity of the endemic species and begin to understand whether hybridization between the two species has occurred. The research is being further developed in collaboration with the University of North Carolina at Chapel Hill, where the application of genomic tools is generating new information on the potential of hybridization between guavas and guayabillos and the effects that this phenomenon might have on the survival of the endemic guayabillo in the archipelago.

References:

1. Urquía D, Gutierrez B, Pozo G, Pozo MJ, Espín A, Torres MdL (2019) Psidium guajava in the Galapagos Islands: Population genetics and history of an invasive species. PLoS ONE 14(3): e0203737. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0203737
2. Lundh JP. Galápagos: A Brief History; 2004 [cited 2017 Sept 20]. Repository: Human and Cartographic History of the Galápagos Islands [Internet].
3. Lundh JP. The farm area and cultivated plants on Santa Cruz, 1932–1965, with remarks on other parts of Galapagos. Galapagos Research. 2006;64: 12–25.
4. Torres MdL, Mena CF. Understanding Invasive Species in the Galapagos Islands: Springer International Publishing AG; 2018.