Genomic analyses reveal three independent introductions of the invasive brown rat (Rattus norvegicus) to the Faroe Islands

Abstract

Population genomics offers innovative approaches to test hypotheses related to the source and timing of introduction of invasive species. These approaches are particularly appropriate to study colonization of island ecosystems. The brown rat is a cold-hardy global invasive that has reached most of the world's island ecosystems, including even highly isolated archipelagoes such as the Faroe Islands in the North Atlantic Ocean. Historic records tell of rats rafting to the southern island of Suðuroy in 1768 following a shipwreck off the coast of Scotland, then expanding across the archipelago. We investigated the demographic history of brown rats in the Faroes using 50,174 SNPs. We inferred three independent introductions of rats, including to Suðuroy, the islands of Borðoy and Viðoy, and onto Streymoy from which they expanded to Eysturoy and Vágar. All Faroese populations showed signs of strong bottlenecks and declining effective population size. We inferred that these founder events removed low frequency alleles, the exact data needed to estimate recent demographic histories. Therefore, we were unable to accurately estimate the timing of each invasion. The difficulties with demographic inference may be applicable to other invasive species, particularly those with extreme and recent bottlenecks. We identified three invasions of brown rats to the Faroe Islands that resulted in highly differentiated populations that will be useful for future studies of life history variation and genomic adaptation.

Travel Tales of a Worldwide Weed: Genomic Signatures of Plantago major L. Reveal Distinct Genotypic Groups With Links to Colonial Trade Routes

“…In the absence of records describing the native flora before the arrival of Europeans, written accounts from early voyagers and colonists dating back to the sixteenth and seventeenth centuries provide evidence of the introduction of European species to continents such as North America; however, such records are scarce ( Fernald, 1900 ). This lack of information hinders our ability to retrace the origins of historical introductions ( Wilson et al, 2009 ; Estoup and Guillemaud, 2010 ; Puckett et al, 2020 ).…”

Insights into invasive species from whole‐genome resequencing

“…Assuming that sequencing is conducted at sufficient depth to accurately call low-frequency alleles, WGR data are therefore more likely to capture rare alleles required to time recent bottlenecks compared to RRS (Hahn, 2019). A study by Puckett et al (2020) demonstrates this limitation. Using RRS, the authors set out to test the hypothesis that a 1768 shipwreck introduced the brown rat (Rattus norvegicus) to the Faroe Islands.…”

The population genomics of invasive species

“…Assuming that sequencing is conducted at sufficient depth to accurately call low-frequency alleles (see Box 1), whole genome sequence data are therefore more likely to capture rare alleles required to time recent bottlenecks (Hahn, 2019). For example, Puckett et al(2020) set out to test the hypothesis that a 1768 shipwreck introduced the brown rat (Rattus norvegicus ) to the Faroe Islands using reduced representation sequencing. Although three introduction events could be inferred, the authors were unable to estimate the timing of each event due to a lack of rare alleles, which were removed through the very bottlenecks they were attempting to date.…”