How did the toad get over the sea to Skye? Tracing the colonisation of Scottish inshore islands by common toads (Bufo bufo)

Abstract

Processes of island colonisation have long been of interest to biologists. Colonisation events themselves are rarely observed, but the processes involved may be inferred using genetic approaches. We investigated possible means of island colonisation by common toads (Bufo bufo) in western Scotland (the Isle of Skye and five neighbouring small islands), using evidence derived from nuclear microsatellites and mitochondrial (mt) DNA. Levels of microsatellite allelic richness for populations on Skye were high and comparable to adjacent mainland populations, but lower for populations on small islands. Pairwise measures of genetic distances between populations and a clustering algorithm were both suggestive of frequent gene flow between Skye and the mainland. For small islands the levels of genetic differentiation were higher, implying stronger isolation and no evidence for inbreeding. The distribution of mtDNA haplotypes broadly mirrored the genetic structure revealed by microsatellites. Reconciled with existing palaeoclimatological evidence, since the last glaciation, our findings rule out the possibility that the B. bufo populations stem from glacial refugia, or that recent anthropogenic transfer of toads is responsible for their current distribution. The most parsimonious explanation of our data is that the studied inshore islands have been repeatedly colonised via rafting from the mainland or neighbouring islands. This may give us insights into the processes likely to take place when ice sheets retreat poleward as a result of climate change. It also has implications for the colonisation of both native and invasive non-native species, and hence the biosecurity of island refugia.

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“…Brede & Beebee, 2004 ; Thörn et al, 2021 ), with the high‐density panel of SNPs unequivocally demonstrating a reduced level of heterozygosity in island populations compared with the mainland, an effect which was less apparent in previous studies based on fewer loci (Roth & Jehle, 2016 ; Seppä & Laurila, 1999 ). The early putative split of the population on the small island (CRO) confirms evidence from microsatellites and mtDNA that supports its natural origin likely through rafting events during the melting of glaciers as opposed to a recent human introduction (O’Brien et al, 2021 ).…”

“…(b) Scatterplot of the first two principal components of DAPC analysis. Each individual is coloured according to its population of origin rafting events during the melting of glaciers as opposed to a recent human introduction (O'Brien et al, 2021).…”

“…Ultimately, however, the pathways underpinning disease can be highly complex, consisting of multiple steps and influenced by the interactions of a large number of potentially pleiotropic genes with small individual effect sizes (Fenton et al, 2012 ; Manolio et al, 2009 ). Originating from separate founder events and with a high degree of divergence (see also O’Brien et al, 2021 ), the populations in our system are expected to lead to sets of study individuals which are partitioned by their distinct genetic architectures. From this view, it appears plausible that the high degree of divergence between our populations alone could have caused the distinct, seemingly idiosyncratic responses to pathogen exposure irrespective of the effects of heterozygosity or the potential contribution of a small number of loci which are linked to disease resistance.…”

“…Despite recent declines, the common toad ( Bufo bufo ) remains one of the most widespread amphibians in northern Europe, where fragmented population structures on islands and adjacent coastlines have been documented (O’Brien et al, 2021 ; Roth & Jehle, 2016 ; Seppä & Laurila, 1999 ; Tuncay et al, 2018 ). As a result of its intermediate susceptibility to Bd (e.g.…”