Strong founder effects and low genetic diversity in introduced populations of Coqui frogs

Abstract

The success of non-native species may depend on the genetic resources maintained through the invasion process. The Coqui (Eleutherodactylus coqui), a frog endemic to Puerto Rico, was introduced to Hawaii in the late 1980s via the horticulture trade, and has become an aggressive invader. To explore whether genetic diversity and population structure changed with the introduction, we assessed individuals from 15 populations across the Hawaiian Islands and 13 populations across Puerto Rico using six to nine polymorphic microsatellite loci and five dorsolateral colour patterns. Allelic richness (R(T)) and gene diversity were significantly higher in Puerto Rico than in Hawaii populations. Hawaii also had fewer colour patterns (two versus three to five per population) than Puerto Rico. We found no isolation by distance in the introduced range, even though it exists in the native range. Results suggest extensive mixing among frog populations across Hawaii, and that their spread has been facilitated by humans. Like previous research, our results suggest that Hawaiian Coquis were founded by individuals from sites around San Juan, but unlike previous research the colour pattern and molecular genetic data (nuclear and mtDNA) support two separate introductions, one on the island of Hawaii and one on Maui. Coquis are successful invaders in Hawaii despite the loss of genetic variation. Future introductions may increase genetic variation and potentially its range.

Cast adrift on an island: introduced populations experience an altered balance between selection and drift

“…Sample sizes within each population ranged from 29 to 372, with more than 50 individuals sampled from the majority of populations. A single population from Maui exhibited a unique colour pattern combination, the interoccular bar and the unstriped morph [12]. Previous studies indicate that this population is the result of an independent introduction and is genetically isolated from other Hawaiian populations, possibly because this introduction was very recent or because of unknown barriers to gene flow [12]; therefore we excluded it from our analyses, which rely on estimates of historical gene flow between populations.…”

“…A single population from Maui exhibited a unique colour pattern combination, the interoccular bar and the unstriped morph [12]. Previous studies indicate that this population is the result of an independent introduction and is genetically isolated from other Hawaiian populations, possibly because this introduction was very recent or because of unknown barriers to gene flow [12]; therefore we excluded it from our analyses, which rely on estimates of historical gene flow between populations. We estimated the most likely genotype of the locus stripes for each individual using the phenotypes, the published model of inheritance [11], and the expectation-maximization algorithm [13].…”

“…For Puerto Rico, we conducted both a global test across all populations and two local tests using clusters of populations previously defined by a STRUCTURE analysis [12]. For Hawaii, we used a single global test because previous analyses indicate extensive gene flow [12], which may largely reflect recent range expansion from the original introduction site(s) to each local population.…”

“…We used point estimates of F ST for the stripes locus. For the Puerto Rico dataset, we structured populations using a hierarchical island model and assigned populations to two groups (eastern and western) based on previous STRUCTURE analyses [12]. For the Hawaii dataset, we used the non-hierarchical island model because there was no evidence of genetic structure among these populations [12].…”

“…We estimated the most likely genotype of the locus stripes for each individual using the phenotypes, the published model of inheritance [11], and the expectation-maximization algorithm [13]. Microsatellite data were obtained from a previous study [12] of population genetics of E. coqui in Hawaii and Puerto Rico. The microsatellite data represent a subset of the individuals scored for colour patterns (HI: n ¼ 343; PR: n ¼ 283).…”