Eric M. O'Neill

Landscape-scale genetic variation in a forest outbreak species, the mountain pine beetle (Dendroctonus ponderosae)

The mountain pine beetle Dendroctonus ponderosae is a native species currently experiencing large‐scale outbreaks in western North American pine forests. We sought to describe the pattern of genetic variation across the range of this species, to determine whether there were detectable genetic differences between D. ponderosae occupying different host trees in common localities, and to determine whether there was molecular evidence for a past demographic expansion. Using a combination of amplified fragment length polymorphism (AFLP) and mitochondrial sequencing analyses, we found evidence of genetic structuring among populations that followed a broad isolation‐by‐distance pattern. Our results suggest that the geographical pattern of gene flow follows the core distribution of the principal D. ponderosae host species, around rather than across the Great Basin and Mojave Deserts. Patterns of haplotype diversity and divergence were consistent with a range‐wide population expansion. This signal was particularly pronounced in the northern part of the species’ range, where outbreak activity is currently increasing. Using AFLP markers, we were unable to detect significant differences among groups of insects sampled from different host trees in common locations. Incidentally, we found that a large proportion of the polymorphic AFLP markers were gender‐specific, occurring only in males. While we did not include these markers in our analyses, this finding warrants further investigation.

Strong Founder Effects and Low Genetic Diversity in Introduced Populations of Coqui Frogs

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 (RT) 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.

Clinal Variation in Calls of Native and Introduced Populations of Eleutherodactylus coqui

We determined whether Eleutherodactylus coqui exhibits clinal variation in male advertisement call parameters across its native Puerto Rico and introduced range of Hawaii. In the laboratory, we determined whether clinal variation in call parameters were a result of body size or temperature. Calls correlate with elevation in both Puerto Rico and Hawaii in the following ways: negative for fundamental frequency of each call syllable (Co and Qui), positive with the duration of each call, negative with call rate (calls per minute), and no relationship with call intensity. In the laboratory, we found the negative relationship between elevation and call frequency was best explained by larger body sizes at higher elevations, and that the positive and negative relationships between elevation and call duration, and elevation and call rate, respectively, were best explained by lower temperatures at higher elevations. While frogs in Hawaii exhibited the same relationships between elevation and call parameters found in Puerto Rico, they exhibited less variation in call frequency with elevation because they had less variation in body size. Differences in call duration and rate between Hawaii and Puerto Rico reflected lower temperatures in Hawaii at similar elevations. While individual frogs are not louder in Hawaii, choruses may appear louder where densities are higher.

Isolation of Microsatellite Loci From the Coqui Frog, Eleutherodactylus Coqui

Thirteen microsatellite loci were isolated from the coqui frog (Eleutherodactylus coqui) and optimized for future research. The loci were screened across 37 individuals from two Puerto Rican populations. Loci were variable with the number of alleles per locus ranging from three to 38. Polymorphic information content ranged from 0.453 to 0.963 and observed heterozygosity for each population ranged from 0.320 to 0.920.

Cast Adrift on an Island: Introduced Populations Experience an Altered Balance Between Selection and Drift

A long-standing question in evolutionary biology is what becomes of adaptive traits when a species expands its range into novel environments. Here, we report the results of a study on an adaptive colour pattern polymorphism (stripes) of the coqui frog, Eleutherodactylus coqui, following its introduction to Hawaii from Puerto Rico. We compared population differentiation () for the stripes locus—which underlies this colour pattern polymorphism—with neutral microsatellite loci to test for a signature of selection among native and introduced populations. Among native populations, for stripes were lower than expected under the neutral model, suggesting uniform balancing selection. Alternatively, among introduced populations, for stripes did not differ from the neutral model. These results suggest that the evolutionary dynamics of this previously adaptive trait have become dominated by random genetic drift following the range expansion.

Cryptic Diversity and Conservation of Gopher Frogs across the Southeastern United States

Identifying cryptic biodiversity is fundamental to evolutionary biology and to conservation efforts. This study investigated range-wide genetic diversity of Gopher Frogs, Lithobates capito, across the southeastern United States coastal plain to determine implications for taxonomy and conservation. We collected data for two mtDNA regions in 21 populations to identify genetic structure across the geographic distribution of the species. Based on population genetic, phylogenetic, and genealogical analyses, we recovered three reciprocally monophyletic mtDNA lineages corresponding to mainland coastal plain populations and two lineages within peninsular Florida. Breakpoints for these lineages did not occur in previously identified hotspots of amphibian phylogeographic breaks and did not follow currently recognized subspecies designations. We recommend these lineages be recognized as separate distinct population segments and be considered separately by the U.S. Fish and Wildlife Service for listing under the Endangered Species Act. Additionally, we propose an evolutionary hotspot for amphibians that deserves further attention.

Body Size and Life History Traits in Native and Introduced Populations of Coqui Frogs

Introduced populations often exhibit rapid phenotypic changes following colonization of new environments. These changes, which often contribute to the successful establishment and spread of introduced species, may result from evolution or phenotypic plasticity. We studied variation in adult body size across elevational gradients in native (Puerto Rico) and introduced (Hawaii) populations of Coquis (Eleutherodactylus coqui). To explore the possible mechanisms underlying variation in adult body size, we conducted a laboratory common-environment experiment to study the effects of temperature (19°C and 25°C), elevation (<300 m and >700 m), and area (two in Puerto Rico and one in Hawaii) on five size-related life history traits: clutch size, egg size, hatching size, size at 30 days, and growth rate. In the field in both Puerto Rico and Hawaii, body size was positively correlated with elevation, which is negatively correlated with temperature, but the magnitude of the slope was greater in Puerto Rico than in Hawaii. In the laboratory, egg size, hatching size, and body size at 30 days were positively correlated with elevation for populations from Puerto Rico and Hawaii. Egg size, hatching size, and body size at 30 days were negatively correlated with temperature for all populations. Clutch size and growth rate were positively correlated with elevation for populations from Puerto Rico but not for populations from Hawaii. Furthermore, both low and high elevation populations from Hawaii had life history traits more similar to low elevation populations than high elevation populations from Puerto Rico. Temperature effects in the laboratory suggest that plasticity in response to temperature contributes to the variation in adult body size with elevation in both Puerto Rico and Hawaii. However, temperature-induced plasticity cannot explain the difference in slopes between Puerto Rico and Hawaii, because temperature varies across elevations to the same degree in both regions. Variation in growth rate paralleled that for adult body size and, if heritable, provides a possible mechanism for the observed differences in adult body size across elevations between Puerto Rico and Hawaii.