DNA insights into species distributions – trilling chorus frogs.

By Steve Lougheed

We have gained many ecological and evolutionary insights from studying variation in DNA markers, from resolving the very base of the tree of life (genealogical affinities of bacteria, archaebacteria/extremophiles, eukaryotes), through overturning received truths about mating systems of birds (most are not in fact genetically monogamous), to quantifying impacts of human activities on connectivity of populations of species of conservation concern. Among the many revelations that come from such DNA studies are those from phenotypically cryptic taxa whose appearances often mask deep phyletic diversity. Indeed an increasing number of studies shows that myriad, traditionally-regarded ‘species’ are in fact complexes of separate, reproductively-isolated species. DNA studies have revealed such cryptic species in many groups, including mammals (Ceballosa & Ehrlich. 2009), birds (e.g. Lohman et al. 2010), amphibians (e.g. Elmer et al. 2007), and insects (e.g. Hebert et al. 2004). Many examples of cryptic diversity come from the tropics, but here are also some intriguing examples from higher latitudes like ours, with implications not only for understanding of evolutionary affinities of taxa in question, but also for their geographical distributions and the forces that have shaped them.

The trilling chorus frogs (a distinct lineage or clade within the treefrog genus Pseudacris) comprise one such group. This clade, distributed broadly across eastern North America, includes at least nine species: the mountain chorus frog (P. brachyphona), Brimley’s chorus frog (P. brimleyi), spotted chorus frog (P. clarkia), Cajun chorus frog (P. fouquettei), New Jersey chorus frog (P. kalmi), upland chorus frog (P. feriarum), southern chorus frog (P. nigrita), boreal chorus frog (P. maculata) and western chorus frog (P. triserieta) (Moriarty & Cannatella 2004).

Two of these species occur in Ontario, P. maculata and P. triserieta. The two are very similar in appearance – both are small (generally < 3.5 cms in snout-vent length), with smooth skin, and a dorsum varying in colour from brown to greenish-gray, and a dark stripe through the eye and longitudinal markings on the dorsum. The calls too are very similar being comprised of a trill that is often likened to running one’s fingers along a plastic comb.  The boreal chorus frog was until recently considered to be distributed from northwestern Ontario to Alberta and north to the NWT, also being found in the USA in the Midwest south to Arizona and New Mexico. The western chorus frog was thought to range from southern Quebec and Ontario/northern New York state west to South Dakota, and south to the states of Kansas and Oklahoma (Harding 1997). In southern Ontario until recently there were considered to be two regional populations of P. triserieta: a “Carolinian population” found south and west of Toronto, and a Great Lakes–St. Lawrence population found east and north of Toronto, with the latter considered as ‘Threatened’ under the Canadian Species at Risk Act.

Boreal chorus frog from QUBS. N.A. Cairns.
Boreal chorus frog from QUBS. N.A. Cairns. Click on thumb for a larger image.

That’s the old view. Mitochondrial DNA evidence suggests that the Great Lakes–St. Lawrence population which was classified as P. triserieta is not in fact western chorus frog at all, but rather is a disjunct population of boreal chorus frog (Lemmon et al. 2007a,b, Rogic et al. 2015). Playbacks by Rogic et al. (2015) seem to affirm this, with eastern Ontario and western Quebec chorus frogs responding to previously recorded calls of P. maculata and not P. triserieta.

All of this has interesting implications for 1. Conservation (Is this western boreal chorus frog population genetically distinct and thus does it merit conservation priority?), 2. Biogeography (How did the species become disjunct and what paths of re-colonization did these distinct populations use?), and 3. Understanding the nature of species (these trilling chorus frogs are cryptic to us, but clearly they can tell each other apart – it is in the domain of mate recognition system and acoustics that the species differences are clear). As always there’s lots more work that can be done, not least of which is more finely mapping genetic diversity across the entire boreal chorus frog distribution.


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