A macroevolutionary study of historical contingency in the fanged frogs of Southeast Asia

A New Limnonectes (Anura: Dicroglossidae) from Southern Thailand

Simple Summary

New species of frogs continue to be discovered at a rapid rate in Southeast Asia, often as a result of reexamining populations of geographically widespread species using new molecular and bioacoustic tools. Here, we show that members of the fanged frog genus Limnonectes from Ko Pha-ngan, Ko Samui, and Ko Lanta Yai Islands in southern Thailand can be distinguished from the morphologically similar species Limnonectes doriae in molecular, advertisement call, morphometric, and qualitative morphological characters. On the basis of these multiple lines of evidence, we describe the insular populations in southern Thailand as a new species, Limnonectes pseudodoriae sp. nov. The new species occurs near small streams in low to mid-elevation forests and breeds in terrestrial nests consisting of moist, clay depressions in which the eggs and larvae develop.

Abstract

A new species in the dicroglossid frog genus Limnonectes is described from Ko Pha-ngan, Ko Samui, and Ko Lanta Yai Islands in southern Thailand. Males of Limnonectes pseudodoriae sp. nov. lack a caruncle on top of the head and very closely resemble L. doriae (Boulenger, 1887) from Myanmar and western and southern Thailand. However, the new species is distinguished from L. doriae and its congeners using an integrative taxonomic approach of morphology, mitochondrial DNA, and bioacoustics. Limnonectes pseudodoriae sp. nov. differs from L. doriae and its congeners by having a unique combination of morphological characters, including body size; skin texture of the interorbital region, dorsum, and shank; toe webbing; relative size of the inner metatarsal tubercle; and coloration of the tympanum, venter, and ova. The advertisement call of the new species is also readily differentiated from that of L. doriae in temporal parameters. Limnonectes pseudodoriae sp. nov. is highly divergent in mitochondrial DNA from L. doriae and its congeners, but its phylogenetic position within the genus is not resolved. The natural history of the new species is presented, and the geographic range of L. doriae in Thailand is clarified.

Contingency and determinism in evolution: Replaying life’s tape

Historical processes display some degree of “contingency,” meaning their outcomes are sensitive to seemingly inconsequential events that can fundamentally change the future. Contingency is what makes historical outcomes unpredictable. Unlike many other natural phenomena, evolution is a historical process. Evolutionary change is often driven by the deterministic force of natural selection, but natural selection works upon variation that arises unpredictably through time by random mutation, and even beneficial mutations can be lost by chance through genetic drift. Moreover, evolution has taken place within a planetary environment with a particular history of its own. This tension between determinism and contingency makes evolutionary biology a kind of hybrid between science and history. While philosophers of science examine the nuances of contingency, biologists have performed many empirical studies of evolutionary repeatability and contingency. Here, we review the experimental and comparative evidence from these studies. Replicate populations in evolutionary “replay” experiments often show parallel changes, especially in overall performance, although idiosyncratic outcomes show that the particulars of a lineage’s history can affect which of several evolutionary paths is taken. Comparative biologists have found many notable examples of convergent adaptation to similar conditions, but quantification of how frequently such convergence occurs is difficult. On balance, the evidence indicates that evolution tends to be surprisingly repeatable among closely related lineages, but disparate outcomes become more likely as the footprint of history grows deeper. Ongoing research on the structure of adaptive landscapes is providing additional insight into the interplay of fate and chance in the evolutionary process.

Sex and weaponry: The distribution of toxin-storage glands on the bodies of male and female cane toads (Rhinella marina)

The parotoid macroglands of bufonid anurans store (and can expel) large volumes of toxic secretions and have attracted detailed research. However, toxins also are stored in smaller glands that are distributed on the limbs and dorsal surface of the body. Female and male cane toads (Rhinella marina) differ in the location of toxin‐storage glands and the extent of glandular structures. Female toads store a larger proportion of their toxins in the parotoids than males as well as (to a lesser extent) in smaller glands on the forelimbs. Males have smaller and more elongate parotoids than females, but glands cover more of the skin surface on their limbs (especially hindlimbs) and dorsal surface. The delay to toxin exudation in response to electrostimulation varied among glands in various parts of the body, and did so differently in males than in females. The spatial distribution of toxin glands differs between the sexes even in toads that have been raised under standardized conditions in captivity; hence, the sexual dimorphism is due to heritable factors rather than developmentally plastic responses to ecological (e.g., habitat, predation risk) differences between the sexes. The selective advantages of this sexual dimorphism remain unclear. A priori, we might expect to see toxin widely dispersed across any part of the body likely to be contacted by a predator; and a wide distribution also would be expected if the gland secretions have other (e.g., male–male rivalry) functions. Why, then, is toxin concentrated in the parotoids, especially in female toads? That concentration may enhance the effectiveness of frontal displays to deter predation and also may facilitate the transfer of stored toxins to eggs.

A case study in evolutionary contingency

Biological evolution is a fundamentally historical phenomenon in which intertwined stochastic and deterministic processes shape lineages with long, continuous histories that exist in a changing world that has a history of its own. The degree to which these characteristics render evolution historically contingent, and evolutionary outcomes thereby unpredictably sensitive to history has been the subject of considerable debate in recent decades. Microbial evolution experiments have proven among the most fruitful means of empirically investigating the issue of historical contingency in evolution. One such experiment is the Escherichia coli Long-Term Evolution Experiment (LTEE), in which twelve populations founded from the same clone of E. coli have evolved in parallel under identical conditions. Aerobic growth on citrate (Cit(+)), a novel trait for E. coli, evolved in one of these populations after more than 30,000 generations. Experimental replays of this population's evolution from various points in its history showed that the Cit(+) trait was historically contingent upon earlier mutations that potentiated the trait by rendering it mutationally accessible. Here I review this case of evolutionary contingency and discuss what it implies about the importance of historical contingency arising from the core processes of evolution.

Protamines in the internally fertilizing neobatrachian frogEleutherodactylus coqui

The internally fertilizing primitive frog Ascaphus truei (family Ascaphidae) from the Pacific Northwest is the only frog with an intromittent organ. The more advanced neobatrachian frog Eleutherodactylus coqui (family Leptodactylidae) from Puerto Rico has secondarily acquired internal fertilization but mates by cloacal apposition. Nonetheless, both frogs have introsperm with an elongated head containing highly condensed chromatin. Characterization of sperm nuclear basic proteins (SNBPs) in E. coqui by acid-urea polyacrylamide gel electrophoresis indicates that, as in A. truei, testes from a single animal contain several protamines. Amino acid analysis indicates a composition for the most rapidly moving protamine of each species as follows: in E. coqui, ARG (35.6 mol %) + LYS (3.8 mol %) + HIS (7.6 mol %) = 47 mol % total basic residues and in A. truei, ARG (42.1 mol %) + LYS (11.1 mol %) = 53.2 mol % total basic residues. Transmission electron microscopy shows that E. coqui introsperm, like those in A. truei, are elongate with highly condensed chromatin. However, E. coqui introsperm lacks an axial perforatorium that extends into an endonuclear canal. These morphological features are plesiomorphic (primitive) and shared by A. truei with urodeles and basal amniotes (Jamieson et al. (1993) Herpetologica 49:52-65). In E. coqui introsperm, the nucleoprotein complex has a cross-sectional axis of 420 + 20 angstroms and shows a knobby chromatin structural organization in TEM. The presence of arginine-enriched protamines in both a basal anuran like the ascaphid A. truei and a more advanced neobatrachian like the leptodactylid E. coqui supports the hypothesis that internal fertilization acts as a constraint on the range of SNBP diversity in animals.