Evidence of morphological homoplasy among large, semi-aquatic species of Desmognathus

The salamander family Plethodontidae is replete with instances of repeated homoplasy. We tested for morphological homoplasy in distantly related species of the plethodontid genus Desmognathus that share similar ecologies. Specifically, we compared species that are large and nearly aquatic. Using morphometric analyses, we compared the respective morphologies of four large, nearly aquatic forms, specifically the Black Mountain Salamander (Desmognathus welteri), the Dwarf Black-bellied Salamander (D. folkertsi), and two phylogenetically divergent lineages of the Black-bellied Salamander (D. quadramaculatus). Morphometric analysis uncovered distinct differences among them. However, all of the large-bodied lineages exhibited the same extent of tail-fin development in spite of D. welteri’s closer phylogenetic relationship to smaller, more-terrestrial species than to the other large, nearly aquatic forms we tested. We hypothesize that large body size is also a consequence of aquatic adaptation. These morphological consequences of a nearly aquatic ecology represent another case of homoplasy within this salamander family.

How Metamorphosis Is Different in Plethodontids: Larval Life History Perspectives on Life-Cycle Evolution

Plethodontid salamanders exhibit biphasic, larval form paedomorphic, and direct developing life cycles. This diversity of developmental strategies exceeds that of any other family of terrestrial vertebrate. Here we compare patterns of larval development among the three divergent lineages of biphasic plethodontids and other salamanders. We discuss how patterns of life-cycle evolution and larval ecology might have produced a wide array of larval life histories. Compared with many other salamanders, most larval plethodontids have relatively slow growth rates and sometimes exceptionally long larval periods (up to 60 mo). Recent phylogenetic analyses of life-cycle evolution indicate that ancestral plethodontids were likely direct developers. If true, then biphasic and paedomorphic lineages might have been independently derived through different developmental mechanisms. Furthermore, biphasic plethodontids largely colonized stream habitats, which tend to have lower productivity than seasonally ephemeral ponds. Consistent with this, plethodontid larvae grow very slowly, and metamorphic timing does not appear to be strongly affected by growth history. On the basis of this, we speculate that feeding schedules and stress hormones might play a comparatively reduced role in governing the timing of metamorphosis of stream-dwelling salamanders, particularly plethodontids.

Induction of Metamorphosis Causes Differences in Sex-Specific Allocation Patterns in Axolotls (Ambystoma mexicanum) that Have Different Growth Histories

We tested the hypothesis that salamanders growing at different rates would have allocation patterns that differ among male and female metamorphic and larval salamanders. We raised individual axolotls, Ambystoma mexicanum, on four food regimes: constant high growth (throughout the experiment), constant low growth (restricted throughout the experiment), high growth switched to low growth (ad libitum switched after 140 d to restricted), and low growth switched to high growth (restricted switched after 140 d to ad libitum). Because axolotls are obligate paedomorphs, we exposed half of the salamanders to thyroid hormone to induce metamorphosis. We assayed growth and dissected and weighed gonads and fat bodies. Salamanders that were switched from restricted to ad libitum food regime delayed metamorphosis. In all treatment groups, females had larger gonads than males and males had larger fat bodies than females. The association between storage and reproduction differed between larvae and metamorphs and depended on sex.

Can parallel diversification occur in sympatry? Repeated patterns of body-size evolution in coexisting clades of North American salamanders

A classic paradigm in evolutionary biology is that geographically isolated clades inhabiting similar selective regimes will diversify to create similar sets of phenotypes in different locations (e.g., similar stickleback species in different lakes, similar Anolis ecomorphs on different islands). Such parallel radiations are not generally expected to occur in sympatry because the available niche space would be filled by whichever clade is diversified first. Here, we document a very different pattern, the parallel evolution of similar body-size morphs in three sympatric clades of plethodontid salamanders (Desmognathus, Plethodon, Spelerpinae) in eastern North America. Using a comprehensive, time-calibrated phylogeny of North American plethodontids from nuclear and mitochondrial DNA sequences, we show that these three clades have undergone replicated patterns of evolution in body size and that this parallel diversification occurred in broad-scale sympatry. At the local scale, we find that coexisting species from these clades are more similar in body size than expected under a null model in which species are randomly assembled into communities. These patterns are particularly surprising in that competition is known to be important in driving phenotypic diversification and limiting local coexistence of similar-sized species within these clades. Although parallel diversification of sympatric clades may seem counterintuitive, we discuss several ecological and evolutionary factors that may allow the phenomenon to occur.