Plasticity and constraint in the evolution of development: Has the aquatic larval stage re-evolved in plethodontid salamanders?
Final Report Abstract
Dollo's Law States that complex structures are unlikely to be regained in the same form, once they are lost during evolution. This notion has been seriously challenged recently by studies indicating a re-evolution of an aquatic larva from direct developing ancestors in plethodontid salamanders. While the phylogenetic evidence seems compelling, we have currently little understanding of the evolutionary and developmental mechanisms that would enable such a re-evolution. For this project development of plethodontid salamanders was investigated, with a particular focus on the development of morphological structures that are assumed to have re-evolved in Desmognathus. The main result is that the larval stage of Desmognathus has indeed re-evolved and that the traditional scenarios about the evolution of this diverse group of salamanders are in need of revision. Desmognathus are a plesiomorphically terrestrial group and a semi-aquatic lifestyle re-evolved within the group. Further evidence suggests that a more or less completely aquatic lifestyle and a corresponding large body size probably evolved repeatedly and independently within the group. The re-evolution of aquatic larvae corresponds to this reinvasion of an aquatic habitat. This is almost the opposite of the traditional scenario, where large, aquatic species were thought to have given rise to smail, more terrestrial species. Embryologically, the evidence for a re-evolution of an aquatic larva ('secondary larva') is compelling. The development of aquatic Desmognathus strongly resembles that of direct-developing, terrestrial plethodontids and has little in common with the developmental patterns seen in plethodontids that are presumed to have retained the plesiomorphically aquatic larva ('primary larva'). The most important finding however, is that morphological structures that were thought to have been lost in the direct developing ancestors and then re-evolved in the secondary larvae of Desmognathus, are actually maintained during embryonic development in at least some direct developing plethodontids. It therefore seems plausible, that the structures in question were never lost in the ontogeny (and evolution) of Desmognathus with a secondary larva, but were still present, albeit in reduced form. Selection did then presumably act upon these vestiges to produce structures similar to the plesiomorphic condition. As such, the re-evolution of an aquatic larva in Desmognathus does not seem to be a clear case ofa violation of Dollo's Law in that morphological structures did not completely disappear and then evolved de-novo again. Rather, these structures were always present during the ontogeny and simply became more elaborate again, which facilitated a secondary invasion of the aquatic habitat. This highlights the importance of ontogenetic studies to fully understand the evolution of morphological structures and the organisms that contain them.
Publications
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(2011) Pseudotriton ruber. Leucism. Herpetological Review 42:406
Larson, j . G. & Müller, H.
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(2012) Do larval traits re-evolve? Evidence from the embryogenesis of a direct-developing salamander (Plethodon cinereus). Evolution 66:252-262
Kerney, R. R., Blackburn, D. C. Müller, H. & Hanken, J.
