Abstract
BackgroundHabitat variation strongly influences the evolution of developmentally flexible traits, and may drive speciation and diversification. The plethodontid salamander Eurycea tynerensis is endemic to the geologically diverse Ozark Plateau of south-central North America, and comprises both strictly aquatic paedomorphic populations (achieving reproductive maturity while remaining in the larval form) and more terrestrial metamorphic populations. The switch between developmental modes has occurred many times, but populations typically exhibit a single life history mode. This unique system offers an opportunity to study the specific ecological circumstances under which alternate developmental and life history modes evolve. We use phylogenetic independent contrasts to test for relationships between a key microhabitat feature (streambed sediment) and this major life history polymorphism.ResultsWe find streambed microstructure (sediment particle size, type and degree of sorting) to be highly correlated with life-history mode. Eurycea tynerensis is paedomorphic in streams containing large chert gravel, but metamorphoses in nearby streams containing poorly sorted, clastic material such as sandstone or siltstone.ConclusionDeposits of large chert gravel create loosely associated streambeds, which provide access to subsurface water during dry summer months. Conversely, streambeds composed of more densely packed sandstone and siltstone sediments leave no subterranean refuge when surface water dries, presumably necessitating metamorphosis and use of terrestrial habitats. This represents a clear example of the relationship between microhabitat structure and evolution of a major developmental and life history trait, and has broad implications for the role of localized ecological conditions on larger-scale evolutionary processes.
Highlights
Habitat variation strongly influences the evolution of developmentally flexible traits, and may drive speciation and diversification
Using Phylogenetic independent contrasts (PICs) we found a significant negative correlation between paedomorphosis and the presence of material in the sand and granule (r2 = 0.7056; p < 0.001; Fig. 3A) and small/medium gravel (r2 = 0.3721; p < 0.002; Fig. 3B) categories
Streambed microstructure and life history of E. tynerensis Previous work has shown the presence of E. tynerensis to be associated with Silurian and Ordovician chert gravel streams in the western Ozark Plateau [33,34,35]
Summary
Habitat variation strongly influences the evolution of developmentally flexible traits, and may drive speciation and diversification. The switch between developmental modes has occurred many times, but populations typically exhibit a single life history mode. For organisms with complex life cycles, local selection involving major developmental shifts can produce rapid changes in ecology, morphology and life history among populations, and may hasten divergence and speciation [7,8,9]. Most amphibians have a free-living aquatic larval stage followed by metamorphosis into a more terrestrial adult form [10,11,12,13] One deviation from this pathway, exhibited by some salamanders, is paedomorphosis, whereby individuals reach adulthood while maintaining many larval structures ( gills) and an aquatic lifestyle [10,14]. In regions where peripheral habitats are extremely harsh or dry, remaining in the aquatic environment may be the only viable strategy [11,26,27,28]
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