SCHLOSSER, G.*: Detection of Dissociated Coevolution of Characters using Heterochrony Plots: Mosaic Evolution of Brain Development in Direct Developing Frogs

Many evolutionary modifications of development are accompanied by heterochronic shifts of character development. Frequently, only some developmental events are shifted relative to others. The detection of such mosaic alterations of character development is increasingly difficult, the more characters have to be compared. Heterochrony plots, in which the timing of developmental events in one species is plotted against the timing of the same events in another species, facilitate the analysis because they allow the simultaneous visualization of multiple events in pairwise species comparisons. In combination with outgroup analysis, heterochrony plots may reveal coordinated temporal shifts of several developmental events relative to others (dissociated coevolution). Such patterns of dissociated coevolution may provide insights into which developmental processes reciprocally constrain each others’ modifiability during evolution (in particular when dissociated coevolution of the same suite of events has occurred repeatedly during phylogeny). Heterochrony plots comparing the direct developing frog Eleutherodactylus coqui with biphasically developing frogs such as Discoglossus pictus illustrate this approach. These plots reveal that heterochronic shifts of several regions of the central nervous system (e.g., lateral motor column, tectum) during the evolution of E. coqui are strongly dissociated from each other, but closely match heterochronic shifts of their targets (limbs and retina, respectively). To what extent the dissociated coevolution of brain regions with (some of) the targets that they innervate is a more general phenomenon remains to be investigated.