Development and evolution of lateral line placodes in amphibians. - II. Evolutionary diversification.

The amphibian lateral line system develops from a series of lateral line placodes. The different phases of development from early induction, to pattern formation, differentiation, morphogenesis, and metamorphic fate were summarized in the first part of this review (Schlosser, 2002a). Here, a survey of the diversity of lateral line systems in amphibians is presented indicating that most phases of lateral line development have been subject to evolutionary changes. Several trends suggest important roles for both adaptive changes and internal constraints in amphibian lateral line evolution. Many of these trends involved the coordinated modification of different derivatives of lateral line placodes suggesting that these placodes are not only autonomous developmental modules, but also units of evolutionary variation that tend to be modified in a coherent and largely context-independent fashion.