Evo-devo perspectives on segmentation: model organisms, and beyond.

Bilaterian animals show a diverse array of segmental patterns and segmentation processes. Differences in pattern and process emerge both in comparisons of taxa and among sets of serial structures within one animal. Diversity in developmental mechanisms of segmentation and their genetic control is reflected in the modes in which segmentation evolves, which are difficult to accommodate within the traditional concept of segments as modular building blocks. Thus, in spite of the apparent simplicity of segmental patterns, studying the evolution of segmentation requires an approach that, in an adequate comparative framework, combines the efforts of researchers of genes, cells, embryos and post-embryonic stages.