Axial patterning in the leech: developmental mechanisms and evolutionary implications.(Genetic Regulatory Networks in Embryogenesis and Evolution)

The phylogenetic history of animal body plans, particularly those of the segmented protostomes (arthropods + annelids), is one of the most important topics in the current study of the evolution of developmental mechanisms. Genetic studies of the fruitfly Drosophila have uncovered a wealth of information about the molecular biology of development, but the degree to which other animals utilize similar or different mechanisms is not entirely clear, nor is it obvious how and when the mechanisms observed in Drosophila first arose during evolution.

Glossiphoniid leeches, such as Helobdella, offer numerous advantageous features for embryological research. Helobdella embryos are highly amenable to 'classical' embryology: the eggs are large, undergo stereotyped cell lineages, and single cells can be identified and manipulated during the developmental stages when segmentation is being established (1). The segmental body plan of the leech is generated through the iterative cell divisions of teloblastic stem cells situated in a posterior growth zone (2) - a process so outwardly different from what is known of segmentation in insects or vertebrates that a detailed comparison is likely to be informative. Comparing the cellular and molecular mechanisms that underlie pattern formation in different taxa can reveal those…