Glossiphoniid leech embryos are well suited for cellular and molecular analyses of embryonic development. They are also one of the best studied representatives of the phylum Annelida (segmented worms). Molecular phylogenies indicate that most bilaterally symmetric animals fall into three major groups whose ancestors had already diverged by the time of the Cambrian explosion, ~600 million yeas ago: Edysozoa (arthropods, nematodes and other "cuticle-shedding" animals); Lophotrochozoa (annelids, mollusks flatworms, and related phyla whose embryos undergo spiral cleavage); and Deuterostomia (echinoderms, hemichordates and chordates). What was the last common ancestor of these three groups like? And how have developmental processes (e.g., gastrulation, axial growth, segmentation) evolved in isolation in the three major groups of animals?
Accordingly, the goals of the research in this lab are twofold: first, to obtain as satisfying as possible an understanding of leech development; and second, to understand how developmental processes are modified during the evolution of different animal taxa. For example, leeches undergo spiral cleavages homologous to those of mollusks, yet generate a segmented body plan, like arthropods. How have cell fates been modified among the different spirally cleaving animal groups to generate different types of embryos and adults? In particular, how is it that leeches make exactly 32 segments during development and cannot make more, either post-embryonically or in response to injury, while closely related annelids (oligochaetes) exhibit wonderful capacities for post-embryonic segmentation, regeneration, and even vegetative reproduction? And how finally, how is it that in annelids, chordates (and many arthropods) segment primordia arise sequentially from a posterior zone of cell migration and/or proliferation, whereas flies generate segments simultaneously within a syncytial blastoderm? Our work entails the use of various cellular, molecular and embryological techniques. Microinjection of cell lineage tracers and various imaging techniques are frequently used in combination with other procedures.
Areas of recent interest include:
(1) the dramatic increase in genome rearrangements in leeches and oligochaetes compared to most other animals; (2) the expression and function of genes in the highly fragmented Hox cluster of the leech; (3) the molecular control of stem cell divisions in the posterior growth zone in leech; (4) dynamic patterns of wnt and notch expression in the 2-cell embryo; (5) the mechanisms of epiboly (cell movements during gastrulation); (6) gangliogenesis and the assignments of specific neuronal fates; (7) developmental comparisons of leeches and oligochaetes.
Given its compact genome and the extensive knowledge of its development that is already available, Helobdella robusta (the leech we study), was an appropriate choice for one of the first lophotrochozoan species for genome sequencing at the Joint Genome Institute in Walnut Creek.
In the News
Leeches, despite the yuck factor, have captured the hearts of two University of California, Berkeley, scientists who are part of a team that this week is publishing the leech’s complete genome sequence.