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Leech/Annelid Development and the Evolution of Developmental Mechanisms

Leeches are regarded by some with a mixture of horror and disgust so intense as to preclude their consideration as objects for biological investigation. But in fact leeches are among the best studied representatives of the phylum Annelida (segmented worms). Glossiphoniid leech embryos in particular are quite beautiful and are well suited for cellular and molecular analyses of embryonic development.

Development and Evolution

One of the most fascinating and fundamental questions in biology is an essentially historical one. How has the remarkable diversity of species arisen during the past ~2 billion years of biological evolution on earth? It seems clear that the immediate causes of changes in body plan during evolution are changes in development. These developmental changes are thought to depend more on changes in the regulation of evolutionarily well-conserved genes than from the appearance of new genes, but it is inconceivable that within the foreseeable future we will be able to predict development or even developmental differences simply by knowing the genomic sequence of an organism. In the meanwhile [and if we are ever to be able to make such predictions], we need to generate detailed understanding of the development of various kinds of animals and correlate those differences with genomic differences.

Thus, to understand the developmental bases of evolutionary change, we must understand the development of extant species and then make comparisons, with respect to the phylogenetic tree by which they evolved. To avoid bias and inherent circularity, the phylogenetic tree should be constructed independent of developmentally derived traits [i.e. using "developmentally neutral" sequence changes.]
Molecular phylogenies now converge on the re-organization of bilaterally symmetric animals into three major groups: Ecdysozoa (including arthropods, nematodes and other cuticle-shedding groups), Lophotrochozoa (including annelids, mollusks and related phyla), and Deuterostomia (including echinoderms, hemichordates and chordates). Assuming that the last common ancestor of these three groups was a relatively simple worm-like creature, it is of interest to determine how processes such as gastrulation and segmentation have evolved in isolation during the 500+ million years separating these three clades.
Most of the species used to study development are either Deuterostomes (vertebrates, ascidians, echinoderms) or Ecdysozoans (insects and other arthropods, nematodes). Thus, the Lophotrochozoa are a relatively unknown territory for the field of development and evolution.

Annelid Development

Among the Lophotrochozoa, annelids are of obvious interest for such studies:
They are a speciose and morphologically/developmentally diverse group and yet include some of the best studied representatives of this clade. In particular the (highly derived) glossiphoniid leeches such as Helobdella robusta are arguably the best understood annelid in terms of cellular and molecular mechanisms of embryonic development, and thus provide a good starting point for comparisons with other annelids.
They contain the only unambiguously segmented lophotrochozoan species (along with unsegmented groups that were previously categorized as separate phyla).
Many species possess extraordinary powers of postembryonic segmentation, regeneration, vegetative reproduction (and thus the ability to re-specify segment identities)
Their spiral cleavages and formation (in polychaetes) of trochophore larvae are important points of comparison with mollusks and flatworms.
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? And how is it that in annelids, chordates (and many arthropods) segment primordia arise sequentially from a posterior growth zone, whereas flies generate segments simultaneously from a syncytial blastoderm?

Research in our lab is currently supported by NSF, NIH and NASA.