We are interested in using non-standard model organisms to explore the diversity of developmental mechanisms and the developmental underpinning of morphological diversity in the animal kingdom. We focus on clitellate annelids, a taxon that is relatively accessible but remains poorly understood. The main experimental organisms in our lab are leeches of the genus Helobdella (Superphylum Lophotrochozoa; Phylum Annelida, Class Clitellata, Subclass Hirudinea, Order Rhynchobdellida, Family Glossiphoniidae). Helobdella spp. are aquatic leeches that feed on small freshwater snails, oligochaetes, and insect larvae. The embryos of Helobdella are highly accessible for embryological experiments. The recently completed H. robusta whole-genome sequencing has endowed us a better capacity to analyze the molecular aspects of leech development. Furthermore, Helobdella is also useful for studying developmental evolution at the species or population level. Helobdella is the most speciose genus within Hirudinea. Their unusual reproductive strategy, self-compatible hermaphroditism (note that all clitellate annelids are hermaphrodites but few can self-fertilize), may have drived rapid diversification within this genus. Interspecies variations in developmental timing, cell lineage, and even patterning mechanism have been reported for Helobdella. Given their experimental tractability, comparative embryology of Helobdella will be a viable approach to explore developmental variations at a species/population level.

Our recent research interests include the following topics:

Animal Body Plan Evolution
Study of leech development helps to understand the development of spiralian/lophotrochozoan embryos. Spiralian taxa are a unique system for studying animal body plan evolution. While the adult body plans vary greatly between molluscs and annelids, for example, their embryos share a common cleavage program and cell lineage pattern. By comparing the development of homologous cells in different taxa, one can hope to see how the diverging developmental programs led to the diversification of body plans. Different from the marine spiralian species, leech and other clitellate annelids undergo direct development. Despite the loss of larval stage, cell lineage patterns are still comparable between clitellates and polychaetes. Thus, it is also possible to reconstruct the 'polychaete-to-clitellate' transition - an important developmental change during the annelid version of 'the invasion of land' - by comparing the major annelid groups.

Evolution of Pattern Formation Mechanisms
Leech development features a stereotyped, invariable cell lineage. This is similar to C. elegans and ascidians, but the cellular complexity of leech is several orders above that of C. elegans and ascidians. Leech embryo may represent the most complicated form of lineage-driven embryogenesis. On the other hand, vertebrates do not have a fixed cell lineage in their early development. They are by-and-large patterned by morphogen-based mechanisms. Given that the biochemical components of molecular pathways involved in cell lineage and morphogen patterning are similar and that all animals share a common origin, these two modes of embryogenesis should be viewed as epiphenomena arising from distinct ways for deploying the conserved molecular pathways during development. One of our major interests lies in understanding the evolvability of developmental mechanisms that enabled the transition between these two modes of patterning mechanisms. We have identified certain 'intermediate form' of patterning mechanism in the leech, and we believe that the leech might be a useful system to bridge these two ideals of embryogenesis.

Development and Evolution of Nervous System and Behavior
The medicinal leeches are a major experimental system for systems neurobiology, as they were mainly studied as 'simple' neural circuits underlying rhythmic behaviors since the 1960s. However, despite all that we have learned about the leech neural circuits, we know very little of how they arise, since embryos of medicinal leeches are fairly small and not accessible for experimental procedures. Studies concerning the development of nervous system in medicinal leeches have since focused on later events in nervous system development, e.g. axon guidance and synapse formation, and the earlier events such as neurogenesis and specification neuronal identities are largely unknown for this system. In the 1970s, Helobdella was introduced as a model system to complement the medicinal leeches for studying neural development, since its early embryo is highly accessible and amenable for experimental manipulations. Unfortunately, because of various technical limits, the two systems were never able to fully complement each other. Now, with the development of new techniques for molecular manipulation in Helobdella, it is possible to fill in that gap. Hence, this combined system of two different leech species should provide a unique opportunity to study the formation of a simple nervous system.

Embryonic Development versus Asexual Reproduction and Regeneration
In additional to Helobdella, we are also studying embryonic development, asexual reproduction and regeneration in other annelid species. One of our main interest concerns a key evolutionary transition between the two major groups of clitellates – the oligochaetes and the leeches. These two groups can be distinguished by features in segmental development. Oligochaetes have a variable number of segments, can undergo postembryonic posterior addition of segments and can regenerate new segments. Leeches, in contrast, have a fixed number (32) of segments, do not grow postembryonic segment and do not regenerate. On the other hand, clitellate embryonic development is highly evolutionarily conserved. For both oligochaetes and leeches, embryonic development is highly stereotyped and follows a very similar cell lineage pattern. In contrast, the addition of new segments in postembryonic development and regeneration of oligochaetes appear to be more similar to that of polychaetes, which implements the gradual addition of new segments from a posterior growth zone containing a highly proliferative stem cell population. Thus, comparing embryonic development and postembryonic development in oligochaetes may bring insights into the evolution of cell-lineage-driven direct development in the clitellate and the loss of axial regeneration in the leech.

Biodiversity of Developmental Patterns
Finally, we are interested in exploring the diversity of lophotrochozoan development by examining "lesser" lophotrochozoan microinvertebrates found in the marine and freshwater environment. Given that their embryonic development has diverged from the ancestral spiralian archetype, we can further appreciate the expanse of evolvability and plasticity in metazoan developmental mechanisms by comparing their embryogenesis to the standard spiralian program. Such investigations can be extended to developmental patterns in asexual reproduction, an area that has been largely ignored.

Prospective students and postdocs are expected to be interested in developmental biology, evolution, and organismal biology. We welcome independent researchers working on the developmental biology of 'non-model' animal species to join us, provided that our facilities and budgets are sufficient to support the proposed study. While this is primarily an embryological laboratory, we also look forward to collaboration with colleagues in other areas of biology (e.g. ecology, genetics, biochemistry & biophysics etc.) using annelids or other local invertebrate species as experimental materials. Please contact the PI to explore the opportunity to study and work with our research group.