Cell-cell interactions in dorsoventral patterning of the segmental ectoderm of the leech Helobdella robusta: differences between the rostral and midbody segments of the same individual and variation among geographical strains

textDevelopmental origins and the cell lineage patterns of segmental ectoderm are evolutionary conserved in clitellate annelids. Recent comparative studies have shown that developmental pathways can be highly variable even if their morphological end product is evolutionarily conserved. To see whether this kind of evolutionary dissociation can be applied to segmentally homologous ‘modules’ in the same individuals, I compared axial patterning pathways in different segments of the leech Helobdella robusta. Distinct cell lineage origins of homologous O and P pattern elements in the ectoderm of the rostral segments and the midbody segments were previously revealed by lineage tracer injection experiments. In the rostral segments, these pattern elements arise from a single OP lineage. In the midbody segments, these pattern elements arise from separated O and Plineages. In the leech, the O and P lineages of the midbody are the members of the O/P equivalence group and are conditionally specified by cell-cell interactions. Ablation experiments revealed that the cell-cell interactions involved in the development of the OP lineage are different from those in the O/P equivalence group, despite the morphological similarity of their descendant tissues. These data suggest that serially homologous structures in different segments of the same individual can develop via distinct developmental pathways. To further explore the dissociability of developmental pathway and its morphological outcome in an evolutionary context, symmetry-breaking pathways in the O/P equivalence group of the midbody segments were compared in three closely-related laboratory strains of Helobdella. It was found that the mesoderm, the M lineage, and the dorsal ectoderm, the Q lineage, are redundantly involved in symmetry breaking of the O/P equivalence group in one particular leech strain, while the Q lineage is both necessary and sufficient for symmetry breaking of the O/P equivalence group in a second strain. Results from this comparative study of the symmetry breaking pathways in closely related leech strains suggest that redundancy may play an important role in the evolution of developmental pathways. Additional evolutionary implications of these findings will be discussed.Biological Sciences, School o

Developmental biology of the leech Helobdella

Glossiphoniid leeches of the genus Helobdella provide experimentally tractable models for studies in evolutionary developmental biology (Evo-Devo). Here, after a brief rationale, we will summarize our current understanding of Helobdella development and highlight the near term prospects for future investigations, with respect to the issues of: D quadrant specification; the transition from spiral to bilaterally symmetric cleavage; segmentation, and the connections between segmental and non-segmental tissues; modifications of BMP signaling in dorsoventral patterning and the O-P equivalence group; germ line specification and genome rearrangements. The goal of this contribution is to serve as a summary of, and guide to, published work

Report A New Molecular Logic for BMP-Mediated Dorsoventral Patterning in the Leech Helobdella

Summary Bone morphogenetic protein (BMP) signaling is broadly implicated in dorsoventral (DV) patterning of bilaterally symmetric animals Results and Discussion Dorsoventral Patterning in Segmental Ectoderm of the Leech: The O-P Equivalence Group Cell lineage plays a predominant role in leech embryogenesis. Segmental ectoderm and mesoderm arise from a posterior growth zone consisting of five bilateral pairs of lineagerestricted segmentation stem cells (M, N, O/P, O/P, and Q teloblasts). Each teloblast divides repeatedly and asymmetrically, giving rise to a column (bandlet) of segmental founder cells (primary blast cells). Near the surface in prospective dorsal posterior territory, the ipsilateral bandlets converge into parallel arrays, forming left and right germinal bands ( Despite the overall determinacy of Helobdella development, the primary o and p blast cells in the lateral ectoderm constitute a developmental equivalence group. They are initially equipotent and assume distinct O (ventrolateral) or P (dorsolateral) fates based on their positions in the germinal band Short-Range BMP5-8 Signaling Is Necessary and Sufficient to Specify the P Fate To test this hypothesis, we first surveyed BMPs and their antagonists in Helobdella (see Also unexpectedly, Hau-gremlin was expressed at markedly higher levels in the p bandlet, immediately adjacent to the dorsalmost q bandlet, rather than in ventral territory To see whether Hau-BMP5-8 is necessary for patterning the O-P equivalence group, we knocked down Hau-BMP5-8 expression by injecting the Q teloblast or a progenitor (proteloblast OPQ or NOPQ) with Hau-bmp5-8 antisense morpholino oligomer (ASMO). In normal embryos, the o blast cell divides with marked asymmetry, giving rise to a larger anterior daughter and a smaller posterior daughter, whereas the p blast cell divides nearly equally, giving rise to two similarly sized daughters

A tale of two leeches: Toward the understanding of the evolution and development of behavioral neural circuits

In the animal kingdom, behavioral traits encompass a broad spectrum of biological phenotypes that have critical roles in adaptive evolution, but an EvoDevo approach has not been broadly used to study behavior evolution. Here, we propose that, by integrating two leech model systems, each of which has already attained some success in its respective field, it is possible to take on behavioral traits with an EvoDevo approach. We first identify the developmental changes that may theoretically lead to behavioral evolution and explain why an EvoDevo study of behavior is challenging. Next, we discuss the pros and cons of the two leech model species, Hirudo, a classic model for invertebrate neurobiology, and Helobdella, an emerging model for clitellate developmental biology, as models for behavioral EvoDevo research. Given the limitations of each leech system, neither is particularly strong for behavioral EvoDevo. However, the two leech systems are complementary in their technical accessibilities, and they do exhibit some behavioral similarities and differences. By studying them in parallel and together with additional leech species such as Haementeria, it is possible to explore the different levels of behavioral development and evolution.Fil: Kuo, Dian Han. National Taiwan University; República de ChinaFil: De Miguel, Francisco F.. Universidad Nacional Autónoma de México; MéxicoFil: Heath Heckman, Elizabeth A. C.. University of California at Berkeley; Estados UnidosFil: Szczupak, Lidia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Fisiología, Biología Molecular y Neurociencias. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Fisiología, Biología Molecular y Neurociencias; ArgentinaFil: Todd, Krista. Westminster College; Estados UnidosFil: Weisblat, David A.. University of California at Berkeley; Estados UnidosFil: Winchell, Christopher J.. University of California at Berkeley; Estados Unido

Insights into bilaterian evolution from three spiralian genomes

Current genomic perspectives on animal diversity neglect two prominent phyla, the molluscs and annelids, that together account for nearly one-third of known marine species and are important both ecologically and as experimental systems in classical embryology(1–3). Here we describe the draft genomes of the owl limpet (Lottia gigantea), a marine polychaete (Capitella teleta) and a freshwater leech (Helobdella robusta), and compare them with other animal genomes to investigate the origin and diversification of bilaterians from a genomic perspective. We find that the genome organization, gene structure and functional content of these species are more similar to those of some invertebrate deuterostome genomes (for example, amphioxus and sea urchin) than those of other protostomes that have been sequenced to date (flies, nematodes and flatworms). The conservation of these genomic features enables us to expand the inventory of genes present in the last common bilaterian ancestor, establish the tripartite diversification of bilaterians using multiple genomic characteristics and identify ancient conserved long- and short-range genetic linkages across metazoans. Superimposed on this broadly conserved pan-bilaterian background we find examples of lineage-specific genome evolution, including varying rates of rearrangement, intron gain and loss, expansions and contractions of gene families, and the evolution of clade-specific genes that produce the unique content of each genome