Regulatory Blueprint for a Chordate Embryo

Abstract

Ciona is an emerging model system for elucidating gene networks in development. Comprehensive in situ hybridization assays have identified 76 regulatory genes with localized expression patterns in the early embryo, at the time when naïve blastomeres are determined to follow specific cell fates. Systematic gene disruption assays provided more than 3000 combinations of gene expression profiles in mutant backgrounds. Deduced gene circuit diagrams describing the formation of larval tissues were computationally visualized. These diagrams constitute a blueprint for the Ciona embryo and provide a foundation for understanding the evolutionary origins of the chordate body plan.

Embryology of a planktonic tunicate reveals traces of sessility

“…Fate restrictions occur considerably earlier, and clonal organization of the cell lineage is even more tightly coupled to tissue fate in O. dioica. For example, tail muscle cells derive from four distinct lineages in the ascidian embryo (A8.16, B7.4, B7.5, B7.8, and b8.17) (13,19), whereas, in O. dioica, no A-cell line gives rise to musculature. Similarly, whereas the nervous system derives from A-, a-, and b-lines in ascidians (19,28,29) (31,32).…”

“…Thus, larvaceans are pivotal for the understanding of chordate evolution and figured prominently in discussions about the role neoteny might have played in the evolution of chordates (9)(10)(11)(12). However, whereas cell lineage and molecular aspects of tissue restriction have been intensely studied in ascidians, culminating in the deciphering of the first metazoan whole-embryo gene regulatory network (13,14), lack of comparable knowledge in larvaceans has hindered a deeper understanding of chordate ontogeny and evolution (2,4,15,16). Here, we report the precise cell lineage and fate map in the embryo of the larvacean Oikopleura dioica, using 4D microscopy technique and transmission electron microscopy.…”

Darwin 200: Beneath the surface

“…It is this problem that Lemaire and others now hope to address in tunicates. Because of the relatively simple body plan and the ease with which tunicate genes can be manipulated, researchers have come closer to building a complete regulatory network for tunicates than for any other model organism, one that shows how each regulatory gene interacts with every other, in every cell, and at every stage of development 10 . Lemaire is hoping to extract a 'network signature' that would characterize the process of gastrulation, perhaps a mathematical expression that describes the density of links between genes or the types of feedback loops used.…”

Cryptic speciation in a model invertebrate chordate

“…A chordate body plan, research amenability, worldwide distribution, and easy sampling are the main reasons for the success of C. intestinalis in biosciences (6,7). In 2002, completion of the whole genome sequence marked an impressive transition in the history of C. intestinalis, transforming it into a model organism of prime importance in integrative genomics (8)(9)(10). Besides progress in culturing, line storage, and formal genetics, research with C. intestinalis is largely based on sampling in nature (like in the 18th century) under the view that natural populations globally belong to the linnean taxon (data not shown and refs.…”