The invertebrate deuterostomes: an introduction to their phylogeny, reproduction, development, and genomics.

The invertebrate deuterostomes comprise approximately 10,000 species of marine animals distributed throughout the oceans of the world. Recent molecular phylogenetic studies suggest that the deuterostomes include only the echinoderms, hemichordates, and chordates. Among the echinoderms, sea urchins are most widely used for developmental studies. Sexual reproduction is by far the most common mode within the group. Although sexual reproduction is predominant, reproduction by asexual budding occurs in a number of groups that have colonial forms, including pterobranch hemichordates and the colonial ascidians. Experimental work with invertebrate deuterostomes has played an important role in developmental biology, cell biology, and genetics. The ability to produce vast numbers of synchronously developing embryos (using methods that are best developed with sea urchins) facilitates a wide variety of biochemical and molecular biological approaches, from protein purification to complementary DNA (cDNA) library construction. Genomic resources are being developed from a variety of these organisms because of the utility of invertebrate deuterostomes for developmental analysis and their close phylogenetic relationship to vertebrates. The draft genomic sequences of two invertebrate deuterostomes, the related ascidians C. intestinalis and C. savignyi, are released. A useful feature of invertebrate deuterostome genomes is that many genes appear to be present in single copies that are found in multiple copies in vertebrate genomes.