The first chordates appear in the fossil record at the time of the Cambrian explosion, nearly 550 million years ago. The modern ascidian tadpole represents a plausible approximation to these ancestral chordates. To illuminate the origins of chordate and vertebrates, we generated a draft of the protein-coding portion of the genome of the most studied ascidian, Ciona intestinalis. The Ciona genome contains ϳ16,000 protein-coding genes, similar to the number in other invertebrates, but only half that found in vertebrates. Vertebrate gene families are typically found in simplified form in Ciona, suggesting that ascidians contain the basic ancestral complement of genes involved in cell signaling and development. The ascidian genome has also acquired a number of lineage-specific innovations, including a group of genes engaged in cellulose metabolism that are related to those in bacteria and fungi.

Genome-wide sequence analysis in the invertebrate chordate, Ciona intestinalis, has provided a comprehensive picture of immune-related genes in an organism that occupies a key phylogenetic position in vertebrate evolution. The pivotal genes for adaptive immunity, such as the major histocompatibility complex (MHC) class I and II genes, T-cell receptors, or dimeric immunoglobulin molecules, have not been identified in the Ciona genome. Many genes involved in innate immunity have been identified, including complement components, Toll-like receptors, and the genes involved in intracellular signal transduction of immune responses, and show both expansion and unexpected diversity in comparison with the vertebrates. In addition, a number of genes were identified which predicted integral membrane proteins with extracellular C-type lectin or immunoglobulin domains and intracellular immunoreceptor tyrosine-based inhibitory motifs (ITIMs) and immunoreceptor tyrosine-based activation motifs (ITAMs) (plus their associated signal transduction molecules), suggesting that activating and inhibitory receptors have an MHC-independent function and an early evolutionary origin. A crucial component of vertebrate adaptive immunity is somatic diversification, and the recombination activating genes (RAG) and activation-induced cytidine deaminase (AID) genes responsible for the Generation of diversity are not present in Ciona. However, there are key V regions, the essential feature of an immunoglobulin superfamily VC1-like core, and possible proto-MHC regions scattered throughout the genome waiting for Godot.

What mechanisms underlie aging? One theory, the wear-and-tear model, attributes aging to progressive deterioration in the molecular and cellular machinery which eventually lead to death through the disruption of physiological homeostasis. The second suggests that life span is genetically programmed, and aging may be derived from intrinsic processes which enforce a non-random, terminal time interval for the survivability of the organism. We are studying an organism that demonstrates both properties: the colonial ascidian, Botryllus schlosseri. Botryllus is a member of the Tunicata, the sister group to the vertebrates, and has a number of life history traits which make it an excellent model for studies on aging. First, Botryllus has a colonial life history, and grows by a process of asexual reproduction during which entire bodies, including all somatic and germline lineages, regenerate every week, resulting in a colony of genetically identical individuals. Second, previous studies of lifespan in genetically distinct Botryllus lineages suggest that a direct, heritable basis underlying mortality exists that is unlinked to reproductive effort and other life history traits. Here we will review recent efforts to take advantage of the unique life history traits of B. schlosseri and develop it into a robust model for aging research.

is a colonial marine invertebrate chordate that utilizes both sexual and asexual modes of reproduction. Colonies of Botryllus can fuse and exchange cells via the bloodstream, including mobile stem cells that are capable of parasitizing the soma and germ line. We use Botryllus as a model for investigating the molecular mechanisms of stem-cell competition and the interaction between the germ line and germ-line support cells.The embryonic and post-embryonic origins of follicle cells are still unclear in Botryllus. Shown here is a fluorescence double-in situ hybridization detecting the transcripts of vitellogenin (red) and transforming growth factor ␤ (green) genes in the follicle cells surrounding a developing Botryllus oocyte; nuclei are visualized by DAPI staining (blue). Interior to the follicle cells are the nuclei of the test cells, an accessory cell type that is retained around the Botryllus embryo following fertilization. Vitellogenin and other yolk proteins secreted by follicle cells play an evolutionarily conserved role in nourishing the developing embryo, and are expressed by Botryllus follicle cells during the final phase of oogenesis, termed vitellogenesis. Oocytes mature outside of the body chamber of developing Botryllus buds in a gonadal niche that may also contain maturing testes. Once the oocyte is fully grown, the outer layer of follicle cells is shed and the egg is ovulated into the body chamber for fertilization.

The colonial ascidian Botryllus schlosseri is an ideal model organism for studying several aspects of germ-line biology, such as germ-line stem cell function, gonad differentiation, and regulation of fertility. Botryllus fertility is synchronized and sequential; colonies always develop testes fi rst (8-10 weeks post metamorphosis), and some genotypes may remain male for long periods of time. Development of oocytes occurs after testes development, resulting in colonies that are hermaphrodites. Adverse changes in environmental conditions (i.e. temperature and nutrients availability) and other signals can lead to an infertile state that can persist for multiple asexual generations. Once conditions improve, however, colonies can regenerate testes and eggs. The mechanisms regulating the ability of Botryllus colonies to cycle in and out fertility are poorly understood, but provide a model in which we can characterize the cellular and molecular mechanisms controlling fertility in either male or female gonads. This bright-fi eld picture depicts a hermaphrodite colony with developing and maturing testes, as well as maturing oocytes and eggs (large, spherical brown cells).