Anthony W. De Tomaso

Isolation and Characterization of a Protochordate Histocompatibility Locus

Histocompatibility—the ability of an organism to distinguish its own cells and tissue from those of another—is a universal phenomenon in the Metazoa. In vertebrates, histocompatibility is a function of the immune system controlled by a highly polymorphic major histocompatibility complex (MHC), which encodes proteins that target foreign molecules for immune cell recognition. The association of the MHC and immune function suggests an evolutionary relationship between metazoan histocompatibility and the origins of vertebrate immunity. However, the MHC of vertebrates is the only functionally characterized histocompatibility system; the mechanisms underlying this process in non-vertebrates are unknown. A primitive chordate, the ascidian Botryllus schlosseri, also undergoes a histocompatibility reaction controlled by a highly polymorphic locus. Here we describe the isolation of a candidate gene encoding an immunoglobulin superfamily member that, by itself, predicts the outcome of histocompatibility reactions. This is the first non-vertebrate histocompatibility gene described, and may provide insights into the evolution of vertebrate adaptive immunity.

Stem Cells Are Units of Natural Selection in a Colonial Ascidian

Stem cells are highly conserved biological units of development and regeneration. Here we formally demonstrate that stem cell lineages are also legitimate units of natural selection. In a colonial ascidian, Botryllus schlosseri, vascular fusion between genetically distinct individuals results in cellular parasitism of somatic tissues, gametes, or both. We show that genetic hierarchies of somatic and gametic parasitism following fusion can be replicated by transplanting cells between colonies. We prospectively isolate a population of multipotent, self-renewing stem cells that retain their competitive phenotype upon transplantation. Their single-cell contribution to either somatic or germline fates, but not to both, is consistent with separate lineages of somatic and germline stem cells or pluripotent stem cells that differentiate according to the niche in which they land. Since fusion is restricted to individuals that share a fusion/histocompatibility allele, these data suggest that histocompatibility genes in Botryllus evolved to protect the body from parasitic stem cells usurping asexual or sexual inheritance.

Striving for normality: whole body regeneration through a series of abnormal generations

Embryogenesis and asexual reproduction are commonly considered to be coordinated developmental processes, which depend on accurate progression through a defined sequence of developmental stages. Here we report a peculiar developmental scenario in a simple chordate, Botryllus schlossenri, wherein a normal colony of individuals (zooids and buds) is regenerated from the vasculature (vascular budding) through a sequence of morphologically abnormal developmental stages. Vascular budding was induced by surgically removing buds and zooids from B. schlossenri colonies, leaving only the vasculature and the tunic that connects them. In vivo imaging and histological sections showed that the timing and morphology of developing structures during vascular budding deviated significantly from other asexual reproduction modes (the regular asexual reproduction mode in this organism and vascular budding in other botryllid species). Subsequent asexual reproduction cycles exhibited gradual regaining of normal developmental patterns, eventually leading to regeneration of a normal colony. The conversion into a normal body form suggests the activation of an alternative pathway of asexual reproduction, which involves gradual regaining of normal positional information. It presents a powerful model for studying the specification of the same body plan by different developmental programs.—Voskoboynik A., Simon‐Blecher, N., Soen, Y., Rinkevich, B., De Tomaso A. W., Ishizuka, K. J., Weissman I. L., Striving for normality: whole body regeneration through a series of abnormal generations. FASEB J. 21, 1335–1344 (2007)

Early lineage specification of long-lived germline precursors in the colonial ascidian Botryllus schlosseri

In many taxa, germline precursors segregate from somatic lineages during embryonic development and are irreversibly committed to gametogenesis. However, in animals that can propagate asexually, germline precursors can originate in adults. Botryllus schlosseri is a colonial ascidian that grows by asexual reproduction, and on a weekly basis regenerates all somatic and germline tissues. Embryonic development in solitary ascidians is the classic example of determinative specification, and we are interested in both the origins and the persistence of stem cells responsible for asexual development in colonial ascidians. In this study, we characterized vasa as a putative marker of germline precursors. We found that maternally deposited vasa mRNA segregates early in development to a posterior lineage of cells, suggesting that germline formation is determinative in colonial ascidians. In adults, vasa expression was observed in the gonads, as well as in a population of mobile cells scattered throughout the open circulatory system, consistent with previous transplantation/reconstitution results. vasa expression was dynamic during asexual development in both fertile and infertile adults, and was also enriched in a population of stem cells. Germline precursors in juveniles could contribute to gamete formation immediately upon transplantation into fertile adults, thus vasa expression is correlated with the potential for gamete formation, which suggests that it is a marker for embryonically specified, long-lived germline progenitors. Transient vasa knockdown did not have obvious effects on germline or somatic development in adult colonies, although it did result in a profound heterochrony, suggesting that vasa might play a homeostatic role in asexual development.

A conserved role of the VEGF pathway in angiogenesis of an ectodermally-derived vasculature

Angiogenesis, the growth and remodeling of a vascular network, is an essential process during development, growth and disease. Here we studied the role of the vascular endothelial growth factor receptor (VEGFR) in experimentally-induced angiogenesis in the colonial ascidian Botryllus schlosseri (Tunicata, Ascidiacea). The circulatory system of B. schlosseri is composed of two distinct, but interconnected regions: a plot of sinuses and lacunae which line the body, and a transparent, macroscopic extracorporeal vascular network. The vessels of the extracorporeal vasculature are morphologically inverted in comparison to the vasculature in vertebrates: they consist of a single layer of ectodermally-derived cells with the basal lamina lining the lumen of the vessel. We found that when the peripheral circulatory system of a colony is surgically removed, it can completely regenerate within 24 to 48 h and this regeneration is dependent on proper function of the VEGF pathway: siRNA-mediated knockdown of the VEGFR blocked vascular regeneration, and interfered with vascular homeostasis. In addition, a small molecule, the VEGFR kinase inhibitor PTK787/ZK222584, phenocopied the siRNA knockdown in a reversible manner. Despite the disparate germ layer origins and morphology of the vasculature, the developmental program of branching morphogenesis during angiogenesis is controlled by similar molecular mechanisms, suggesting that the function of the VEGF pathway may be co-opted during the regeneration of an ectoderm-derived tubular structure.

The significance and scope of evolutionary developmental biology: A vision for the 21st century

Evolutionary developmental biology (evo‐devo) has undergone dramatic transformations since its emergence as a distinct discipline. This paper aims to highlight the scope, power, and future promise of evo‐devo to transform and unify diverse aspects of biology. We articulate key questions at the core of eleven biological disciplines—from Evolution, Development, Paleontology, and Neurobiology to Cellular and Molecular Biology, Quantitative Genetics, Human Diseases, Ecology, Agriculture and Science Education, and lastly, Evolutionary Developmental Biology itself—and discuss why evo‐devo is uniquely situated to substantially improve our ability to find meaningful answers to these fundamental questions. We posit that the tools, concepts, and ways of thinking developed by evo‐devo have profound potential to advance, integrate, and unify biological sciences as well as inform policy decisions and illuminate science education. We look to the next generation of evolutionary developmental biologists to help shape this process as we confront the scientific challenges of the 21st century.

The significance and scope of evolutionary developmental biology

Evolutionary developmental biology (evo‐devo) has undergone dramatic transformations since its emergence as a distinct discipline. This paper aims to highlight the scope, power, and future promise of evo‐devo to transform and unify diverse aspects of biology. We articulate key questions at the core of eleven biological disciplines—from Evolution, Development, Paleontology, and Neurobiology to Cellular and Molecular Biology, Quantitative Genetics, Human Diseases, Ecology, Agriculture and Science Education, and lastly, Evolutionary Developmental Biology itself—and discuss why evo‐devo is uniquely situated to substantially improve our ability to find meaningful answers to these fundamental questions. We posit that the tools, concepts, and ways of thinking developed by evo‐devo have profound potential to advance, integrate, and unify biological sciences as well as inform policy decisions and illuminate science education. We look to the next generation of evolutionary developmental biologists to help shape this process as we confront the scientific challenges of the 21st century.

Molecular mechanisms of allorecognition in a basal chordate

Allorecognition has been described in many metazoan phyla, from the sponges to the mammals. In vertebrates, allorecognition is a result of a MHC-based recognition event central to adaptive immunity. However, the origin of the adaptive immune system and the potential relationship to more primitive allorecognition systems is unclear. The colonial ascidian, Botryllus schlosseri, has been used as a model organism for the study of allorecognition for over a century, as it undergoes a natural transplantation reaction controlled by a single, highly polymorphic locus. Herein we will summarize our current understanding of the molecular mechanisms that underlie this innate allorecognition reaction.

Regeneration and Stem Cells in Ascidians

Understanding and utilizing the ability of stem cells to expand and differentiate into tissues and organs is a major goal of biomedical science. Ascidians are basal chordates (Tunicata) which offer unique opportunities to investigate the biology of stem cells. These marine organisms begin their life as a larva with a typical chordate body plan, including a notochord, dorsal hollow nerve tube and a striated musculature. After a swimming phase, the larvae settles and undergoes an extensive metamorphosis during which most of the chordate characteristics are resorbed, leaving a filter feeding sessile invertebrate adult. Due to its small size (in many species a larva consists of ca. 2,500 cells) and rapid development (a fertilized egg can complete development in less that 24 h in Ciona species), the study of ascidian larvae has a long history and continues to be an outstanding model for studying specification and differentiation events which occur during chordate embryogenesis. In comparison, the adult body plan is relatively unstudied at a molecular level, but several examples of extensive regeneration following surgical ablation of different tissues have been described. In addition, within this chordate subphylum two distinct adult body plans exist: solitary and colonial. Following larval metamorphosis, solitary species grow into an adult that can range from several millimeters to tens of centimeters in length. In addition, colonial species grow not by increasing in size, but by asexually propagating, eventually creating a colony of genetically identical individuals that can cover areas of several square meters. 1 Institute for Stem Cell Biology and Regenerative Medicine, Stanford University, Stanford, CA 94305, USA 2 Biology Department, Center for Developmental Biology and Institute for Stem Cell and Regenerative Medicine, University of Washington, Seattle, WA 98195, USA 3 Friday Harbor Laboratories, University of Washington, Friday Harbor, WA 98250, USA 4 Department of Biological Sciences, Stanford University, Hopkins Marine Station, 120 Oceanview Blvd, Pacific Grove, CA 93950, USA * Author for correspondence Tel.: 831 655 6206, Fax: 831 3750793 E-mail: tdet@stanford.edu T.C.G. Bosch (ed.), Stem Cells: From Hydra to Man, 95

Botryllus schlosseri allorecognition: tackling the enigma ☆

Allorecognition has been well-studied in the context of vertebrate adaptive immunity and recognition of the Major Histocompatibility Complex (MHC), which is the central event of vertebrate immune responses. Although allorecognition systems have been identified throughout the metazoa, recent results have shown that there is no apparent conservation or orthologous relationship between the mechanisms underlying this phenomenon in different organisms. Thus the origin of the vertebrate adaptive immune system as well as these other complex recognition systems is a complete mystery. This review will focus on allorecognition in Botryllus schlosseri, a basal chordate which undergoes a natural transplantation reaction following contact between two individuals, and, analogous to vertebrates, is controlled by a single locus. We will summarize each of the known candidate genes within this locus and their potential roles in allorecognition, and speculate on how these findings may in fact be revealing potential functional relationships between disparate allorecognition systems.