Lymphatic regulation in nonmammalian vertebrates

All vertebrate animals share in common the production of lymph through net capillary filtration from their closed circulatory system into their tissues. The balance of forces responsible for net capillary filtration and lymph formation is described by the Starling equation, but additional factors such as vascular and interstitial compliance, which vary markedly among vertebrates, also have a significant impact on rates of lymph formation. Why vertebrates show extreme variability in rates of lymph formation and how nonmammalian vertebrates maintain plasma volume homeostasis is unclear. This gap hampers our understanding of the evolution of the lymphatic system and its interaction with the cardiovascular system. The evolutionary origin of the vertebrate lymphatic system is not clear, but recent advances suggest common developmental factors for lymphangiogenesis in teleost fishes, amphibians, and mammals with some significant changes in the water-land transition. The lymphatic system of anuran amphibians is characterized by large lymphatic sacs and two pairs of lymph hearts that return lymph into the venous circulation but no lymph vessels per se. The lymphatic systems of reptiles and some birds have lymph hearts, and both groups have extensive lymph vessels, but their functional role in both lymph movement and plasma volume homeostasis is almost completely unknown. The purpose of this review is to present an evolutionary perspective in how different vertebrates have solved the common problem of the inevitable formation of lymph from their closed circulatory systems and to point out the many gaps in our knowledge of this evolutionary progression.

The painted turtle, Chrysemys picta: a model system for vertebrate evolution, ecology, and human health

Painted turtles (Chrysemys picta) are representatives of a vertebrate clade whose biology and phylogenetic position hold a key to our understanding of fundamental aspects of vertebrate evolution. These features make them an ideal emerging model system. Extensive ecological and physiological research provide the context in which to place new research advances in evolutionary genetics, genomics, evolutionary developmental biology, and ecological developmental biology which are enabled by current resources, such as a bacterial artificial chromosome (BAC) library of C. picta, and the imminent development of additional ones such as genome sequences and cDNA and expressed sequence tag (EST) libraries. This integrative approach will allow the research community to continue making advances to provide functional and evolutionary explanations for the lability of biological traits found not only among reptiles but vertebrates in general. Moreover, because humans and reptiles share a common ancestor, and given the ease of using nonplacental vertebrates in experimental biology compared with mammalian embryos, painted turtles are also an emerging model system for biomedical research. For example, painted turtles have been studied to understand many biological responses to overwintering and anoxia, as potential sentinels for environmental xenobiotics, and as a model to decipher the ecology and evolution of sexual development and reproduction. Thus, painted turtles are an excellent reptilian model system for studies with human health, environmental, ecological, and evolutionary significance.