Immune Reactions in the Vertebrates' Closest Relatives, the Urochordates.

Currently existing urochordates (sea squirts) and vertebrates diverged in evolution around 570 million years (myr) ago. Phylogenetic analyses based on molecular data provide compelling evidence that these animals are the closest living relatives of vertebrates. Urochordares, therefore, are of critical importance for understanding the origin of vertebrate immune system. For a number of species a large body of molecular data is now available. An extensive EST project and the draft genome sequences of Ciona intestinalis and C. savignyi allow rapid "in silico" searches for immunorelevant molecules. Recent data convincingly demonstrate that urochordates possess nearly full repertoire of vertebrate innate immune system, but totally lack most of the components of an adaptive immunity such as MHC, TCRs and antibodies. In this review we show that knowledge of immunity in lower vertebrate and invertebrate species is now rapidly increasing. Elucidating the details of the origin of the immune systems from a comparative point of view in vertebrate's closest relatives, may finally lead to a better understanding of our own immune system. [ABSTRACT FROM AUTHOR]