Urochordates are at the Root of Vertebrate Evolution

Urochordates (tunicates), cephalochordates (lancelets), and vertebrates (including lamprey and hagfish) constitute the three extant groups of chordate animals (Fig. 1). Phylogenetic analyses based on molecular data provide compelling evidence that urochordates represent the closest living relatives of vertebrates (Delsuc et al. 2006). Currently existing urochordates and vertebrates diverged in evolution around 570 myr ago. Urochordates, therefore, are of critical importance for understanding the origins of vertebrates. They comprise approximately 2800-3000 species and, beside their phylogenetic position at the root of vertebrate evolution, are valuable objects because of the availability of a large body of molecular data.

Chondrichtyes

Agnatha Petromyzon

Mammalia

Cephalochordata

Urochordata Ciona Botryllus Halocynthia

Echinodermata

Chondrichtyes

Agnatha Petromyzon

Mammalia

MHC

-

TCR

-

NK cell receptors

+

Complement (Bf,C3, C6-C9)

+

Complement receptors (SCR)

+

Fig. 1 Urochordates separated from the predecessors of a vertebrate lineage around 570 million years (myr) ago and share many features of the innate immune system with vertebrates. However, they do not possess the molecular machinery of the adaptive immune system found in jawed vertebrates (Gnathostomata). The emergence of a recombinatorial adaptive immune system featuring immunoglobulins, TCR and MHC took place sometime during the transition from Agnatha to Gnatostomata. A search for these cardinal elements of a recombinatorial immune system in lymphocyte-like cells of the sea lamprey, Petromyzon marinus, has not been fruitful, hence the agnathans remain a gray zone in the evolution of adaptive immunity in vertebrates. Blue background: phylogenetic groups with both adaptive and innate immune systems

A large expressed sequence tag (EST) project (Satou et al. 2002) and the draft genome sequences of Ciona intestinalis (Dehal et al. 2002) and Ciona savignyi allow extensive "in silico" searches for immunorelevant molecules.

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