ABSTRACT
Living jawless fish diverged from a common vertebrate ancestor over 500 million years ago. They comprise two groups, lampreys and hagfishes, which form the monophyletic group Cyclostomata based on molecular phylogenetic analyses. Cyclostomes are important model organisms for understanding early vertebrate evolution because they retain many features that ancient jawless vertebrates had. However, it should be noted that since they are not “ancestral animals”, cyclostomes lived independently from the jawed vertebrate (or gnathostome) lineages following divergence and thus possess independently evolved traits. Therefore, careful comparison of each trait among lampreys, hagfishes and jawed vertebrates would allow us to determine which traits are primitive and which are derived and thus depict the ancestry of early vertebrates. Until recently, lampreys have been used as model organisms of jawless vertebrates, especially in developmental biology. Recently, however, it has become possible to obtain fertilized eggs from inshore hagfish species and study their developmental mechanisms. In this chapter, the characteristics of both lampreys and hagfishes are described as model organisms for the evolution of vertebrates, and challenging questions are suggested from genomic and developmental perspectives.
