Subcellular Positioning : Unstable Filaments On the Move

late 1990s, in particular through the progress in molecular sequence-based phylogenetics. Despite some methodological reservations, these molecular approaches provided increasingly strong support for cyclostome monophyly [12,13] (Figure 1A). Perhaps the most convincing evidence for this came from the analysis of microRNAs (miRNAs) [14], which provided evidence for strong cyclostome phylogenetic ‘signatures’. In addition, miRNA expression in various organs of gnathostomes and lampreys indicated that the latter may have lost many phenotypic traits since the two groups diverged, between 360 million years ago at the latest, and probably much earlier, about 500 million years ago. Although expression of these miRNAs could not yet be studied in hagfish embryos [15], these new data strongly support the hypothesis that cyclostome ancestors were probably more complex than the living forms [14]. Recently, Ota et al. [4] have revisited the question of the alleged lack of vertebral elements in the hagfish Eptatretus burgeri. They investigated a series ofminute cartilaginous nodules that line the notochord ventrally in the caudal region of the embryo and had been mentioned long ago but overlooked since [16]. These nodules form from mesenchyme of the ventromedial part of the somites, which express Pax 1/9 and Twist genes. Homologues of these genes are expressed in the corresponding part of the sclerotomes in jawed vertebrates, from which the ventral elements of the vertebral column arise — that is, the basiventrals and interventrals that persist in the adults of most early gnathostomes. This is strong evidence for the homology of these elements between hagfish and gnathostomes Yet, unlike lampreys and gnathostomes, hagfishes show no sign of corresponding dorsal elements. Therefore, Ota et al. [4] suggest that both the dorsal and ventral series of vertebral elements were initially present in the common ancestor of all vertebrates, with the dorsal one having been lost in hagfishes, and the ventral one in lampreys. Among fossil jawlessvertebrates, only the 380 million year-old Euphanerops (Figure 1B,C) seems to display dorsal and ventral series of vertebral elements. Its dorsal elements strikingly resemble the basidorsals and interdorsals of lampreys, with the ventral ones extending ventrally to partly surround the dorsal aorta, as also observed in the hagfish embryo [4,17]. Contrary to ‘ostracoderms’,Euphaneropsproduced no bone, and all the skeletal elements observed in large individuals are still preserved because they consist of calcified cartilage (Figure 1B). Its head also displays a peculiar ring-shaped cartilage, which is suggestive of the annular cartilage that arms the oral disc of lampreys (Figure 1C). This cartilage is practically the only character of Euphaneropswhich suggests a relationship with lampreys, except for a vaguely similar overall body shape. However, the structure of its axial skeleton is strikingly similar to that suggested by Ota et al. [4] for the last common ancestor of vertebrates. The phylogenetic relationships of Euphanerops are still obscure, but it is tempting to consider it as either an early lamprey relative or a stem cyclostome. Whether cyclostomes are monophyletic or paraphyletic, their stem, or that of either hagfishes or lampreys, is still out of reach for palaeontologists. Thus, any developmental data, such as those provided by Ota et al. [4], will be invaluable for comparative anatomists whose goal is to elucidate vertebrate phylogeny and reconstruct the vertebrate ancestor.