The neural crest (NC) emerges from combinatorial inductive events occurring within its progenitor domain, the neural border (NB). Several transcription factors act early at the NB, but the initiating molecular events remain elusive. Recent data from basal vertebrates suggest that ap2 might have been critical for NC emergence; however, the role of AP2 factors at the NB remains unclear. We show here that AP2a initiates NB patterning and is sufficient to elicit a NB-like pattern in neuralized ectoderm. In contrast, the other early regulators do not participate in ap2a initiation at the NB, but cooperate to further establish a robust NB pattern. The NC regulatory network uses a multistep cascade of secreted inducers and transcription factors, first at the NB and then within the NC progenitors. Here we report that AP2a acts at two distinct steps of this cascade. As the earliest known NB specifier, AP2a mediates Wnt signals to initiate the NB and activate pax3; as a NC specifier, AP2a regulates further NC development independent of and downstream of NB patterning. Our findings reconcile conflicting observations from various vertebrate organisms. AP2a provides a paradigm for the reiterated use of multifunctional molecules, thereby facilitating emergence of the NC in vertebrates.he neural crest (NC), a vertebrate embryo multipotent population, gives rise notably to peripheral nervous system, melanocytes and craniofacial structures (1). Combined Wnt, FGF, and BMP signals emanating from the paraxial mesoderm, neural plate, and nonneural ectoderm activate NC specifiers (e.g., snail2, foxd3, sox10) that are expressed in the premigratory NC and essential for further NC development (2-5). NC induction starts during gastrulation with the establishment of a broad competence domain at the neural border (NB) (6-8). The NB specifiers (e.g., pax3, msx1, zic1, hairy2) are essential for NC induction, but usually are not maintained in the NC cells after induction (5, 6, 9). They integrate FGF, Wnt, and BMP signals into a coherent induction of NC specifiers by diverse actions (6, 9-11). The hierarchical organization of this network is conserved across vertebrate evolution (12). Our understanding of this complex network remains preliminary, however; defining the roles of early actors is essential to understanding how this network might have emerged in vertebrates. Comparative analyses in amphioxus and lamprey have highlighted ap2 transcription factor (tfap2) up-regulation at the NB in vertebrates (13). AP2 transcription factors are well conserved in vertebrates and are essential for both nonneural ectoderm development and NC induction (14-16). Expression in the ectoderm is shared among chordates, whereas the up-regulation at the NB is a hallmark of vertebrates (13). AP2a depletion or mutation in mice or zebrafish results in a variety of NC defects (16)(17)(18). Recent data in a basal vertebrate (lamprey) suggest its role in NB formation, whereas results in zebrafish embryos identified a function during NC postspecification steps (8,19,2...

In this study, the phylogenetic relationships of 164 species of the family Drosophilidae are discussed, using the Amyrel gene, a member of the a-amylase multigene family. This study focuses on numerous species groups in the subgenera Sophophora and Drosophila of the genus Drosophila but also includes other closely related genera. Nucleotide data were analysed by several methods: maximum parsimony, neighbour joining, maximum likelihood and Bayesian inference. Heterogeneity of base composition (mainly low GC contents in the species groups willistoni and saltans) has been addressed. In all analyses, the genus Drosophila appeared paraphyletic. The subgenus Sophophora clearly appeared to be a monophyletic group, showing well-resolved clades, with the Neotropical groups arising in a basal position. Here, it is proposed to raise the species subgroups ananassae and montium to the rank of species group, and to restrict the melanogaster species group to the melanogaster subgroup plus the ÔOrientalÕ subgroups, among which the suzukii subgroup is polyphyletic. Some related genera such as Zaprionus, Liodrosophila, Scaptomyza and Hirtodrosophila are clustered with, or inside the subgenus Drosophila, which is therefore paraphyletic and should be reviewed.