Anteroposterior axis patterning by early canonical Wnt signaling during hemichordate development.

The Wnt family of secreted proteins has been proposed to play a conserved role in early specification of the bilaterian anteroposterior (A/P) axis. This hypothesis is based predominantly on data from vertebrate embryogenesis as well as planarian regeneration and homeostasis, indicating that canonical Wnt (cWnt) signaling endows cells with positional information along the A/P axis. Outside of these phyla, there is strong support for a conserved role of cWnt signaling in the repression of anterior fates, but little comparative support for a conserved role in promotion of posterior fates. We further test the hypothesis by investigating the role of cWnt signaling during early patterning along the A/P axis of the hemichordate Saccoglossus kowalevskii. We have cloned and investigated the expression of the complete Wnt ligand and Frizzled receptor complement of S. kowalevskii during early development along with many secreted Wnt modifiers. Eleven of the 13 Wnt ligands are ectodermally expressed in overlapping domains, predominantly in the posterior, and Wnt antagonists are localized predominantly to the anterior ectoderm in a pattern reminiscent of their distribution in vertebrate embryos. Overexpression and knockdown experiments, in combination with embryological manipulations, establish the importance of cWnt signaling for repression of anterior fates and activation of mid-axial ectodermal fates during the early development of S. kowalevskii. However, surprisingly, terminal posterior fates, defined by posterior Hox genes, are unresponsive to manipulation of cWnt levels during the early establishment of the A/P axis at late blastula and early gastrula. We establish experimental support for a conserved role of Wnt signaling in the early specification of the A/P axis during deuterostome body plan diversification, and further build support for an ancestral role of this pathway in early evolution of the bilaterian A/P axis. We find strong support for a role of cWnt in suppression of anterior fates and promotion of mid-axial fates, but we find no evidence that cWnt signaling plays a role in the early specification of the most posterior axial fates in S. kowalevskii. This posterior autonomy may be a conserved feature of early deuterostome axis specification.

Consequences of hoxb1 duplication in teleost fish.

Vertebrate evolution is characterized by gene and genome duplication events. There is strong evidence that a whole-genome duplication occurred in the lineage leading to the teleost fishes. We have focused on the teleost hoxb1 duplicate genes as a paradigm to investigate the consequences of gene duplication. Previous analysis of the duplicated zebrafish hoxb1 genes suggested they have subfunctionalized. The combined expression pattern of the two zebrafish hoxb1 genes recapitulates the expression pattern of the single Hoxb1 gene of tetrapods, possibly due to degenerative changes in complementary cis-regulatory elements of the duplicates. Here we have tested the hypothesis that all teleost duplicates had a similar fate post duplication, by examining hoxb1 genes in medaka and striped bass. Consistent with this theory, we found that the ancestral Hoxb1 expression pattern is subdivided between duplicate genes in a largely similar fashion in zebrafish, medaka, and striped bass. Further, our analysis of hoxb1 genes reveals that sequence changes in cis-regulatory regions may underlie subfunctionalization in all teleosts, although the specific changes vary between species. It was previously shown that zebrafish hoxb1 duplicates have also evolved different functional capacities. We used misexpression to compare the functions of hoxb1 duplicates from zebrafish, medaka and striped bass. Unexpectedly, we found that some biochemical properties, which were paralog specific in zebrafish, are conserved in both duplicates of other species. This work suggests that the fate of duplicate genes varies across the teleost group.

A new time-scale for ray-finned fish evolution.

The Actinopterygii (ray-finned fishes) is the largest and most diverse vertebrate group, but little is agreed about the timing of its early evolution. Estimates using mitochondrial genomic data suggest that the major actinopterygian clades are much older than divergence dates implied by fossils. Here, the timing of the evolutionary origins of these clades is reinvestigated using morphological, and nuclear and mitochondrial genetic data. Results indicate that existing fossil-based estimates of the age of the crown-group Neopterygii, including the teleosts, Lepisosteus (gar) and Amia (bowfin), are at least 40 Myr too young. We present new palaeontological evidence that the neopterygian crown radiation is a Palaeozoic event, and demonstrate that conflicts between molecular and morphological data for the age of the Neopterygii result, in part, from missing fossil data. Although our molecular data also provide an older age estimate for the teleost crown, this range extension remains unsupported by the fossil evidence. Nuclear data from all relevant clades are used to demonstrate that the actinopterygian whole-genome duplication event is teleost-specific. While the date estimate of this event overlaps the probable range of the teleost stem group, a correlation between the genome duplication and the large-scale pattern of actinopterygian phylogeny remains elusive.