BackgroundThe experimental approach to the evolution and development of the vertebrate skeleton has to a large extent relied on “direct-developing” amniote model organisms, such as the mouse and the chicken. These organisms can however only be partially informative where it concerns secondarily lost features or anatomical novelties not present in their lineages. The widely used anamniotes Xenopus and zebrafish are “indirect-developing” organisms that proceed through an extended time as free-living larvae, before adopting many aspects of their adult morphology, complicating experiments at these stages, and increasing the risk for lethal pleiotropic effects using genetic strategies.ResultsHere, we provide a detailed description of the development of the osteology of the African mouthbrooding cichlid Astatotilapia burtoni, primarily focusing on the trunk (spinal column, ribs and epicentrals) and the appendicular skeleton (pectoral, pelvic, dorsal, anal, caudal fins and scales), and to a lesser extent on the cranium. We show that this species has an extremely “direct” mode of development, attains an adult body plan within 2 weeks after fertilization while living off its yolk supply only, and does not pass through a prolonged larval period.ConclusionsAs husbandry of this species is easy, generation time is short, and the species is amenable to genetic targeting strategies through microinjection, we suggest that the use of this direct-developing cichlid will provide a valuable model system for the study of the vertebrate body plan, particularly where it concerns the evolution and development of fish or teleost specific traits. Based on our results we comment on the development of the homocercal caudal fin, on shared ontogenetic patterns between pectoral and pelvic girdles, and on the evolution of fin spines as novelty in acanthomorph fishes. We discuss the differences between “direct” and “indirect” developing actinopterygians using a comparison between zebrafish and A. burtoni development.

Wnt signaling pathways are involved in many important cellular processes including proliferation and differentiation. Wnt ligands are released by source cells and signal to target cells by binding to the Frizzled receptor family and triggering changes in downstream target gene expression. Wnt signaling appeared at the base of metazoans and there was an early expansion in the repertoire of Wnt ligands to the 13 known subfamilies. However, little is known about functionality of these ligands in many animal lineages. Understanding the roles of these important signaling molecules in a wider range of animals is crucial to understand the regulation and evolution of cell fate during development and how this can lead to diversification. Here, we analyzed the Wnt repertoire among lepidopterans, where the embryological functionality of these ligands is understudied compared to other insect orders. To be able to explore Wnt gene roles during butterfly embryogenesis we first established a staging system for the butterfly model, Bicyclus anynana, and assayed the expression pattern of all eight lepidopteran Wnt genes during early butterfly development. We detected expression of Wnt1, Wnt10, and WntA in several expression domains, such as segmental stripes as well as expression of Wnt7 in the nervous system and Wnt11 in several head structures. Overall, our study provides, a basis for future research into butterfly embryogenesis and much needed new insights into the potential roles of Wnt genes in specifying cell fate in these animals as well as how this compares to other animals.

Whole genome duplications have occurred multiple times during animal evolution, including in lineages leading to vertebrates, teleosts, horseshoe crabs and arachnopulmonates. These dramatic events initially produce a wealth of new genetic material, generally followed by extensive gene loss. It appears, however, that developmental genes such as homeobox genes, signalling pathway components and microRNAs are frequently retained as duplicates (so called ohnologs) following whole-genome duplication. These not only provide the best evidence for whole-genome duplication, but an opportunity to study its evolutionary consequences. Although these genes are well studied in the context of vertebrate whole-genome duplication, similar comparisons across the extant arachnopulmonate orders are patchy. We sequenced embryonic transcriptomes from two spider species and two amblypygid species and surveyed three important gene families, Hox, Wnt and frizzled, across these and twelve existing transcriptomic and genomic resources for chelicerates. We report extensive retention of putative ohnologs, further supporting the ancestral arachnopulmonate whole-genome duplication. We also found evidence of consistent evolutionary trajectories in Hox and Wnt gene repertoires across three of the six arachnopulmonate orders, with inter-order variation in the retention of specific paralogs. We identified variation between major clades in spiders and are better able to reconstruct the chronology of gene duplications and losses in spiders, amblypygids, and scorpions. These insights shed light on the evolution of the developmental toolkit in arachnopulmonates, highlight the importance of the comparative approach within lineages, and provide substantial new transcriptomic data for future study.