Alejandro Reyes-Bermudez

Although gastrulation is a conserved morphogenetic process in animals, the cellular mechanisms underlying it are highly variable. This raises important questions regarding the extent to which conserved or divergent gene regulatory programs (GRNs) control gastrulation in phylogenetically distant taxa. We compared gene expression profiles during gastrulation of Acropora digitifera and Acropora tenuis , species that diverged ∼50 million years ago. Despite the morphological similarity, each species uses divergent GRNs, supporting the concept of developmental system drift. Consistently, orthologous genes showed significant temporal and modular expression divergence, indicating GRN diversification rather than conservation. Yet, we identified a subset of 370 differentially expressed genes that were up-regulated at the gastrula stage in both species, with roles in axis specification, endoderm formation, and neurogenesis, suggesting a conserved regulatory “kernel” for the process. We also identified species-specific differences in paralog usage and alternative splicing patterns that indicate independent peripheral rewiring of this conserved module. Interestingly, although A. digitifera exhibits greater paralog divergence, consistent with neofunctionalization, A. tenuis shows more redundant expression, suggesting the regulatory robustness of developmental programs in this species.

Due to their pluripotent nature and unlimited cell renewal, stem cells have been proposed as an ideal material for establishing long-term cnidarian cell cultures. However, the lack of unifying principles associated with “stemness” across the phylum complicates stem cells’ identification and isolation. Here, we for the first time report gene expression profiles for cultured coral cells, focusing on regulatory gene networks underlying pluripotency and differentiation. Cultures were initiated from Acropora digitifera tip fragments, the fastest growing tissue in Acropora. Overall, in vitro transcription resembled early larvae, overexpressing orthologs of premetazoan and Hydra stem cell markers, and transcripts with roles in cell division, migration, and differentiation. Our results suggest the presence of pluripotent cell types in cultures and indicate the existence of ancestral genome regulatory modules underlying pluripotency and cell differentiation in cnidaria. Cultured cells appear to be synthesizing protein, differentiating, and proliferating.

Corals belong to the most basal class of the Phylum Cnidaria, which is considered the sister group of bilaterian animals, and thus have become an emerging model to study the evolution of developmental mechanisms. Although cell renewal, differentiation, and maintenance of pluripotency are cellular events shared by multicellular animals, the cellular basis of these fundamental biological processes are still poorly understood. To understand how changes in gene expression regulate morphogenetic transitions at the base of the eumetazoa, we performed quantitative RNA-seq analysis during Acropora digitifera’s development. We collected embryonic, larval, and adult samples to characterize stage-specific transcription profiles, as well as broad expression patterns. Transcription profiles reconstructed development revealing two main expression clusters. The first cluster grouped blastula and gastrula and the second grouped subsequent developmental time points. Consistently, we observed clear differences in gene expression between early and late developmental transitions, with higher numbers of differentially expressed genes and fold changes around gastrulation. Furthermore, we identified three coexpression clusters that represented discrete gene expression patterns. During early transitions, transcriptional networks seemed to regulate cellular fate and morphogenesis of the larval body. In late transitions, these networks seemed to play important roles preparing planulae for switch in lifestyle and regulation of adult processes. Although developmental progression in A. digitifera is regulated to some extent by differential coexpression of well-defined gene networks, stage-specific transcription profiles appear to be independent entities. While negative regulation of transcription is predominant in early development, cell differentiation was upregulated in larval and adult stages.

Background: The coral skeleton consists of CaCO₃deposited upon an organic matrix primarily as aragonite. Currently galaxin, fromGalaxea fascicularis, is the only soluble protein component of the organic matrix that has been characterized from a coral. Three genes related togalaxinwere identified in the coralAcropora millepora.Results: One of theAcroporagenes (Amgalaxin) encodes a clear galaxin ortholog, while the others (Amgalaxin-like 1andAmgalaxin-like 2) encode larger and more divergent proteins. All three proteins are predicted to be extracellular and share common structural features, most notably the presence of repetitive motifs containing dicysteine residues. In situ hybridization reveals distinct, but partially overlapping, spatial expression of the genes in patterns consistent with distinct roles in calcification. Both of theAmgalaxin-likegenes are expressed exclusively in the early stages of calcification, whileAmgalaxincontinues to be expressed in the adult, consistent with the situation in the coralGalaxea.Conclusion: Comparisons with molluscs suggest functional convergence in the two groups; lustrin A/pearlin proteins may be the mollusc counterparts of galaxin, whereas the galaxin-like proteins combine characteristics of two distinct proteins involved in mollusc calcification. Database searches indicate that, although sequences with high similarity to the galaxins are restricted to the Scleractinia, more divergent members of this protein family are present in other cnidarians and some other metazoans. We suggest that ancestral galaxins may have been secondarily recruited to roles in calcification in the Triassic, when the Scleractinia first appeared. Understanding the evolution of the broader galaxin family will require wider sampling and expression analysis in a range of cnidarians and other animals.