As esponjas têm uma zona de crescimento preferencial? Evidências in situ, morfológicas e moleculares.

Abstract

In Porifera, growth is considered indeterminate, in contrast to the growth zones of many modular marine invertebrates. But, are sponges really that simple? Understanding whether there is regionalization in sponges is important for comprehending the evolution of body plans in metazoans. Here, we are comparing the transcriptome (RNAseq, de novo), morphometrics, and histology of two regions of the sponge body: proximal (with developed aquiferous system modules) and distal (with the possible growth zone). The absence of oscula distally, creeping branches, and a positive relationship between length and the number of oscula made Cladocroce caelum (Haplosclerida) the model species for this study. We obtained 167,551,552 reads from the eight sequenced fragments, recovering 99% of the transcripts for Eukarya and 94% of the predicted genes in Methazoa. All the genes characteristic of sponges involved in reproduction and stem cell maintenance were expressed in both regions, as well as many genes from the Hedgehog, Wnt, TGF-beta, Notch-Delta, JAK-STAT signaling pathways, nuclear receptors, RTK, and CAM. Out of these 140,614 transcripts, 60 were differentially expressed (FDR < 0.96) between the regions: 21 transcripts were upregulated in the proximal region, and 39 in the distal region, including Frizzled-2 (related to the Wnt pathway). By monitoring the growth of 37 specimens in situ for a month, we observed that growth predominantly occurs in the distal region (28.2% increase in length compared to 12.8% in width). Cellular analyses also showed differences in composition, with a predominance of archaeocytes (pluripotent cells, 65.3%) in the distal region, and choanocytes (body maintenance cells, 47.6%) in the proximal region. Therefore, the gene expression profile, growth pattern, and cellular composition of the two regions exhibit differences, suggesting regionalization in the body. Thus, despite the simplicity of sponges, these animals may share with others a preferred growth zone.