Fadl AEA et al. (2017), New biomarkers of post-settlement growth in the...

ECB-ART-45813

Heliyon 2017 Oct 03;310:e00412. doi: 10.1016/j.heliyon.2017.e00412.

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New biomarkers of post-settlement growth in the sea urchin Strongylocentrotus purpuratus.

Fadl AEA , Mahfouz ME , El-Gamal MMT , Heyland A .

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Some sea urchins, including the purple sea urchin Strongylocentrotus purpuratus, have been successfully used in aquaculture, but their slow growth and late reproduction are challenging to overcome when developing efficient aquaculture production techniques. S. purpuratus develops via an indirect life history that is characterized by a drastic settlement process at the end of a larval period that lasts for several weeks. During this transition, the bilateral larva is transformed into a pentaradial juvenile, which will start feeding and growing in the benthic habitat. Due to predation and other ecological factors, settlement is typically associated with high mortality rates in juvenile populations. Additionally, juveniles require several days to develop a functional mouth and digestive system. During this perimetamorphic period, juveniles use up larval resources until they are capable to digest adult food. Mechanisms underlying the onset of juvenile feeding and metabolism have implications for the recruitment of natural populations as well as aquaculture and are relatively poorly understood in S. purpuratus. The insulin/insulin-like growth factor signalling (IIS)/Target of Rapamycin (TOR) pathway (IIS/TOR) is well conserved among animal phyla and regulates physiological and developmental functions, such as growth, reproduction, aging and nutritional status. We analyzed the expression of FoxO, TOR, and ILPs in post-settlement juveniles in conjunction with their early growth trajectories. We also tested how pre-settlement starvation affected post-settlement expression of IIS. We found that FoxO provides a useful molecular marker in early juveniles as its expression is strongly correlated with juvenile growth. We also found that pre-settlement starvation affects juvenile growth trajectories as well as IIS. Our findings provide preliminary insights into the mechanisms underlying post-settlement growth and metabolism in S. purpuratus. They also have important implications for sea urchin aquaculture, as they show that pre-settlement nutrient environment significantly affects both early growth trajectories and gene expression. This information can be used to develop new biomarkers for juvenile health in sea urchin population ecology and aquaculture aquaculture.

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Species referenced: Echinodermata
Genes referenced: foxo1 LOC100887844

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	Fig. 1. Summary of Ill/TOR signaling. The diagram emphasizes the role of four target genes analyzed in this study (ILPs 1 and 2, TOR and FoxO) in response to nutrient changes in the environment. This scheme was adapted from work done on different organisms: Drosophila melanogaster (Edgar, 2006), Daphnia pulex (Boucher et al., 2010) and Drosophila (Benmimoun et al., 2012).
	Fig. 2. Juvenile morphology measured in our experiments (A, B). A is a juvenile at 6 days post settlement, while the juvenile in B is a different individual at 13 days post settlement. Different settlement substrates result in different test (C) and spine (D, E) growth in juvenile S. purpuratus. The response of spine growth and test growth is not directly correlated however. Specifically, relative spine length (the ratio of spine length over test area − E) is largest when juveniles are grown on biofilm (BF) and biofilm + food (F). We did not find a statistical interaction between food and biofilm on relative spine length (F). Stars indicate statistically significant difference between substrate and the control (FASW) based on a simple contrast post-hoc comparisons.
	Fig. 3. Survey of target genes expression levels in juvenile S. purpuratus as a function of food (F) and biofilm (BF) 2 and 6 days post-settlement. All expression levels are expressed as fold change relative to ubiquitin and post-induction juveniles (day 0).
	Fig. 4. Target gene fold change of juvenile S. purpuratus on different substrates. All expression levels are in reference to non-competent larvae A) Average baseline gene expression levels of pre-induction and juveniles (0–6 days). Asterisks indicate significance relative to reference stage (expression level of 1) in one sample t-test. B) Average gene expression levels in response to natural inducer (coralline alga: Calliarthron tuberculosum) decreases for ILP2 and increased for TOR in juveniles compared to non-competent larvae at the same age.
	Fig. 5. Juvenile growth of sea urchins (A) and spine growth (B) positively correlate with gene expression patterns of all target genes across experiments. For correlation analysis see Table 1.
	Fig. 6. Competent larvae that are starved before settlement grow the same test size (A) but longer spines (B, C) post-settlement. Starved larvae show significantly higher mortality 2–8 days post-settlement (D).
	Fig. 7. Gene expression levels for FoxO, TOR ad ILP2 as a consequence of pre-settlement starvation. We tested gene expression levels using qRT-PCR for three target genes as a consequence of a five-day starvation of competent larvae pre-settlement. Larval gene expression of FoxO and TOR was reduced post-starvation while ILP2 expression was increased post-starvation. We detected a decrease in FoxO expression and an increase in TOR and ILP2 expression post-settlement as a consequence of starving larvae pre-settlement.

References [+] :

Adams, Rapid adaptation to food availability by a dopamine-mediated morphogenetic response. 2011, Pubmed, Echinobase