Anguilla anguilla Transcriptome or Gene expression

Understanding the mechanisms underlying morphological/phenotypic dimorphism and plasticity, including the genomic basis and links to ecological influences, remains one of the major challenges in evolutionary biology. One prime example of phenotypically plastic species distributed along various environmental gradients is the European eel, Anguilla anguilla. At the juvenile (yellow eel) stage of the European eel (Anguilla anguilla), two morphotypes are found: broad- and narrow-heads. This dimorphism has been linked to dietary differences, with broad-heads feeding on harder and larger prey than narrow-heads. However, recent research showed that both morphotypes could already be distinguished at the glass eel stage, the non-feeding predecessor of the yellow eel stage, implying that factors other than trophic segregation play a role in the development of this head shape dimorphism. Here, we used gene transcription profiling (RNAseq) to identify significantly differential expressed genes between broad- and narrow-headed glass eels. Identification of these genes would strongly increase our understanding of the mechanism behind this dimorphism and its ecological implications. We found 260 genes that were significantly differentially expressed between the morphotypes, of which most were related to defense and immune responses. Interestingly, two genes involved in growth (soma and igf2) were significantly up-regulated in narrow-heads, while also nine genes involved in chemotaxis showed a significant differential expression. As such, we found support for the observation that head shape is also associated with somatic growth, with fast-growing eels developing a narrower head. Additionally, observations in the wild have shown that slow-growers prefer freshwater, while fast-growers prefer brackish water. The differential expression of genes involved in chemotaxis indeed seems to indicate that glass eel growth rate and habitat choice are related to each other. We hence hypothesize that two levels of segregations could take place in the European eel: first according to habitat choice, secondly according to feeding preference.