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The annelid worm Platynereis dumerilii (Lophotrochozoa) exhibits ancestral developmental, body plan and genomic characteristics and possesses two types of eyes: adult pigment cup eyes and larval two-celled eyes. Platynereis therefore represents a useful model organism for the study of eye evolution in annelids. My research goal has been to characterize the differentiating Platynereis adult and larval eyes on the molecular level in order to explore the evolutionary history of these two types of eyes and their cell types: rhabdomeric photoreceptor cells (rPRCs) and pigment cells (PCs). This aim has been addressed by using the ‘molecular fingerprint’ (MFP) approach for the comparative study of cell types. I first identified specific molecular markers for each of the cell types in both types of eyes. These were then used to establish a comprehensive MFP of these cell types that included both effectors genes (differentiation genes expressed in eye and neuronal cell types) and transcription factors which play a role in eye and neuronal specification. This was achieved by means of gene expression studies, using wholemount double in-situ hybridization and 3D in-silico alignments. The data obtained reveal that Platynereis adult and larval eyes are composed of six cell types, based on MFP comparisons: adult eyes ventral and dorsal rPRCs, adult eyes ventral and dorsal PCs, larval eye rPRCs and larval eye pigment cells. The distribution of the adult rPRCs and PCs into two (ventral and dorsal) cell types relates to the fact that Platynereis develops two pairs of adult eyes that appear to differ in terms of their molecular regulation. It also revealed that many transcription factors regulating eye development in Drosophila and/or vertebrates are also expressed in the differentiating Platynereis eyes. Surprisingly, some of these are adult eye-specific and some are larval eyespecific, meaning that the adult and larval eyes of Platynereis show a distinct MFP, corroborating that they represent different types of eyes. On the other hand, some shared effector genes were identified between the rPRCs and PCs of the adult eyes, as well as of the larval eyes. This finding implicates that the rPRCs and PCs of Platynereis are sister cell types that can be traced back to a single ancestral multifunctional cell type precursor. Hierarchical clustering analysis based on the MFP results mirrors the ‘phylogeny’ of the different eyes cell types, in which the larval eyes cell types cluster together as do the two types of adult eyes rPRCs and PCs. In order to gain more insight into the developmental regulation of both eyes in Platynereis, I chose to assess the role of the conserved hedgehog (Hh) signaling pathway in Platynereis eye development. By using the antagonist cyclopamine to inhibit the hedgehog pathway in Platynereis embryos, I found out that Platynereis Hh pathway plays a role in adult but not in larval eye development. This adds another key distinction between the adult and larval eyes of Platynereis. These results support the view that annelid eyes originated from one multifunctional single cell prototype eye that bore characteristics of both PRCs and PCs. It was first duplicated to give rise to adult and larval eye precursors to then diversify into the PRCs and PCs present in today’s annelid eyes. |
