Embryonic and larval development of ascidians: alterations by organotin compounds

For their phylogenetic position and the peculiar mosaic development, ascidians represent model organisms for developmental and molecular biology. Moreover, the draft of the complete genome sequencing of one ascidian species (Ciona intestinalis) was recently released. Ascidians are also important bioindicators of environmental pollutants as benthonic filter-feeding invertebrates. They reproduce by oviparity or viviparity and the fertilized egg develops into a chordate free swimming larva, which adheres to substrate and metamorphoses in a sessile oozooid. In order to understand the underlying mechanisms by which widespread pollutants can affect ascidian fitness, we studied at the light and electron microscopy, the effects of the exposure to tributyltin (TBT), the most important organotin compound, on the development of the solitary Styela plicata and the colonial Botryllus schlosseri. The results show that TBT affects all developmental stages in a dose and time-dependent manner and the most sensitive stages are gastrula and neurula. TBT blocks development giving rise to anomalous embryos with irreversible effects. Moreover, it acts on swimming larvae causing severe malformations, which prevent the adhesion to substrate, and interferes with metamorphosis through developmental delays and mortality. A parallel morphofunctional study on Botryllus schlosseri hemocytes, for understanding the cytological mechanisms responsible of toxic effects, supports our hypothesis that the TBT alters the calcium homesostasis by direct interaction with calmodulin, causing cytoskeletal damage and apoptosis and altering cell adhesion. Moreover, the occurrence of electrondense precipitate in the inner membrane of mitochondria, suggests an inhibitory effects on oxidative phosphorilation, particularly conspicuous in the gastrula stage.