Accurately estimation of chloroplasts’ longevity inside the animal cells is a difficult task. Temperature, light quality and intensity, aeration, water quality, previous life history, are just a few variables that are likely to affect chloroplasts functionality. Therefore, it is of paramount importance to evaluate and compare the performance of chloroplasts, within the species being studied, under controlled conditions. Additionally, control treatments must be implemented and sea slugs under those conditions need to display photosynthetic activity at maximum rates, otherwise the maintenance/culturing conditions may not be acquitted for chloroplasts loss. The first months of the project were spent on improving and expand culturing facilities of Elysia crispata (tropical species) and Elysia timida (Mediterranean species) as well as the maintenance facilities for the local (temperate species) Elysia viridis. While the first two species have a direct development, meaning the sea slug leaves the egg already ready to feed on the macroalgae, the latter species requires a more complex culturing procedure, with a larval phase feeding on microalgae, prior to metamorphose and settlement on the macroalgae.
Having reached exceptional facilities for breeding sea slugs in a controlled environment, we then tested the shift in algal food sources. Among several combinations tested, the most interesting identified was a common alga as donor of photosynthetically efficient chloroplasts to both highly specialized monophagous and polyphagous sea slugs capable of long-term retention, which opens new experimental routes. From this exhaustive work, it became clear that different algal sources induce drastic changes in kleptoplasty competence. The different animal-alga associations successfully achieved were characterized regarding long-term kleptoplasty, photosynthesis (carbon fixation using the energy of the sun) and photoprotection/photoinhibition (ability for protecting and repairing the chloroplast during light stress) performance. We have discovered that chloroplasts deriving from the same algae but hosted in different animal species do not perform equally inside the different host animal cells. It then became clear as well that the chloroplast source alone does not determine the success of kleptoplasty.
We are now investigating the specificity of each animal-algae combination that makes it successful. The next ongoing steps are the analysis of which genes and lipids are playing a role in maintaining the chloroplast when these are no longer able to be replaced – this approach is paralleled in the different algal-animal associations for elucidating what works and what fails in the different combinations. From these broad overviews, we expect to bring new hypothesis to the field, to be tested by ourselves but also colleagues across the globe.
