Spontaneous evolution of heterotrophy in cultured dinoflagellate algae

Acquisition of chloroplasts by an early eukaryote and their subsequent spread across eukaryotes is well understood. However, evolution can be undone, and many important lineages (especially in the Apicomplexa) have lost photosynthesis, returning to heterotrophy. Spontaneous transition to heterotrophy remains difficult to study, as most examples happened on an evolutionary timescale. Here, we describe frequent transition to heterotrophy in a dinoflagellate alga under laboratory conditions. Dinoflagellates, close relatives of Apicomplexa, have an unusual chloroplast genome, highly reduced and fragmented into subgenomic "minicircles," each typically with a single gene encoding a component of the photosynthetic electron transfer chain. This extraordinary genome organization may predispose to loss of gene(s) essential for photosynthesis. We found that growing the dinoflagellate Symbiodinium microadriaticum (a strain able to form symbioses with corals and other Cnidaria) on medium supplemented with glucose and amino acids allowed the ready isolation of multiple strains with spontaneous partial or complete loss of photosynthetic growth, resulting from the loss of a chloroplast subgenomic minicircle. Different strains showed independent loss of a minicircle encoding one of the photosystem II (PSII) components PsbE or PsbI. Spectroscopic analysis confirmed loss/impairment of PSII and retention of photosystem I (PSI) and cyclic electron flow (CEF), probably providing ATP. This system should allow the study of the spontaneous transition to heterotrophy and the role of intermediate stages in maintaining CEF. The ready loss of photosynthesis in dinoflagellates resulting from their chloroplast genome organization may explain why dinoflagellates lost or replaced chloroplasts several times during evolution.