Secondary Plastids of Euglenophytes

Euglenophytes obtained their plastids from a primary green alga related to extant genus Pyramimonas . The relatively recent establishment of this new organelle is an intriguing evolutionary phenomenon worth studying and comparing with other secondary plastids with a regard to their similarities and differences. A remarkably fast evolution driven by rapid intron gain and diversification is observed in euglenid plastid genomes which often tend to swell in size and rearrange while keeping the gene content stable. As a result of the secondary endosymbiosis, the plastid is wrapped in an additional membrane which makes any protein, metabolite, or ion transporting routes more complicated. In the case of protein import, secretory pathway-derived, signal peptide-dependent mechanism involving the endoplasmic reticulum, Golgi, and vesicular transport were recruited. The plastid endosymbiosis also served as a source of various oddities concerning metabolic pathways as the new organelle contained some of the enzymes and pathways already present in the host. Thus, several cases of division of labour and specialization can be observed, as well as simple redundancies which might be in fact just transitory and will eventually disappear in the future course of evolution. Endosymbiotic and lateral gene transfers were quite common in the ancestors of euglenophytes, especially in the case of plastid proteins many of which were demonstrated to have originated not only from the green-algal endosymbiont but also from a spectrum of nongreen lineages. The circumstances of the nongreen-algal gene gains are unclear. Another evolutionary phenomenon occurring in euglenophytes is the secondary loss of plastid or its photosynthetic capacity. This process gave rise to a number of distinct species which no longer possess the ability to photosynthesize. Interestingly, this “bleaching” process can be induced in the laboratory, enabling to study the process of plastid loss in vitro.