Diatom genes originating from red and green algae: Implications for the secondary endosymbiosis models.

Previous phylogenomic analyses of diatoms have discovered some plastid-targeted genes apparently coming from green algae. Number of these genes varied from less than a half of EGT-compatible genes to an overwhelming majority, and their presence was treated as an evidence of cryptic green plastid. We have performed such an analysis with a novel weighted approach on an extended dataset of diatom genomes and proteomes. Approximately equal evidence was found for red and green algal origins for diatoms genes. Considering that very similar results were obtained on other secondary photosynthetic groups whose endosymbioses were independent from that of the diatom ancestors, we consider the serial plastid replacements unparsimonious. A better explanation of these data can be provided by the shopping bag model, where a future host switches numerous endosymbionts and acquires some genes from each of them. Eventually the host loses the ability to replace endosymbionts (e.g. through the loss of phagotrophy) and whatever symbiont was present at the moment gets fixed and reduced to an organelle.