On the origin of plastids

The dearest wish of any evolutionary biologists probably is for a time machine to travel through prehistoric times and to investigate species that either have gone extinct or have developed. The obvious lack of such a device means that the possible evolution of organisms has to be inferred from circumstantial evidence. The evolution of plastids from free-living cyanobacteria that first were taken up by a host cell and then converted to organelles is such a case. As this process occurred more than a billion years ago, we can only speculate about howabacteriumpossibly could have been integrated permanently into a eukaryotic cell. Investigating the rhizarian photosynthetic amoeba Paulinella chromatophora, it seems that Debashish Bhattacharya and colleagues may have less need for a time machine. Although this species was first described in 1895 by Lauterborn,[1] only recent analyses revealed that its photosynthetic organelles did not originate from the original primary endosymbiosis that led to all other eukaryotic algae, but rather from an independent primary endosymbiosis that took place much later – about 120 million years ago. [2] P. chromatophora possesses plastid-like organelles called cyanelles which still have cyanobacterial properties, for example, a bacterial peptidoglycan cell wall, carboxysomes, and phycobilisomes, revealing their cyanobacterial ancestry. However, based on their inability to survive outside of the host cell, their location in the host cytoplasm (and not within a food vacuole), and their strongly reduced genome, they nevertheless are considered to be organelles. [3] In this issue, Bhattacharya et al.[4] now revisit theoretical discussions about post-endosymbiotic plant and algal evolution by studying P. chromatophora and its very few known close relatives in more detail. They took advantage of the presence of a heterotrophic Paulinella cell line that might be very closely related to the ancestral host cell that acquired the cyanobacterial endosymbiont. By examining the speciation within the Paulinella group based on genomics and morphology, Bhattacharya et al. conclude that endosymbiosis and the successful integration of plastids, including the required cellular adaptations, occurred rather quickly in evolutionary terms and under strong selective pressure. This eventually led to sympatric speciation, pushing recently diverged lines apart from each other. This view is based on the concept of Muller’s ratchet,[5] a mechanism originally described for bacteria, that leads to the extinction of small asexual populations due to fixation of slightly deleterious mutations in the absence of recombination. Mullert’s ratchet can also be applied to plastids, as the organelles are excluded from any sexual recombination process of the host cell. The hypothesis of an “endosymbiotic ratchet” as described by the authors may help to understand why the successful establishment of plastids seemingly is such a rare event in evolution, but also might serve to explain the huge diversity of the Archaeplastida. Based on observations on Paulinella described here, there is strong evidence that the Archaeplastida (including red algae, green alga and glaucophytes, and thus the lineages that evolved after the first knownprimary endosymbiosis) had to face similar selective pressures, leading to a strong divergence of these evolutionary lines. This article comments on the hypothesis paper by Debashish Bhattacharya et al., https://doi.org/10.1002/bies.202200165