rial genome. A second case involves a freshwater diatom (Rhopalodia gibba) that again has an obligate association with a cyanobacterium. At first sight this is a bit odd because the diatom is photosynthetic, so it hardly needs a photosynthesizing guest. In point of fact the cyanobacteria is involved with nitrogen reduction (as cyanobacteria do in the free-living colonies using specialized cells called heterocysts), and these provide essential nutrients to the diatom. The process of nitrogen fixation takes place in the dark.
d) Two complex endosymbioses in single-celled protistans are particularly interesting. In the ciliate Myrionecta rubra there are symbiotic plastids (from a group of protistans known as the cryptophytes), but the ciliate also needs to “steal” nuclei from the cryptophyte in order for the chloroplasts to continue to function. One nucleus can serve up to eight plastids, and function for up to a month until it needs to be replaced. This sophisticated genomic interaction indicates that complex endosymbiotic arrangements are possible to evolve. The second example is a flagellate (Hatena arenicola) that has an extraordinary symbiosis with a protistan belonging to the group known as the prasinophyceans. The symbiont is largely reduced to a plastid, so evidently the flagellate benefits from the photosynthetic products of the symbiont. Extraordinarily, however, the symbiont retains its eye-spot and this is evidently used by the flagellate for navigation, as it crawls across sand-grains. This symbiosis, however, is not obligate because when Hatena undergoes cell division the symbiont cleaves to one cell, leaving the other bereft. What happens? The “lucky” daughter cell is self-sufficient with the photosynthetic symbiont, but the other then transforms itself into a heterotroph and starts to feed.
e) In addition to the symbiotic associations desribed above it is also clear that secondary (or even tertiary!) endosymbiosis has occurred whereby one eukaryotic cell (or part thereof) enters into symbiotic association with another eukaryote. There has been discussion as to how many times such secondary endosymbioses may have occurred, but a minimum estimate is twice. Interestingly there are also other convergences, notably in comparison between two endosymbioses involving a cryptomonad and a chloroarachniophyte and in both cases not only has there been a startling reduction in genome size but also in a highly convergent fashion.
Finally, in the wider context of evolutionary convergence it is worth noting that dramatic decreases in genome size are also seen in other types of endosymbiont, notably in the α-proteobacteria that are associated with sap-feeding insects, and also pathogenic bacteria. In these cases there are also parallelisms within the specific groups.
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Map of Life - "Chloroplast and mitochondrial plastid origins"
October 16, 2019