Popular Science: Evolution of endosymbiosis in action
The newly found endosymbiosis between protist Novymonas esmeraldas and bacterium Pandoraea novymonadis is a relatively recent event. The protist is a typical member of its group Kinetoplastida: Trypanosomatida. Its body is consists of a single cell with a flagellum. Novymonas is a close relative of disease-causing members of Trypanosomatida including sleeping sickness in Africa, Chagas disease in South America and leishmaniosis in all tropical and subtropical regions. All of the three diseases can be fatal for humans, other ones cause serious loss in animal husbandry. Our newly found protist is a parasite as well, but its host is insect only and it was discovered during an expedition in Ecuador.
Source: photo from original scientific article
Endosymbiosis isn’t a completely new phenomenon in Trypanosomatida, within its one group it was already once discovered. How does a typical endosymbiosis look like within this group? There is only one bacterial cell in each cell of protist. The protist and bacterium have synchronized their cell division: when the protist divides into two cells, the bacterium divides into two cells as well, and each of them moves into a different of newly independent daughter protist cells. The bacterium lost its cell wall, so there is no barrier preventing an exchange of nutrients and other substances between the protist and the bacterium. In comparison to its relatives, the protist has relatively larger mitochondria, since it provides energy not only for the protist, but for the bacterium as well. In exchange, the bacterium provides nutrients for the protist’s growth and reproduction – nutrients which the protist can’t create by itself or it would be disadvantageous for it. If we artificially remove the bacterium from the protist, it can live, however, it is not capable to continue in further growth and reproductive cycle.
But the newly discovered endosymbiosis of Novymonas esmeraldas is a wholly different case. In each protist cell, there is a varying number of bacteria – from none up to over ten. This is most likely a consequence of desynchronized cell division. We can easily imagine a case when both protist and bacteria divide, but daughter protist cells receive a different number of bacterial cells, possibly the first receives several, the second none. Despite this factm the presence of bacteria is somehow important for protists – scientists weren’t able to keep a colony of protists completely free of bacteria in a long-time scale. The protists gather bacteria in their cells (especially under unfavorable circumstances), although we yet don’t know what for. Moreover, the bacteria still have their cell wall, so the exchange of nutrients is restricted.
All of these mentioned differences indicate that the relationship between Novymonas and its bacteria Pandorea novimonadis is a relatively recent event without strictly determined rules. We thus have a unique opportunity to watch the evolution of endosymbiosis in action! The situation is even more interesting thanks to the fact that no inclination for a symbiosis-like lifestyle was found in close relatives of Novymonas esmeraldas, nor close relatives of Pandoraea novymonadis. Last but not least, there is the question of the purpose of bacterial presence. Every parasite should be capable to get all nutrients necessary for its growth from its host. Why would they have endosymbiotic bacteria then? This is the question of the highest priority for the authors’ team right now.
Iveta Štolhoferová
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