Evolution of subcellular structures from symbioses

We have seen in this chapter that there is remarkable variety in the types of association that may be regarded as symbiotic - many of them shown clearly to be mutualistic. They extend from patterns of behavior linking two very different organisms that spend parts of their lives apart, through the microbial communities of the vertebrate gut (strictly external to the body tissues), to the intercellular ectomycorrhizas and lichens, and the intracellular dinoflagellate algae of corals and mycetocyte bacteria of insects. We end this chapter by examining how an ecological interaction - mutualism - may lie at the heart of biological patterns operating on the longest evolutionary timescales.

It is now generally accepted that the origin of the various sorts of eukaryotes from more primitive ancestors has progressed at least in part through the inextricable merging of partners in a symbiosis. This view was championed especially by Margulis (1975, 1996) in the 'serial endosymbiosis theory' (Figure 13.23a). The aim is to understand the relationships between the three 'domains' of living organism: the archaebacteria or Archaea (many of them now 'extremophiles', living at high temperatures, low pHs and so on), the 'true' bacteria (Eubacteria) and the eukaryotes (Katz, 1998). One suggested first step (estimated to have occurred around 2 billion years ago) was the merger of archaeal and bacterial (spirochete) cells in an anaerobic symbiosis. The former brought its nucleocytoplasm and the latter brought its swimming motility, thus explaining the the serial endosymbiosis theory

Archaeon with Spirochaete Endosymbiosis: Eukaryote with nucleoid membrane motility eukaryotic flagellum

Eukaryote Aerobic Endosymbiosis: Eukaryote with bacterium respiration mitochondria

Bacteria

Eukaryota

Archaea

Bacteria

Eukaryota

Archaea

Origin of nucleus, microtubules and mitochondria

Figure 13.23 (a) The first two steps in the serial endosymbiosis theory for the origin of the eukaryotic cell. Ef, eukaryote flagellum; Mt, mitochondrion; N, nucleus. (b) A model for the origin of eukaryotes indicating a symbiosis between archaeal and bacterial lineages, and the possible simultaneous origin of nuclei, microtubules and mitochondria in eukaryotes. Bold lines represent lineage boundaries; pale lines are gene genealogies; broken arrows are possible lateral transfers of individual genes. (After Katz, 1998.)

Origin of nucleus, microtubules and mitochondria

Figure 13.23 (a) The first two steps in the serial endosymbiosis theory for the origin of the eukaryotic cell. Ef, eukaryote flagellum; Mt, mitochondrion; N, nucleus. (b) A model for the origin of eukaryotes indicating a symbiosis between archaeal and bacterial lineages, and the possible simultaneous origin of nuclei, microtubules and mitochondria in eukaryotes. Bold lines represent lineage boundaries; pale lines are gene genealogies; broken arrows are possible lateral transfers of individual genes. (After Katz, 1998.)

chimeric nature - the mix of archaeal and bac-terial features - of the proteins and the genetic material of even the most primitive eukaryotes. Subsequently, some of these chimeras incorporated aerobic bacteria that were the forerunners of mitochondria, to become aerobic eukaryotes from which all other eukaryotes have evolved. Some of these later acquired phototrophic cyano-bacteria that were the forerunners of chloroplasts, providing the stock from which the algae and higher plants evolved.

In fact, the serial endosymbiosis theory is merely one of several seeking to link the three domains and recreate the origins of the eukaryotes (Katz, 1998). A suggestion, for instance, that the most primitive eukaryotes have lost mitochondria, rather than never having had them, calls into question the whole sequential nature of eukaryote origins. It may also be that the 'lateral transfer' of individual genes (from one evolutionary lineage to another) has been more pervasive over evolutionary time than was previously imagined, so that the branching tree of life is in fact much more of a tangled web (Figure 13.23b). No doubt, as further evidence accumulates, these competing theories will themselves evolve further, both through progression and the lateral transfer of ideas. What they share, however, is the idea that mutualistic symbioses, beyond their ecological importance, lie at the heart of some of the most fundamental steps in evolution.

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