Evolution In Progress - Complex Cells

Tiny Genomes May Offer Clues to First Plants and Animals | Quanta

When we think of evolution in progress we almost always think of higher animals evolving by changing over time or of bacteria and viruses adapting to environmental changes like antibiotics or host resistance, more rarely of a new species having been found to have arisen by hybridization like the Italian sparrow which I described recently in Evolution In Progress - A Tale Of Three Sparrow. It's not often that we get to witness a stage in life's evolution which we think of as having happened a long time ago as complex (eukaryotic) cells evolved out of cooperative alliances of simple (prokaryotic) cells, so it's exciting to find an example of this very process occurring right now, and in a species which is all too familiar to gardeners and farmers, since it involves a common sap-sucking insect pest - the mealybug.

It was only in the 1960 that we began to understand the process of endosymbiosis which is now accepted by mainstream biology as how complex cells arose. This was first proposed in 1966 by the late Lynn Margulis and is an example of how a radical new theory can be adopted by science despite being initially greeted with scepticism, even hostility by the scientific establishment, if the arguments in favour of it are overwhelming, just as happened with plate techtonics.

However, the evidence for endosymbiotic evolution was always circumstantial, being inferred from the appearance of cell organelles like mitochondria and chloroplasts and their resemblance to free-living organisms like rickettsia and cyanobacteria and the fact that some organelles have retained a small genome of their own.

Recently, however, scientists have discovered a diminutive bacteria, Tremblaya princeps living inside specialised cells in the body of the sap-sucking mealybug. This tiny organism now replaces the bacterium Mycoplasma genitalium which lives in the human genital tracts, as the organism with the smallest known genome. T. princips has a genome of a mere 121 protein-coding genes - a size previously considered far too small for a living organism. In comparison, the genome of M. genitalium is about four times the size. It is believed that T. princips derives some of it's essential enzymes from its host and gives something in return - typical of a cell organelle, in facts.

However, T. princeps throws us another surprise. Unlike cell organelles, T. princeps is itself host to yet another bacteria which appears also to be an endosymbiont, Moranella endobia which, although it is obviously smaller than it's host, never-the-less, has a genome almost three times as large as that of T. princeps. The specialised cells of the mealybug, T. princeps and Moranella endobia together appear to form an interdependent complex which work to produce some essential amino acids lacking in the mealybug's diet.

Given its extreme minuteness and the fact that it must get many essentials from both its host and resident microbes, some suggest that Tremblaya blurs the boundaries between cellular organisms and organelles, specialized structures within cells such as the energy-producing mitochondria. It has been officially designated an endosymbiont, an organism that lives within the cells of another organism. But its genome size resembles that of some organelles. “When do these things stop being bacteria?” asked John McCutcheon, a biologist at the University of Montana in Missoula who studies these organisms.

Indeed, scientists now know that some organelles evolved from endosymbiont bacteria, raising hopes that studying tiny endosymbionts like Tremblaya could shed light on the evolution of those organelles. “There is no bright line between endosymbionts and organelles,” McCutcheon said. “We might be looking at something pretty darn similar to the endosymbiont-to-organelle transition.”

Tiny Genomes May Offer Clues to First Plants and Animals; Emily Singer, June 20, 2013

This may not be the first time this has happened in the mealybug either. The same team found remnants of DNA from up to four other bacteria in it's genome suggesting at least a horizontal transfer of genes from bacteria to the insect similar to what is known to happen between bacteria.

In fact, in the long history of life on earth, complex cells, and multicellular organisms are relative newcomers.

Fling your arms wide in an expansive gesture to span all of evolution from its origins at your left fingertip to today at your right fingertip. All the way across your midline to well past your right shoulder, life consisted of nothing but bacteria. Multi-celled invertebrate life flowers somewhere around your right elbow. The dinosaurs originate in the middle of your right palm, and go extinct around your last finger joint. The whole story of Homo sapiens and our predecessor Homo erectus is contained in the thickness of one nail-clipping.

Richard Dawkins, Unweaving The Rainbow

So perhaps the surprising thing is that we have so far discovered so few examples of this form of evolution in progress.

References:

Filip Husnik, Naruo Nikoh, Ryuichi Koga, Laura Ross, Rebecca P. Duncan, Manabu Fujie, Makiko Tanaka, Nori Satoh, Doris Bachtrog, Alex C.C. Wilson, Carol D. von Dohlen, Takema Fukatsu, John P. McCutcheon; Horizontal Gene Transfer from Diverse Bacteria to an Insect Genome Enables a Tripartite Nested Mealybug Symbiosis;
Cell - 20 June 2013 (Vol. 153, Issue 7, pp. 1567-1578)
Tiny Genomes May Offer Clues to First Plants and Animals; Emily Singer, June 20, 2013

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