Crimes of evolution

Volume 9 Number 1 January 14 - February 11 2013

While the recent red tides at Victorian and NSW beaches were visually striking, the microscopic algae that caused them have a criminal past. Nerissa Hannink reports.

In order to evolve into a new and more powerful species, microscopic animals held algae captive and stole their genes for energy production, scientists have revealed.

The results provide a ‘missing link’ in evolution because the tiny animal thieves (protozoa) couldn’t completely hide all evidence of the captive algae, and have been effectively frozen in time and caught in the act by genetic sequencing.

The protozoa captured genes for photosynthesis – the process of harnessing light to produce energy used by all plants and algae on Earth – so the study also provides insight into the origin and repurposing of these genes and may be useful in algae biofuel production.

The work was conducted by an international team of researchers led by Dalhousie University in Canada, which included Professor Geoff McFadden from the University of Melbourne.

Professor McFadden from the School of Botany says scientists have long suspected that quantum leaps of evolution occurred by one organism cannibalising another, but there was not much hard evidence.

But when the team looked at two specific algae – Guillardia theta and Bigelowiella natans – they realised the evolution was not quite complete. They could see the organisms’ cells had two nuclei (like the control centre of the cell that contains DNA). This is unusual because plant and animal cells have only one, so the genes were sequenced to find out more. 

“We think the genes for photosynthesis originally evolved only once about three billion years ago. So all plants, algae and blue green bacteria are able to produce their own energy from light because they’ve acquired these genes for photosynthesis,” Professor McFadden says.

Like prisoners in Alcatraz, the captive algae appear to have been nurtured by their enslavers, and the precious sugars produced from photosynthesis became a vital part of the protozoan slave-keeper’s diet. The captives lived inside the protozoan cell and, under the right conditions, the pair gradually became unified as a single organism – a process called endosymbiosis, literally living inside each other.

 “We discovered the captors were initially able to keep many separate clones of their slaves and occasionally pillage one or two for most of the essential genes. However, at some point in time, the number of captives reduced inside each gaoler to just one individual.

“So if they broke into the alga’s cell to steal the last essential genes, they would destroy it in the process and would not then be able to use the genes to run photosynthesis. So the two cells, one captive and one captor, had apparently reached an evolutionary stand-off situation where both are dependent on each other to survive.”

This process of endosymbiosis created tremendous new diversity during the early evolution of life. By capturing and cannabilising algae, protozoa evolved into radical new life forms, some good like the plankton of the oceans, and some not so good, such as red tide algae or parasites like malaria. The latter went full cycle, first converting to photosynthesis then converting to parasitism to steal food from larger organisms like us. 

“The genome sequences from these two missing links tell us how this enslavement and cannabilisation happens. We’re watching the process of BIG evolution”, says Professor McFadden.

www.botany.unimelb.edu.au