Decoded genomes shed light on algae with a curious past

Two unusual algae species are the latest life forms to have had their entire genomes decoded.

A scanning electron microscope image of the alga G. theta.

A scanning electron microscope image of G. theta, a microscopic alga that turns waters red. Credit: Dr. Geoff McFadden, University of Melbourne, Australia.

Too tiny to be seen with the naked eye, Guillardia theta and Bigelowiella natans are microscopic algae that turn water red or green, respectively. Assistant Professor of Biology Scott Roy was part of a team of scientists that sequenced the genomes of these single-celled organisms.

These particular algae are unique because their cells have an unusually complex architecture. While most plant cells have a nucleus, which serves as the cell's command center, these algae also have a second, miniature nucleus. It's a remnant of a curious turn of events in the species' evolutionary past.

The algae are believed to have evolved millions of years ago, when a tiny marine organism swallowed an unrelated algae cell through a process called endosymbiosis, in which one organism lives inside another.

"Many algal species were created by a host organism engulfing an algae cell," Roy said. "Usually the extra nucleus from the captive cell disappears over time. But these two species have kept this little remnant nucleus with just a couple hundred genes compared to the thousands of genes found in the cell's main nucleus."

 A scanning electron microscope image of the alga B. natans.

A scanning electron microscope image of B. natans, a green alga. Credit: Dr. Geoff McFadden, University of Melbourne, Australia.

It is unusual that these algae kept a second nucleus, albeit in a reduced, simplified form. However, the process of endosymbiosis is not uncommon in nature. It is believed to be a driving force in the diversification of plant life and the evolution of chloroplasts, the special compartments in plant cells that turn sunlight into sugars through photosynthesis.

The newly sequenced algae genomes, published Dec. 6 in the journal Nature, provide insights into how host organisms and their captives become one over time.

Inside a typical cell, genes are stored in the nucleus and provide the blueprint for the creation of proteins that the cell needs to survive. Roy helped the research team explore how that process works in the algae cells given that they have a main nucleus and a smaller, simplified counterpart, known as a nucleomorph.

"We wanted to understand how the transport of genes and proteins is coordinated in such a complex cell," Roy said. "Some kind of traffic control is needed."

That traffic control system comes through the way each gene is turned into a protein, Roy found. He is an expert on a process called alternative splicing, in which "filler" sections of DNA -- the stretches of DNA which don't encode for proteins -- are edited out during the manufacture of proteins.  

 Photo of Assistant Professor of Biology Scott Roy.

Assistant Professor of Biology Scott Roy, who joined the faculty in spring 2012.

"Alternative splicing is a mix and match process that means the same stretch of DNA can be used to create different proteins," Roy said.

In the case of the two algae species, Roy found that this phenomenon allows a single gene to create two slightly different versions of the same protein: one with a signal instructing the protein to stay in the nucleus and one directing it toward the outpost nucleomorph.

"It's almost like the same gene creates proteins with different zip codes attached to them," Roy said. "It hints at the question of why alternative splicing evolved and what function it serves."

Roy and his lab are now investigating the role of alternative splicing in fungi, where the phenomenon may have a similar coordination role.

--Elaine Bible