October 2013

On mitochondrial DNA, mating and mussels

Mussels and barnacles
Mussels and barnacles in the intertidal near Newquay, Cornwall, England. Image credIt: Wilson44691 of Wikimedia Common.jpg.

In humans and most other animals, offspring get all their mitochondrial DNA from their mothers. But in mussels and other related bivalves, fathers also give their offspring their mitochondrial DNA. In a recent paper in the journal Molecular & Cellular Proteomics, researchers propose a new model to explain this mechanism of mitochondrial DNA inheritance, which is called doubly uniparental inheritance, or DUI. The model also puts forward a possible explanation for sex determination in mussels, the mechanisms of which are not known.
“We thought that study of DUI might lead to deeper understanding of the function and evolution of mitochondrial DNA in general, with implications in a variety of areas,” says David Skibinski at Swansea University in the U.K. “This could include areas of benefit to humans, for example, in understanding genetic conditions caused by mitochondrial DNA or assisted reproduction. In evolution, this could include understanding of evolutionary forces and even the endosymbiotic theory of mitochondrial origin.”

David Skibinski

Mussels are an intriguing case study for DUI. “DUI is present in about 40 bivalve species and could have an origin as old as 400 million years ago. It is a mystery why it exists in some species but not others,” says Skibinski.
Besides learning more about mitochondrial DNA evolution and function, Skibinski explains that DUI in mussels also plays into sex determination in the animals. “At fertilization, embryos destined to be male must also pass mitochondria into an aggregate in the gonadal tissue and then into their own sperm. Precisely how this is all achieved is not known at the moment,” he says.
Skibinski and colleagues decided to look into the proteomic differences between eggs destined to become males (showing the sperm mitochondria aggregation phenomenon) and eggs destined to become females (not showing the mitochondrial aggregation phenomenon). The small proteomic differences they found confirmed a hypothesis they had about DUI: A maternal effect is involved. Mussel dads may be generous in passing along their mitochondrial genomes, but moms still have the final say.

Online extra

computer clipartJoin Geoff Hunt, ASBMB’s public outreach coordinator, and Raj Mukhopadhyay, ASBMB’s science writer, for an ASBMB Journal Club about this MCP paper. They will speak with David Skibinski and some of his coauthors to understand how studying mitochondrial inheritance in mussels informs our understanding of mitochondrial DNA evolution in general. Please periodically check ASBMB’s Facebook, Twitter or Google+ sites to get the day and time announcement for this exciting conversation.

The maternal effect seems to involve proteasome proteins. In fertilized eggs that go on to become male, the proteasome may inactivate the cellular machinery that normally results in mitochondrial dispersal. The sperm mitochondria presumably remain as an aggregate ready for passage into the sperm of the next generation. In fertilized eggs that go on to become female, the proteasome may be less active, and the sperm mitochondria presumably are dispersed among the cells of the embryo.
Thus, the proteasome in the eggs from the females seems to determine whether the resulting offspring are male or female. This is now an area the investigators are looking into in more detail.

Rajendrani MukhopadhyayRajendrani Mukhopadhyay (rmukhopadhyay@asbmb.org) is the senior science writer and blogger for ASBMB. Follow her on Twitter (www.twitter.com/rajmukhop), and read her ASBMB Today blog, Wild Types.

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