On mussels, mating and mitochondrial DNA

July 29, 2013 § Leave a comment

Mussels on rocks. Image credit: Angel P. Diz, University of Vigo, Spain

Mussels on rocks. Image credit: Angel P. Diz, University of Vigo, Spain

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, 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.”

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

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.

UPDATE on November 20, 2013: We did an ASBMB Journal Club on this paper where we discussed the work in the MCP paper with some of the authors. Check it out!

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