Happy Mother's Day & Cheers to Maternal Inheritance of the Mitochondria

Happy Mother's Day! 

Don't forget to thank her for all she has done for you including giving you your mitochondrial DNA (mtDNA).

Let's talk about maternal inheritance of mtDNA (This post is slight adaptation from my original post which can be found here).

For most organisms (living things), including plants, animals, and fungi, mtDNA is inherited from a single parent (uniparental inheritance). In animals that reproduce sexually (make offspring/babies by... well I think you get it) the mtDNA is normally* inherited from the mother (maternal inheritance). 

*Like in most science, there are almost always exceptions. And in this case, there are examples of certain species having paternally inherited mitochondria such as Plymouth Rock chickens [1] or organisms that get "leakage" and have mtDNA from both mom and dad such as fruit flies [2], honeybees [3] cicadas [4], mice [5], sheep [6] and even humans [7, 8]. 

Back to human mtDNA, why does Mom's mtDNA beat out Dad's? There are two main ideas on how this happens; the dilution model and the active elimination model [9]. In the case of dilution, a human egg has ~200,000 mtDNA molecules whereas sperm has maybe 5 and I'm sure you can do that math (this model also allows for "paternal leakage" or some mtDNA from the dad to get through as seen in the above *exceptions). Also most mitochondria in the sperm are in the tail (mitochondria like to hang out where they are needed to make energy and the tail needs a lot since it is the motor for the sperm to swim) and the tail is often lost during fertilization. And lastly, there is evidence that mitochondria in mammalian sperm are destroyed by the egg after fertilization, active elimination [10].

[9] Carelli V. (2015).

Why does mtDNA usually only come from one parent? To be honest, we don't really know but there are plenty of theories out there and scientists are working on it (Possible future post? I don't know. Maybe. Tell me in the comments if you want me to write about this).

Why care where mtDNA comes from? Well, for genealogy (study of the family tree), it let's us trace back maternal lineage. We can do that for the paternal lineage using Y chromosome DNA. Also mtDNA is highly conserved with relatively slow mutation rates (doesn't change much generation to generation) so that also let's us study our evolutionary relationships to other species.

While on the topic of maternal inheritance of mtDNA, I should mention mitochondrial replacement therapy (MRT). MRT is an in vitro fertilization (IVF) technique where the mitochondria from a donor egg is moved to the mother's egg and results in a baby with mtDNA from a donor female and nuclear DNA from the mother's egg and father/donor's sperm - this procedure is used when a woman with genetically defective mitochondria wants to have a baby with healthy mitochondria but have the baby be genetically similar to her (she could also use a donor egg). Wait, so is this the three parent baby I hear about in the news? Yes. And you can read more about it here. Some people think it's controversial but I personally find it no more controversial than egg or sperm donation. mtDNA contributes such minimal DNA (37 genes, when there is an estimated 20,000 genes in the nucleus) to have a major impact on the child's identity (this is what most of the controversy centers on) other than allowing them to be healthy.

________________________________________

References

[1] Alexander M et al. (2015). Mitogenomic analysis of a 50-generation chicken pedigree reveals a rapid rate of mitochondrial evolution and evidence for paternal mtDNA inheritance. https://doi.org/10.1098/rsbl.2015.0561

[2] Wolff JN et al. (2012). Paternal transmission of mitochondrial DNA as an integral part of mitochondrial inheritance in metapopulations of Drosophila simulans. https://doi.org/10.1038/hdy.2012.60

[3] Meusel MS, Moritz RFA. (1993). Transfer of paternal mitochondrial DNA during fertilization of honeybee (Apis mellifera L.) eggs. https://doi.org/10.1007/BF00351719 

[4] Fontaine KM et al. (2007). Evidence for Paternal Leakage in Hybrid Periodical Cicadas (Hemiptera: Magicicada spp.). https://doi.org/10.1371/journal.pone.0000892

[5] Gyllensten U et al. (1991). Paternal inheritance of mitochondrial DNA in mice. https://doi.org/10.1038/352255a0

[6] Zhao et al. (2004). Further evidence for paternal inheritance of mitochondrial DNA in the sheep (Ovis aries) https://doi.org/10.1038/sj.hdy.6800516

[7] Schwartz M, Vissing J. (2002). Paternal Inheritance of Mitochondrial DNA. https://doi.org/0.1056/NEJMoa020350

[8]  Luo S et al. (2018). Biparental Inheritance of Mitochondrial DNA in Humans. https://doi.org/10.1073/pnas.1810946115 

[9] Carelli V. (2015). Keeping in Shape the Dogma of Mitochondrial DNA Maternal Inheritance. https://doi.org/10.1371/journal.pgen.1005179

[10] Sutovsky P et al. (1999). Ubiquitin tag for sperm mitochondria. https://doi.org/10.1038/46466

________________________________________

If you liked learning about mitochondria, you might also like the following posts:

Mitochondria: The Powerhouse of the Cell & Wait, what do you do? I study RNA processing in the mitochondria