Thank you Mom for my Mitochondrial DNA (PSA to not forget Mother's Day)

Don't forget Mother's day is this Sunday! (Do people celebrate Mother's day outside of the U.S.? Is it on the same day as the U.S.? Okay, I just Googled it - 40+ countries recognize it and most celebrate this weekend but some celebrate on different days. Read more on Wiki.)

Since I don't live by my mother to celebrate with the typical brunch, I like to send her a card - the nerdier the better. I can't share the card I sent this year because I don't want to ruin it but I promise it is funny. Anyway, here is one of my favorites that I sent two years ago. Why is it my favorite? Because it mentions the mitochondria, specifically mitochondrial DNA (mtDNA). Go figure!

Side note: If you're looking for nerdy or niche cards I highly recommend checking out Etsy. 

Photo courtesy of my mom's FB, Card courtesy of Nerdy Words Inc

As this card implies, mtDNA is inherited maternally aka you get your mtDNA from your mom (Thanks Mom!). I also mentioned that mtDNA is maternally inherited in this past post. So let's talk about maternal inheritance of mtDNA. 

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.

Honestly, I could easily write a whole blog post on the ethics and the different methods behind MRT - let me know if you're interested below in the comments. 

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Sources

[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