Friday, 21 August 2015

Maternal transmission, sex ratio distortion, and mitochondria

http://www.pnas.org/content/112/33/10162.full

Steve J. Perlman, Christina N. Hodson, Phineas T. Hamilton, George P. Opit, Brent E. Gowen

Most (but not all) multicellular organisms have uniparental inheritance of mitochondrial DNA, which is thought to prevent invasion by a more competitive lineage, and maintain compatibility between mitochondrial and nuclear genomes. Mitochondrial DNA is also subject to evolutionary pressures, and in this review the authors discuss potential (deleterious) side-effects of uniparental inheritance. 

An immediate consequence of uniparental inheritance is that one sex is an evolutionary dead-end. Thus, any selective pressure on mtDNA is principally exerted on the transmitting sex (females), and so deleterious mutations for the non-transmitting (males) sex can accumulate. This explains why male infertility is often attributed to mutations in mtDNA. A study by Innocenti et al. [1], established fly lines with variation in their mtDNA. A large fitness variation was observed, but only in males; this caused significant differential expression across the nucleus of male flies. 

The authors suggest three possible consequences of maternal inheritance of mtDNA:
  1. MtDNA mutations which are detrimental to males may fixate, if they do not affect female fitness
  2. MtDNA mutations which cause the frequency of females to increase, may fixate
  3. Nuclear symbionts carried on the nuclear female chromosome which fixate, will also cause their associated mtDNA haplotype to fixate


[1] Innocenti P, Morrow EH, Dowling DK (2011) Experimental evidence supports a sex-specific selective sieve in mitochondrial genome evolution. Science 332(6031):845848.



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