Eukaryotes, though they carry their own DNA in their nucleus, which
is passed on every generation, also harbor a stew of DNA from their
evolutionary past within their cytoplasmic soup. This DNA, which
includes mitochondria in animals and plastids in plants, are the
powerhouses of the cell, and are normally only passed down maternally.
But do these cytoplasmic DNA also undergo adaptation within the
confines of the cell? And if so, how do they pull off this feat while
seemingly having one evolutionary arm tied behind its back? You see,
cytoplasmic DNA can't easily swap out harmful mutations like nuclear DNA
(side note: neither can the male Y chromosome, which is why it is
‘Using computational tools scientists have helped understand cytoplasmic DNA adaptation and how they avoid harmful mutations which could become like Trojan horses to affect the whole cell.’
Only nuclear DNA can maintain the best mutations and remove bad ones
by reshuffling its genome like a deck of cards during reproduction
after sperm meets egg. This reshuffling allows nuclear DNA to adapt. For
mitochondria and plasmids, there is only asexual reproduction.
Now, University of Sydney School of Life and Environmental Sciences
researchers Joshua Christie and Madeleine Beekman, have used
computational tools to better understand cytoplasmic DNA adaptation and
how they promote beneficial mutations - and more importantly, avoid
harmful mutations which could become like Trojan horses to affect the
whole cell, and thereby, the health of an organism.
"Why is the mitochondrial genome still going strong after 1.5-2
billion years?" asked co-author Beekman. "It all comes down to the
fundamental difference between male and female sex cells, or gametes.
While each sperm cell contains one sex-chromosome (either an X or a Y),
each egg cell contains many mitochondria. Because mitochondria are
essential to cell function, after all they provide the energy, egg cells
that contain faulty mitochondria are selected against. As a result,
only egg cells that contain a full complement of healthy mitochondria
stand a chance to produce a zygote."
The study shows how efficiently the biology of cytoplasmic
genomes - specifically their organization into host cells and their
uniparental inheritance - can allow them to accumulate beneficial
substitutions and to purge deleterious substitutions faster than
free-living sexual genomes.
"So, while males are happy to pass on any Y-chromosome to their
sons, females appear to take the future of their offspring a little more
seriously," said Beekman.
In addition, the general insights gained from the study can be
applied broadly to understanding adaptation for other cytoplasmic
genomes. In addition to mitochondria, these include plastids and
obligate endosymbionts such as Rickettsia
and the Wolbachia