14 November 2013
In a letter to the Reverend J S Henslow dated 16th July 1860, Charles Darwin posed one of the biggest questions in evolutionary biology; why do the sexes exist?
There is no greater mystery in the whole world, as it seems to me, than the existence of the sexes, – more especially since the discovery of Parthenogenesis. The origination of the sexes seems beyond all speculation.
153 years later, research lead by Prof John Allen at Queen Mary offers an answer to Darwin’s mystery by revealing the extraordinary mechanisms in female gametes used to preserve the powerhouse of cells: the mitochondria.
A fluorescent reporting reactive oxygen species (green) in image of three consecutive stages of oogenesis in the fruit fly Drosophila melanogaster.
Image by Wilson de Paula
Prof. Allen and his team observed “template mitochondria” in the female gametes of jellyfish, fruit flies, and zebra fish. The research demonstrates that a special line of mitochondria in egg cells are left inactive to ensure that their DNA remains intact and genetic mutations are not passed to future generations.
It is believed that mitochondria originated as separate cells that were engulfed by a single celled organism in early evolutionary history. As a result, they carry their own DNA essential for replication. However, hosting DNA presents a serious problem.
The role of mitochondria is to produce ATP, the energy currency used in almost all cellular mechanisms through respiration. However, this process also produces free-radicals; reactive molecules that corrupt DNA leading to damaging mutations. It is theorised that these mutations are associated with diseases of aging.
So in their normal course of work, mitochondria are extremely prone to damaging their own genetic material over time.
The mitochondrion is about the worst imaginable place in the cell to keep genetic information, It’s like keeping all your precious documents next to an open incinerator.
Prof. John Allen
However, in spite of this predisposition towards accumulated genetic damage, very few mutations are observed from generation to generation; offspring seem to inherit a fresh set of mitochondrial DNA.
Prof Allen’s team examined the activity of mitochondria in egg cells and found evidence of those with reduced respiratory function, therefore protecting their DNA from damage and preserving the genome intact for the next generation. Inactive examples were not found in the male gamete; sperm cells, as the energy needed for swimming demands the operation of all available mitochondria.
This key difference offers an insight into why two sexes might have evolved; to ensure that fragile mitochondrial DNA is transmitted intact.
Prof Allen’s and his co-authors’ research is featured in the Journal of Genome Biology and Evolution and on the UCL Genetics, Evolution and Environment Blog.
See also: jfallen.org - Mitochondria, ageing, separate sexes