an autodidact meets a dilettante…

more Neanderthal eye candy

To continue with my Neanderthal confusion, I’ll quote from the brief Cosmos article and try to tease out some meanings and investigate further.

Mitochondria reproduce asexually – without mixing of genes – so if they’re replicating a lot, as they need to do in continuously dividing sperm cells, they may build up deleterious mutations that are potentially lethal to their ongoing reproduction.

Okay, some questions. What exactly are mitochondria and why do they reproduce asexually? What are they doing in our sperm cells?  Aren’t sperms ‘male’ as opposed to egg cells? But apparently mitochondria are found in female germ cells too. What’s going on?

Mitochondria are membrane-enclosed organelles found in eukaryotic cells. Above all, they generate ATP, providing the cell with chemical energy. And they do many other things – in fact, you could clearly spend a lifetime or several in investigating the properties of and the various functions of these wee beasties within cells, so I’ll try not to get too distracted. The number of mitochondria within particular cells varies enormously, from one to thousands, depending on the organism or the tissue. They have their own genome, quite separate from the DNA in the nuclei of their host cells. This genome is substantially similar to bacterial genomes, and this has famously prompted a hypothesis about endosymbiosis which is now, I think, generally accepted. But I digress. Or do I?

Presumably this symbiotic relationship between essentially bacterial mitochondria and eukaryotic cells means that the mitochondria exist in all cells of the eukaryotic organism, including the germ cells. ‘Continuously dividing sperm cells’ is a term that has thrown me – I was at first thinking of the continuously dividing germ cells of the blastula, but no, these are sperm cells continuously reproduced within the adult male. Nature being nature, this reproduction isn’t absolutely perfect every time and there’s the occasional mutation. This is a problem, as the mutations don’t just die off as they might in a different environment – but if a certain percentage of the replicated cells are mutant, wouldn’t that figure be constant as the cells multiply? But presumably these cells don’t just divide endlessly into hugely huge numbers… I’m thinking too much? Let’s return to the article:

Female germ cells, however, are held in a prolonged phase of dormant suspension from the moment of formation in late foetal life until just prior to ovulation. And so mitochondrial DNA in oocytes – female germ cells – is protected from mutations. For this reason, female parents pass on their mitochondrial DNA but male parents do not.

I get at least some of this. The female germ cells, with their mitochondria, are formed in late female life, and they don’t replicate ‘until just prior to ovulation’. What exactly is ovulation, then? By the word itself, I’d have thought ‘the formation of eggs’. So oocytes, female germ cells, aren’t themselves eggs. Forget that, they are eggs, and ovulation is about the discharge of an oocyte [or ovum] from a ruptured ovarian follicle. So the oocyte doesn’t change or replicate at all? It’s just discharged into – where? Down the fallopian tube, one egg per month [usually] in human females. By month, I mean, per cycle. If it doesn’t get fertilized, from its new position in the lining of the womb, it’s discharged through menstruation. Sorry, just getting this clear. What I do get is that the ova are ‘fully formed’ even before their female ‘owner’ is born. The female’s ovum becomes fertilized when a sperm cell penetrates the nucleus of the ovum, and its chromosomes combine with those of the ovum. The mitochondrial DNA remains the same, and of course is only carried on through the female line. I think I’m clear about this, more or less, for the first time.

Let’s go back again to the article:

Since Neanderthal mitochondrial DNA is absent in living humans, it suggests that any hybrid offspring who carried the mitochondrial DNA from Neanderthals didn’t contribute to the 4% of shared human-Neanderthal DNA.

The article then goes on to canvas a few theories as to why this might be so. But whatever the explanation, the fact is that the modern human genes traceable to Neanderthals are solely the result of male Neanderthals mating with female humans. And now we get to Haldane’s Law, which relates to the sex ratio of the offspring of hybrid animals. Basically, XY [male] progeny in such cases will be likely to be non-functional, non-existent or sterile, due to the high mutation rate of Y chromosome genes. So, more female progeny would be produced, but they have not turned out to be contributors to our gene pool