Molecular Clocks & Generation Time

Dr Jim Cummins cummins at POSSUM.MURDOCH.EDU.AU
Thu May 4 20:42:47 EST 1995

>In article <3o9q5g$rl6 at owl.und.ac.za>, daceh at shrike.und.ac.za (Halford
>Dace) writes:
>|> Just a brief addition to the molecular clock debate.  I don't know whether
>|> it'll be useful or not.
>|> It's probably worth bearing in mind that mutation rates per se probably
>|> aren't that important - a mutation will only enter a population's gene
>|> pool if it happens in a germ line cell.  I would think that this would
>|> tend to `speed up' the molecular clocks of organisms with shorter
>|> generatioal times, since more generations (ie more gametogenesis &
>|> fertilizations) in a given period would allow more mutations to
>|> accumulate, since the genes comprising the population's gene pool
>|> would have spent a greater proportion of time as gametes.
>|> I think the idea makes sense but I'm sure that there's a better way of
>|> expressing it.
>I'm not quite sure I understand Hal's idea, but if the significant part
>of the argument is the proportion of time spent as gametes, surely the
>mutation rate would be much higher in females of a species than males,
>since gametogenesis takes place very early in females, while in males,
>gametogenesis happens close to the time of fertilization.

Hmmm.. Not that simple.  In (human) females meiosis commences in fetal life
after a limited number of mitotic divisions to colonise the ovary with
around 7 million primordial oocytes. The oocytes then enter a prolonged
dictyate resting stage and meiosis recommences at ovulation to be completed
(ie reach the haploid stage) after sperm entry.  Errors accumulate largely
as chromosomal factors (eg Downs syndrome) By contrast, spermatogenesis
starts at puberty and is continuous.  Thus a sperm produced by a 50 year
old man (do I give myself away here?) has been through many more division
cycles than the equivalent gamete in a female.    This leads to genetic
errors primarily through DNA breakage and replication errors.

Jim "Spermatology rules o~ o~ o~ o~" Cummins

Associate Professor in Veterinary Anatomy
Murdoch University, Western Australia 6150
Tel +61-9-360 2668, Fax +61-9-310 4144
E mail <cummins at possum.murdoch.edu.au>
URL <http://Numbat.murdoch.edu.au/spermatology/spermhp.html>

"There are two kinds of people in the world, those who believe there are
two kinds of people in the world and those who don't."

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