Re: RC: Re: Breeding theories: long!

[Peggy Rinehart <rubyjwl@cts.com>]

Hi Paula,

Great post!  I think you did an excellent job of clarifying what several of
us have been trying to say about this subject. Aged horses CAN and DO
successfully reproduce in controlled breeding situations.  But the mere fact
that the stallion or mare is elderly does not, in and of itself, IMPROVE the
outcome.

I might also add that natural selection does not favor reproduction of
elderly mares or stallions.  I realize that there is very little that
resembles natural selection in our current breeding programs for any captive
species.  My point is that natural selection FAVORS animals with the best
breeding potential.  For all the reasons you (and Heidi and others,
including myself) have discussed, elderly horses, despite impeccable genetic
blueprints, may not have the best breeding potential.

Peggy Rinehart
Ramona, CA

At 01:45 PM 9/16/99 -0700, you wrote:
>
>This got longer than I intended when I started, so if you're not into
>biology lectures, you can delete!
>
>OK, here are my inclinations on the subject. Keep in mind that breeding is
>really a numbers game. As we've seen on multiple posts, old stallions and
>mares can be successfully bred. If you divide mares into age groups,
>though, very young mares and old mares have lower conception and birth
>rates than young and middle aged mares. 
>
>> That is interesting.  What physical mechanism do the researches think
>> comes into play here? The genetic blueprint should be the genetic
>> blueprint.  
>
>Well, yes, and no. It's the blueprint all right, but the paper and ink are
>subject to insult that make it hard to read! 
>
>>I understand that an older mare may not be a good as growing
>> the fetus as a younger one, but what physics explains the aging of the
>> genes?
>
>At it's core, the explanation is certainly a matter of physics, but for
>us, the reality is in the biology. DNA is a molecule with chemical
>properties, but it doesn't exist in isolation and even in an ovum, which
>is fairly quiescent, the metabolic reactions that sustain the life of a
>cell still occur. And DNA isn't just tucked away in a bag within the cell
>for safekeeping. In combination with a number of enzymes and other
>molecules, DNA is directly involved with day to day cell function as well
>as overall identity. You might say that your identity stems from the sum
>total of all the interactions of your DNA with all the molecules that were
>originally made from the instructions within your DNA and which are
>continually, to the day you die, made from your DNA. The DNA in every cell
>in your body is continually in use along multiple points along its length.
>Since we already have metaphors going, DNA is sort of like a pattern book
>that tells a cell how to make proteins that will form and do the work of
>the cell and through its life, the cell continually refers to the DNA.
>
>As Heidi said in a recent post, the DNA in an ovum is pretty vulnerable.
>In addition naked and relatively unprotected DNA, the passage of years
>also brings on a reduction in the efficiency of DNA repair mechanisms. To
>build on Heidi's sniper metaphor, you could add some MDs (sorry, I'm a PhD
>in an animal science department of a  research university, so I delineate
>between MDs, DVMs and PhDs <g>), construction guys, etc., who take care of
>fixing the problems caused by the sniper, whether they be hits to
>buildings or people. Even a hit to a building can cause a problem down the
>road if the building collapses and kills people. 
>
>In the male, there is the opposite situation. The germ cells that give
>rise to sperm cells are continuously dividing, which brings on a different
>set of risks. A sperm cell is really nothing but a bag of DNA with an
>outboard motor and a few emergency supplies, and the testes are geared
>toward making as many as possible as quickly as possible, and
>continuously. It's up to the egg to provide all the rest of the
>stuff that the embryo will need to get started. Like other rapidly
>dividing cells of other tissues, germ cells in the testes are vulnerable
>to a number of different things. As a stallion ages, there is an
>accumulation of genetic "hits" and a decrease in the effectiveness of DNA
>repair mechanisms, so there is an increase in the number of mutations that
>get propagated in sperm cells, and which increase the probability of a
>problem in a resulting embryo once the DNA from both egg and sperm get
>together and the embryonic genome (DNA) is activated. I could go on and
>on forever, but you get the picture.
>
>When it comes to breeding animals, which is a crap shoot anyway, the
>*odds* are better with younger animals. It isn't that old mares and/or
>studs *can't* be bred, but they do require better management and there's a
>greater probability of failure. It isn't any one thing, it's just that
>the probability that a problem will occur increases over time, while the
>ability to respond effectively to problems decreases over time. Together,
>this means that compared to young animals, old animals have a harder time
>of it. 
>  
>So, back to the idea of old studs or mares bred to young mares or studs,
>you can see that by having one of the pair be young, you help your odds a
>little bit. And unfair it may be, but the numbers would work better
>for the owner of the old stallion, because the chances of viable
>offspring are greater. An old stud can still breed more than one
>mare, while a mare can still only have one foal a year. From a
>reproductive management standpoint as well, it's easier to take care of a
>stud. 
>
>----------------------
>Paula C Gentry, PhD
>pgentry@U.Arizona.EDU
>
>It is not knowing a lot but grasping things intimately and savoring them
>which fills and satisfies the soul. 
>
>--St. Ignatius--
>
>
>
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