Loren Bolinger, boss TBMitotyper
By Loren Bolinger, Tuesday, August 02, 2005
Loren Bolinger said...
Diversity and Bottlenecks
Thoroughbred horses were created from a small founder population.
Throughout their history [the limited founder gene pool and the
possibilities of genetic bottlenecks threatening genetic diversity have
been been ongoing topics of discussion. The usual meaning of a genetic
bottleneck is when a dramatic reduction in the size of a population
causes a reduction in its genetic diversity.
A
genetically prepotent individual can temporarily overwhelm the balance
of bloodlines and the diversity of the gene pool of the Thoroughbred in
such a way that its popularity upsets the diversity and distribution of
patrilines in the gene pool. Less successful bloodlines [both tail-male
and tail-female] by virtue of their lack of success, accelerate their
own decline. Once past a certain threshold in its decline, it becomes
very improbable that a bloodline can return to major influence in
patrilineal or matrilineal descent. It remains in pedigrees as a
diffuse and distant force usually in the interior of pedigrees where it
undoubtedlly does contribute, although indirectly.
I find an
interesting parallel between the subject of the discussion "Diversity"
and a similar situation in England that begun 124 years ago. St. Simon
and his descendants dominated the Thoroughbred gene pool to such an
extent, that their lasting effects created what I belive was a genetic
bottleneck. Similar loss of [questionably] valuable tail male lines may
have been lost due to St. Simon swamping the breeding population.
John P.
Sparkman recently wrote in the TBHeritage e-mail list, in answer to a
question by "Elles Prins",” How big is the St. Simon influence on
today's horses?”
"Subject: Re: St. Simon, influence & linebreeding"
"Hi Elles,
According to modern statistical studies, roughly 8% of the genes of the modern Thoroughbred
come from St. Simon, the highest influence of any horse since the Eclipse/Herod era."
Since the
main method we have to determine the relative success or failure of our
selective mating schemes IS "the test of the winning post," those that
win succeed and those that fail fall by the wayside. Performance
ultimately determines the fate of all individuals. It is a harsh and
unforgiving reality - this performance testing. An individual's
"value," his economic "worth," his prestige and reputation are
intimately tied to performance. In the Thoroughbred world, survival of
the fittest revolves around racing accomplishments.
St. Simon
[and later, his sons and grandsons], arguably the most significant and
dominant sire [and sireline] in the history of the Thoroughbred,
saturated the gene pool with his heritable characteristics. His lasting
influence has permeated the breeding population such that in most
modern pedigrees the number of strains of St. Simon exceeds all other
ancestral influences. St. Simon became the most obvious of the forces
that created excess-male inbreeding in the gene pool [in the profile of
the ongoing, historical breeding population].
Survival
of bloodlines depends on many factors, but the most important is their
kinship to winning performance among their descendants. In a
performance-based meritocracy, those individuals and families failing
to win descend the economic ladder until they become extinct.
I'm sure
tbpedix members can cite other influences that upset the balance or
profile of the breeding population through dominance causing similar
bottlenecks.
I'm a firm
believer in maintaining diversity of the gene pool of the Thoroughbred,
but what a conundrum for breeders. Winner win, losers lose, and unless
they remain competitive, bloodlines will fall along the way as they
have since the origin of the Thoroughbred. As one who believes in the
possibility of improvement through the principles of selective matings,
I should be able to help preserve the diversity of rare or less
successful bloodlines through clever matings. While it's extremely
difficult to figure out how to let altruism trump economics while
preserving my own little breeding operation, at some point preservation
becomes economically completely untenable. Just as with the elite, my
stock needs to win. The "holy grail" among breeders would be some
equation of bloodline affinities combined with selective breeding
patterns [genotype], biomechanical proportions [phenotype], parental
and familial past performances, environmental factors, and intangible
courage/heart factors. Of course, many have tried to condense
performance into a formula, yet few have succeeded in even defining all
of the variables or demonstrate consistent, formulaic success.
According
to research, however, the dominance of one sire and his descendants
such as St. Simon or in the cases of extreme inbreeding such as
practiced by M. Marcel Boussac may not represent true genetic
bottlenecks of major importance or be able to cause jeopardy to
diversity of the gene pool. They can have a more immediate effect on
performance profiles of certain pedigrees both closely related to the
bottleneck and more distant relatives. As inbreeding receeds into the
past it becomes far less significant rapidly dropping into the range of
background noise. Expression of performance traits is more likely to
occur with inbreeding within six generations or middle distant
inbreeding 6 - 10 generations than it is with far distant inbreeding.
Far distant inbreeding forces are more likely to be usually matrilineal
in origin [female-family related].
Yet when a
dominant patriline causes the distribution of patrilines within a
population to bottleneck to any degree, usually the overdominance of
the dominant patriline is eventually compensated to some degree through
the subsequent rise of other patrilines that soon overtake the
previously dominant patriline. Patrilines rise and fall in a wave-like
manner [perhaps along the slopes of a normal curve].
There are
two inevitable irresistible general groups of genetic forces having
distinctly different long term effect upon the Thoroughbred gene pool..
The forces that increase genetic diversity are heterozygous
(outcrossing) they tend to dilute inheritance make it more variable
over a wider range of traits, less predictable. The other forces are
those that reinforce through narrowing the range of inheritance usually
through inbreeding or through an outside catastrophic event that causes
a genetic bottleneck (sudden elimination of a portion of the gene pool
through catastrophy would cause a severe reduction of the gene pool,
reducing diversity). Separately are the effects upon the mitochondrial
DNA, maternal inheritance or femnale families. Here, reduction in
female families has a long term adverse effect upon the gene pool and
genetic diversity of the Thoeroughbred. Sire lines come and go in a
wave-like effect lasting 4 6 generations and therefore are fugitive or
impermanent. Dam lines are the true heart of the gene pool of the
Thoroughbred. mtDNA is passed from mother to progeny through her ova,
but since mtDNA resides in the cytoplasm of the female germ cell only
through the daughters (female line) are they passed along to future
generations of descendants. They are replicated not re-created and must
be preserved – this is the true diversity of the Thoroughbred.
St. Simon
has had a profound influence on the modern Thoroughbred. As one of the
most successful sire in the history of the breed, he and his progeny
became so popular that his influence overwhelmed (suffocated) the gene
pool of the breed - in my opinion, his genes gained a far greater share
of the gene pool than might be otherwise healthy to the longevity of
the breed. A type of genetic bottleneck or narrowing of the gene pool
was established through the high frequency of his crosses (the same
also applies to his sire, Galopin) appearing in modern Thoroughbreds.
His and his descendants' popularity, disproportionately overwhelmed
other bloodlines among breeders of the day and onto modern times. St.
Simon is but one contributing factor to the overwhelming excess male
inbreeding characterizing the genetic profile of the modern
Thoroughbred.
While
lovers of Thoroughbred horses may decry the apparent lack of diversity
in sires and/or sirelines at the elite level of the potential starters
for the 2005 Vidaphone Epsom Derby, by no means are they similar in the
choices of selective mating strategies made by their breeders. The
individual mating strategies chosen vary significantly indicating a
type of genetic diversity in spite of the similarities of patrilines.
We see outcrossing [within six generations], inbreeding [within six
generations], various opposite-sex inbreeding [toward specific
ancestors, patriline reinforcement, pattern reinforcement]– all
the strategies we in this forum believe in.
I disagree
with many of the conclusions of Rachel Pagones, bloodstock editor of
the Racing Post and the author of the article, “Derby: It May
Prove Best to Stick with Recent Form.” Pedigree analysis as a
betting tool tends to work quite well when the selective mating
patterns used for each horse as well as their family trees are included
in handicapping competitors in elite races such as the Breeders’
Cup and international classic races.
Narrowing
of successful patrilines may represent a bit of a historical
mini-bottleneck or it may indicate dominance. Demonstrable performance
is a combination of genotype and phenotype interacting with various
environmental factors and also includes a bit of luck. The “test
of the winning post” as it always has been, should be the
ultimate arbitrator of bloodline success or failure. Winners win and
losers fail as it has always been. If the breeding farms incorporate
proven selective breeding schemes they, their sires, and the bloodlines
they favor will tend to dominate. In a meritocray one may choose to use
any blood a breeder wishes, but can it win? Does Sadlers Wells, as with
St. Simon, represent a form of a genetic bottleneck? – I
don’t know for sure. Will the breed known as the Thoroughbred
survive such peril to its genetic diversity? Unquestionably, yes! The
scientific literature is full of such ruminations over breeds and
species in far greater jeopardy than the Thoroughbred, yet extensive
research from geneticists and molecular biologists tends to refute such
fears.
As
pedigrees are examined, the breeder is quite puzzled to see obvious and
pronounced pedigree patterns in some elite runners while also finding,
equally obvious, heterozygous pedigrees with no internal patterns
apparent within six generations. One is forced to conclude that both
types; inbred and outbred pedigrees are equally capable of elite
performance.
I
don’t think the apparent narrowing of successful bloodlines is
cause for concern. There is a self-fullfilling nature to a meritocracy
[prophesy at work in a meritocracy]– when the success of the ND /
MrP cross saturates or overwhelms the overall gene pool, it will begin
to fail to dominate. Other lines, perhaps some more heterozygous, will
rise up to supersede [supplant, usurp] the previously dominant blood.
All: horses, breeders, owners, trainers must supplicate to the
Thoroughbred’s historical precedent– Vow allegiance to the
test of the winning post or be forgotten.
What is the difference between a taxidermist and a tax collector? The taxidermist takes
only your skin. Mark Twain - Notebook, 1902
-Loren Bolinger