CYTOGENETIC CONSIDERATIONS IN ANIMAL BREEDING*
N.S. FECHHEIMER
Department of Genetics, University of Edinburgh, Edinburgh EH9 3JN
SUMMARY
Some fundamental aspects of cytogenetics, a brief review of work done with farm animals, and an assessment of the known effects of chromosomal aberrations on animal population are
summarized in this paper.
The basic chromosome number and other detailed features of the karyotype have been reliably established for all the farm animals within the past 10 years. Intra-species variants, i.e. chromosomal polymorphisms have been found in cattle, pigs, and goats.
Two primary types of aberrations are recognized. The first, an alteration of numbers of whole chromosomes is known as heteroploidy. The second type, rearrangement of chromosomal segments, results following physical breaks of chromosomes.
Factors affecting the frequency with which aberrations occur include both genetic and non-genetic elements. Among the most important in the latter category are certain virus groups, some drugs and other chemical compounds, ionizing radiations, age of gametes, parti- cularly ova, and in some animals, maternal age.
Presence of chromosomal aberrations are apparently responsible for a large proportion
of the embryonic deaths in mammals and birds. In man, perhaps as many as one-third of
spontaneously expelled abortuses are afflicted. Early indications from work with pigs indicates
that the situation may be similar. Very little cytogenetic work has been done with congenitally
malformed livestock, although a number of aberrations have been reported. In man, somewhat less than 1 p. 100 of new born babies carry chromosomal abnormalities.
Some aberrations, although they cause only slight if any somatic alteration, can be the
source of reduced fertility or sterility in their bearers. Isolated cases of this phenomenon have
been reported but a quantitative assessment of its importance has not yet been made.
The presence of centric fusions as chromosomal polymorphisms in cattle, pigs, and goats has been reported, and work is being conducted to ascertain how they effect fitness of the popu- lations in which they occur.
It is concluded that the occurrence of chromosomal aberrations causes a serious reduction of potential productivity. The extent of this loss should be accurately determined and means
found to reduce it.
(
*
) Invited report presented in the Study Meeting of the European Association for Annual Production Genetic Commission, Gbdbllb, Hungary, August 27th, 1970.
(
**
) Permanent address: Department of Dairy Science, the Ohio State University, Columbus, Ohio 43210, U.S.A.
1. - INTRODUCTION
Interest in the chromosomal
complements
of farm animals has beenhigh
since
early
in the zoth century when it was first established that the chromosomeswere the vehicles on which the genes were carried.
Particularly European
andJapanese
workerspursued
thequestion
of the normal chromosome numbers of the domesticatedspecies.
It is a tribute to the astuteness and perseverance of thepioneers
thatalthough techniques
were laborious andinadequate, they managed,
sometimes with uncanny accuracy to describe thekaryological make-up
of a number of vertebrates.
(For
a historical review of ruminants see HULOT andI,AUVERGN!, 1967.)
Three technical
developments
of the195 o’s
enabledcytological
studies of mammals to be made much morereadily, accurately,
and onlarger samples
ofspecimens.
These were( I )
Hsu and POMERAT’S( 1953 ) discovery
of thesalutary
effect of the treatment of cells in
hypotonic
solutionprior
tofixation; ( 2 )
theuse of mitosis
arresting agents,
mostnotably colchicine,
to arrest cell division at astage
most suitable for observation ofchromosomes; ( 3 )
thedevelopment
of
simplified techniques
for cell and tissueculture, particularly
thestartling finding
of MOORHEAD et al.( 19 6 0 )
that certain substances would induce mitosis oflymphocytes.
Although
various modifications of these new found methods have beenwidely
used to
study
variations in the chromosomalcomplement
of man,they
haveonly quite recently
been usedby
workers interested inincreasing productivity
of farm animals.
II. - CHROMOSOMES OF LIVESTOCK
The normal
karyotype
of all domesticated mammals and most of the birdshas, by
now been well established(Hsu
and BENIRSCHKE,1970 ).
In Table ithis information is summarized for the more common
ungulates.
There are anumber of
interesting
features characterizedby
the data on this table. First note the extremeevolutionary stability
in the number of chromosomal arms in a widespectrum
ofspecies
of Bovidae. Thispoint
has been well madeby
the studies of a number of taxa
by
Wuxs’r!R and BENIRSCHKE(ig68).
In contrast is the widevariability
exhibitedby karyotypes
of the variousequine species.
This fact would be even more notable if I had listed data for some of the zebras
(see
Hsu andBE NIRS C HK E, ig 7 o). Secondly,
it isinteresting
to note thatalthough relatively
few animals of any of thesespecies
have beenexamined, already
chro-mosomal
polymorphisms
have been found. The centric fusion in cattle is in reference to the exhaustivestudy
madeby
GUSTAVSSON( 19 6 9 )
of Swedish Red and White Cattle. A similartype
ofpolymorphism
isbeing
studiedby
MCFEEand others
( 19 66)
in a group of wildpigs
descended from animportation
to theU.S. from
Germany.
It appears that Zebu cattle possess an acrocentric Y chromosome while the Y ofEuropean
cattle is submetacentric(K IEFFER
andC ARTWRIGHT
, 19 68). Undoubtedly
as more animals are looked at from a widergeographical spectrum
more variants will be found.They
areinteresting
intheir own
right
and becausethey
shed some additionallight
on theevolutionary history
of our livestock.III. - TYPES OF ABERRATIONS
Before
proceeding
with a discussion of the causes and effects of aberrations ofchromosomes,
itmight
be wellbriefly
to review thetypes
ofthings
that canhappen
to chromosomes and to establish someterminology.
A morecomplete
discussion was
previously published (FE CHH E IM E R , 19 68).
Ingeneral
there aretwo
types
of alterations that can occur. The first is an alteration in the number of whole chromosomespresent
in a cell and the second is a structural rearran-gement
of chromosomalsegments following
the occurrence ofphysical
breaks.Alteration of the normal 2 N somatic number
by
wholemultiples
of agametic
set is known as
euploidy.
The twoprimary categories
arehaploidy
andpolyploidy,
of which
triploidy ( 3 N)
andtetraploidy ( 4 N)
are the mostfrequently
seentypes.
Aneuploidy
refers to achange
in the genome of one or a few chromosomes. The mostfrequent types
ofhypodiploidy
andhyperdiploidy
are monosomy andtrisomy respectively.
These terms refer tokaryotypes
with onemissing
chromosome andone additional chromosome. In mammals all
heteroploid
conditions with theexception
of some trisomics and monosomic X are lethal and result in thepremature
death of theanimal, usually prior
to birth.Structural aberrations may be classified upon the number of breaks that
necessarily preceeded
therearrangement
of material. Thus one break resultsultimately
in the loss of a terminalsegment.
If the break isthrough
the centro-mere, an
isochromosome,
i.e. one with two identical arms, may arise.Two breaks may occur in the same chromosome. This can lead to an inter- stitial
deletion,
aninversion,
or formation of aring.
Should one break occur in each of twochromosomes,
the mostlikely
recoveryproducts
would be trans-location chromosomes each of which
possessed
a centromere. Productsresulting
from other
types
of refusions of broken ends aremostly
unstable.Quite obviously
it ispossible
to havemultiple aberrations,
either in thesame cell line or in
separate
cell lines of the sameorganism.
It may seenunlikely, given
the lowfrequency
of occurrence of mostaberrations,
that one would findmultiple aberrations,
but such is not the case. Thephysical
eventgiving
riseto one error in many instances
produces
two new aberrant cells at the same time.An
example
of this would benondisjunction
of a chromosome at anearly cleavage
division. In this case two new
lines,
one monsomic and one trisomic for the chromosome inquestion
would begenerated simultaneously by
thesingle
event.Finally,
indiscussing organisms
which possess more than one cellline,
it is useful to
distinguish
between mosaics and chimerae. The former is the of two or more cell lines derived from the samezygote.
Chimerism is the presence presence of an additional cell line in anorganism,
derived from anotherzygotic product.
Freemartins are therefore chimerae.IV. - VARIABLE EFFECT OF ABERRATIONS
The effect that any of these chromosomal aberrations will exert on an
organism
or on a
population depend
upon a number of interrelatedfactors,
some of whichare
briefly
discussed.a) Quite obviously
if alarge
chromosomalsegment
were lost(or perhaps duplicated)
theexpected
effect would begreater
than if a smallsegment
were involved. This would beparticularly
so if the lostsegment
werecomposed primarily
of heterochromatin and thus contained littlegenetic activity.
b)
Someaberrations,
e.g. inversion ortranslocation,
when carried as hetero-zygotes,
will exhibit no somatic manifestation on their bearers.However,
at meiosiscomplications
do arise and aberrantgametes
will beproduced. Secondary
aberrations are, as a consequence to be
expected
in some of the progeny ofparents carrying
a structural aberration.c)
The later inembryogenesis
that an aberration occurs, the more attenuated will be its influence on theorganisms. If,
for instance it occurs at the firstcleavage
division then all(or
at leasthalf)
of thezygotes
cells willeventually
contain an
aberrant
genome. On the other hand if it first occurs in one cell in analready
differentiatedtissue, only
a smallproportion
of theorganisms
cellswill contain an altered genome and no noticable alteration of
phenotype
needbe
produced.
’
d)
Some aberrations are selflimiting
in thatthey produce
suchprofound
effects that
they severely
alter the mitoticactivity
of the cellcontaining
them.In this case the aberrant cell line will remain small and may not manifest itself at all.
Very
little is in fact known aboutcompetition
and selection amonggenetically
different cell lines.e)
Otherphenomena
ofembryogenesis
can serve to attenuate thephenotypic expression
of an aberration.( I
)
It waspointed
outby
BEATTY( 1957 )
that if an event occursduring
the first few
cleavage divisions,
the aberrant cell linemight
beentirely
included in the
extra-embryonic tissues,
becauseonly
a few cells in the inner cell mass are destined to contribute to theembryo
itself.( 2
)
The presence ofdosage compensation
mechanisms is another factor that alters the effect of an aberration. Inactivation of one X chromosome in each cell ofdeveloping
mammals is such a mechanism that was firstpostulated by
I,YON( 19 6 2 ).
When an aberrant X ispresent,
it has been shown that it ispreferentially
inactivated so that most cell lines are left with the normal X as thefully
functional one.f) Just
as with genemutations,
the same event even in otherwisegenetically
identical
populations
willproduce
variable effectsdepending
upon the environment in which thepopulation
is found. The fact has beenelegantly
demonstratedby
DOBZHANSKY and his group.They
have observed that thefrequency
withwhich
particular
inversions are observed inpopulations
ofDrosophila
is influencedby
a number of environmental factors. Someinversions,
detrimental to thepopulation
in oneecological niche,
enhance its fitness in others.V. - CAUSATIVE FACTORS OF ABERRATIONS
Explicit
in thepreceding
discussion is theconcept
that mitosis and meiosisare not
invariably regular
processes thatalways give
rise to theexpected
endproducts.
Both aresubject
to avariety
ofirregularities,
each of which has itsown
expected
array of aberrant endproducts.
In Table 2 are listed a number of
agents
that have beenimplicated
asetiological
factors of chromosomal aberrations. It will be seen
that,
like any other cha- racteristic ofliving organisms
bothgenetic
andnon-genetic
factors are involved.Let me very
briefly
relate the nature of some of the evidenceimplicating
thefactors listed. In
regard
togenetic control,
the evidence isquite
clear that incidence ofheteroploidy
differs ingenetically
distinct stocks of mice. Further- more, selection for increased incidenceyielded
apositive
response(see B EATTY ,
1957for review).
In cattle inbred lines derived from a common source have been shown to differ in thefrequency
of cells withhypodiploid
orhyperdiploid
chromosome
complements (Z AR T MAN
andF!CHH!IM!R, ig6 7 ).
At least one gene is known in man thatproduces
chromosome breaks and alsobrings
about a rela-tively high
rate of somaticpairing (GERMAN
andA R C HIBALD , 19 65).
Thatmeiosis is
primarily
agenetically
controlled process is clear from studies of actionsof many mutant genes in
bringing
about variations that lead toproduction
ofaberrant
gametes.
Most of such work has so far been done withplants
andlower animals
(S ANDLER
etal., 19 68).
Primary aberrations,
both numerical andstructural,
causecomplications
atmeiosis that result in new
secondary
aberrations in thegametic
genomes. Fre-quently
however therelationship
cannot be discovered because theprimary
aberration will have been
cryptic.
Anexample
of this would beparacentric
inversions which can be discovered
only by linkage
studies orby viewing
germ line cells at fist meioticprophase. They
cannot be seen in somatic cellpreparations.
The
relationship
between autoimmune disease and aberration is notyet
clear. It has been noted thatparents
of babies with monosomic or trisomic conditions have ahigher
incidence of autoimmune disease thanpaired
controls(BuRCH
etal., 19 66;
Vnr,!,oTOrr andFORBES, 19 67).
A multitude of
specific
externalagents appropriately applied
to cells willbring
about aberrations. The entirespectrum
ofionizing radiations,
many chemicalcompounds (Fvnrrs, 1970 ),
certain virus groups(NrcHOr,s, 19 66)
andheat shock
(BE AT T Y , 1957 ) produce
the indicated effects. The action of someof these
agents
isfairly
well understood while for othersnothing
at all is knownexcept
that results arerepeatable.
The maternal age effect manifests itself in man where there is a
precipitously
increased
frequency
of trisomic children born to mothers over 40 years of age(P!ENROS!, 19 66).
While the samephenomenon
was not observed in mice(G
OODLIN
, 19 65),
it may be that an unfortunate choice of lines was used for thestudy.
EDWARDS( 197 o)
has shown that chiasmafrequency
is lower inoocytes
of olderfemales,
both human and mice. He argues that this fact could be acausative
agent
for increasedtrisomy.
When
mating
isdelayed
muchbeyond
the time ofovulation,
mammalianova either lose their mechanism of defense
against penetration by
more than onesperm or do not
undergo
a second meiotic division. In both instances thezygote
contains more than twohaploid
sets ofchromosomes,
i.e. ispolyploid. Polyploidy resulting from delayed mating
has been demonstrated in mice (V I C K E R S, 1 9 6 9 ),
rats
(BUTCHER
andFuGO, 19 6 7 )
rabbits(SHAVER
andC ARR , 19 6 7 )
andpigs (HArrcocx,
zg5g;HuNT!R, ig6!).
It has notyet
been demonstrated in a mono-tocous
species
but therelatively high
occurrence oftriploidy
in man, where there is no estrusexhibited, might
be considered indirect evidence.S A I,
ISBURY
( 19 68)
hasrepeatedly
observed a correlation between time ofstorage
of spermprior
to use in A.I. and estimated death rate of theembryos resulting
from its use. Such observations arecertainly suggestive
of the increased presence of aberrations in storedgametes.
Finally,
it can be shown that aberrant cells accumulate in certain mammalian tissues atspecific
times in the life of the individual(S WARTZ , 195 6)
or can beinduced to accumulate
(or
notto) by appropriate
hormone treatment. Thephysiological
function ofpolyploid
cells inliver,
pancreas and other tissues is not known. Their appearance can be hastenedby
sex hormone treatment andcan be
repressed by
castration orthyroidectomy (S WARTZ
etal., ig6o;
SWARTZand
FORD, ig6o).
’It is clear from the
foregoing
thatenough
is now known about theetiology
of various
types
of aberrations that a researcher canproduce,
almost atwill,
any
type
of aberration that he wishes tostudy.
Notnearly enough
is knownhowever to be able to discern how the
spontaneous
incidence of chromosomal aberrationsmight
be reducedby
the animal breeder or clinicalpractitioner
foreconomic or humanitarian purposes.
VI. - EFFECTS OF CHROMOSOME ABNORMALITIES
Just
as with genemutations,
the effect of chromosome aberrations is harmful in theoverwhelming majority
of instances in thatthey
reduce the fitness of their bearer in one or more of avariety
of ways.Embryo
and fetaldeaths,
congenital malformation,
intersexualphenotype,
other formssterility
or reducedfertility
are the costs of chromosome aberrations.Following
is a brief reviewwhat is known
regarding
the incidence of chromosomal abnormalities and their effects in animalpopulations.
A. - Chromosome aberrations and
embvyonic
deathIn Table 3 are shown some
representative
estimates of thefrequency
ofembryos
afflicted with one or anothertype
of chromosomal aberration. In man, well over 1,000spontaneous
abortions have been examinedcytologically
withthe very
surprising finding
that aboutz / 5 ,
i.e. 20p. 100 possess an aberrationthat in all likelihood was the cause of death and abortion
(Geneva Conference, ig66).
Thefrequency
ofembryos
with aberrations ishigher
in youngerembryos (C
ARR
, 1970 )
and it is nowthought
that among the veryearly
abortions(those occurring
in the first month ofpregnancy)
the incidence may be ashigh
as 30-q.o p. 100. If it is
supposed
that 20-25 p. 100 ofsuccessfully
fertilized human eggs aresubsequently aborted,
then it can be calculated 4-5p. 100 of allzygotes
carry a lethal chromosome
abnormality.
BE A TT Y
and FISCHBERG
(BEATT Y ,
1957 forreview)
looked at more than3 ,
000mouse
embryos
and found about 2.5 p. 100 wereheteroploid
at3 i !2 days.
Similar
findings
werereported by
VICKERS( 19 6 9 ).
All of these die and are resorbed. In the rabbit(SHAVER
andCA RR , 1 9 6 7 )
andpig (McFEEi v Y, 1967)
although
the scale of theexperiments
so far conducted has beensmall,
it appears that about 10p. 100 ofpreimplantation embryos
arechromosomally
defectiveand destined to die as a result. From
preliminary
studies of cattleby
McFEEi<Y( 19
68)
and HARPER( 1970 )
it appears that the situation is similar. Themeaning
of all this is that about one third of the
prenatal
pregnancywastage
of our farm animals is attributable to the presence of chromosome abnormalities.We have found the chicken to be a very useful
organism
for more detailed studies of thisphenomenon.
Different strains have different incidences of chro- mosomal aberrations in veryearly embryos,
one broiler stockhaving
an incidenceof z2 p. 100
(MILLER
etal., 1970 ).
Aberrantembryos
in this stock include monosomy,trisomy, ZZW, haploidy, triploidy, tetraploidy
andrearrangements.
In addition
simple
andcomplex
mosaics are common occurrences in this stock.In one
experiment
15 of 3q. aberrantembryos
all derived from one hen. This observationplus
the clear difference infrequency
among lines leads one to suppose there is astrong genetic component
in theetiology.
The distribution of
types
of abnormalities seen inembryos
of diff erentspecies
of animals is shown in Table 4. It is seen that in aborted fetuses o f man
trisomy
is the
predominant abnormality
and monosomy(mostly XO)
andpo lyploidy
areabout half as
frequent.
In the mouse,pig
and rabbitpolyploidy
causes mostof the trouble while in
chickens, euploid
mosaics areby
far the mostimportant.
Some of these differences reflect the
type
of material collected and thetechniques
used but there is no doubt that different mechanisms are involved in
causing cytological
errors in the differentspecies. Very probably,
animportant job
forthose interested in livestock
production
islearning
about the causes of and howto control the induction of
spontaneously occurring polyploidy.
B. - Chvomosome aberrations in neonates
Estimates of the
frequency
of neonatal animals with chromosome abnormalitiesare not available for any
species
other than man.Representative
estimates of incidence of newborn babies with aberrations are listed in Table 5. As ageneral
statement, the overall incidence can be taken as somewhere between o.5 p. 100andi.o p. 100. For technical reasons this can be
thought
of as the minimalfrequency.
The afflictions are divided rather
evenly
among threecategories:
sex chromosomeaneuploidy, trisomy
andrearrangements.
Sex chromosomeaneuploidy
ishigher
in males
owing
to the fact that XXY is more viable and therefore has agreater probability
ofsurviving
to birth than does XO. Also XYY is notinfrequent
in neonatal
samples properly
ascertained. For the rest, there does not appear to be a difference between the sexes.No similar surveys appear to have been conducted in animals. It is
thought
that the incidence would be lower because
trisomy
does not seem to be sofrequent
in animal
populations.
From therelatively
few laboratoriesengaged
incytogenetic
research with domestic
animals,
the whole array ofexpected
sex chromosome aberrations arebeing
turned up(see
Table 6 forlist). XX /XY
chimerism has been seen in all theLlngulatas. XXX, XO,
XXY and various mosaicscontaining
these cell
types
have beenreported by German,
Scottish and New Zealand workers.what is
causing
them and what are the effects. Acomprehensive
review of sexchromosome
aneuploidy
in animals was writtenby
BEATTY(i 9 6q).
Along
this line it should be noted that a number of aberrations can be expe-rimentally produced
in mammals now. A number of structural aberrations werefound in live born
pigs
derived from sperm that had been x-irradiated(Z ARTMAN ,
FEC HH EI M E
Rand
BAKER, r 9 6 9 ).
A total of 4opigs
was born of which 8 containedaberrations including translocations,
inversions and deletions. This sort ofstudy
is very
promising
both in terms of thegenetic information
that can begained
and
by being
able toproduce
at will aberrant animals for detailed anatomic andphysiological study.
C. - Chromosome aberrations and
infertility
From studies of man it has been learned that numerical aberration of the
sex chromosomes are
reasonably
well tolerated in thatthey
arecompatible
withsurvival
although,
with fewexceptions,
the bearers are sterile. Thosecausing complete sterility,
e.g. XXY and itsvariants,
areunlikely
to be sources of seriousloss of fitness in animal
populations. However,
carriers of structural aberrationsare
expected
to have reducedfertility,
so if such mistakeshappen
at allrequently, they
can be animportant
source of loss ofreproductive
fitness in thepopulation.
In
mice, semi-sterility
of males is aregular
concomitant of translocation hetero-zygosis
and most other chromosome anomalies(1 / y ON
andME R E DI T H , 19 66).
In man, about 10per cent of individuals
presenting
themselves toinfertility
clinics where
karyotyping
has been done have been found to carry a chromosome aberration(KJ!ss!,!x, ig6 5 ;
Mdi/TREE etal., zg66).
It is notknown,
of course,what
proportion
of the totalpopulation
thisrepresents.
In the datapresented earlier,
it was seen that about 0.14per cent
of neonatal babies are carriers of structural aberrations. Alarge proportion
of these will have a reducedfertility
or
sterility.
The
picture
is even less clear in farm animals. It has been shownby
KNUns!rr(1958, ig6ia, z 9 6zb)
and other Swedish workers(H!Errxrcsorr
andBÄ CKSTR Ö M , 19
6 4
)
that some of theinfertility
of cattle andpigs
is attributable to structural aberrationspossessed by
otherwise normal animals. Recent observations oninfertile rams have disclosed translocations
(B RU E RE , 19 6 9 ) as well as the sex
chromosome
aneuploidy
referred to earlier(BRUi;R!
etal., 19 6 9 ).
It is tooearly
toattempt
an assessment of theimportance
of chromosome aberrations incausing
reducedfertility
of farm animals. It is clear however thatthey
area factor to be taken into account.
VII. - POLYMORPHISMS
The number of chromosome
polymorphisms
that have been found in farm animals is verysurprising
in view of therelatively
small numbers of animals that have been studied. Centricfusions,
i.e. translocation and loss of a centro- mere, have beenreported
in cattle(GusTAvsso N , 19 6 9 ;
RIECK etal., 19 68), pigs (McFEE
etal., 19 66;
RARY etal., 19 68;
McFEE andBANNER, 19 6 9 )
andgoats (So!,!!x
etal., ig66).
Each of these variants isthought
to bepresent
as a stablepolymorphism
in thepopulation
where it was found. It isprobable,
but notat all
certain,
thatthey
became establishedthrough genetic
drift but to be maintainedthey
must not beseverely
detrimental. In any case, from thisplus
otherevidence,
it does appear that theungulate
chromosomes areparticularly
prone to centric fusion and it is to be
anticipated
that more will be found.They
should be
thoroughly
studied notonly
becausethey
are of interest in their ownright
but becauseimportant insights applicable
to similar conditions in man can begained.
Animportant
source ofduplication
in man is unbalancedgametes
produced by
translocation carriers. Much can be learned in our animalsregarding segregation
mechanisms in translocationheterozygotes
that would beextremely
useful to human
geneticists.
Need
f o y
moye workIt is clear from this brief review that
cytogenetic
research in farm animals hasonly just begun
butpromises
to contribute answers to someimportant problems
of livestock
breeding
andgenetics.
There are a number ofthings needing
to bedone
immediately.
Surveys
should be conducted in a number ofplaces
to determine thefrequencies
andtypes
of chromosome aberrations in domestic animals.Attempts
should be made to ascertain theexpected
effects ofgiven
aberrationson their carriers and on
populations
as a whole.The
importance
of the variouspossible
causes of aberrations should becarefully
determined for each of the groups of farm animals. When this isdone,
it isprobable
that theirfrequencies
can bematerially
reducedby adopting appropriate husbandry practices.
Inaddition,
when the causes areknown,
various aberrations can be
experimentally produced
and theirpossible
beneficialaspects
viewedby geneticists.
Chromosome abnormalities in domestic animals will be very useful as models of their
counterparts
in man. The mouse, for technical reasons, has a limited usefulness in thisrespect.
Once this
background
of information isestablished,
it no doubt will be found thatcytogenetic investigations
of farm animals have much to contribute toenhancing efliciency
of animalproduction.
Reçu pour publication en octobre r97o.
ACKNOWLEDGEMENTS
Preparation of this manuscript was aided by a grant from the Ford Foundation to Dr.
R.A. BEATTY and by NIH Grant No. HD 03822. Gratitude is extended to Professor D.S.
FALCONER and Dr. R.A. BEATTY for the provision of facilities.
RÉSUMÉ
CONSIDÉRATIONS
CYTOGÉNÉTIQUES
EN SÉLECTION ANIMALECet article résume quelques aspects fondamentaux de la cytogénétique, les travaux faits
sur les animaux domestiques et une évaluation des effets connus des aberrations chromosomiques
sur les populations animales.
Ces dix dernières années le nombre chromosomique de base ainsi que des caractéristiques précises du caryotype ont été établis avec sûreté pour tous les animaux domestiques.
Des variants intra-spécifiques, c’est-à-dire des polymorphismes chromosomiques ont été
mis en évidence chez le Bceuf, le Porc et la Chèvre.
Il existe deux types primaires d’aberrations. Le premier type est une altération du nombre des chromosomes, on l’appelle l’hétéroploïde. Le second type, qui consiste dans le réarrangement
de segments chromosomiques, intervient après le bris des chromosomes.
Les facteurs responsables de la fréquence d’apparition des aberrations sont à la fois géné- tiques et non génétiques. Dans la seconde catégorie, les plus importants sont certains groupes de virus, de produits pharmaceutiques ou chimiques, les radiations ionisantes, l’âge des gamètes, principalement l’ovule et, chez certains animaux, l’âge de la mère.
La présence d’aberrations chromosomiques rend apparemment compte, pour une large part, de la mortalité embryonnaire des mammifères et des oiseaux. Chez l’homme, la proportion
des avortons expulsés spontanément et porteurs d’anomalies chromosomiques serait peut-être
d’un tiers, ce qui est énorme. Chez le porc, des études préliminaires semblent indiquer que la situation est sans doute fort semblable.
Très peu de travaux de cytogénétique ont été faits sur le bétail présentant des défauts congénitaux, bien qu’un certain nombre d’aberrations aient été signalées. Chez l’homme, un peu moins de 1 p. 100 des nouveaux-nés portent des anomalies chromosomiques.
Quelques aberrations à effets somatiques légers, voire nuls, peuvent être cause de fertilité réduite ou de stérilité des porteurs. Des cas isolés ont été décrits sans qu’une évaluation sta-
tistique de leur incidence ait été faite.
Chez le Boeuf, le Porc et la Chèvre on a trouvé des fusions centriques et des travaux sont
en cours pour préciser leur effet sur les valeurs adaptatives des populations où elles sont apparues.
Pour conclure, on peut dire que les aberrations chromosomiques réduisent sérieusement la productivité des animaux qui les portent. Il faudra encore préciser l’ampleur de ces pertes et trouver le moyen de les réduire.
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