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EXPERIMENTAL INDUCTION OF CHROMOSOME ABNORMALITIES
Induction experimentale d’anomalies chromosomiques
Experimentelle Induktion von chromosomalen Anomalien
Ondine BOMSEL-HELMREICH *
I ntroduction
As chromosomes are the vectors of genetic inform ation transm
itted from one generation to the other, it is to be expected that
any alteration in the num ber or organisation of the chromosomes
may be of importance.
Chromosome aberrations are specially effective in the perform
ance of the reproductive system: reduced fertility or complete
sterility, embryonic or fetal m ortality, congenital m alform
ations and perinatal death may all result from chromosomal
variation.
Many w orkers have contributed in the last 20 years to the
exploration of the effects of chrom osom al variation in many
fields and some very good reviews are mentioning the different ways
of inducing them experimentally (see Fechhei- mer, 1972 for
review).
Experiments were m ainly directed in three directions:
— Induction of chromosomal aberrations of any type by exogenous
factors,
Induction of physiological situations which are known to
increase embryonic m ortality and therefore eventually chromosomal
aberrations,
— Induction of program m ed aberrations of one or a complete set
of chromosomes.
de ph7 siologie Animale, Centre National de Recherches
Zootechni- ques (CNRZ), Institut National de la Recherche
Agronomique (INRA), Domaine de Vilvert, 78350 Jouy-en-Josas,
France.
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I nduction of chromosomal aberrations of any type by exogenous
factors
a) Ionizing radiations
It is well known that ionizing radiations induce chromosome
breaks; their effect has been extensively studied on different cell
types both in vivo and in culture. The effect of X radiations on m
am m alian germ cells is, qualitatively at least, sim ilar to its
effects on somatic chromosomes. X rays have been used on testes or
samples of semen to produce chrom osom ally abnorm al young (see
Russell, 1962 for review).
Griffen and Bunker (1967) applied X rays to the testes of male
mice and used them subsequently as sires. A variety of aberrations
appeared in the descendants such as translocations, deficiencies or
trisomy. But the relative sensitivity of the various
spermatogenetic cell types are not the same in all species (Lyon
and Smith, 1971) which is also the case for oogenic cells (B aker,
1971). Besides, animals of different genotypes within the same
species react also differently to a same X ray dosage (Searle et a
l, 1970).
b) Chemicals
Many substances have been used and induce various chromosomal
aberrations. Those which excert an effect a t very low
concentrations are the ones of prim ary interest. Between many
compounds Rohrborn et al. (1971) recently tried mutagens such as
triethyleneimminobenzoquinone or triazaquinone (H ansmann and
Rohrborn, 1973). They were injected into mice shortly before
ovulation. A very high percent of preim plantation embryos had
chromosomes w ith structu ral anomalies or were hyper- or
hypodiploids.
The interest of these chemicals rem ains in w hat these experim
ents show about the structure and functioning of chromosomes and
cells. But their action is not necessarily lim ited to DNA or
chromosomes bu t many other cellular constituants: perm eability of
membranes, enzyme activity, protein synthesis, etc., all of which
play a role in embryonic development. Another bu t negative in
terest shows the large possibilities of most chemical compounds
used in form of food additives, drugs, pesticides or w ater contam
inants to induce indesired chromosome aberrations.
I nduction by preferential physiological situations
Another line to induce aberrations is to recreate experimentally
physiological situations which are known to increase embryonic m
ortality. Aging of gametes is certainly the m ost largely and
recently explored situation of these.
The importance of such experiments are confirm ed by the
possibility of these situations occuring either in farm animal or
hum an reproduction. The use of progestagens to induce ovulation in
cows or sheep, the induction of ovulation of eggs for transplant
purposes, the im portance of the right timing of mating or insem
ination are all involved.
In man, long or irregular cycles appearing in very young or
premenopausal women are often related to spontaneaous abortion and
chromosomal abnormalities; perhaps through the process of delayed
ovulation. Delayed fertilization
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is evidently bound to occur often in the hum an where the
possibility of coitus is not lim ited by an oestrus.
a) The aging o f the egg
Aging of the ovum in a preovulatory stage by delaying ovulation
causes a threefold increase in chromosomal aberrations of the post
im plantation embryos in the ra t (B utcher and F ugo, 1967).
Follicular aging of the oocyte of the rabbit (Bomsel-Helmreich,
1972) increases likewise aberrations in the blastocyst: triso-
mics, triploids, mosaics, the last being the most frequent, and
embryonic m ortality; bu t in a higher proportion as they are
observed earlier in development.
Aging of the egg in the Fallopian tube by delayed mating,
affects the embryos in the same way; bu t the aging process appears
already after several hours, whereas it takes more than 40 hours
for intrafollicular aging of the oocyte.
The aging of eggs has been mostly observed immediately after
fertilization. For this reason digyny and dispermy are the most
often described abnormalities, in that they are the easiest to
observe. Both have been extensively studied m different species.
Their percentage may be quite high: 34 % in the ham ster (Chang and
Fernandez-Cano, 1958) and 21 % in the sow for digyny, 10 % for
disperm y (Thibault, 1959). The observation of later stages of
development shows not only that these eggs are able to develop but
also that lesser m itotic anomalies occur in a m uch higher
proportion. Austin (1967) obtained 25% triploids and 65% trisom ics
and monosomies in the rabbit as did Shaver and Carr (1967) and
Vickers (1969) in the mouse. Mixoploids apear as well. In the sow
at preim plantation stage, besides a high embryonic m ortality, 25
% of the embryos are morphologically abnorm al and retarded. A
certain number are triploid bu t the large m ajority diploid (B
omsel-Helmreich, 1967). The relative part played by the environm
ental factors which change (age) with delayed mating and the aging
of the e§8 per se could be efficiently studied by using in vitro
techniques. In the rabbit Thibault (1967) aged eggs in vitro and
fertilized them w ith aged sperm. Digyny was observed in m ore than
half of the eggs and dispermy in 20 %. This shows a direct effect
of aging on the gametes. Fraser and Dandekar (1973) — also in the
rabbit — aged the eggs in vitro but fertilized them with fresh
sperm and cultivated them for two days. They observed no reduction
in the fertilization ra te and no increase in triploid embryos;
nevertheless there was a m uch reduced vitality after the first
cleavage. The authors supposed that the effects of delayed mating
were mainly due to tubal factors. The involvement of tubal
environment in impairing development has been recently dem
onstrated (Bomsel-Helmreich and S zollosi, 1974).
b) Aging of sperm
The time dependant decrease in the capacity of spermatozoa to
fertilize and support norm al embryogenesis is known since a long
time. Tesh (1966) dem onstrated both pre-and post im plantation
losses; with increasing age of the spermatozoa the losses appeared
at an earlier stage. Maurer et al. (1969) reported reduced cleavage
rates in cultivated embryos taken from rabbits insem inated 20 hrs.
before ovulation. Thibault (1967) described digyny in 40% of
freshly ovulated eggs fertilized w ith aged sperm. Martin and
Shaver (1972) observed the chromosomal constitution of rabbit
blastocysts when sperm was aged in utero for 14-28 hrs.
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Almost ten percent had abnorm al caryotypes: mixoploids (2n/4n),
mosaics (2n / 2nd) or chim aeras (XX/XY). I t is interesting tha t
aged sperm induces in a young egg digyny which is an anomaly of the
egg and may be considered as an incomplete activation.
Non-disjunction as late as the first cleavage is also observed.
Dispermy appears when the interval between insem ination and
ovulation is abnorm ally long in the rabbit (H arper, 1970); it may
be related w ith a higher num ber of sperm present being then, a t
the site of fertilization. Post-ejaculatory storage of m am m alian
spermatozoa before use in artificial insem ination leads to a
reduction in fertility and to an increase in embryonic m ortality,
in the bull (review by Salisbury and H art, 1970) and in the rabbit
(S tranzinger, 1970; Ko- foed-Johnsen et a l, 1969). Storage
possibly reduces fertility in such a way, tha t it leads to delayed
fertilization, and tha t chromosome aberration occur in the waiting
ova.
c) Polyspermy
Penetration of the ooplasm by more than one spermatozoon has
been described in many species including the rat, mouse, ham ster,
guineapig, rabbit and pig (for review see Austin and B ishop, 1957;
P iko, 1961). The domestic pig seems specially favourable for these
studies as polyspermy has been found to reach a striking incidence
under a variety of experim ental conditions. Through delayed m
ating or insemination (P itkjanen, 1955; H ancock, 1959; Thibault,
1959) an alteration of the egg mem brane seems to occur by which
the aged egg is not able any m ore to perform a block to
polyspermy. Polyspermy occurs also after injecting progesterone
systematically (Day and Polge, 1968) or locally (H unter,1972)
shortly before ovulation or after provoking ovulation during the
luteal phase of the oestrus cycle (H unter, 1967). The relaxation
of the isthm us and uteral- tubal junction occurs after local
injection of progesterone. Polyspermy is then thought to originate
principally from the increased num ber of sperm atozoa reaching the
site of fertilization and an alm ost sim ultaneous penetration of
the egg m em brane by two or more spermatozoa before com pletion of
the zona reaction. But the direct effect of progesterone on the m
em branes of the gametes cannot be excluded.
In the pig surgical resection of the isthm us is followed by the
same high percentage of polyspermic eggs (34 %) (H unter et
Leglise, 1971) and also bu t to a much lesser extent, in the
rabbit. This shows once more a species - specific difference.
Logically, the direct deposition of excessive num bers of
spermatozoa a t the site of fertilization induces polyspermy as
well (H unter, 1972).
But in both dispermic and trisperm ic eggs, a single m ale
pronucleus is uniting w ith the female one, the accessory male
element (s) remaining in the opposite hem isphere of the egg. This
may leave a doubt about the possibility of induction of polyploid
embryos, as up to now the descendant of these polyspermic zygotes
have not been studied.
I nduction of definite chromosomal aberrations
The obtention of a specifically induced aberration concerning
either one chromosome or a whole set should allow genetically b e
tte r defined observations.
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a) Aneuploidy
Genetic factors have been used in an extensive line. Recently an
experimental system in the mouse using crosses between Mus
poschiavinus (tobacco mouse) with the laboratory mouse, perm its
the study of the lethal effects of zygotic aneuploidy for certain
chromosomes (Tettenborn and Gropp, 1970). Structural heterozygosity
of the Robertsonian type may enhance irregular segregation in
meiotic anaphase I; as a consequence monosomic and trisom ic
gametes occur. Back-crosses of heterozygotes w ith normal
laboratory mice are used. It is possible to study fetal m ortality
related to the individual specific m etacentrics involved and
eventually to the sex of the heterozygous parental carrier
dispensing aneu- ploid gametes (Gropp and al, 1972). The efficiency
of the system is very high: less than 50 % of preim plantation
embryos are eusomic; but aneuploid embryos gradually die off. Hypo-
and hypersomic blastocysts are present in equal numbers, which
correspond well to the frequency of aneuploids in secondary sperm
atocytes of the sires observed a t metaphase (S toll and Gropp,
1974). A small num ber of haploids and triploids occur also. In
early post-implantation, monosomies become alm ost completely
eliminated while trisom ics and even double trisomics survive
distinctively longer, almost to birth. Offspring of female carriers
of heterozygosity have a much higher incidence of aneuploidy than
that of heterozygotic males.
Trisomic embryos display surprisingly sim ilar pattern of
development in all the system of single m etacentric heterozygotes
so far studied: retardation of development and general hypotrophy
(external and at level of organs) and placenta. Gross m alform
ations in contrast occur very rarely (Gropp, 1973).
These experim ents show eventually that in aneuploids one
chromosome in excess is less detrim ental to development than the
loss of one chromosome and that these losses determ ine in mice a
crisis quite early, around implantation. Further, in regard to
answer the question: does a spermatozoon transport its genetic load
w ith equal efficiency regardless the content, there seems to be no
selective process operating against unbalanced genomes between
second m etaphase in the male and three- day embryos. A sperm atid
with an extra chromosome or w ith one chromosome missing is as
likely to m ature into a spermatozoon and take part in
fertilization as one with a balanced genome; an egg is not
sensitive to an abnorm al genome in the first three days of
development (Ford and E vans 1973; Ford and E vans, 1974; Ford,
1972). However, this seems not to apply to all! for instance to sex
chromosomes: Morris (1968) has shown that in mice 2n X0 eggs
develop norm ally where as 2n Y0 do not develop beyond the first
division.
The overwhelming m ajority of trisomic embryos sired by
heterozygous males have received the fa ther’s m etacentric
chromosomes. Trisomy of another chromosome seems therefore
«spontaneous» trisomy and occurs as frequently as in controls (4 %
). This shows tha t the presence of a trivalent does not influence
the frequency of o ther abnorm al events; which is different to
what seems to happen in other circum stances i.e . in aging eggs or
hereditary translocations.
There are once m ore species-specific differences: for instance
only in mice do XO’s develop normaly; Ford and E vans (1973)
obtained ten time m ore mosaics in their born mice than appear in
human.
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b) Parthenogenesis
The induction of an alteration of chromosome num ber may concern
not only one bu t a complete set of chromosomes. The induction of
parthenogenesis, tri- ploidy or tetraploidy by exogenous factors
has been largely studied.
Parthenogenic development may appear after exposure to various
stimuli, such as hyper- and hypotonic solutions, cooling or heating
(for review see Beatty, 1957, 1967). Many experiments were made on
the rabbit as suppression of polar body form ation is easily
obtained by m oderate cooling; Chalmel (1962) even obtained
fertilization (though abnorm al) of the parthenogenetically
activated egg.
In the mouse Braden and Austin (1954) employed heat shock as the
activating agent but did not continue observation after 70 hrs.
Recently Graham (1970) obtained development of mouse eggs up to the
blastocyst stage by cultivating cumulus-free eggs in vitro ;
Tarkowski et al. (1970) obtained development beyond the stage of im
plantation as a result of activation in vivo w ith a electric
current. Komar (1973) activated eggs by heat in vitro; fewer eggs
developed to morula and blastocyst stage (15%) compared to the two
preceding ways of induction (50%).
Whereas Graham (1970), Tarkowski et al. (1970), Witkowska (1973)
obtained mostly haploid parthenogenons, Komar (1973) obtained some
diploids but her low percentage of success may be explained by the
induction of many hypohaploids which seem to be unable to cleave. W
hatever the means of obtention, partheno- genetic eggs developped
more slowly than controls.
c) Triploids
Triploid embryos have been produced at will by using colchicine
or a related compound which suppresses cytokinesis; E dwards (1958;
1961) in the mouse, P iko and Bomsel-Helmreich (1960) in the ra t
injected the compound into the m other during ovulation or
fertilization. A less damaging way is to use colcemid during a
short and precisely defined time i. e. at expulsion of the second
polar body, if in vitro fertilization’in the rabbit is used
(Bomsel-Helmreich and Thibault, 1962; Bomsel-Helmreich, 1967). The
success is alm ost 100 % so tha t embryos which are all of dygynic
origin could be studied extensively. They are macroscopically norm
al but delayed in development. Most of them die before mid
gestation, but some were found in the last quarter of gestation.
This shows that under very favourable conditions (genetic and horm
onal) a triploid embryo may eventually survive till parturition.
This is in accordance w ith the triploid hum an embryos, very num
erous in spontaneous abortion; they neither survive till birth ,
exception made for mosaics 2n/3w.
The genetic pathway to obtain triploids was used by Wroblewska
(1971) who crossed inbred females with males of a different strain.
They showed all a specific disturbance of embryogenesis, an
inhibition of the embryonic part of the egg- cylinder to form;
their origin seems to be digynic. But the genetic background seems
to be able to modify the morphological expression of the chromosome
aberration; since the m ajority of triploids obtained either
spontaneously or by the ways mentioned above were retarded in
development but otherwise normal.
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Another m anner to induce triploidy in excellent in vivo
conditions is suggested by Beatty and Fechheimer (1972); in certain
individual rabbits as well as in other species diploid sperm appear
quite often in semen and could be well separated from haploid sperm
by centrifugation; these could induce triploid zygotes after
artificial insemination.
d) Tetraploids
In opposition to triploids or haploids a balanced polyploidy
seems to be compatible w ith complete fetal development in mammals;
recently Snow (1973) obtained tetraploid mouse embryos by
cultivating 2- cell eggs in cytochalasine which prevents
cytokinesis w ithout influencing nuclear division. The proportion
of treated embryos which develop normally is im portant: they are
of norm al dimensions and some develop to term. They go much fu
rther than the tetraploids obtained by cell-fusion by Graham
(1971). Tarkowski (personal communication) uses also a fusion
technique and obtains tetraploid blastocysts. However, most of the
tetraploid embryos are abnormal; but as the size of tetraploid
cells is larger than diploid ones it is likely tha t the cause of
embryonic m alform ations is in the reduced num ber of cells of the
blastocysts.
Mechanisms of occurrence of chromosomal aberrations
Despite the num erous ways of induction, not much is known yet
about the causal relation of chrom osom al anomalies and abnormal
development of m ortality of embryos.
The use of exogenous agents such as X-rays or drugs generally
induce chromosome aberrations a t random . When the physiological
ways of induction are used in a same situation, alm ost all kinds
of abnorm alities occured: digyny as well as dispermy, monomy,
trisom y or mosaics. Even the time of occurence of the abnorm ality
is not easy to define. Trisomies or monosomies may occur at the
first or second meiotic division of the gametes. Triploidy may be
induced as well by nonexpulsion of the first or second polar body
or dispermy. Mosaics arise at early cleavage but not necessarily at
the first. There appears also a notion of connected anomaly, which
is the occurence of two different types of aberration occuring
subsequently such as 2>z-l/3n-2. This is understandable as well
from a genetic point of view than a physiologic one.
Further, the way of induction does not necessarily induce the
only chromosomal aberration, but also yet insufficiently explored
anomalies of the whole cell. This is easily understood for
different exogenous agents but occurs as well in the case of
physiological aging. I t explains different aberrations to be
determ ined by a some inducer as well as the high embryonic m
ortality observed in m ost of the experiments, and which is not
always linked to the chromosomal picture.
It is equaly possible tha t a different time of induction as
well as a different m anner which produces a same abnorm ality does
not allow an equivalent development. This may be the case in
dispermy induced in norm al eggs of the pig where the three
pronuclei do not fuse, whereas dispermy in an aging egg leads to
the syngamy of the three pronuclei and therefore to a triploid
embryo.
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The use of identification of the origin of individual
chromosomes by the banding technique should be a great help in
these problem s. The induction of a specific anomaly by genetic
factors or of polyploidy by in vitro experim ents gives a much be
tte r control over these variations. Nevertheless they cannot
suppress genetic regulations which are still not undestood such as
the preferential survival of 3nXXY embryos seem specially
morbid.
A m ost puzzling topic remains the alm ost complete lack of
inform ation of the causes of embryonic m ortality of chromosomally
abnorm al embryos. In some cases the embryo shows a gross
developmental m alform ation, but in m ost cases the embryo is only
delayed in growth which determ ines death at different periods of
embryogenesis. M ortality is also surprising, e. g. 90 % of 2nXO
embryos die in utero, 10 % live w ith only m inor anomalies. Very
few causal studies have been realized up to now. The m itotic cycle
of aneuploids seems to be abnormally long (Cure et al., 1973). Some
enzymatic activities of tripoid embryos may be higher than in
diploids e. g. glucose-6-phosphodehydrogenase and other X linked
enzymes, some may be lower, some of equal activity (B
omsel-Helmreich, 1970). But these are only the very first
explorations in this direction.
Conclusion
The different ways of inducing experimental anomalies bring all
new lights on physiological and genetic research. They give inform
ation on behaviour of nuclei and cells with an abnormal genetic
constitution, they show the precise circumstances of induction of
errors of meiosis or mitosis, which should perm it to avoid these
accidents to occur spontaneously; also the critical periods of
embryonic m ortality related with abnorm al chromosome num bers.
Finally the obtention of planned abnormalities in zygotes which
have been little stressed by the induction provides a convenient
tool to explore the genetic and metabolic causes of this m
ortality, which for the moment is a field still little
explored.
SUMMARY
Experiments on the effects of induced chrom osom al variations
are directed after three m ajor lines:
1) Exogenous factors such as X-radiations or chemicals induce
any type of chromosomal variation, but w ithout possibility of
choosing precisely one type.
2) Some physiological situations which may occur spontaneously
in farm anim al or hum an reproduction produce a high percentage of
embryonic m ortality and chromosomal anomalies in embryos or
new-born. The best known situation is the aging of gametes. Aging
of sperm before insem ination, aging of intrafollicu- la r oocytes
before ovulation, aging of egg or sperm in the fallopian tubes
before fertilization induces in all mammals studied the same
anomalies: digyny, dispermy and non-disjunction at meiosis or first
m itosis after cleavage. The embryos present various anomalies such
as monosomies, trisomies, triploidies, mosaics, mixoploids or
chimaeras. But on inducing of these preferential physiological
situations, once m ore one does not obtain a specific anomaly.
3) There exist some ways to induce definite chrom osom al
variations. Embryos
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aneuploid for a known chromosome may be obtained by using
genetic factors. Aneuploid sperm for the chromosomes explored up to
now is as fertile as euploid sperm. After fertilization, trisom ics
and monosomies are present in equal num bers; but monosomies die
off before implantation, wheras trisom ics survive after im
plantation. The sex of the anomaly-inducing parent is related to
the survival of the abnorm al embryo. Nevertheless individual
chromosomes (such as sex chromosomes) play also a role in
survival.
Parthenogenesis, if relatively easy to induce, allows no
development later than m id -gestation. Triploids may be obtained
in large numbers, specially by in vitro fertilization techniques,
but die before the end of gestation. Tetraploids eventually are
able to survive till a fter birth.
Genetic origin such as digyny or dispermy, or cytological
factors such as abnorm al cell size, determ ine in polyploids very
different survival possibilities. In general, polyploids have no
malformation, but seem to die of delay in em- bryogenesis.
The possible m echanisms of occurance of chromosomal aberrations
are described, keeping in m ind tha t in general different types of
aberration occur simultaneously, but may not necessarily allow an
identic development of the embryo. The causes of embryonic m
ortality related to chromosomal aberrations are actually alm ost
totally unknown. First explorations of this phenomenon are
described. But m ost genetic and metabolic causes are still to be
discovered.
RESUME
Les experiences destinees a m ettre en evidence les effets d
’une variation induite du nom bre des chromosomes sont menees dans
trois directions principales:
1. Les facteurs externes tels que les rayons X ou les substances
chimiques sont suceptibles de provoquer tous les types de
variations chromosomiques, mais sans possibility de choisir
precisem ent un type particulier.
2. Quelques situations physiologiques, qui peuvent se produire
spontanem ent au cours de la reproduction des animaux domestiques
ou de Thomme provoquent 1’apparition d ’un pourcentage eleve de m
ortalite embryonnaire et d ’anomalies chromosomiques chez les
embryons ou nouveaux-nes. La mieux connue de ces situations est le
vieillissement des gametes. Le vieillissement des spermatozoides
avant l’insemination, le vieillisement intrafolliculaire des
ovocytes avant l’ovula- tion, ou le vieillissement des
spermatozoides ou de 1’oeuf dans les trom pes de Fallope avant la
fecondation, provoque chez tous les mammiferes etudies les memes
anomalies: digynie, dispermie et non-disjonction lors de la meiose
ou de la prem iere m itose de segmentation. Les embryons presentent
diverses anomalies telles que des monosomies, des trisomies, des
triploidies, des mosaiques, des mixoploidies, des chimeres. Mais,
ici encore, l'induction de ces situations physio- logiques
preferentielles ne provoque pas 1’apparition d ’une anomalie
specifique.
3. Quelques moyens existent cependant pour induire une variation
definie du nombre de chromosomes. On peut obtenir des embryons
aneuploides pour un chromosome determ ine en m ettan t en oeuvre
des facteurs genetiques. Les spermatozoides aneuploides pour les
chromosomes etudies jusqu’a ce jou r sont aussi fertiles que les
sperm atozoides euploides. Apres la fecondation, les
trisomiques
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et les monosomiques sont presents en nom bre egal; mais les
monosomiques meu- ren t avant l’implantation, tandis que les trisom
iques survivent a celle-ci. Le sexe du parent inducteur de l
’anomalie est en relation avec la survie de l’embryon. II n ’en
reste pas moins que des chromosomes determ ines (tels que les
chromosomes sexuels) jouent un role aussi dans la survie.
La parthenogenese, si elle est relativam ent facile a induire,
ne perm et pas de developpement au dela, de la mi-gestation. Les
triploides peuvent etre obtenus en grand nombre, specialement au
moyen de techniques de fecondation in vitro, mais m eurent avant la
fin de la gestation. Les tetraploides sont parfois capables de
survivre ju squ ’a la naissance et au dela.
Les polyploides ont des possibility de survie tres differente
suivant leur origine genetique (digynie ou dispermie) ou en
fonction de facteurs cytologiques tels une taille cellulaire
anormale. En general, les polyploides n ’ont pas de m alform ation
mais il semble qu’ils m eurent d ’un retard de l’embryogenese.
Les mecanismes possibles de l’apparition d 'aberrations
chromosomiques sont decrits en gardant en memoire que le plus
souvant plusieurs types d ’aberrations se produisent simultanement
mais qu'ils ne perm ettent pas obligatoirement le meme
developpement de l’embryon. Les causes de la m ortalite em
bryonnaire liee a des aberrations chromosomiques sont pratiquem ent
totalem ent inconnues. Les prem ieres explorations de ce phenomene
sont decrites, mais la p lupart des causes d’ordre metaboliques ou
genetiques restent a decouvrir.
ZUSAMMENFASSUNG
Die Experimente betreffend die Effekte von induzierten
chromosomischen Variationen fiihren in drei hauptsachlichen
Richtungen:
1. Exogene Faktoren wie X Strahlen oder chemische Faktoren
fiihren zu alien Typen von chromosomischen Variationen, aber ohne
die Moglichkeit irgend einen speziellen Typus zu wahlen.
2. Gewisse physiologische Situationen welche spontan bei der
Fortplanzung von Haustieren oder vom Menschen auftreten erzeugen
einen hohen Prozentsatz embryonaler Sterblichkeit und abnormale
chromosomische Effekte in Embryonen und Neugeborenen.
Die am meisten bekannte Situation ist das Altern der Gameten.
Das Altern der Spermatozoiden vor der Besamung, das Altern in
trafollikularer Ovozyten vor der Ovulation, das Altern von
Spermatozoiden und oder Eiern in den Tubae Fallopiae vor der
Befruchtung, fiihren bei alien untersuchten Saugetieren zu den
gleichen Anomalien: Dygynie, Dispermie, und Non-disjunktion bei
Meiose und der ersten Mitose nach der Teilung. Die Embryonen zeigen
verschiedene Anomalien wie Monosomien, Trisomien, Triploidien,
Mosaiken, Mixoploide oder Chimaeren. Aber bei der Induktion dieser
vorziiglichen physiologischen Situationen erhielt man wiederum
keine spezifischen Anomalien.
3. Es bestehen verschiedene Moglichkeiten um prezise
chromosomische Variationen zu erhalten: aneuploide Embryonen, ein
bekanntes Chromosom betreffend konnen erhalten werden indem man
genetische Faktoren beniitzt. Aneuploide Spermatozoiden die
Chromosomen betreffen die bis je tz t untersucht wurden sind ebenso
fruchtbar wie euploide Spermatozoiden. Nach der Befruchtung sind
Tri-
210
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somen und Monosomen in gleicher Zahl vorhanden. Aber die
Monosomen gehen vor der Einpflanzung zu Grunde, wahrend die
Trisomen bis nach der Einpflan- zung fortbestehen. Das Geschlecht
des die Anomalie induzierenden Vorfahrs ist im Zusammenhang m it
dem Uberleben des abnormalen Embryos, Nichtsdesto- weniger spielen
individuelle Chromosomen (wie z. B. Sex-chromosomen) eine Rolle
beim Uberleben.
Parthenogenese ist verhaltnism assig leicht zu erreichen,
erlaubt aber keine Entwicklung spater als bis zur Mitte der
Tragzeit. Triploide erhalt man in grosser Zahl speziell durch
Techniken m it in vitro Befruchtung; sie sterben aber vor dem Ende
der Tragzeit. Tetraploide konnen moglisherweise bis nach der Geburt
uberleben.
Genetisches Entstehen wie Digynie oder Dispermie oder
zytologische Faktoren wie abnoim ale Zellgrosse, bestim m en in den
Polyploiden sehr verschiedene Mo- glichkeiten des Uberlebens. Bei
Polyploiden bestehen im Allgemeinen keine Missbildungen abr sie
scheinen zu Grunde zu gehen durch Verzogerung der Embryogenese.
Der mogliche M echanismus des Erscheinens chromosomischer
Abweichungen wird beschrieben und m an legt nahe dass im
Allgemeinen verschiedene Typen von Abweichungen gleichzeitig
auftreten; aber sie fiihren nicht notwendiger- weise zu identischer
Entwicklung. des Embryos. Die Ursachen der Embryonen sterblichkeit
die m it chromosomischen Abweichungen in Zusammenhang stehen, sind
gegenwattig fast vollig unbekannt. Erste Untersuchungen dieses
Phenomens sind beschrieben aber die meisten genetischen und
metabolischen Ursachen miissen erst gefunden werden.
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C07-EXPERIMENTAL INDUCTION OF CHROMOSOME ABNORMALITIESOndine
BOMSEL-HELMREICH