|
|
|
Original
Article About Dexters
"The
Significance of an Achondroplasia-Like Condition Met With in Cattle"
Part IX
by F.A.E. Crew
in Proceedings of the Royal Society of London,
volume 95, 1923, pages 228-255.
Page 250 (Cont'd)
IX - The Possible Bearing of the Case of the Monstrous Calf of the
Dexter Upon the Species Question
The species-cross Bos americanus x Bos taurus is complicated by the
occurrence of hydramnios during the pregnancy, which results in the
production of the F.1 generation, and by dystocia in the case of the F.1
male, which renders the birth of a male almost impossible. Moreover, if an
F.1 male is born alive it is invariably sterile. The case resembles that of
the Dexter in that hydramnios and dystocia are involved.
Certain breeders (Boyd, Goodnight) became impressed by the advantages which
would follow the production of the “cattalo.” Bison bulls were mated with
Hereford and Aberdeen Angus heifers (the reciprocal cross could not be made,
though the reason is not stated), and in every case the pregnancy was
complicated by severe hydramnios, with the result that the majority of the
females aborted. In fact, only about one in thirty produced a living calf
and of these a male was a rarity, for the size and shape of the male hybrid
were such that the mother died in labour in the great majority of cases. It
was found that the size and shape of the head of the male and the length of
his neural spines were such as could not be accommodated by the birth
passages of the cow, and the calf or the cow or both died.
In this case analogous lethal factors are involved, complementary lethals
resulting in the production of a degree or of a kind of development of the
skeleton of the offspring which renders its natural birth impossible and in
the production also of hydramnios which in a great number of instances ends
in abortion. In this case, however, the factors concerned in the production
of the conditions leading to dystocia are sex-linked and are not linked with
Page 251
those which are concerned in the production of hydramnios. In the case of
the Dexter monster the dissociation of hydramnios and dystocia has been
recorded but very rarely, and has not been critically observed. In the great
bulk of the cases the presence of a monster and of hydramnios in association
is the rule so that the factors concerned are the same or they are so
closely linked that their dissociation is extremely rare. The cattalo case
differs also in that the surviving F.1 male is sterile.
The conditions met with in the bison x cattle cross can be interpreted in
terms of the factorial hypothesis as follows:
A and A’ are complementary autosomal factors, and together result in the
production of hydramnios.
B is a dominant autosomal factor complementary to d a sex-linked
recessive, and the combination Bd results in the production of
certain skeletal characters which lead to dystocia.
C is a dominant autosomal factor complementary to e, a sex-linked
recessive, and the combination Ce results in sterility.
The series de is balanced by DE.
The bison male according to this scheme is AA BB CC (DEX)Y, the female AA BB
CC (DEX) (DEX). The cattle male is A’A’ bb cc (deX)Y, the
female A’A’ bb cc (deX) (deX). The F.1 male will be AA’ Bb
Cc (deX)Y, the female AA’ Bb Cc (DEX) (deX).
There will be hydramnios in all cases. Of the foetuses which continue to
term there will be dystocia in the case of the male but in the female, since
de is balanced by DE, parturition will be possible. Similarly, the
female will be fertile, the male sterile. The fact that a few F.1 males are
produced is to be explained by the variation in the maternal musculature, by
differences in the size of the birth passages in cows of different ages and
sizes, by differences in the foetal presentation, and by differences in the
management of labour.
The females of F.1 were then back-crossed to the bison or to the bull, and
this procedure was continued for several generations. It was found that, as
time went on, the incidence of hydramnios became less and less and that the
proportion of dead-born calves was steadily becoming reduced. Fertile males
were obtained. These facts are easily accommodated by the scheme outlined
above. It will be found that a backcross of an F.1 female to the bison sire
will yield a generation in which the incidence of hydramnios is reduced by
50 per cent as are also those of dystocia and of sterility in the case of
the males. After a few years of breeding in this way several genotypes would
exist and chance selection, guided to some extent by the breeder’s art,
would
Page 252
surely in time completely remove hydramnios, dystocia and sterility. The
peculiar results of this species-cross can be interpreted genetically, and
looking at the problem broadly the case of the Dexter is very similar.
Hydramnios characterises another species-cross - Bos americanus and
Bos indicus - and one case of this was treated by Lewis, who performed
hysterectomy in order to obtain a living hybrid.
The case of the monstrous calf of the Dexter suggests that in certain
instances the factor of the geneticist may be endocrinal in nature,
affecting the development of the tissues in such a way that conditions
unfavourable to the foetus are produced. Dystocia may be the result of an
abnormal size or proportion which cannot be accommodated by the maternal
birth passages, and there is much circumstantial evidence which shows that
proper growth is regulated through the mechanism of the endocrine system,
and that abnormality of a component member of this system is followed by
abnormality in the proportions of the individual. A “lethal” factor may be
one which affects the proper and timeous functioning of a ductless gland.
A species, as recognised by the extreme systematist, is an association of
individuals all of which exhibit a common morphological character complex;
such species are commonly defined without any first-hand knowledge as to
their behaviour as breeding units. It is perhaps due to this fact that most
workers of the Mendelian school, with the notable exception of Bateson, have
turned their attention from the historic problem of the origin of species to
the more immediate question as to the origin of characters. Nevertheless, as
Bateson has rightly pointed out, the fundamental discontinuity of species in
the Linnean sense as breeding units has still to be interpreted in terms of
the factorial hypothesis, which hitherto has shed much light on the
discontinuity of animal structure but not upon the discontinuity of the
breeding unit. Until further light has been shed on this issue the validity
of the assumption upon which the evolution theory rests will not have found
a satisfactory basis in experimental enquiry.
The species may be a gene-complex, as suggested by Morgan, and the
morphological characters may be linked with physiological characters which
really separate unit from unit. The case of the Dexter provides an
opportunity for offering a suggestion as to the manner in which a discrete
breeding unit may have its origin.
A mutant lethal factor may be such that in the simplex state its action is
balanced by its normal allelomorph, but in the duplex state the combined
action of the two results in the production of anatomical anomaly and physio-
Page 253
logical derangement, of a kind that render the further development of the
zygote impossible or profoundly abnormal. Such is the case of the homozygous
yellow mouse. Or else, a mutant factor may be of such a nature that alone,
either in the simplex or in the duplex condition, it produces no evident
effects; but combined with a complementary factor of the same nature it
results in a non-viable condition, and the mating of two individuals which
carry such complementary lethal factors is rendered abortive.
A mutant appears in a stock; a lethal factor is present in the simplex
state, and in the course of time there will make their appearance also
individuals with this factor in the duplex state. The nature of the factor
is such that alone it results in no appreciable effect. Synchronously, or at
a different time and in a different race of the same stock, another mutation
occurs and a factor appears whose action is complementary to that of the one
referred to above, and in time individuals with this factor in the duplex
state will be produced. Two distinct breeding units may thus arise within a
common stock: each can successfully mate within its own group and with the
parent stock, but the mating between the groups is rendered abortive. The
expression of the action of such, complementary lethal factors may take the
form of incompatibility in the form of the copulatory apparatus, or in the
physiological relationship between the male and female, or between the ovum
and the sperm, of anatomical anomaly or of physiological derangement leading
to an abnormal development of the zygote and its death, of hydramnios or of
dystocia, or of sterility of the F.1 heterogametic sex. The two groups,
however, have had their origin in a common germplasm and so, in the light of
“return” and “regional” mutations, it is to be expected that parallel
mutations will occur. Members of the two groups will exhibit characters
which were borne by the common stock from which the groups arose, and
characters which, have resulted through mutation since the groups became
distinct, some of these being the result of parallel mutation and some of
mutation which has occurred in one group only. Such mutant morphological
characters as are linked with the respective complementary lethals cannot be
brought into genetic association and will become the distinguishing
characters of the group.
On to Summary and
Bibliography
|
