VOX Sang. 12: 25-31 (1967)

Irregular Transmission of a Blood-Group Complex in One Family of Pigs Following Irradiation*

E. ANDRESEN

Iowa State University, Ames, Iowa

Males of the Hampshire and Duroc breeds were exposed to gonadal X-irradiation in a study of the genetic effects of irradiating swine [6]. Female parents were not irradiated but were descendents of exposed and unexposed males. The genetic consequences of the paternal irradiation were measured in the first filial generation. Polymorphic systems of red cell antigens controlled by genes at 11 loci were de- tected in all parents and progeny by blood-typing reagents isolated from isoimmune sera [l, 2, 4, 121 and were used to study mutations at these loci.

The exceptional event considered here was an irregular trans- mission of a blood-group complex in the E system. The four antigenic complexes (= phenogroups) El = Edbg, E, = Edeg, E, = Eseg, E, = Edef have been observed in the two breeds of pigs used for the experimental work. The blood factors &-Ed, Eb-Ee, and ErE, occur as three sets of reciprocal blood factors in each breed. Hence, the genic basis for the E blood-group system might be visualized as a series of alleles of a single gene, or, as three or more sets of closely linked genes (Fig. 1) [l, 21.

An E1ES Hampshire male was mated with an E1E1 Hampshire female. The male had been irradiated, while the genetic material of the female had not been exposed to X-rays. Thirteen pigs were born (Fig. 2). Six pigs were ElE1, six were E1E3, and one appeared to be E I E a . Repeated bloodtyping confirmed that the exceptional pig had the probable genotype EIEa according to the usual criteria. Adoption or superfecundation seemed unlikely, and there is so far no

Journal Paper No. J-5275 of the Iowa Agricultural and Home Economics Experiment Station, Ames, Iowa. Project No. 1424. This work has received as- sistance from Contract AT(l1-1)-707 with the U.S. Atomic Energy Commission.

26

g g f

e e e

d a d

ANDRESEN Irregular Transmission of a Blood-Group

Rh E E' E 2 3 E E4 chromosomes chromosomes

E4

Eb

Ed

g

b

d

Fig. 1. Diagram of the human Rh chromosomes depicted according to the original Fisher-Race conception, and the E chromosomes in pigs based on the assumption of closely linked genes [from page 172 in Reference 21. The dotted lines indicate crossing-over possibilities. Four E chromosomes which might be postulated in

Hampshire and Duroc pigs are shown.

Ek3 E'E'

1/3 111 111 1/3 1/2 1/3 1/3 111 111 1 / 1 I/3 I / l 1/3 Fig. 2. Pedigree of a family of Hampshire pigs observed in a study of the genetic

effects of irradiating swine.

evidence that this exceptional event is causally associated with the minus-minus (-/-) E,Ed phenotype observed in Hampshire pigs [3].

CHOWN, LEWIS and KAITA [5] and STEINBERC [19] have observed anomalies of inheritance in the MNSs and Rh blood-groups which are comparable t o that observed in the E system of pigs. In the first of these reports evidence is presented for the assumption that cros- sing-over during oogenesis has taken place between the hypothetical

Complex in One Family of Pigs Following Irradiation 21

subloci M N and Ss. The legitimacy of the child was supported by numerous tests, and results of using a dosage anti-s reagent sup- ported the hypothesis of recombination. The authors refer to other reports of possible crossing-over affecting the MNSs system, but in neither case was illegitimacy ruled out, and they considered in a t least one of them that illegitimacy was the more probable explanation.

In STEINBERG'S report reasons are presented for the assumption that mutation or crossing-over of paternal origin rather than gene interaction is responsible for an exceptional event observed in the Rh-system. There was circumstantial evidence for the legitimacy of the child, and this evidence was supported by results of laboratory tests involving 8 genetic loci. Unfortunately, a dosage reagent (the still missing anti-d) was not available to support one or the other of the two hypotheses favored by STEINBERG. However, the third pos- sibility of gene interaction deserves more emphasis, because the re- cessive suppressor gene, if it exists, probably has a low frequency and, therefore, is expected to occur in homozygous state at rare occasions, despite genetic isolation and inbreeding. Moreover, only certain mat- ing types are expected to be informative with respect to the effect of the homozygous state of the suppressor gene. In fact, evidence for suppressor genes affecting the Rh locus has been reported previously [7, 10, 11, 14, 15, 21, 221. On the other hand, the mere occurrence of multiple alleles doubtless reflects germinal mutations at the Rh locus, and a recent report also gives evidence for somatic mutation affecting this locus [9].

Several genetic and non- genetic interpretations of the irregular blood-group transmission in pigs are possible. The three discussed here are that the exceptional phenotype is a result of mutation, interallelic recombination or gene interaction.

Mutation. A possible mutation could be of paternal or maternal origin ; that is, either of the events E -+ E a or E 3 -+ E According t o a linkage hypothesis for the E-system, the change of E l (Edbg) t o E 2 (Edeg) would require the mutation Eb -+ Ee, whereas a change of E 3 (Eaeg) to E 2 would require the mutation Ea +Ed (Fig. 1). Red cells from the exceptional pig reacted with the Ee reagent in a typical manner. Under the linkage hypothesis, the possible mutation of E* to 'not Ea' is assumed to include the possibility of a deletion. Any of these events might cause a phenotype corresponding to the genotype E1E2. Unfortunately, the zygosity of the exceptional progeny with respect to Ed could not be determined, because an E d dosage reagent

28 ANDRESEN Irregular Transmission of a Blood-Group

has not been produced, and, more importantly, because this pig was not available for further inheritance studies.

Approximately 10000 gametes derived from parental males and females in both breeds have been tested where an E gene mutation, if it had occurred, would have been detected. About 25% of these genes were derived from irradiated spermatogonia. Consequently, the estimate of the rate of spontaneous plus induced mutations a t the E locus is 1 in 10000, if the exceptional pig is regarded as a mutant.

Approximately 17 500 gametes have been recorded relative to the 10 additional blood-group loci where a mutation, if it had occurred, would have been detected. Also in these data, about 25% of the genes were derived from irradiated spermatogonia. No mutation has been noted. For further comparison, besides the possible mutation of an Rh gene already referred to, only one example of a probable germinal mutation of a blood-group gene has been reported in man [8].

Recombination. Recombination of genes belonging to the same blood-group system has been accepted as one of several possible causes for irregular transmission of certain blood-groups in mice, chickens, and cattle [13, 201, but not in pigs [17].

Crossing-over between the hypothetical subloci EaEd and EbEe would imply that the exceptional progeny carried the E2 complex besides E l ; the latter being transmitted from the mother. Conse- quently, the pig should be homozygous with regard to E d . The rea- sons this possibility could not be tested were given in the preceding section.

The distance between the hypothetical subloci EaEd and EbEe may be measured if we assume tha t the exceptional pig was a bonafide recombinant. The four gene-complexes El , E2, E3, and E 4 give rise to 10 genotypes. These genotypes will produce 55 different mating types if reciprocal mating types are combined. Each of the 55 parental genotype combinations has been observed among 1171 matings with 10506 progeny. The E genotype of each parent and progeny was determined. Only eight of the 55 mating types are informative relative to possible crossing-over between the two hypothetical subloci. The eight informative mating types included 273 double backcrosses (2372 progeny), 93 single backcrosses (775 progeny), and 124 double inter- crosses (1113 progeny). The exceptional pig was observed among a total of 1357 progeny from the E'E' x E1E3 mating type. The binomial estimate of the recombination frequency is 112372 or 0.04%, based on the double backcross data.

Complex in One Family of Pigs Following Irradiation 29

According to the linkage hypothesis, eight specific combinations of genes may be predicted because of interallelic recombination at the E-locus. Six of these gene-complexes have been found [l, 2, 171. The missing ones are Eabg and Eab*. If the estimated recombination fre- quency reflects any reality, it is not astonishing that the two predicted gene combinations still are missing and that pig breeds exhibit pronounced linkage disequilibrium.

Gene interaction. Besides the examples previously referred to in man, there are also examples of both allelic and nonallelic gene inter- action affecting blood groups in animals, including pigs [13, 16, 181. Nonallelic gene interaction might be disclosed in connection with several blood-group systems because in a t least some blood-group systems, the antigenic determinants are carbohydrates rather than proteins or polypeptides and, therefore, are not primary gene pro- ducts.

The present results could be due to gene interaction. Several models of gene interaction may be in accord with the data, but merely one model shall be mentioned here. Both parents might be heterozygous with respect to an independently inherited dominant activator gene, X, which is necessary for the phenotypic expression of E3. The allele of X acts as a recessive suppressor gene, x. The genotype of the ex- ceptional pig would be E1E3, xx which simulates the genotype E I E B as deduced from its phenotype and previous knowledge about the genetics of the E system. Accordingly, the six E, positive progeny would have the genotype ElE3, XX or ElE3, Xx, and the remaining six pigs would have EIE1 combined with X X , Xx, or xx.

This report shows some of the problems of interpretation in the studies of mutation rates at specific loci. The genetic basis of a new trait cannot be unravelled without further breeding tests. In the case cited here, the exceptional progeny was not available for inheritance studies; thus, the distinction of a point mutation from an interallelic recombination was not possible. However, distinguishing between these two hypotheses combined and the third hypothesis of gene interaction may be possible, because such a distinction does not de- pend on the availability of the exceptional progeny.

Summary

Pigs of the Hampshire and Duroc breeds were subjected to bloodtyping. An anomaly of inheritance affecting the E blood-group system was detected in one pig among 10056 progeny from 1171 matings. An E*ES Hampshire male was mated with an

30 ANDRESEN Irregular Transmission of a Blood-Group

E'E' Hampshire female. The male had been exposed to 300 r of gonadal X-irradia- tion. Thirteen pigs were born. Six pigs were E'E', six were E1E3, and one appeared to be E1E2. The exceptional bloodtype might be a result of mutation, interallelic recombination or gene interaction. These hypotheses are discussed.

References

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Complex in One Family of Pigs Following Irradiation 31

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Author’s address: Dr.Erik Andresen, Department of Animal Genetics and Breeding, The Royal Veterinary and Agricultural College, Copenhagen (Denmark).