J. Vet. Med. A 47, 31–36 (2000)© 2000 Blackwell Wissenschafts-Verlag, BerlinISSN 0931–184X

Institut fur Tierzucht und Vererbungsforschung der Tierarztlichen Hochschule, Hannover, Germany

Association Analysis between Bilateral Convergent Strabismus

with Exophthalmus and Milk Production Traits in Dairy Cattle

O. DISTL1 and M. GERST

Addresses of authors: Institut fur Tierzucht und Vererbungsforschung, Tierarztliche HochschuleHannover, Bunteweg 17p, 30559 Hannover, Germany; 1Corresponding author

With 3 figures and 4 tables

(Received for publication August 6, 1997)

Summary

Bilateral convergent strabismus with exophthalmus (BCSE) is characterized in cattle by a symmetricalantero-medial rotation and protrusion of the eyeballs. This eye defect is caused by an inherited, centrallyinsufficient function of the eye muscles recti laterales and retractores. In German Brown cattle a monogenic,autosomal dominant inheritance proved to be most probable in complex segregation analysis. In our studyrunning between October 1993 and May 1995 a total of 200 affected German Brown cows was studied.The investigation of the association between milk production traits and BCSE was based on 10 960German Brown cows. The analysis revealed no significant differences between affected and non-affectedcows, nor between cow families with and without affected members. There was no indication of anassociation between milk production traits and occurrence of BCSE within cow families. Linkage orpleiotropy of the BCSE locus with quantitative trait loci for milk production traits may be rather unlikely.

Introduction

Bilateral convergent strabismus with exophthalmus (BCSE) was found to be a congenitalanomaly of both eyes caused by a dominant single autosomal gene (Distl, 1993). The symptomsdevelop progressively, with onset by the age of 1–2 years. The first signs of BCSE are usuallythe convergence of both eyeballs to the median. The motoric insufficiency of the N. abducensis manifested as the animals are no longer able to move their eyeballs. As the course of theanomaly continues the symptoms of the paralytic strabismus are strengthened, whereby anincreasing bilateral protrusion of the eyeballs can be observed. The symmetrical antero-medialrotation of both eyeballs can lead to the disappearance of the pupils at the medial angle of theeyes, followed by blindness. Schutz-Hanke et al. (1979) concluded from their histological studiesthat the motoric insufficiency of the N. abducens is caused by an inborn defect of the centralnervous system. The economic losses through BCSE arise from decreased market value andthe exclusion of progeny as breeding animals. In cubicle housing and grazing systems inparticular, animals with BCSE have to be handled with more care, because they move morecautiously.

The objective of our study was the analysis of the association between milk productiontraits and BCSE in dairy cows. We mainly collected data from German Brown cattle, however,this eye defect seems to be widespread in many cattle breeds.

Materials and Methods

The analysis could be based on 260 affected cows belonging to different breeds. All herds includedin the investigation are under the official milk recording system of Bavaria (Landeskuratorium der

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32 DISTL and GERST

Table 1. Survey of the available data

Non-affected cows Affected cows

Ascertainment Number of herds Total DBV Total DBV

Group I 93 2848 2132 177 123Group II 254 7958 7288 73 67Group III 41 1570 1540 10 10Total 388 12 376 10 960 260 200

Erzeugerringe fur tierische Veredelung in Bayern), so pedigrees and milk production data are available onelectronic data files. Affected animals were ascertained between October 1993 and May 1995 in threegroups (Table 1). Animals in group I were diagnosed as being affected at a slaughtherhouse or by veterinarypractitioners. In group I we found the following breed distribution: 123 cows belonged to German Brown,six cows to German Fleckvieh, 31 to German Holsteins and 21 cows to other breeds or cross-breeds.Through these affected animals their herdmates were brought into the study. Animals of group II wereascertained by sampling of first and second lactation daughters of 15 unproven German Brown sires inmilk-recorded herds. About 25 German Brown daughters per unproven sire and all their herdmates of thesame breed were examined for BCSE. The third group was recruited from herdbook farms with GermanBrown cattle sampled through bull dams identified in the data of group I. The recordings were primarilyfrom the area where German Brown cattle (DBV) is traditionally bred. Hence, the high percentage ofGerman Brown animals sampled in group I as compared to other breeds, e.g. German Fleckvieh orGerman Holsteins, can be explained by the location of the slaughterhouse chosen. Milk production wasanalysed by using breeding values of a multi-trait animal model. These breeding values are routinelypredicted for all dairy breeds in Germany using a model including herd–year–season, the additive geneticeffect of the cow and, after pre-corrections of the trait values for age at calving, intercalving interval anddays in milk. The breeding values analysed are summed over the three periods of first lactation (1–100,101–200, 201–305 days), the second and third lactations giving a specific weight for each breeding valueaccording to its relative economic value. Breeding values were chosen for analysis because a possible effectof the BCSE gene locus is interpreted as a random effect and the influence of selection due to BCSE maybe more evident in the genetic merit of the cows. It is very unlikely that the reliabilities of the breedingvalues of cows are influenced by BCSE through a direct effect on milk performance and/or culling policies(Gerst, 1996).

The association analyses were restricted to the breed German Brown because for the other breedstoo few cow families were available. Cow families were defined by the founder cow which was traced backeight generations on the maternal path.

The following generalized linear models were employed.

Model I

Model I was for comparison of breeding values among cow families with at least one affected cowand cow families without any affected cow within the breed German Brown (DBV):

Yij =m+Si + eij

where: Yij is the breeding value of the ij-th DBV cow, m is the model-dependent constant; Si is the fixedeffect of affection statusi (i=1 or 2) of the cow family; and eij is the remainder.

A significant effect of the affection status in model I can be caused by selection effects and/or verytight linkage among the BCSE gene locus and an important quantitative trait locus for milk performance.

Model II

Model II, for comparison of breeding values among affected and non-affected cows within cowfamilies with at least one affected cow for the breed German Brown, is used to detect allelic associationwithin segregating families. When linkage is not so close this type of association cannot be observed byapplying model I.

Model II:

33Strabismus with Exophthalmus and Milk Production

Yijk =m+Fi +Bij + eijk

where: Yijk is the breeding value of the ijk-th DBV cow; m is the model-dependent constant; Fi is the fixedeffect of the affected cow familyi (i=1–91); Bij is the fixed effect of affection statusij ( j=1 or 2) withincow familyi ; and eijk is the remainder.

In the association analysis according to model II only cow families of the breed German Brown withat least one affected and one non-affected cow could be included in order to reach the required size ofcow family with at least three members. In total, 91 cow families with 171 affected and 849 non-affectedGerman Brown cows could be regarded in the analysis. The least-square means for each cow family wereaveraged over all cow families, but within affection status. In one case the reciprocal standard errors ofthe least-square means were used as weights and in the other case the least-square means were notweighted.

Results

Among affected and non-affected cows of all breeds no significant differences (P ¾ 0.1)could be estimated using model I (Table 2). Non-affected cows in groups I and II had higherbreeding values for fat and protein yield as compared to affected cows. In group III, similardifferences were only found for the breeding value in protein yield. With the exception of groupIII, differences between affected and non-affected cows were rather small for the breedingvalues in fat and protein content. The comparison of the frequency distribution of breedingvalues among affected and non-affected German Brown cows revealed no differences (Figs 1to 3).

The estimated means (model II) for breeding values of milk traits were not significantlydifferent (P ¾ 0.1) among cow families with affected and non-affected cows (Table 3). Formilk and protein yield the cow families with affected members showed lower breeding valuesthan cow families free of BCSE.

The unweighted results of the association analysis within cow family (model II) reveal thataffected cows were superior by 4 kg in the breeding value for milk yield and by 0.2 kg in thebreeding values for fat and protein yield as compared to non-affected cow family members(Table 4). Weighting the least-square means by their standard errors diminished the differencesbetween groups to a large extent. Significant differences for breeding values of milk productiontraits (P ¾ 0.1) could not be detected for the effect of the affection status within affected cowfamily in the analysis of variance nor for the least-square means averaged over affected cowfamilies, even in the case of using unweighted means.

Discussion

German Brown cows having high breeding values for milk production were not prone toan increased risk of being affected by BCSE. Therefore, the presence of BCSE in cows may

Table 2. Comparison of breeding values among affected and non-affected DBV cows

Breeding values for:

Ascertainment Milk (kg) Fat (kg) Fat (%) Protein (kg) Protein (%)

Group Iaffected 82.22 277.4 5.1 2 12.2 0.03 2 0.2 3.5 2 9.1 0.022 0.1non-affected 109.0 2 263.8 5.4 2 13.6 0.02 2 0.2 4.32 8.9 0.01 2 0.1

Group IIaffected 120.22 240.1 4.9 2 11.6 0.01 2 0.1 4.3 2 8.9 0.012 0.1non-affected 116.4 2 249.9 5.7 2 12.4 0.02 2 0.2 4.72 8.7 0.02 2 0.1

Group IIIaffected 253.22 150.1 14.5 2 5.2 0.08 2 0.06 5.72 4.8 −0.05 2 0.1non-affected 253.9 2 277.2 13.3 2 15.1 0.06 2 0.2 10.7 2 9.8 0.04 2 0.1

34 DISTL and GERST

Fig. 1. Distribution of breeding values for milk yield according to affection status in German Brown cattle(range of classes: 120 kg milk using an open lowest and highest class).

Fig. 2. Distribution of breeding values for fat content according to affection status in German Browncattle (range of classes: 0.06% fat using an open lowest and highest class).

not be related to a selection advantage for milk production. The genetic analysis assumed thatthe defective allele was segregating within cow families and was only expressed in affectedcows. The inheritance of BCSE has been proved by studies in different breeds (Regan et al.,1944; Holmes and Young, 1957; Schutz-Hanke et al., 1979; Distl, 1993; Gerst, 1996). In theGerman Brown breed the dominant effect of the autosomal allele causing BCSE could bedemonstrated, when data covering several generations were used (Distl, 1993). The comparisonbetween cow families with affected and non-affected members should show if selection favours

35Strabismus with Exophthalmus and Milk Production

Fig. 3. Distribution of breeding values for protein content according to affection status in German Browncattle (range of classes: 0.04% protein using an open lowest and highest class).

Table 3. Least-square means and their standard errors of breeding values for cow families withaffected (AFF) and non-affected (NAFF) members in DBV cows

Breeding values for:

Status Milk (kg) Fat (kg) Fat (%) Protein (kg) Protein (%)

AFF 126 2 9 6.82 0.4 0.032 0.005 5.1 2 0.3 0.022 0.003NAFF 133 2 3 6.72 0.1 0.032 0.002 5.4 2 0.1 0.022 0.001

Table 4. Averaged least-square means with their standard errors of breeding values for milk pro-duction traits in affected and non-affected DBV cows within affected cow families

Breeding values for:

Analysis/status Milk (kg) Fat (kg) Fat (%) Protein (kg) Protein (%)

Weightedaffected 1322 162 6.72 8.2 0.03 2 0.11 5.12 5.3 0.022 0.03non-affected 132 2 76 7.1 2 3.8 0.04 2 0.05 5.42 2.5 0.012 0.07

Unweightedaffected 1192 179 5.92 9.03 0.02 2 0.12 4.7 2 3.2 0.01 2 0.07non-affected 115 2 98 5.7 2 4.97 0.02 2 0.07 4.5 2 5.9 0.022 0.04

families carrying the defective allele for BCSE. However, a selection difference was not evidentfor cow families with affected members. The analysis within those families segregating for thedefective allele should enable us to prove the allelic association between milk production traitsand the BCSE locus. We can conclude from our association analysis within cow families that

36 DISTL and GERST

the probability for linkage among loci important for milk production traits and the BCSE locusis rather low. Nevertheless, there may be loci for milk production traits on the same chromosomewhere the BCSE locus is located, but the effects of these loci may be too small to be detectableby an association analysis. The probability for a large number of quantitative trait loci for milkproduction traits may be rather low (Georges et al., 1995; Weller et al., 1995), so that the a prioriprobability for significant results decreases. Chromosomes 1, 6, 9, 10, and 20 gave strongevidence for quantitative trait loci controlling milk production traits. Our ascertained pedigreesshow that the BCSE allele is segregating over many generations in cow families with affectedmembers. Even if the defective allele and some alleles desirable for high milk production areinitially arranged on the same strand of a chromosome in the coupling phase (gametic phasedisequilibrium), in later generations recombinations occur between those loci and thus theassociation among phenotypes tends to resolve. The recombination rate (r) decreases thegametic phase disequilibrium between the BCSE allele and other alleles influencing the milkproduction by a factor (1 − r) in each generation, so that loci at a distance of 5 cM andmore are recombining more often than tightly linked loci. In later generations (t) the linkagedisequilibrium (D) between two loci on the same chromosome is changed to the amountDt = (1 − r)tD. On the other hand, tight linkage among loci keeps the association amongphenotypes over many generations and across families. Therefore, very close linkage withquantitative trait loci for milk production traits can be excluded in our data, as shown by theanalysis across families. Then chromosomes other than 1, 6, 9, 10 and 20, which were alreadymapped for quantitative trait loci for milk production traits, may be the first candidate chromo-somes to be searched for the BCSE locus by the means of a systematic genome scan.

Our results from the genetic association study should not be influenced by selectionprocesses. Intensive selection of BCSE carriers could pretend to a genetic association ormaintain an existing linkage disequilibrium, as in the case of the Weaver locus on milk yield inthe US–Brown Swiss sire population (Hoeschele and Meinert, 1990), especially if the defect haseconomic implications for the breeders. A selection advantage could be clearly excluded forBCSE in our material and thus, this influence plays no role for the genetic association analysis.Pleiotropic effects on quantitative trait loci for milk production arising from the BCSE locusor nearby loci on the same chromosome may be unlikely. Also in this case, large effects onmilk production traits had to be shown in the association analysis. Therefore, the occurrenceof BCSE should be neutral to selection for milk production traits.

Acknowledgement

M. Gerst was supported by a graduate scholarship from the Ludwig-Maximilians-Universitat, Mun-chen, for the promotion of the younger generation in sciences and arts.

References

Distl, O., 1993: Analysis of pedigrees in dairy cattle segregating for bilateral strabismus with exophthalmus.J. Anim. Breed. Genet. 110, 393–400.

Georges, M., D. Nielsen, and M. Mackinnon et al., 1995: Mapping quantitative trait loci controlling milkproduction in dairy cattle by exploiting progeny testing. Genetics 139, 907–920.

Gerst, M., 1996: Populationsgenetische Untersuchungen zum bilateralen Strabismus convergens mitExophthalmus beim Rind. Diss. med. vet., Munchen.

Hoeschele, I., and T. R. Meinert, 1990: Association of genetic defects with yield and type traits: the weaverlocus effect on yield. J. Dairy Sci. 73, 2503–2515.

Holmes, J. R., and G. B. Young, 1957: A note on exophthalmus with strabismus in Shorthorn cattle. Vet.Record 69, 148–149.

Regan, W. M., P. W. Gregory, and S. W. Mead, 1944: Hereditary strabism in Jersey cattle. J. Hered. 35,233–234.

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