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* Lehrstuhl für Tierzucht, Technische Universität München, 85354 Freising, Germany; and
and
Institut für Tierzucht und Haustiergenetik der Justus-Liebig-Universität, 35390 Gießen, Germany
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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Key Words: Cattle • Gene Expression • Gene Mapping • Milk Production • Characteristics • Substitution
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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) several genome-wide mapping experiments were conducted in all major dairy breeds to detect QTL with an impact on economically important traits (Bovenhuis and Schrooten, 2002). Although these enterprises were successful in determining the approximate chromosomal regions most likely harboring QTL variation, the mapping intervals of the QTL were large (Zhang et al., 1998). However, the efficient use of QTL information in selective breeding requires precise mapping within a few centimorgans, or preferably, knowledge about the molecular basis of the QTL variation. The latter was recently achieved for a QTL on chromosome 14 with a pronounced effect on milk fat content. It was shown that the QTL variation is most likely caused by a nonconservative base substitution in the candidate gene DGAT1 changing lysine to alanine (K232A) in the enzyme diacylglycerol O-acyltransferase (DGAT) (Grisart et al., 2002; Winter et al., 2002). In particular, the allele encoding the lysine 232 variant proved to be more efficient with regard to milk fat synthesis. The nucleotide variation underlying the K232A substitution can be diagnosed by an RFLP assay. Before integrating such molecular information in breeding schemes, a rigorous characterization of the alternative variants with respect to improvement of genetic gain is necessary (Sonstegard et al., 2001). The objective of the present study was to determine the allele frequencies and to estimate the effects of the two DGAT variants on milk production traits for production levels typically found in Germany using two existing granddaughter designs for Fleckvieh and German Holstein cattle. We also investigated possible differences in effects across lactations and between breeds. Finally, the potential use and impact of the first cloned QTL in dairy cattle are discussed in light of current breeding goals. |
Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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Granddaughter designs (Weller et al., 1990) previously used for QTL mapping studies (Thomsen et al., 2001; Winter et al., 2002) were available for German Holstein and Fleckvieh, the major dairy/dual-purpose breeds in Germany. The Fleckvieh granddaughter design consisted of 23 paternal half-sib families (i.e., widely used AI sires representing the major sire lines in Fleckvieh and their sons). In total, 833 sons born from 1988 to 1995 were genotyped; the family size varied from 14 to 56 sons with an average of 36.2. The German Holstein granddaughter design consisted of 16 paternal half-sib families with a total of 858 sons. The sons were born between 1988 and 1993. The family sizes ranged from 18 to 128 with an average of 53.6 sons per family. The family structures for both breeds are presented in more detail in Table 1.
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Genomic DNA was genotyped by an RFLP for the locus responsible for the DGAT1 K232A substitution assay as described in Winter et al. (2002). Briefly, PCR products of 411 bp (PCR reaction contained 5% dimethyl sulfoxide, forward primer 5'-G C A C C A T C C T C T T C C T C A A G-3' and reverse primer 5'-G G A A G C G C T T T C G G A T G-3') were digested with CfrI restriction enzyme (MBI Fermentas, St. Leon-Rot, Germany) and separated on a 2% agarose gel. The uncut fragment represents the lysine variant, whereas the CfrI RFLP fragments of 203 and 208 bp represent the alanine variant.
Phenotypic Data
Daughter yield deviations (DYD; VanRaden and Wiggans, 1991) of sons for milk, fat, and protein yield were obtained from the national breeding evaluation centers. The DYD are average phenotypes of daughters corrected for fixed effects, such as herd, season, and calving interval, and corrected for genetic contributions of the daughters’ dams. Unlike breeding values, DYD are not regressed to the mean. Fixed effects and breeding values of the dams for correction of measured phenotypes were previously estimated in routine breeding evaluations.
For Fleckvieh, the evaluations are carried out at the Bayerische Landesanstalt für Tierzucht in Grub, Germany. The current breeding evaluation method is a multitrait BLUP animal model including five different part or whole lactations (1 to 100 d, 101 to 200 d, 201 to 305 d, second lactation, third lactation) as correlated traits. Milk, fat, and protein yield are evaluated separately (i.e., no correlations between these phenotypes are accounted for).
The Vereinigte Informationssysteme für Tierzucht in Verden, Germany, administers the national breeding evaluation for German Holsteins, which is based on a fixed test day animal model. Separate estimates for milk, fat, and protein yield are obtained based on measurements of the first three lactations as correlated traits, respectively.
Because no evaluations are carried out for fat and protein content, DYD could not be obtained from the evaluation centers for these traits. Therefore, DYD for fat and protein content, measured as percentages, were calculated by the same procedure applied to compute breeding values for content traits indirectly, using the following formula (VIT, 2001) (here demonstrated for DYD of fat content):
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where DYD for milk and fat or protein yield as well as the population means (PM) of milk yield and the respective yield traits are included. To avoid a potential bias due to selection of superior bulls after progeny testing, DYD of sons were based only on phenotypes of daughters recorded during the progeny-testing period. Furthermore, a minimum of 10 daughter observations per lactation was required. Total number of sons and basic statistics of DYD of sons included in the analysis are given in Table 2. In addition to DYD, genetic parameters used in current breeding evaluations were supplied by the evaluation centers, which allowed us to calculate appropriate weighting factors for DYD.
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Allele frequencies of DGAT1 alleles were estimated based on the maternal alleles of sons. If sires were homozygous, the maternally inherited allele of sons could be determined directly from the genotypes of sons. However, if sires were heterozygous, the maternal alleles of sons could only be determined unequivocally if sons were homozygous. Therefore, a maximum likelihood procedure was applied for estimating allele frequencies in Fleckvieh and German Holstein using the following formula:
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where p represents the allele frequency of the lysine variant and 
and 
are the numbers of homozygous KK and AA sons within heterozygous sires, respectively; and nKA and nAA are the numbers of hetero-zygous KA and homozygous AA sons within homozygous AA sires, respectively. The total number of sons with alternative genotypes across all homozygous and all heterozygous sires, respectively, used for estimating allele frequencies is given in Table 3.
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). Instead, the gene substitution effect of the lysine variant was estimated with the following fixed model:
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and
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where yij is the DYD of son j within sire i, µ is the overall mean, sirei is the fixed effect of sire i, xij is the number of lysine alleles (0, 1, or 2) of son j within sire i, b is the regression coefficient representing half of the gene substitution effect (
/2) as defined by Falconer and Mackay (1996), eij is the random residual effect consisting of polygenic and environmental effects, h2 is the heritability and 
the phenotypic variance of the trait under consideration, respectively, and nij is the number of daughters of son j within sire i included in the analysis. Sires were included as fixed effects since they represent highly selected animals from the sire population and cannot be considered to be a random sample. Weights wij for DYD were calculated for each trait and each lactation separately as 1/Var(eij). Additional relationships of sons within and across sires were ignored.
In addition, correlations between residuals of milk yield, fat, and protein content were calculated within each lactation for the model of analysis as introduced above and applied a reduced model neglecting DGAT1 effects by omitting the regression part.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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The two major cattle breeds for milk production in Germany strongly differed with respect to the distribution of DGAT1 genotypes (Table 1
). Only three out of 23 Fleckvieh sires were heterozygous for K232A, all other sires were homozygous for the alanine variant (AA). In German Holstein however, eight sires were heterozygous and eight sires were homozygous AA. Accordingly, differences in the distribution of genotypes of sons within sires were observed for the respective granddaughter designs (Table 3). Only five sons in Fleckvieh, but a total of 148 sons in German Holstein were homozygous for the lysine variant at DGAT1. Estimated allele frequencies for the lysine variant were 0.072 ± 0.009 for Fleckvieh and 0.548 ± 0.020 for German Holstein, respectively.
Statistics of DYD in Granddaughter Designs
Descriptive statistics of DYD included in the analysis for estimating effects of DGAT1 are presented in Table 2. The number of sons included in the analysis of milk production traits in the second and third lactation decreased for Fleckvieh but not for German Holstein. The reason is that the granddaughter design for German Holstein was set up earlier, and most of the sons were born from 1991 to 1993. For Fleckvieh, however, sons were mainly born from 1993 to 1995, and thus not all of them have sufficient daughter records in the second or third lactation to enter the analysis. Average DYD for yield traits were positive throughout lactations and substantially higher in German Holstein. A continuous increase of average DYD with higher lactations can be observed for all yield traits in German Holstein but not in Fleckvieh. Here, a less pronounced increase is only evident for the second relative to the first lactation, and even a slight decrease for fat and protein yield in the third relative to the second lactation. Considering fat and protein content, average DYD are close to zero for Fleckvieh across all lactations. In German Holstein, however, negative values were found for these traits, with fat percentages in the third lactation showing the most negative values. The standard deviations of DYD were similar for both breeds and lower for the first lactation but nearly equal for the second and third lactation across all traits, respectively.
Effects of DGAT1 Variants
Table 4 shows estimated effects of the lysine variant (/2) and their respective standard errors. For German Holstein, effects were highly significant for all yield and content traits across all lactations. In Fleckvieh, effects were highly significant as well, with the exception of protein yield in the second and third lactation.
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The antagonistic effects of the lysine variant on milk yield and content traits in connection with the higher impact on fat relative to protein content led to different results concerning fat and protein yields. The enormous positive effect on fat percentage more than compensated for the negative effect on milk yield and resulted in positive effects of lysine on fat yield for all lactations in both breeds. Effects on fat yield were lowest in the first lactation and increased with higher lactations, especially for Fleckvieh. The opposite effect of lysine could be seen for protein yields, which were reduced as a consequence of negative effects on milk yield superimposing on only slightly positive effects on protein content. In German Holstein, the negative effect of the lysine variant on protein yield was similar in size throughout lactations, whereas a reduced but significant effect in the same direction was found only for the first lactation in Fleckvieh. No significant effects of DGAT1 variants on protein yield of the second and third lactation were observed in Fleckvieh. Standard errors of estimates were generally higher for Fleckvieh due to the lower number of sons and the higher unbalancedness of their genotype distribution compared to German Holstein. Standard errors increased across all lactations for Fleckvieh, but only from the first to the second lactation for yield traits in German Holstein.
Residual Correlations Among Traits
The consequences of antagonistic effects found for DGAT1 variants on the genetic relationship between milk yield, fat content and protein content were investigated by comparing correlations of DYD residuals, mainly comprising polygenic effects, between these traits from the analysis model and those calculated without accounting for the DGAT1 effect. Results are given in Table 5. Correlations of residuals between milk yield and content traits were negative and substantially higher in German Holstein compared to Fleckvieh. Furthermore, the relationships weakened for higher lactations in Fleckvieh but not for German Holstein. The correlations of residuals between fat and protein content were positive in both breeds, but more pronounced in German Holstein. Stronger residual correlations were found throughout breeds and traits compared to when effects of DGAT1 were not included in the analysis. Differences between residual correlations of models with and without accounting for DGAT1 were larger in German Holstein for relationships between milk yield and content traits but equal in size between fat and protein content.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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Allele frequencies estimated for the fat content-enhancing lysine variant at DGAT1 are in good agreement with those previously published. Winter et al. (2002) reported an estimate of 0.07 for the lysine variant in a random sample of Fleckvieh bulls. A comparable value of 0.06 was found in a sample of Fleckvieh cattle encluding both sexes (our unpublished observations). In the Holstein breed, allele frequencies reported for the lysine variant range from 0.30 in the New Zealand to 0.63 in the Dutch populations (Bovenhuis and Schrooten, 2002; Grisart et al., 2002). Winter et al. (2002) estimated a lysine frequency in German Holstein of 0.35, and in another independent sample of German Holstein animals, the value was 0.44 (our unpublished observations).
The lower frequency of lysine in Fleckvieh vs. German Holstein is somewhat unexpected since Fleckvieh have been superior in content traits in the past, whereas German Holstein have a higher performance in milk yield. Reasons for the low lysine frequency in Fleckvieh could be genetic drift or reinforced selection on milk yield. The comparatively high lysine frequency in German Holstein could be caused by special emphasis set on content traits when selecting bull dams in recent years. A stronger impact on fat content in selection decisions was postulated by breeding organizations in German Holstein as countermeasures in light of negative genetic trends for fat content, which amounted to 0.2% in the last 10 yr (F. Reinhardt, personal communication). However, in this context, it is interesting that none of the grandsires is homozygous for the lysine variant and that the lysine frequency within these elite sires deviates significantly from the estimate based on the maternal alleles of their sons. Selection of bull sires is based mainly on the relative breeding value for milk production, which is actually a combination of protein and fat yield with weighting factors of four to one for protein. Applying current weighting factors to estimated effects for the alternative DGAT1 variants (see Table 4) shows that the alanine variant is preferable in German Holstein. Selecting bull sires from the extremes with respect to relative breeding values in milk production leads to a low lysine frequency in these highly selected animals as observed. This reasoning is supported by the fact that a higher proportion of AA genotypes was evident for proven bulls with more than 10,000 daughters produced in German Holstein. It seems that farmers inadvertently preferred AA bulls when selecting sires for their cows. It can be questioned whether the favorable effects of the alanine variant sufficiently explain this finding or whether there might be positive effects on other traits like fertility or type traits, which should be investigated in future studies. In Fleckvieh, the low frequency of the lysine variant and the fact that proven bulls do not yet have sufficient daughters prohibit making statements about the consequences of farmers decisions on distribution of DGAT1 genotypes within intensively used sires.
Effects of DGAT1 Variant
Although effects of estimated DGAT1 variants were generally in good agreement between Fleckvieh and German Holstein cattle, some differences became apparent in this study. Whereas a decrease of the negative effect of the lysine variant on milk yield in higher lactations was found for Fleckvieh, the opposite was true for German Holstein. The latter might be explained by a scale effect since the increase of the negative effect of the lysine variant corresponds with the increases observed for the population mean and the average DYD for the three lactations, respectively. On the other hand, milk yield might be the predominant selection criteria for daughters entering higher lactations in Fleckvieh. If so, the higher proportion of daughters from lysine-carrying sons being culled would result in decreasing negative effects of the lysine variant on milk yield observed.
The effect of the lysine variant on milk fat and milk protein content is higher in Fleckvieh vs. German Holstein. Assuming similar additive and dominance values across the two breeds, a possible explanation for the differences in gene substitution effects could be a partly recessive mode of inheritance of the lysine variant in combination with the differences in allele frequencies found. This mode of inheritance for the lysine variant is similar to the impact of DGAT1 on intramuscular fat deposition (our unpublished observations). However, Grisart et al. (2002) did not report dominance effects for milk production traits significantly deviating from zero in the New Zealand cow population. To further clarify the mode of inheritance, it is necessary to estimate additive and dominance effects simultaneously by association studies using phenotypic records of genotyped daughters directly. Other reasons for differences in effects of content traits between breeds could be that effects of DGAT1 variants depend on the genetic background due to interactions with other genes or that differing haplotypic environments of the K232A substitution cause different effects. Indeed, additional polymorphisms have been reported by Winter et al. (2002) in both breeds and subsets of haplotypes for both, the lysine and alanine encoding variants were derived. In particular, Winter et al. (2002) found two lysine encoding haplotypes in Fleckvieh grandsires, one apparently of Fleckvieh origin and another most likely introgressed from the Holstein breed. These findings motivate a more detailed study of the variants in different haplotypic environments. Furthermore, Winter et al. (2002) have found that QTL effects for fat content estimated within the Fleckvieh grandsires were different, although not significantly so.
Estimated effects for fat and protein yield, the economically important traits in dairy cattle, tended to be inconsistent across breeds and lactations. The higher impact of the lysine variant on fat content compared to milk yield results in positive effects on fat yield. Estimates were similar for Fleckvieh and German Holstein in the first lactation since the higher effects of the lysine variant on fat percentage in Fleckvieh were compensated for by the higher production level in German Holstein. Larger effects for Fleckvieh in later lactations are mainly due to less negative effects on milk yield and a higher production level at nearly constant effects for fat content. The less pronounced increase in German Holstein is a consequence of stronger negative effects of the lysine variant on milk yield, despite an increased effect on fat content. In contrast to estimates of fat yield, estimates for the lysine variant on protein yield were generally negative, which is due to the lower impact on protein content compared to fat content. In German Holstein, the negative effect of about 2.5 kg was nearly constant across lactations. However, in Fleckvieh a reduced negative effect of about 1.8 kg was estimated in the first lactation and effects found for the second and third lactation were practically zero. Consequences of these aspects on breeding decisions have to be considered in light of the current emphasis of protein and fat yield in the breeding goal in Germany. Current weights of four to one in favor of protein yield relative to fat yield applied in both breeds suggest the interesting possibility that superiority of a variant varies between breeds. The alanine variant has economically beneficial effects in German Holstein across all lactations. In Fleckvieh, the alanine variant performs better in the first lactation, whereas the lysine variant is preferable in later lactations. However, the latter finding might also be a consequence of selection on milk yield among sires possessing the lysine variant and has to be investigated more thoroughly. Furthermore, it has to be stated that results for higher lactations in Fleckvieh have to be interpreted with caution due to less phenotypic information available (Table 2).
Effects of K232A variants based on DYD of a granddaughter design of the Dutch Holstein population and based on lactation records on a daughter design within the New Zealand Holstein population have already been reported by Grisart et al. (2002). Estimates for yield traits in their granddaughter design were nearly identical with the results found for German Holstein, with half of the gene substitution effects for the lysine variant of -158, 5.2, and -2.8 kg for milk, fat, and protein yield, respectively. Slightly stronger effects of 0.17% on fat content and 0.04% on protein content were found by Grisart et al. (2002) compared with about 0.14 and 0.03% for fat and protein content in German Holsteins when averaging across lactations. The very similar effects confirm the substantial impact of DGAT1 on milk production traits. The finding that effects might be influenced by different genetic background, population level, or selection in later lactations is supported by the results found by Grisart et al. (2002) in the New Zealand population. Compared with the granddaughter designs, estimates from the New Zealand daughter design were in the same direction but less than half the size for milk and protein yield, and reduced for fat yield. Content traits, however, were somewhat higher.
The strong effects of the DGAT1 variants on milk production traits become evident when comparing these effects with other single-gene effects. Genotypes of the casein genes have been reported in different studies to be associated predominantly with milk yield, as well as protein content and protein yield (Bovenhuis et al., 1992; Ojala et al., 1997; Ikonen et al., 2001). Although the size of effects on these traits was mostly in the range of DGAT1 effects on protein traits, results were inconsistent across studies. It was concluded that linked genes might be responsible, and thus casein genes do not possess the same candidate status as DGAT1. Renaville et al. (1997) investigated the effects of PIT-1 gene variants in Italian Holstein and reported positive estimates for half of the gene substitution effects of about 75 kg in milk yield and -0.04% in fat content for the same allele, resulting in negligible negative effects for fat yield. Protein content was virtually unaltered, but in total a positive effect of about 3.0 kg was reported for protein yield, which is similar in size to the effect of the alanine variant of DGAT1 in German Holstein.
Application and Future Aspect
There are apparent differences on how DGAT1 variants are acting with respect to milk production traits. The comparison of residual correlations between milk yield and percentage traits analyzed with and without accounting for DGAT1 (Table 5) also indicates the possible influence of DGAT1 on genetic correlations. Including DGAT1 in models for estimating (co)variances among traits and for breeding evaluation would therefore allow for more precise estimation of polygenic effects. Considering all of the aspects shown, breeding schemes can now be evaluated carefully to decide on whether and how to integrate DGAT1 variants.
Well-established effects and diagnostics of DGAT1 variants in individual animals will in addition be extremely helpful to further 1) investigate biochemical pathways involved in the expression of milk production traits, 2) to study possible interactions with other causal genes, and 3) to increase our understanding of the lactation process. From the breeder’s perspective, extra benefits could be achieved by offering special products designed for consumer demands, such as low-fat milk or butter that can be spread more easily.
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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Footnotes |
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2 Correspondence: Alte Akademie 12 (phone: 49-8161-713743; fax: 49-8161-713107; E-mail: Georg.Thaller{at}tierzucht.tum.de).
Received for publication December 16, 2002. Accepted for publication April 22, 2003.
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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