Figure 1.
P. Dimsoski, H.C. Hines, and K.M. Irvin
Department of Animal Sciences
Microsatellite analysis was performed to determine the genetic distances between Large White and Yorkshire pig populations. The results of this study indicated that Large White and Yorkshire (LW and Y) are very closely related lines; however, some alleles have a different distribution in the two lines. Based on the results of this study, development of a test for line identification is possible. This study further shows that microsatellites are very useful in the study of between-breed (line) genetic variations, even in closely related lines or breeds.
Historical evidence suggests that the Large White (LW) breed of pigs was developed in England 200 years ago (Plumb, 1920). In the U.S. the Large White is known as Yorkshire, and it was introduced in 1893 (Day, 1916). While Yorkshire swine evolved through selection schemes practiced in the United States and Canada, the Large White was developed under selection schemes practiced in the United Kingdom. The Large White and Yorkshire (LW and Y) lines had little cross-contacts until recently when pure Large White pigs were imported into the U.S., where the lines are considered to be one breed and are registered within the same breed association. The lines cannot be easily distinguished phenotypically; however, there might be some differences in the production characteristics. Recently, significant heterosis values in F1 crosses between LW and Y had been reported for some production traits (Neal, 1994). Thus, expectations are that the two populations have different gene pools. If this is true, both lines could be included in the crossbreeding selection schemes commonly applied in the pig breeding industry.
As an addition to the classical population genetics study where data on F1 crosses were collected for several years to estimate heterosis effects, the microsatellite analysis was performed in the current study to determine the genetic distances between LW and Y populations.
Animals. Large White and Yorkshire lines were selected from six different farms in Ohio. Complete pedigree information for six generations was available so relationship coefficients could be calculated. All animals belonged to different families (different parents). Out of this group, animals with the smallest relationship coefficients entered the microsatellite study.
Microsatellite Analysis. Eighteen polymorphic microsatellites randomly chosen from over 400 available were distributed along the pig genome. After DNA extraction from white blood cells and PCR amplification, the band pattern was read from the polyacrylamide gels that were silver stained.
Statistical Analysis. Nei's genetic distance (Nei, 1972, 1978), Wahlund coefficients (Wright, 1978), and other standard genetic parameters were calculated based on microsatellite frequencies.
The genetic measurements were calculated from allele frequencies of 18 microsatellite loci genotyped on 30 animals per line (LW or Y). All but one of the microsatellites were polymorphic. The results of the pooled test for the deviation from Hardy-Weinberg equilibrium are presented in Table 1. Half of the alleles for both populations are according to Hardy-Weinberg proportions with a pooled test. A possible reason for this could be the high selection pressure commonly applied in pig breeding. Nei's genetic distance (Table 2) between LW and Y was 0.16, with genetic identity being 0.88. A maximum value of 1 for genetic identity means that all alleles over all loci have the same frequencies. Considering the fact that the alleles in this study were nonfunctional nucleotide repeats that had been reported to have high mutation rate, it can be concluded that the identity between the two populations is high.
Wahlund coefficients were above 0.15 in 9 alleles and 6 loci (Figure 1). Thus, despite the fact that both lines have a common origin, different selection schemes that were applied caused differences in allelic distributions between the two lines.
| Table 1. The 2 and P values for pooled Hardy-Weinberg test. | |||||
| Yorkshire | Large White | ||||
| Microsatallite | 2 | P | 2 | P | |
| S0008 | 6.7 | 0.009 | 6.7 | 0.009 | |
| SW24 | 0.6 | 0.415 | 0.6 | 0.415 | |
| SW38 | 8.9 | 0.000 | 19.3 | 0.000 | |
| SW159 | 2.3 | 0.000 | 26.6 | 0.000 | |
| SW703 | 3.2 | 0.022 | 5.3 | 0.001 | |
| SW749 | 0.6 | 0.576 | 1.9 | 0.573 | |
| SW787 | 0.1 | 0.715 | 0.1 | 0.715 | |
| SW835 | 0.4 | 0.484 | 0.5 | 0.484 | |
| SW874 | 3.1 | 0.077 | 3.1 | 0.077 | |
| SW859 | 29.0 | 0.000 | 29.0 | 0.000 | |
| SW973 | 31.6 | 0.000 | 19.6 | 0.000 | |
| SW995 | 19.9 | 0.000 | 19.9 | 0.000 | |
| SW1026 | 19.2 | 0.000 | 19.3 | 0.000 | |
| SW1041 | 0.5 | 0.484 | 0.5 | 0.484 | |
| SW1067 | 2.6 | 0.106 | 2.6 | 0.106 | |
| OPN | 31.3 | 0.000 | 31.3 | 0.000 | |
| IGF1 | 0.025 | 0.874 | 0.100 | 0.874 | |
| Table 2. Genetic distances or similarities between Yorkshire and Large White populations. | ||||||
| 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| 0.064 | 0.054 | 0.253 | 0.157 | 0.134 | 0.875 | 3.15 |
| 1 = Nei minimum distance.
2 = Nei unbiased minimum distance. 3 = Roger distance. 4 = Nei genetic distance. 5 = Nei unbiased genetic distance. 6 = Nei unbiased genetic identity. 7 = distance. | ||||||
Figure 1.
Day, G. E. 1916. The Breeds of Livestock. C.W. Gay, Ed., The Macmillan Comp., New York.
Neal, S.M. and K.M. Irvin. 1994. Evaluation of Large White and Yorkshire breeds and reciprocal crosses. Proc. 5th World Congress on Genetics Appl. to Livestock Prod., 17:436.
Nei, M. 1972. Genetic distance between populations. Am. Nat. 106:283.
Nei, M. 1978. Estimation of average heterozygosity and genetic distance from a small number of individuals. Genetics 89:583.
Plumb, C. S. 1920. Types and Breeds of Farm Animals. Ginn and Company, Boston.
Wright, S. 1978. Evolution and the Genetics of Populations, Vol. 4. Variability within and among natural populations. University of Chicago Press, Chicago.