B. J. Isler*,
K. M. Irvin1*,
S. M. Neal,
S. J. Moeller*,
M. E. Davis*,
and D. L. Meeker*
*The Ohio State University Department of Animal Sciences
The Ohio State University Agricultural Technical Institute
1For more information, contact at: The Ohio State University,
110F Animal Science Building, 2029 Fyffe Road,
Columbus, OH 43210; 614-292-6407; 614-292-2929 fax;
The identification of genes or markers associated with reproductive traits in swine is an important area of research, due to the large economic impact that these discoveries could have on the swine industry. It has previously been reported that one of these genes, the estrogen receptor (ESR) gene, is associated with increased litter size in pigs. There has been little research, however, on the association between the ESR gene and other litter traits such as number of piglets alive at weaning. The purpose of this study was to investigate the association between ESR genotype and these litter traits in a population of 322 Large White, Yorkshire, and crossbred pigs. Two hundred twelve litter records from these animals were collected and analyzed for associations between ESR genotype and the following litter traits - number born, number born alive, litter weight of animals born, litter weight of animals born alive, number of mummies, number of stillborn animals, number of overlaid animals, number of animals at weaning, and litter weight at weaning. Data were analyzed using a model that included the effects of ESR genotype of dam, parity, farrowing month, dam breed, sire breed, and significant two-way interactions. Some litter traits displayed favorable, but not statistically significant, trends with respect to ESR genotype - litter weight born alive, litter weight born, number of stillborn pigs, number of pigs at weaning, and total litter weight at weaning. Paternal and maternal breed effects were also found for several of the litter traits studied. Dams with Large White fathers had an increased number of stillborn piglets (P = 0.08) and an increased number of mummies (P = 0.07). Dams with Large White mothers had an increased number of piglets alive at weaning (P = 0.10) and an increased litter weight of piglets alive at weaning (P = 0.001).
There has recently been intense effort within the field of genetics to find the genes that control all aspects of physiology. Special attention has been focused on those genes that are thought to control the physiological pathways that influence the ability of our common livestock species to produce meat and milk. In the past, these pathways were manipulated by selecting superior animals for use in traditional breeding programs. Unfortunately, many of these physiological traits cannot be quickly or easily improved using traditional methods of selection. In these cases, the newly expanded field of molecular genetics presents a solution. If specific regions of the genome (termed "markers") that have a direct effect on physiological traits are isolated, animals containing these markers can then be selected for using marker-assisted selection schemes. Marker-assisted selection holds great promise to help improve the rate of selection for traits that are reproductive in nature. In swine, special attention has been placed on the discovery of markers that influence the ability of a mother to successfully give birth and care for her young. Traits that influence this ability are termed the litter traits. Litter traits, such as number born and number of piglets alive at weaning, are characteristically of low heritability (Rothschild and Bidanel, 1998). This makes their improvement using marker-assisted selection techniques especially attractive.
One of the first markers shown to have a significant association with litter traits in swine was the estrogen receptor (ESR) gene (Rothschild et al ., 1991). Previous studies have shown that females with the favorable ESR B allele have an advantage of +0.3 (Large White) to +1.2 (Meishan) pigs born per litter over those females that do not have the favorable allele (Rothschild et al ., 1994; Short et al ., 1997). Little research has focused on the association between the ESR gene and other litter traits (such as number of pigs at weaning and litter weight at weaning), however. The purpose of this current study was to determine the relationship between these uninvestigated litter traits and the ESR gene.
Three hundred twenty-two purebred Yorkshire (Y x Y), purebred Large White (LW x LW), and crossbred (LW x Y, Y x LW) animals were selected for use in this study. All animals were raised at the Western Branch of The Ohio State University's Ohio Agricultural Research and Development Center (South Charleston, Ohio). Animals consisted of related and unrelated animals of both sexes and varying ages
For each animal, DNA was extracted from lymphocytes and the ESR gene amplified using a polymerase chain reaction protocol. This protocol has been outlined previously (Short et al ., 1997). Amplified products were digested with PvuII restriction endonuclease, separated on a 4% agarose gel, and visualized under UV light after ethidium bromide staining. Two ESR alleles (A and B) were identified and each animal was classified as either AA, AB, or BB with respect to ESR genotype.
Litter data for all dams with a known ESR genotype were obtained and included in litter data analysis. Dams consisted of all four breed combinations and a variety of parities. Sires of litters were either purebred Yorkshire, purebred Large White, or purebred Hampshire in origin. Litter data were collected for farrowing seasons ranging from August 1994 to July 1998. In total, 212 litter records were collected and included in the analysis. Data subsequently included in the analysis were ESR genotype, farrowing month, sire breed, dam breed, number born, number born alive, litter weight of animals born, litter weight of animals born alive, number of mummies, number of stillborn animals, number of overlaid animals, number of animals at weaning, number of days to weaning, and litter weight at weaning. Weaning age ranged from five to 26 days, with an average weaning age of 20 days. Number of overlaid animals was defined as the number of piglets crushed by the dam. Number born was defined as the number of viable animals born plus the number of stillborn animals. Number born alive was defined as the number of viable animals born.
All reproductive tract data were analyzed using the General Linear Model Procedures of SAS (1990). Data were analyzed using a model that included the effects of ESR genotype of dam, parity, farrowing month, dam breed, sire breed, and significant two-way interactions. Weaning age was included as a covariant in all models involving weaning data. Linear contrasts were used to determine the presence of individual heterosis, maternal breed effects, and paternal breed effects.
The fit of the model for most litter traits was poor, with low R2 values and non-linear normal probability plots (data not shown). This is probably a result of the interaction between complex environmental effects and fetal genotypes on litter traits. The poor fit of these models seriously hampers the interpretation of results from the litter data analysis. Increasing the number of animals in the study would assist in the determination of the validity of all trends and their true significance.
The main effect of ESR genotype was not significant (P < 0.05) for any of the traits analyzed. There was an ESR genotype x breed of dam interaction for both litter weight of pigs born alive (P = 0.05) and litter weight of pigs born (P = 0.08). In both of these cases, Y x Y animals with the BB genotype were found to have lower values than other ESR genotype x breed of dam combinations.
The ESR genotype also displayed notable, but not statistically significant, trends with respect to certain litter traits (Table 1). Animals with additional B alleles tended to have heavier litters at both birth and weaning. The hypothesized Chinese origin of the ESR B allele would explain this result, as Chinese pigs are known to have greater litter size and postnatal survival than domestic breeds (Haley et al ., 1995).
Paternal breed effects (Table 2) were observed for both number of stillborn piglets (P = 0.09) and number of mummies (P = 0.07). Dams with Large White fathers had both a larger number of stillborn piglets and number of mummies than dams with Yorkshire fathers. But, the poor fit of the number of stillborn piglets (R2 = 0.103) and number of mummies (R2 = 0.062) models seriously reduces the interpretability of these results.
Maternal breed effects (Table 2) were observed for both number of piglets alive at weaning (P = 0.10) and litter weight of piglets alive at weaning (P = 0.001). Dams with Large White mothers had a larger number of piglets alive at weaning and a larger litter weight of piglets alive at weaning than dams with Yorkshire mothers. Individual heterosis was not detected for any of the traits studied (data not shown).
|Table 1. Least-Squares Means, Stabndard Errors, and P-Values for Selected Litter Traits.|
|Litter Traita||N||Least-Squares Means and Standard Errors
for Dams With Specified ESR Genotype
|ALIVEWT||212||14.64 ± 0.78||13.23 ± 0.57||12.60 ± 1.18||0.16|
|BORNWT||211||15.35 ± 0.69||14.77 ± 0.52||14.32 ± 0.91||0.53|
|NOSTILL||212||1.36 ± 0.28||1.11 ± 0.23||0.98 ± 0.35||0.49|
|NOWEAN||204||8.12 ± 0.48||8.20 ± 0.37||8.61 ± 0.59||0.70|
|WEANWT||203||47.4 ± 2.5||47.6 ± 1.9||50.3 ± 3.1||0.59|
|a ALIVEWT = total litter weight of animals born alive (kg), BORNWT = total litter weight of animals born (kg), |
NOSTILL = number of stillborn animals, NOWEAN = number of piglets alive at weaning, WEANWT = total litter weight at weaning (kg)
b Significance level of effect of ESR genotype on specified trait
|Table 2. least-Squares means, Standard Errors, and P-Values Used in the Determination of|
Breed Effects for Litters Traits.
|Breed of Dama||Least-Squares Means and Standard Errors for Selected Litter Traitsb|
|YxY||0.71 ± 0.22||0.043 ± 0.071||7.82 ± 0.37||43.3 ± 2.1|
|LWxLW||1.31 ± 0.26||0.284 ± 0.082||8.41 ± 0.43||53.3 ± 2.6|
|YxLW||1.16 ± 0.38||0.090 ± 0.121||8.89 ± 0.63||52.1 ± 3.5|
|LWxY||1.41 ± 0.40||0.137 ± 0.126||8.11 ± 0.67||44.9 ± 3.7|
|Maternal Breed Effect P-valuec||0.50||0.22||0.10||0.001|
|Paternal Breed Effect P-valued||0.09||0.07||0.82||0.59|
|a YxY = Yorkshire sire x Yorkshire dam, YxLW = Yorkshire sire x Large White dam,|
LWxLW = Large White sire x Large White dam, LWxY = Large White sire x Yorkshire dam.
b NOSTILL = number of stillborn animals, NOMUM = number of mummified animals,
NOWEAN = number of piglets alive at weaning, WEANWT = total litter weight at weaning (kg).
c For the linear contrast for maternal effects, where H0 = no differences in the indicated trait between animals with the same breed of dam.
d For the linear contrast for paternal effects, where H0 = no differences in the indicated trait between animals with the same breed of sire.
From this study, it appears that the ESR gene may be associated with several new litter traits, such as number of piglets at weaning and litter weight at weaning. ESR genotype did not significantly affect any of the traits studied. However, several of the traits did show favorable, but nonsignificant, trends with respect to ESR genotype. The addition of more litter records to this analysis in the future will help validate and explain the significance of these trends.
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