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a USDA, ARS, RLH US Meat Animal Research Center, Clay Center, Nebraska 68933
ABSTRACT
Endocrine and testicular responses to unilateral castration on 1, 10, 56, or 112 days of age were characterized in 132 Chinese Meishan (MS) x White composite (WC) crossbred boars in which testicular size associates with a quantitative trait locus (QTL) on X chromosome. At 220 days of age, testicles of boars unilaterally castrated on Day 1 or 10 weighed more and had greater total daily sperm production (DSP) than one testicle of bilaterally intact boars (P < 0.05); compensation did not double these two responses. Boars with MS alleles at the X chromosome QTL had smaller testicles, darker colored parenchyma, and lower total DSP than boars with WC alleles (P < 0.05). The MS alleles engendered greater (P < 0.05) plasma FSH and LH during puberty than WC alleles. Plasma FSH increased (P < 0.05) within 48 h of unilateral castration on Days 1, 10, and 56. Subsequent increases occurred earlier during puberty (P < 0.05) after unilateral castration at younger ages than after unilateral castration at older ages. Pubertal increases in plasma FSH and LH were greater (P < 0.05) in boars with MS alleles than in those with WC alleles for the X chromosome QTL. Breed of Y chromosome had no effect on testicular traits, FSH, testosterone, or estrone. For LH, boars with an MS Y chromosome had greater (P < 0.01) plasma LH across all ages than boars with a WC Y chromosome. We conclude that a gene or groups of genes that reside on the porcine X chromosome regulate testicular development and pubertal gonadotropin concentrations.
follicle-stimulating hormone, luteinizing hormone, pituitary, sperm, testis
INTRODUCTION
Sperm production in boars correlates positively with testicular size [1, 2]. Moreover, number of Sertoli cells in other species establishes testicular size and determines sperm-producing capacity [3–6]. In boars, rapid proliferation of Sertoli cells occurs during the first 3 wk of life in association with an early postnatal increase in plasma FSH concentrations [7–10]. The greatest compensatory response to unilateral castration transpires during this period [11]. In a line of boars produced by crossing Chinese Meishan (MS) and White composite (WC) breeds, considerable variation exists for testicular weight [12–14]. Previously, subpopulations of these MS x WC crossbred boars were selected at 4–6 mo of age for maximal differences in plasma FSH concentration and then evaluated at a year of age [13]. Boars with high plasma FSH had much smaller testicles and greatly lower sperm production than boars with low FSH concentrations. For the current study, the initial objective was evaluation of all boars produced within a farrowing season for variation in testicular weight and sperm production.
After initiation of the present study, a quantitative trait locus (QTL) for plasma FSH and testicular size was identified on the porcine X chromosome [14]. Existence of a gene or group of genes on the X chromosome that affected testicular size contrasts findings in mice in which the Y chromosome contributed a greater impact on testicular size than the X chromosome [15]. Therefore, a second objective was to investigate the effects of breed of origin of X chromosome QTL and Y chromosome on testicular traits and endocrine patterns. We hypothesized that the MS X chromosome QTL would reduce testicular weight and increase endocrine profiles based on previous findings in MS boars [8]. A third objective was to conduct unilateral castration at different ages and evaluate the influence of age at unilateral castration on testicular and endocrine responses.
MATERIALS AND METHODS
Boars
Boars were the first generation of inter se matings of 1/2 MS x 1/2 WC parents. These boars were produced as part of a resource population for swine genome-mapping projects and are comparable to the F4 generation described by Rohrer et al. [16]. A QTL for FSH and testicular size was identified on the porcine X chromosome [14], but boars of the F4 generation were not used in that study. Nine sires were selected from four litters for maximal differences in plasma FSH concentrations within litter, and dams were selected from litters that contained boars with large differences in plasma FSH. The objective of this mating scheme was to maximize the potential for variation in testicular size based on the observation that plasma FSH concentrations in boars correlated negatively with testicular size [12]. At birth, all boars were assigned within sire family to one of five treatments, intact controls or unilateral castration of the right testicle on Days 1, 10, 56, or 112. The Day 112 group was underrepresented to insure that at least 25 boars were assigned to the other groups. These ages were selected based on timing of the neonatal increase in FSH secretion (Days 1 and 10), nadir in prepubertal FSH secretion (Day 56), and formation of the blood-testis barrier (Day 112) [12, 17, 18]. Care of boars was in accordance with the Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching [19]. Boars were weaned at 17–20 days of age, reared in confinement buildings, and fed standard diets that changed to accommodate changing nutritional requirements with increasing age.
Unilateral castrations on Days 1 and 10 were conducted with a standard management protocol without anesthesia. On Day 56, boars received lidocaine hydrochloride (Western Veterinary Supply, Porterville, CA) as multiple, s.c. injections in the scrotal area before unilateral castration. On Day 112, boars were administered sodium thiopental (Rhone Merieux, Athens, GA) i.v. and placed on closed circuit halothane (Halcarbon Laboratories, North Augusta, SC) and oxygen. At 219 ± 0.34 days of age, all boars were moved from their pens and transported to a surgical room. Their left testicle was then removed after induction of anesthesia with sodium thiopental and halothane. Some control boars (n = 26) were retained after unilateral castration on Day 220 until 
14.5 mo of age when their remaining right testicle was removed at slaughter.
Testicular Evaluations
Testis and epididymis were trimmed and weighed immediately after removal. The left testis removed at Day 220 was cut longitudinally, assigned a color score, and a portion was frozen for subsequent evaluation of sperm production by counting homogenization-resistant, elongated spermatid nuclei [20, 21]. Daily sperm production (DSP) per g of testis and total DSP per testis were calculated for each boar. Color score was assigned by matching the parenchyma to a 10-point scale developed to range from light pink (1) to dark maroon (10). This trait was evaluated because color of testicular parenchyma of straight-bred MS boars is considerably darker than observed in WC boars.
Blood Collection and Hormonal Evaluations
Blood was obtained by venipuncture from all boars on Days 1, 3, 10, 12, 19, 26, 42, 56, 65, 72, 93, 112, 135, 156, and after induction of anesthesia before removal of the left testicle on Day 220. An additional sample was collected on Day 58 from controls and boars assigned to the 56-day group plus a sample on Day 114 from controls and boars assigned to the 112-day group. Plasma was frozen and subsequently evaluated for concentrations of FSH, luteinizing hormone (LH), testosterone, and estrone by RIA as described previously [13, 22, 23]. For RIA of FSH, antiserum AFP-C5288113 was used in conjunction with iodinated ovine FSH and porcine USDA-B1 as the reference preparation. For RIA of LH, antiserum AFP-151031194 was used with porcine LH (AFP-10714B) for iodination and pLH-B1 as the reference preparation. For the testosterone and estrone RIAs, plasma aliquots were extracted with diethylether and quantified with anti-testosterone-19-conjugate (ICN Biomedicals, Carson, CA) or anti-estrone-6-conjugate (DSL, Webster, TX) in competition with appropriately iodinated steroids. All assays were validated for parallelism with porcine samples; repeatability was monitored by including reference pools of plasma or serum within each assay. Minimum sensitivity of these assays was 40 ng/ml, 4 ng/ml, 50 pg/ml, and 12 pg/ml for FSH, LH, testosterone, and estrone, respectively. Interassay coefficients of variation ranged from 4% to 18% for the four pools in the FSH assay, 11% to 18% for the four pools in the LH assay, 4% to 18% for the three pools in the estrone assay, and was 13% for the pool in the testosterone assay.
Microsatelite Genotypic Data
Microsatelite markers (n = 20) were used to scan the X chromosome [14]. Seven of these markers (current marker order: SW2476, SY11, SW1346, SW259, SO117, SW1426, and SW1943) spanned a 15-cM region that contains a QTL affecting FSH and testicular weight in boars. The most likely position of the QTL is on the q arm of porcine X chromosome located next to SW1426 in the interval between SW1426 and SW1943. However, the gene(s) responsible for the observed variation could actually be positioned anywhere within the interval flanked by SW2476 and SW1943 due to uncertainties inherent in QTL mapping. The breed of origin (MS or WC) for the testicular size QTL was determined by following transmission of alleles for these markers from the purebred parents to the animals in the present study (four generations). Breed of origin of the Y chromosome was assigned from pedigrees based on transmission from father to son.
Statistical Evaluations
Responses to unilateral castration, origin of X chromosome QTL, and origin of Y chromosome were evaluated by mixed model procedures of SAS [24]. The model for testicular and epididymal weight, color of testicular parenchyma, DSP/g, and total DSP included fixed effects of age at unilateral castration, origin of X chromosome QTL, origin of Y chromosome, and interactions of all of these. Sire and sire within origin of Y chromosome were random effects. Age at removal of the left testicle was a covariate. After breed of origin of Y chromosome was determined to have no effect (P > 0.30) on these traits, the Y chromosome was deleted from the model, and sire was the random effect. Treatments were compared to the control with planned contrasts. Linear regression was used to correlate weight of the first testicle to weight of the second testicle, within boar [25]. Testicular weight at 220 days of age was also correlated to epididymal weight by linear regression.
Mixed model procedures were used to evaluate hormone concentrations as repeated measures within sire family. Treatment, origin of X chromosome QTL, origin of Y chromosome, day, and all interactions were the main effects. Origin of Y chromosome had no effect (P > 0.25) on FSH, testosterone, or estrone and was subsequently removed from the model. Effects of treatment were compared to the control by planned contrasts. When the interaction of treatment with day or the interaction of origin of X chromosome QTL with day were significant (P < 0.05), means on specific days were compared by the paired difference procedure of SAS. For estrone, data obtained on Day 1 were not included in the analysis. Newborn piglets contain high concentrations of estrogen of maternal origin, and time of sampling on Day 1 was not standardized to the time of birth. Origin of Y chromosome was significant in analysis of LH data; for this analysis, interactions with an F value less than 1 were deleted from the final model.
RESULTS
Testicular and Epididymal Traits
Mean body weight was 97.2 ± 1.6 kg when the left testicle was removed at approximately 220 days of age, and body weight at this age was not influenced by age of unilateral castration (P > 0.8). Weights of the left testicle and epididymis at 220 days decreased (P < 0.01) with increasing age at unilateral castration (Fig. 1A and Table 1), and epididymal weights were correlated positively with testicular weights (R2 = 0.75, P < 0.01). Neither DSP/g nor parenchymal color of testes were influenced by unilateral castration (P > 0.2, Table 1). However, total DSP was greater in boars that were unilaterally castrated on 1 or 10 days of age (P < 0.01) relative to control boars (Fig. 1B). Notably, control boars had two testicles at 220 days of age; thus, their total testicular weight, epididymal weight, and total DSP were greater (P < 0.01) than observed in any of the unilaterally castrated groups.
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For individual boars, weight of the testicle removed on Day 1, 10, or 56 was not correlated (P > 0.10) with weight of the testicle removed on Day 220 (R2 
0.06). This contrasts with observations in boars that were unilaterally castrated on Day 112 in which weight of right testicle correlated positively with weight of left testicle (R2 = 0.41; P < 0.01). In control boars that were retained to 14.5 mo of age, weight of their remaining testicle increased to 273 ± 26 g and was highly correlated within each boar with weight of the previously removed left testicle (R2 = 0.89; P < 0.01).
Of the 132 boars from 32 litters, 60 inherited the Y chromosome from the MS breed; whereas the other 72 boars inherited their Y chromosome from the WC breed. Origin of the Y chromosome had no effect (P > 0.30) on testicular and epididymal traits. For the X chromosome QTL, 71 boars had MS alleles and 61 had WC alleles; alleles from both breeds were present in littermate boars of 14 litters; 8 litters contained boars with only the WC alleles, and 10 litters had boars with only MS alleles. The effect of MS alleles for the X chromosome QTL was not significant for testicular weight at the time of unilateral castration, Day 1, 10, 56, or 112 (P > 0.30). However, at 220 days of age, boars that inherited their X chromosome alleles from the MS breed had smaller testicles (P < 0.01) than boars that received this region of X chromosome from the WC breed (Fig. 1A). Similarly, boars with MS alleles had reduced total DSP, smaller epididymal weight, and darker parenchymal color (P < 0.01) than boars with the WC alleles (Fig. 1B and Table 1).
Endocrine Profiles
Plasma concentrations of FSH, LH, testosterone, and estrone were characterized in all boars by increases during early postnatal life followed by reductions and subsequent pubertal increases (Figs. 2, 3, and 4; P < 0.01 for effect of day). Origin of Y chromosome had no effect (P > 0.25) on FSH, testosterone, or estrone. There were no interactions of age at unilateral castration with breed of origin of X chromosome QTL or with breed of origin of Y chromosome (P > 0.20) for plasma FSH, testosterone, or estrone.
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Plasma FSH concentrations were influenced by day of unilateral castration (P < 0.05), and the interaction of day of unilateral castration with day of age (P < 0.01; Fig. 2). Unilateral castration on Days 1, 10, and 56 increased (P < 0.05) plasma FSH acutely (Table 2). Sustained and maximal increases occurred earlier when unilateral castration was conducted on Day 1 or Day 10 than when conducted on Day 56 or 112. In boars unilaterally castrated on Day 1, plasma FSH was greater (P < 0.05) than in controls on Days 72–156 (Fig. 2A). In boars unilaterally castrated on Day 10, FSH was greater (P < 0.05) than in controls on Days 42–156. In comparison, boars that were unilaterally castrated on Day 56 had FSH concentrations greater than controls on Days 93–156 (P < 0.05; Fig. 2B). Moreover, boars unilaterally castrated on Day 112 were similar to controls except after Day 114 when their plasma concentrations were greater (P < 0.05; Fig. 2B). Origin of the X chromosome QTL also affected plasma FSH but only after 42 days of age (Fig. 3A). Boars with MS alleles had greater plasma FSH concentrations (P < 0.05; Fig. 3A) than boars with WC alleles on Days 42–220. A significant pubertal increase in FSH was observed by Day 56 in boars with MS alleles but not until Day 72 in boars with WC alleles.
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Plasma LH concentrations were influenced by both breed of origin of X chromosome QTL and of Y chromosome (P < 0.05), but there was no interaction between these two genetic effects (P > 0.40). Similar to FSH, a pubertal increase in plasma LH was observed in boars with either X chromosome QTL, but the increase was greater (P < 0.05) in boars with MS alleles than with WC alleles (Fig. 3B). Effect of breed of origin of Y chromosome on LH concentrations did not interact with age (P > 0.40). Across all ages, difference due to breed of origin of Y chromosome was 14% (0.96 ± 0.04 ng/ml in MS versus 0.84 ± 0.03 ng/ml in WC boars; P < 0.05). After the pubertal increase in plasma LH, Days 93–220, boars with the MS Y chromosome had 19% greater plasma LH concentrations than boars with the WC Y chromosome (1.06 ± 0.07 versus 0.89 ± 0.05 ng/ml). During this same time, boars with MS alleles for the X chromosome QTL had 33% greater plasma LH than boars with WC alleles (1.11 ± 0.06 versus 0.84 ± 0.07 ng/ml; P < 0.05). Pubertal LH when summarized based on the combined effects of both X and Y chromosomes revealed the additive effects of the MS breed. Boars that received both regions from MS had greater (P < 0.01) pubertal LH concentrations (1.19 ± 0.07 ng/ml, n = 40) than boars that received both regions from WC the breed (0.74 ± 0.07, n = 41). Boars that received either the X chromosome QTL or the Y chromosome from MS and the other of these two regions from WC were intermediate (0.99 ng/ml; n = 51).
Plasma concentrations of testosterone and estrone were not affected by unilateral castration at any age (P > 0.25; data not presented). Plasma testosterone (P > 0.06) and estrone (P > 0.09) were not influenced by breed of origin of the X chromosome QTL (Fig. 4).
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DISCUSSION
The current study clearly documents that testicular weight and endocrine profiles of this population of boars depends substantially upon alleles from the MS breed that map to the X chromosome [14]. Boars with MS alleles had smaller testicles than those with the WC alleles and had an 11% reduction (P > 0.07) in sperm production per g of testis (DSP/g). Combining this slightly reduced DSP/g with significantly lower testicular weights produced a 42% reduction in total DSP. Breed of origin of the Y chromosome had no effect on any of the testicular traits investigated. Based on the findings of Zanella et al. [13], MS x WC crossbred boars with small testicles have a disproportionate reduction in volume of seminiferous tubules compared with boars with large testicles. This population of crossbred boars with an identifiable QTL for testicular size provides unique resources to investigate mechanisms that affect Sertoli cell proliferation and ultimate testicular development. The ability to select, early in life, boars with the greatest potential for sperm production would reduce costs associated with artificial insemination which, in the United States, has increased from <2% of the female pigs mated in 1990 to >65% in 2000. In boars, testicular size continues to increase beyond 7 mo of age. Nevertheless, the high correlation of testicular weight at 7 mo with that at 14.5 mo substantiates that observed differences in testicular weight due to the X chromosome QTL were not the result of variation in rate of pubertal development.
In mice, Chubb [5] determined that few genes had major effects on testicular weight; at least one of these is located on the Y chromosome [15]. These later investigators observed effects of the X chromosome on testicular size early in life, but the X chromosome effects became minor in postpubertal males. Conversely, a region of the X chromosome that greatly influences testicular size in mice was identified in recent genome mapping studies [26]. Additionally, 10 male-specific genes that are expressed in mitotic stages of spermatogenesis in mice were cataloged on mouse and human X chromosomes, confirming the substantial conserved synteny between these two species for this chromosome [27, 28]. Thus, understanding of the mouse X chromosome has advanced it to share a significant role with the Y chromosome in determination of testicular size and sperm production. This should also be the case in swine because there is minimal exchange or modification of genes on the X chromosome across mammals [28].
As predicted, boars with MS alleles for the X chromosome QTL had greater pubertal increases in plasma FSH; pubertal FSH was the determining phenotype for this QTL [14]. Concentrations of LH during pubertal development also varied synchronously with FSH. Similarly, straight-bred MS boars have greater plasma concentrations of FSH and LH than straight-bred WC boars [23, 29]. Furthermore, MS x WC boars with high FSH concentrations have greater thyroid-stimulating hormone (TSH) concentrations than boars with low FSH concentrations (unpublished results). Genes that reside on the X chromosome and could account for such differences in secretion of pituitary glycoprotein hormones remain unknown. Androgen receptor maps near this region of porcine X chromosome [30], and in humans, mutations in the structure of the gene encoding androgen receptor can be associated with small testicular size, low sperm production, and elevated gonadotropin secretion [31, 32]. However, to date we have been unable to identify mutations in porcine androgen receptor that parallel the conditions described in humans.
Boars with MS alleles for the X chromosome QTL had similar plasma concentrations of estrone and testosterone as boars with WC alleles. This contrasts straight-bred MS boars that have much greater testosterone concentrations than straight-bred WC boars [23, 29]. Of particular note, in boars with MS alleles for the X chromosome QTL and/or with the MS Y chromosome, greater LH concentrations were not accompanied by substantial increases in testosterone. Also, MS x WC crossbred boars with greater concentrations of TSH had similar plasma concentrations of T3 and T4 (unpublished results). These findings point to the possibility of reduced biological activity of LH and TSH in boars with MS alleles. Altered patterns of glycosylation alter biological activity of pituitary glycoprotein hormones and may account for decreased potency [33, 34]. The temporal relationship of LH with testosterone secretion, due to pulsatile secretion of each, is missed with single blood samples obtained at specific ages. This issue plus additive effects of MS alleles for the X chromosome QTL with the MS Y chromosome must be examined in greater detail with frequent sampling. Boars selected for differences in pubertal FSH concentrations had significant differences in plasma LH and testosterone when blood was collected at 20-min intervals for 2 h [13].
The magnitude of the increase in testicular weight and total DSP in boars that were unilaterally castrated on Day 1 or 10 was less than expected. The hypothesis under investigation was that compensatory hypertrophy would produce a doubling of total DSP due to increased FSH secretion during the period of Sertoli cell proliferation [7, 10, 11, 35]. Unilateral castration on Day 1 or 10 caused the predicted increases in FSH secretion; however, at 220 days of age, the remaining testicle of these boars had only 71% of total DSP as two testicles of control boars. Furthermore, this lack of complete compensation occurred in boars with either X chromosome QTL. At the end of pregnancy and during the first 3 wk of life, seminiferous tubules in boars experience their greatest rate of increase in length [36]. Kosco et al. [35] reported a doubling of seminiferous tubule mass and length as well as Sertoli cell mass 85 days after unilateral castration of boars at 10 days of age. These investigators did not enumerate their observed changes into number and volume of Sertoli cells nor did they examine boars after pubertal development. In a subsequent study, daily treatment with exogenous FSH from 8 to 40 days of age failed to increase testicular weight or length of seminiferous tubules at 100 days [37], but again number of Sertoli cells was not determined nor were treated boars evaluated after puberty. Boars in the current study with X chromosome alleles from MS or WC breeds had similar patterns of FSH secretion during their first 3 wk of life but differed greatly in testicular size at 220 days of age. Straight-bred MS boars and straight-bred WC boars have similar testicular size and number of Sertoli cells at 1 day of age [10]. However, as adults, MS boars have fewer Sertoli cells and smaller testicles than WC boars [38]. Thus, some component other than elevated FSH secretion dictates magnitude of Sertoli cell proliferation during neonatal development. Intratesticular regulation of activin and follistatin seem likely candidates for a role in determining testicular size [39], although the gene for neither of these is located on the X chromosome.
The increase in testicular size of boars unilaterally castrated on Day 56 or 112 in the absence of an effect on total DSP likely reflects compensatory response within the Leydig cells. After unilateral castration at any age, plasma estrone and testosterone concentrations remained constant, indicative of an immediate increase in testicular steroidogenesis within the remaining testicle. These compensatory increases within the interstial compartment in response to unilateral castration have been described in detail [40].
An increase in plasma FSH after unilateral castration on Day 1 or 10 supports previously reported negative feedback regulation of FSH secretion [41]. This differs from ineffective negative feedback regulation of LH secretion during this period [41–43]. With increasing age, MS alleles for the X chromosome QTL identified boars with greater concentrations of FSH and LH and smaller testicles of darker color. Higher concentrations of iron, much of which is bound to ferritin, form the basis for this darker color [44]. A hypothesis that accounts for the array of correlated traits all associated with this X chromosome QTL currently is not obvious. However, endocrine evaluations combined with identification of genomic regions that account for observed differences provide an additional methodology to expand current understanding of testicular development.
Additional studies must determine if this same region of X chromosome encodes testicular size in genetic lines of pigs others than those developed from MS breeding. In straight-bred WC boars, elevated plasma FSH concentrations occur in boars with small testicles [12]. However, correlation of these two phenotypic traits may be regulated by regions of the porcine genome other than those currently found on the X chromosome. Identification of major genes involved in regulation of testicular development will allow boars to be selected early in life based on their genetic potential for greater sperm production.
ACKNOWLEDGMENTS
The authors thank D. Griess, S. Hassler, M. Judy, A. Kruger, S. Parr, and K. Simmerman for technical assistance and U.S. MARC Swine Personnel for care and assistance with pigs. We thank U.S. Department of Agriculture/National Institute for Diabetes and Digestive and Kidney Diseases for FSH antisera, FSH and LH reference preparations, and ovine FSH for iodination; and Dr. A.F. Parlow for LH antisera and porcine LH for iodination.
FOOTNOTES
1 Names are necessary to report factually on available data; however, the U.S. Department of Agriculture (USDA) neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable. 
2 Correspondence: J.J. Ford, USDA, ARS, RLH US Meat Animal Research
Center, P.O. Box 166, State Spur 18D, Clay Center, NE 68933.
FAX: 402 762 4382; ford{at}email.marc.usda.gov
Accepted: April 30, 2001.
Received: March 5, 2001.
REFERENCES
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