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GENETICS |
Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg 24061-0306
1 Corresponding author: ewong{at}vt.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key Words: nutrient transporter • gene expression • chicken • intestine
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Selection for 56-d BW led to a correlated response of feed intake. The HWS exhibits hyperphagia, whereas the LWS exhibits hypophagia with a certain percentage of the population exhibiting anorexia (Dunnington and Siegel, 1996), with 5 to 20% mortality during the first week of life (Noble et al., 1993; Liu et al., 1995). O’Sullivan et al. (1992) reported that by d 5 posthatch, the ad libitum feed consumption of the HWS chicks was 122% of that for the LWS chicks, and by d 10 and 21 posthatch, the feed consumption of the HWS chicks was 189 and 279% of that of the LWS chicks. The physiological mechanisms causing the changes in eating behavior remain to be fully elucidated.
There are differences in feed efficiency between the HWS and LWS. Lepore et al. (1963) revealed that HWS embryos were more efficient than LWS embryos in utilizing energy and certain amino acids, particularly the sulfur-containing amino acids. The HWS embryos are larger, yet the relative weight of the residual yolk sac (g/100 g of BW) is greater in the HWS at hatch, indicating that the HWS embryos are more efficient (Nitsan et al., 1991). The difference in relative residual yolk sac weight between the lines is gone by d 3, which may be due to the LWS using the yolk sac as a primary source of energy and nutrient intake to compensate for their hypophagia (Siegel and Wisman, 1966). Owens et al. (1971) and Barbato et al. (1983a, b) found that the HWS had a better feed efficiency than the LWS. Increased oxygen consumption (Owens et al., 1971), increased rate of feed passage (Cherry and Siegel, 1978), improved temperature regulation (Dunnington and Siegel, 1984), and superior intestinal glucose absorption capabilities (Walker et al., 1981) in the HWS are linked to the better feed efficiency observed in the HWS. Further, the small intestine is larger per gram of BW in HWS than LWS (Katanbaf et al., 1988). This indicates that the HWS chickens may have a larger capacity to absorb nutrients than LWS chickens.
Although many studies have evaluated several correlated responses to selection in these lines at the animal, organ, and tissue level, there has been a dearth of research examining the correlated responses to selection at the molecular level. In the present study, the correlated response to selection in the developmental gene expression of the brush border membrane nutrient transporters was evaluated in progeny from the 48th generation of the selected lines. The transporters examined were the peptide transporter, PepT1; the glutamate-aspartate transporter, EAAT3; the glucose transporter, SGLT1; and the fructose transporter, GLUT5.
Nutrients are transported into enterocytes by transporters located in the brush border membrane. Glucose is transported from the lumen of the small intestine across the brush border membrane into the enterocyte primarily by the sodium-dependent glucose transporter, SGLT1 (Hediger and Rhoads, 1994; Wright and Turk, 2004). The facilitative glucose transporter, GLUT5, mediates the passive transport of fructose into enterocytes (Kayano et al., 1990; Gould et al., 1991; Burant et al., 1992). Amino acids are transported as free amino acids or small peptides by a variety of amino acid transporters or the peptide transporter, PepT1, respectively. Proteins are broken down into peptides and free amino acids by enzymes in the small intestine. Free glutamate and aspartate are transported across the brush border membrane of the enterocytes by the excitatory amino acid transporter, EAAT3 (Kanai and Hediger, 1992). This transporter is important to enterocytes, because glutamate is a primary fuel for these cells (Newsholme et al., 2003) and may play a significant role in the metabolic processes of other cells (Kanai and Hediger, 1992). The di- and tripeptides are transported across the brush border membrane by the proton-dependent peptide transporter, PepT1 (Fei et al., 1994).
Regulation and expression of these specific nutrient transport systems in the brush border membrane therefore affect the nutrient and energy availability to the animal for growth and development. Therefore, our first objective was to determine the effect of sex on the gene expression of these nutrient transporters in the small intestine of these divergently selected lines of chickens. Our second objective was to examine the correlated response to selection in the gene expression of these nutrient transporters in the small intestine of the selected lines and their reciprocal crosses.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Pooled semen was used to inseminate females from the 48th generation of LWS and HWS to obtain HWS x HWS (HH), LWS x LWS (LL), and their reciprocal crosses, LWS males x HWS females (LH) and HWS males x LWS females (HL). Eggs from the 4 mating combinations were incubated for 21 d. On day of hatch (DOH, without access to feed and water) chicks were wing-banded for identification and placed in batteries with 10 to 15 chicks per pen under continuous light. Chicks had ad libitum access to water and a corn-soybean mash diet (20% crude protein and 2,685 kcal of ME/kg), on which these lines of chickens have been maintained since the initiation of the long-term genetic selection experiment.
Tissue Collection
Tissues were collected on embryonic d 20 (E20), DOH, and d 3, 7 (D7), and 14 (D14) posthatch. On E20, 12 to 16 eggs per mating combination (MC) were weighed and embryos were removed. A liver sample to obtain DNA for chick sexing was rinsed in ice-cold PBS and frozen on dry ice. The small intestine was separated into duodenum (portion extending from gizzard to end of duodenal loop), jejunum (portion from the end of the duodenal loop to Meckel’s diverticulum), and ileum (portion from Meckel’s diverticulum to the ileal-cecal junction). All segments were rinsed in ice-cold PBS and minced. One 20- to 30-mg tissue aliquot was placed in a microfuge tube and frozen on dry ice and stored at – 80°C.
Chick Sexing by PCR
The chicks were sexed using PCR to amplify a W chromosome-specific fragment to identify females (R. Okimoto, Cobb-Vantress Inc., Siloam Springs, AR, personal communication). Deoxyribonucleic acid was extracted from liver samples using the DNeasy kit (Qiagen, Valencia, CA) according to the animal tissue protocol and quantified spectrophotometrically using optical density at 260 nm. A PCR reaction was performed using primers (MWG, Charlotte, NC) to amplify a 1,200-bp amplicon of the W chromosome (forward primer: 5'-CTGTGATAGAGACCGCTGTGC-3' and reverse primer: 5'-CAACGCTGACACTTCCGAT-GT-3') to identify females and a 376-bp amplicon of the PepT1 gene (forward primer: 5'-TTGTCTCCCTGTC-CATTGTCTATAC-3' and reverse primer: 5'-GTTCT-TCAACTGATCCCCACCAAA-3') as a positive reaction control. The PCR reaction used the following program: 2 min at 94°C followed by 35 cycles of 30 s at 94°C, 1 min at 55°C, and 1 min at 72°C and ended with a final extension step of 7 min at 72°C. The PCR product was run out on a 1% agarose gel. Females were identified by the presence of 2 bands, one corresponding to the 1,200-bp amplicon of the W chromosome and one corresponding to the 376-bp amplicon of PepT1. Males were identified by the presence of only the 376-bp band.
Total RNA Extraction and Reverse Transcription
Sample sizes were 4 males from all 4 mating combinations at all time points, except E20 HL (n = 3) for which there was only 3 male HL chicks in the group sampled, and 4 females from HH and LL at all time points, except D7 LL (n = 2) for which there was only 2 female LL chicks in the group sampled. Total RNA was extracted using the RNeasy Kit (Qiagen) according to the animal tissue protocol using a homogenizer. Ribonucleic acid purity and concentration were determined spectrophotometrically using optical densities at 260 and 280 nm. Ribonucleic acid integrity was determined by gel electrophoresis on 1% agarose-formaldehyde gels. Extracted RNA was stored at –80°C. Then, cDNA was synthesized from total RNA using the high-capacity cDNA archive kit (Applied Biosystems, Foster City, CA).
Quantitative Real-Time PCR
Quantitative real-time PCR was conducted using a 7300 Real-Time PCR System (Applied Biosystems) in a 96-well plate format. Each reaction contained the following: 2 µL of cDNA (diluted 1 to 30), 12.5 µL of 2X SYBR Green Master Mix (Applied Biosystems), 0.5 µL each of the forward primer (5 µM) and reverse primer (5 µM), and 9.5 µL of nuclease-free water. The following real-time PCR reaction was run: 95°C for 10 min followed by 40 cycles of 95°C for 15 s and 60°C for 1 min. Genes analyzed were PepT1, EAAT3, GLUT5, and SGLT1. The endogenous control was glyceraldehyde-3-phosphate dehydrogenase. Primer sequences are listed in Table 1
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Two independent real-time PCR experiments were conducted using the same cDNA samples. The cDNA from the male and female HH and LL chicks at all ages were used for the first experiment. The cDNA from the male LL, LH, HL, and HH chicks at all ages were used for the second experiment.
All plates were analyzed individually using the software provided with the 7300 Real-Time PCR system using the Auto function. Average gene expression relative to the endogenous control for each sample was calculated using the 2–
CT method (Livak and Schmittgen, 2001). The CT value for glyceraldehyde-3-phosphate dehydrogenase varied by approximately 1 cycle across MC, intestinal segment, sex, and time point and therefore was considered to be an appropriate endogenous control. The calibrator for each gene in both experiments was the average CT value of male E20 LL duodenum.
Statistical Analysis
Data were analyzed using the PROC MIXED procedure of SAS Institute (Cary, NC). The model included main effects of MC, age, sire line, sex, intestinal segment, and all appropriate 2- and 3-way interactions. Significant MC and intestinal segment differences were further evaluated using pairwise linear contrasts. The main effects of age were further tested for linear, quadratic, and cubic responses using orthogonal contrasts in PROC MIXED. The contrast coefficient matrix for unequal spacing was generated using PROC IML.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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In comparing male and female HH and LL chicks, eggs from HH were heavier than eggs from LL (P < 0.001), with no difference between males and females in either MC, with HH eggs averaging 34.1 ± 1.0 g and LL eggs averaging 27.0 ± 1.0 g. Male and female chicks from HH were heavier throughout the experiment and grew faster than chicks from LL (P < 0.001), with no difference between males and females in either MC (Figure 1A). By D14, there was approximately a 4.5-fold difference in BW between HH and LL birds.
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). Chicks from HH were heaviest and grew fastest, while LL chicks were lightest and grew the slowest. The BW on DOH of reciprocal crosses reflected the BW of the dam line; however, by D14, the HL and LH had BW that were intermediate of the HH and LL (P < 0.001). On D14, there was a 5.4-fold difference in BW between males of the HH and LL mating combinations. Effect of Sex on Nutrient Transporter Gene Expression
For the amino acid transporter, EAAT3, females had approximately 2-fold greater gene expression than the males (P < 0.05; Table 2). For the peptide transporter, PepT1, LL had approximately 6-fold greater PepT1 gene expression than HH (P < 0.001; Table 2). There was a MC x age x sex interaction (P < 0.001) for PepT1 gene expression (Figure 2). Females had peak PepT1 gene expression 7 d earlier than males for both LL and HH. For LL females, peak expression occurred on DOH, whereas for LL males, peak expression occurred on D7 (Figure 2A). For HH females, peak expression occurred on D7, whereas for HH males, peak expression occurred on D14 (Figure 2B). For both males and females, peak expression in HH birds occurred 7 d later than in LL birds.
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). There was a MC x intestinal segment interaction for SGLT1 gene expression (P < 0.001; Figure 3). Regardless of MC, females induced SGLT1 gene expression on DOH and maintained a high level through D14. In contrast, males gradually increased SGLT1 gene expression through D7 and then decreased expression by D14. There was no effect of sex or MC on GLUT5 gene expression (Table 2).
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There was an effect of MC on PepT1 gene expression (P < 0.001; Table 3), with greatest expression in LL, intermediate expression in LH and HL, and least expression in HH. The greatest gene expression of PepT1 was seen in the ileum (P < 0.001; Table 3). Gene expression of PepT1 in the duodenum, jejunum, and ileum is shown for male LL, LH, HL, and HH chicks in Figure 4A. Gene expression of PepT1 increased from duodenum to ileum in LL, with no segment difference in any other MC (P < 0.001). There was also an effect of sire line on PepT1 gene expression, with chicks from LWS sires (LL and LH) having greater expression than chicks from HWS (HH and HL) sires (P < 0.001; Table 3). There were no differences among intestinal segments in progeny from HWS sires; however, the greatest PepT1 gene expression was seen in the ileum of progeny from LWS sires (P < 0.001; Figure 5).
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). In all MC except HH, EAAT3 gene expression increased from duodenum to ileum (P < 0.001; Figure 4B).
Expression of SGLT1 was developmentally regulated with an increase in expression through D7 and then a decrease by D14 (P < 0.001; Table 3). Overall, the greatest expression of SGLT1 was in the jejunum and ileum (P < 0.001; Table 3). There was an intestinal segment x MC interaction with gene expression of SGLT1 being greatest in the jejunum and ileum in LL, LH, and HL but greatest in the jejunum of HH (P < 0.05; Figure 4C).
The gene expression of GLUT5 increased linearly with age (P < 0.001; Table 3). The greatest gene expression of GLUT5 was in the jejunum and ileum (P < 0.001; Table 3). Gene expression of GLUT5 in the duodenum, jejunum, and ileum of male LL, LH, HL, and HH chicks is shown in Figure 4D. There was a MC x intestinal segment interaction (P < 0.05). Gene expression was greatest in the jejunum and ileum of LL, LH, and HL chicks, whereas expression was greatest in the jejunum of HH chicks.
The profiles of gene expression within the ileum for all 4 genes were similar (Figure 4). The LL had greater expression than HH, and the reciprocal crosses had intermediate levels of gene expression.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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There was a difference in the time of peak expression between LL and HH birds. In LL, PepT1 gene expression was induced on DOH in both sexes, with DOH being the peak expression for the females. This peak in PepT1 gene expression is similar to previous studies in chickens (Chen et al., 2005; Gilbert et al., 2007). In contrast, no increase in PepT1 gene expression was seen in HH until D7 in females and D14 in males. The induction of PepT1 gene expression in LL, which was absent in HH, before feed intake suggests that genetic selection has altered the regulatory factors for PepT1 gene expression. The earlier induction in LL as well as overall greater PepT1 gene expression may be necessary to maximize nitrogen assimilation for survival of these chicks, whereas delayed and overall less PepT1 gene expression in HH chicks may be reflective of their greater feed efficiency (O’Sullivan et al., 1992).
There were also differences in nutrient transporter expression between males and females in chickens. Lacking are published data suggesting a difference between males and females of any species in the developmental gene expression of PepT1, EAAT3, and SGLT1 in the small intestine. Females had an earlier peak in PepT1 gene expression, and we observed a 7-d difference in peak expression between males and females in each line. Females are known to be more metabolically efficient (Frisch, 2002) in an effort to prepare for reproduction. Earlier expression of PepT1 may be reflective of this improved metabolic efficiency or an earlier need for nutrients. Lu and Klaassen (2006) reported no sexual dimorphism in the level of PepT1 gene expression in the small intestine of rats and mice. They used 8-wk-old rats and analyzed PepT1 expression at a single time point. Because of the experimental design, they may have not detected a difference in PepT1 gene expression between males and females simply because the rodents were too old.
Females of both lines had a greater overall SGLT1 gene expression and an earlier induction of SGLT1 gene expression than males. Female turkeys and broiler-type chickens have an average of a 7.7% less energy requirement than male turkeys and broiler-type chickens (Shalev and Pasternak, 1998). If greater SGLT1 gene expression in females ultimately correlates with greater glucose absorption, then the earlier and greater SGLT1 gene expression in female chickens may begin to explain, at the molecular level, a reason for the difference in energy requirements of male and female chickens and turkeys. The upregulation and earlier expression of SGLT1 in females may also reflect a need for females to be more metabolically efficient to reach the minimum BW and fat percentage needed to achieve sexual maturity in humans (Frisch, 2002) and chickens (Zelenka et al., 1987) at the same time that the males become sexually mature. A positive energy balance is needed to shift energy expenditures from growth to reproduction (i.e., increased fat deposition). Therefore, having a greater capacity to assimilate glucose, the primary fuel of the body, early would facilitate the generation of energy stores and thus a positive energy balance earlier in life.
Sex-specific gene expression may be controlled by the sex steroids. Renal SGLT1 gene expression, protein expression, and transport capacity are greater in both intact and ovarectomized female rats compared with intact or castrated male rats (Sabolic et al., 2006). Furthermore, castrated male rats had greater renal SGLT1 gene expression, protein expression, and transport capacity than intact male rats, indicating a negative effect of androgens. There was no difference in renal SGLT1 gene expression between the intact and ovarectomized females, indicating that progesterone and estrogen did not affect renal SGLT1 expression. The rat SGLT1 gene sequence has 2 androgen response elements (Sabolic et al., 2006). If the chicken also has an androgen response element in the SGLT1 gene, perhaps androgens in male chickens could cause a down-regulation of the SGLT1 gene in all tissues where it is expressed, in particular the small intestine.
Females had greater expression of EAAT3, the transporter for glutamate, which is the primary fuel source for enterocytes. To facilitate greater gene expression of nutrient transporters, such as SGLT1 and PepT1 and possibly growth and development of the small intestine, females may upregulate gene expression of EAAT3, which may lead to increased protein expression and increased glutamate-aspartate assimilation.
Mating combination had a profound effect on the expression profiles of the nutrient transporters. The LL chicks had greater PepT1 gene expression than HH chicks, with greatest levels in the ileum. In interpreting these results, differences in food intake between the mating combinations must be considered. The LL chicks exhibit hypophagia, whereas HH chicks exhibit hyperphagia. This hypophagia in LL leads to a suboptimal nutrient intake, which can affect their growth. The pattern of increased PepT1 gene expression seen in LL is comparable to PepT1 expression observed in starvation and decreased feed intake studies conducted in rats (Ihara et al., 2000; Naruhashi, et al., 2002; Howard et al., 2004). Ihara et al. (2000) reported an increase in both mRNA and protein expression of PepT1 in response to starvation, decreased feed intake, and total parenteral nutrition treatment. Naruhashi et al. (2002) and Howard et al. (2004) observed increased PepT1 mRNA expression in starved rats, with greatest levels in the distal small intestine. Naruhashi et al. (2002) further demonstrated that activity levels of PepT1, as measured by cefadroxil transport, were greatest in the proximal intestine and, despite greater PepT1 gene expression levels, lowest in the mid and distal intestine. They suggested that the increase in distal PepT1 gene expression was a compensatory mechanism used to counteract the increased pH of the distal small intestine, which hinders PepT1 activity by affecting the proton gradient. They concluded that the increased distal PepT1 gene expression would allow the nitrogen-starved animal to maximize its nitrogen absorption. An unstirred water layer, which is maintained by mucins (Smithson et al., 1981), surrounds the luminal side of the brush border membrane and helps maintain a constant pH (Shiau et al., 1985). Smirnov et al. (2004) demonstrated in chickens that starvation depletes this mucin barrier, particularly in the ileum. If this barrier is responsible in part for maintaining the unstirred water layer and thus pH of the environment surrounding PepT1, mucin barrier depletion may alter the activity of the transporter through disruption of the environment that normally allows maximal activity. Thus, an increase in gene expression of PepT1 in the distal portion of the small intestine may be a mechanism to allow maximal nitrogen absorption by compensating for decreased transport activity due to an increase in pH.
Shimakura et al. (2006) reported that increases in PepT1 gene expression and protein levels in response to fasting were mediated by the peroxisome proliferator-activated receptor 
(PPARa). The PPAR is a nuclear hormone receptor that plays a vital role in the adaptive response to starvation in the liver as well as other tissues. Its function is to aid in regulating fatty acid metabolism, which helps the body switch from metabolizing carbohydrates and fats in the fed state to only fat in the starved state (Kersten et al., 1999). The finding that regulation of PepT1 expression, in particular in the fasted state, by PPAR provides more evidence that upregulation of PepT1 gene expression is an adaptive response to suboptimal feed intake.
Despite a maternal line effect, as demonstrated by egg size, between LL and HH chickens, there was a sire line effect in the gene expression of PepT1. Although the reason for the upregulation in the progeny from L sires is not known, it may be an adaptive response to maximize the survival of both sexes as both males and females inherit a copy of the Z chromosome from the sire. The PepT1 gene is located on chicken chromosome 1, and it is possible that LL males contain a modified gene on the Z chromosome that is involved in upregulating PepT1 gene expression.
Expression of the EAAT3 gene was greatest in the ileum, which is consistent with the findings reported by Iwanaga et al. (2005) and Gilbert et al. (2007). They also found the greatest levels of EAAT3 gene expression as well as protein in the ileum and concluded that most glutamate transport occurs in the ileum. That EAAT3 gene expression was greater in the LL ileum than the HH ileum is consistent with the findings of Howard et al. (2004) of an increase in ileal gene expression of EAAT3 in starved rats. Therefore, the MC x segment interaction is biologically significant with respect to adaptations to decreased feed intake. Like the MC difference seen in PepT1, this difference in EAAT3 gene expression may be an adaptive response to a decreased feed intake in LL, thus maximizing assimilation of glutamate, which is the primary fuel for the enterocytes (Newsholme et al., 2003).
There was a linear increase in GLUT5 gene expression despite a lack of fructose in the feed. The increase in GLUT5 expression may indicate that this expression is a genetically hard-wired event that would maximize nutrient absorption in wild birds, which have access to fructose-containing foods such as fruit. This demonstrates that although the domesticated chicken has undergone intense selection pressures, there still remain genetically hard-wired events, which are a throwback to its jungle fowl ancestor. Because GLUT5 has a low efficiency of glucose transport (Matthews, 2000), increasing gene expression of the transporter may ultimately allow for more GLUT5 protein to be made and thus maximize total glucose transport. It is also possible that sugars, such as sucrose, in the soybean and corn as well as other dietary ingredients may be broken down into glucose as well as fructose by enzymes, such as sucrase. Fructose would then cause substrate-induced upregulation of GLUT5 similar to what is seen in mice and rats (Ferraris, 2001).
Interestingly, MC influenced ileal expression of PepT1, EAAT3, SGLT1, and GLUT5 in a similar fashion. The LL had the greatest gene expression and HH had the lowest, with LH and HL being intermediate. This apparent additive genetic effect may point to a correlated response to selection in these mating combinations with respect to the ability of the small intestine, in particular, the ileum, to upregulate gene expression of nutrient transporters. In particular, there may be a gene in LL that causes upregulation of these genes or conversely a gene in HH that causes downregulation of these genes. If the former is true, the greatest gene expression of these nutrient transporters is seen in LL chicks because they have 2 copies of this upregulating gene, intermediate expression is seen in LH and HL chicks because they have 1 copy of this upregulating gene, and HH chicks would have the least expression because they do not have any copies of this upregulating gene. The reverse would hold true if the HH had a gene that caused a downregulation of these nutrient transporter genes.
Another possibility is that the MC effect for gene expression in the ileum is an adaptation to food intake and thus nutrient availability. The ileum is the last segment of the small intestine in chickens; therefore, it is the last place where any appreciable nutrient absorption can occur, because chickens do not possess much of a large intestine. Upregulation of nutrient transporter gene expression in the ileum may be a mechanism to maximize nutrient absorption in situations of decreased nutrient availability (i.e., the LL chicks).
Results from this experiment demonstrate that expression of nutrient transporters in males and female chicks is not the same during the first 2 wk posthatch, a critical period in the maturation of the thermoregulatory and immunological systems, and thus has implications for the poultry industry. Altering the nutritional guidelines to compensate for these differences may allow for a larger or faster-maturing female. These results also reveal differences in expression of nutrient transporters between chickens that have been selected for high or low juvenile BW. It is important to note, however, that only mRNA levels were examined and not protein levels. Further investigation into the transporter proteins and their functionality would be valuable to understand the nutrient absorption capacity of these selected lines.
Received for publication March 4, 2008. Accepted for publication June 17, 2008.
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