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GENETICS |
,2
* Department of Poultry Science, College of Agriculture and Life Sciences, North Carolina State University, Raleigh 27695-7608; and Department of Animal Sciences, The Ohio State University, Ohio Agriculture and Development Center, Wooster 44691
2 Corresponding author: velleman.1{at}osu.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key Words: turkey • heart weight • heart rate • myocardial energy metabolism • embryo survival
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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It was hypothesized that genetic selection of modern turkey sire and dam lines for economically important traits may have affected embryo heart growth or the development of cardiac function during the plateau stage in embryo development. Knowledge of changes in heart functions that have taken place following long-term selection may assist in improving the level of poult embryo survival during the plateau stage which occurs primarily between 24 and 28 d of embryo development (Christensen et al., 1993).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Another randombred control line (RBC2), the base population of the F line, was started in 1966 from reciprocal crosses of 2 commercial strains that were representative of commercial turkeys at that time (Velleman and Nestor, 2004), and has been maintained without intentional selection using a paired mating system. The F line was started as a subsample of the RBC2 line in 1966 and was selected only for increased 16-wk BW over the past 38 yr, and has been maintained using procedures previously reported (Nestor, 1977). The pureline and reciprocal cross embryos and neonates examined in the current study were from the 43rd and 44th generations of selection for the F line.
In each of 2 yr, approximately 400-d-old poults from the Ohio State University were delivered to the North Carolina State University Turkey Educational Unit. The poults were brooded and grown using standard procedures in each year. At 37 wk of age, the hens were moved to a curtain-sided house and hens from each of the 4 lines (E, RBC1, F, and RBC2) were distributed randomly by line to each of 12 pens of 6 birds each. The hens were exposed to long days of 15.5 h of light per day to stimulate egg production. Approximately 15 toms from each line were moved to a totally enclosed environmental house located near the hen house and were distributed to 3 pens with 5 birds each. The toms were exposed to 14 h of light per day to stimulate semen production.
When the first eggs were produced, half of the hens for the E and F lines were artificially inseminated with pooled semen from sires of the same line to reproduce the pure lines. The remaining hens in each of the 2 selected lines were inseminated with semen from the randombred control line from which the selected line had been derived. Likewise, hens from the RBC1 and RBC2 lines were inseminated with semen from the same lines and semen from the corresponding selected line. Weekly inseminations were performed for a 20-wk experimental period. Embryo survival data were recorded at weekly intervals by examining eggs set in the cabinets using standard incubation conditions (Christensen et al., 2007a).
At 16, 17, and 18 wk of production, embryo survival data were not collected, but embryos from the pure and reciprocal crosses were sampled for cardiac growth and physiologic studies. In each of 3 trials within each year, 3 embryos (25, 26, or 27 d of development) or hatched poults (28 d of development) were sampled as described previously (Christensen et al., 2007b). Sampled embryos were selected randomly from each treatment at 25, 26, 27, and 28 d of development using the morphological criteria of Hamburger and Oppenheim (1970) to represent the following stages of development. These criteria were identical to those used to categorize eggs that did not hatch. Day 25 of incubation was utilized to represent the time when the embryo is tucking its head beneath its wing. Day 26 was used as the time when internal pipping occurs with the beak penetrating the air space, and d 27 was utilized as the time when external pipping occurs or when the embryo’s beak penetrates the eggshell. On the 28th d of incubation, newly hatched poults were selected that had completely freed themselves from the shell, and whose down feathers were nearly dry. Embryo morphology was visualized using a candling light prior to selection for sampling. Each hatch included a comparison of each selected line, its randombred control, and their reciprocal crosses. Heart rates were measured using an oscilloscope (Avitronics, Truro, Cornwall, UK) manufactured for measuring embryo heart rates in eggs. At each trial and stage, 5 embryos were selected randomly and heart rates were measured within 5 min of removal from the incubator. The analysis for heart rates had a total sample size of 15 per treatment combination. Within each of the 3 sampling hatches, 3 embryos or neonates were selected randomly from each pure line and from each reciprocal cross at each stage of development, for a total sample size of 9 for each line or cross subgroup.
Blood samples were collected from embryos and neonates following decapitation. The carcass was weighed with and without the yolk. The blood was centrifuged (700 x g at 4°C), and the plasma was decanted and frozen (–20°C) for later analysis. Hearts and livers were dissected quickly and placed in cold perchloric acid. Following immersion they were weighed (nearest 0.01 g). The tissues were stored in perchlorate under refrigeration (4°C) until assayed for glycogen and lactate concentrations (Trinder, 1969; Dreiling et al., 1987). Plasma was analyzed for activities of creatine kinase (CK) using the method of Oliver (1955) and lactate dehydrogenase (LDH) as described by Wacker et al. (1956). Both enzymes are indicators of myocardial damage (Zimmerman and Henry, 1979).
Weekly data for hatchability and deaths of embryos were collected for 17 wk of production. The data for percentage of dead embryos at internal pipping were not normally distributed; and therefore, the internal pipping mortality data were pooled with the tucking mortality for analysis. Both stages constitute the plateau stage for turkey embryos (Christensen et al., 1993). Percentage data for each pen were subjected to arc sine transformation prior to analysis. The data were sorted by the genetic history so that each selected line was compared with its randombred control line and their reciprocal crosses, i.e., the E line, the RBC1, and their reciprocal crosses were compared, and the F line, the RBC2 and their reciprocal crosses were compared separately. Thus, each analysis was done as a 2 x 2 factorial arrangement of treatments when the pure lines were used as sires and as dams in both pure line and cross line combinations. All main effects and interactions (dam, sire, and trial) were tested and means determined to differ significantly were separated using the least square means procedure (SAS Institute, 1998). Additive genetic variation (line effect) was estimated by the orthogonal contrast of the pure lines. The values for heterosis were obtained by dividing the average of the reciprocal crosses by the average of the parental lines and multiplying by 100. The significance of the heterosis was obtained by contrasting the average of the parental lines and the average of the reciprocal crosses. The significance of reciprocal effects (a confounded measure of sex linkage and maternal effects) was obtained by contrasting the reciprocal crosses.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The week of lay when the hatching eggs were collected was included as a fixed factor in all analyses. No significant week of lay by treatment interactions were observed. Crossing of the E dams, which had been long-term selected for increased egg production, with RBC1 sires (from which the selected E line had been derived) produced embryos that survived better than either pure line or the other reciprocal cross (Table 1
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). The E line had higher fertility than the RBC1 line. A large difference (> 15 %) was also observed between the fertility of the 2 reciprocal crosses. There were highly significant sire line (P < 0.0001), dam line (P = 0.0032), and sire x dam line interaction effects (P < 0.001) for percentage fertility for the E and RBC1 data set. Embryo mortality at embryo tucking and internal pipping was also significantly affected by the sire line (P = 0.0427), the dam line (P = 0.0007), and by the sire x dam line interaction (P < 0.0001). Embryos from E line sires died at a higher rate than embryos from RBC1 sires at both of these stages but the difference was significant only at tucking and internal pipping. Embryos from E line dams had higher mortality (P = 0.0007) than embryos from RBC1 dams at both ages. The orthogonal contrasts for embryo survival for the E and RBC1 pure lines and their crosses indicated the presence of additive gene effects on overall fertility (P = 0.0133), and the presence of maternal or sex-linked gene effects on fertility (P < 0.0001), and on survival at tucking and internal pipping (P < 0.0001), as well as on survival at external pipping (P = 0.0128). The orthogonal contrasts showed that a significant level of heterosis (4.2%) was present for overall fertility (P 
0.0001), as well as for survival at external pipping (–46.1%). Significant (P < 0.001) sire line by dam line effects were also present for overall fertility and for embryo survival at all stages tested indicating that the effects of using the 2 lines as sires or as dams were line and sex dependent.
Crossing of the F line (Table 1), which had been long-term selected for increased 16-wk BW, with the RBC2 from which it had been derived resulted in improved overall fertility of the RBC2 sire by F dam embryos (75.9%) in comparison with the fertility of the F line sire by RBC2 dam cross (71.3%) or for either of the pure lines (67.2 and 71.3% fertility for the F and RBC2 embryos, respectively). On average, the embryos from the F line sires survived slightly, but not significantly, poorer (69.2%) than the embryos from the RBC2 sires (73.6%). The sire line x dam line interaction (P = 0.0065) for overall fertility of these crosses indicated that the difference between the survival of embryos from the 2 lines when used as dams was significantly smaller (1.8 %) than when used as sires (4.4%). The orthogonal contrasts indicated the presence of significant additive gene effects (P = 0.019) for overall fertility, and for embryo survival (P = 0.018) at the 2 stages studied. Significant maternal or sex-linked effects (P = 0.0469) were also present for overall fertility. A significant level of negative heterosis (6.3%; P = 0.0059) was also present for overall fertility.
Heart Weights at Head Tucking, and Internal and External Pipping
Many significant effects were observed for absolute heart weights (data not shown). Significant dam effects and additive genetic variation were noted for heart weight at tucking, internal pipping, and external pipping in both comparisons. Reciprocal effects were significant in all comparisons except for external pipping in the F x RBC2 cross. There was no significant heterosis for heart weight at any age in either cross.
When heart weights were adjusted for BW by expressing them as a percentage of BW, most of the significant differences observed with absolute heart weight were not present (Table 2). Only the additive genetic variation for relative heart weight was significant at internal and external pipping.
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As was observed for heart weights, many significant differences were observed for absolute liver weights (data not shown). Additive genetic variation and reciprocal effects were significant for absolute liver weights at head tucking and internal and external pipping. Heterosis was not significant in any comparison. Dam effects were significant in all comparisons. There were no significant sire effects. When liver weights were expressed as a percentage of BW, few significant differences were observed (Table 3). Dam effects and additive genetic variation were significant for percentage liver weight at external pipping only in the F x RBC2 crosses.
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Heart rates at head tucking (Table 4) were significantly (P = 0.0011) faster in embryos from E line dams (230 bpm) than in embryos from RBC1 line dams (219 bpm); and, embryos from E line sires (228 bpm) had significantly (P = 0.0147) faster heart rates than did embryos from RBC1 sires (220 bpm). At internal pipping, embryos from E line sire embryos had faster heart rates (233 bpm) than did the embryos from RBC1 sires (226 bpm, P = 0.0355). At external pipping, both the sire line effects (P = 0.0572) and the dam line effects (P = 0.0113) were significant, and in both cases the embryos from the E line had faster heart rates than did the embryos from the RBC1 line. The orthogonal contrasts for heart rate indicated the presence of significant additive effects at tucking and at external pipping.
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). Embryos from the F line dams had slower heart rates (P 0.0266) at tucking and at internal pipping, but not at external pipping, than did the embryos from the RBC2 dams. Embryos from the F line sires had slower heart rates (P 0.0211) at all 3 stages than did the embryos from the RBC2 sires. A significant sire x dam interaction (P = 0.0096) was present at internal pipping, indicating that the difference in heart rate between the embryos from the F and RBC2 sires (16 bpm) was significantly larger than between the offspring from the F and RBC2 dams (8 bpm). Orthogonal contrasts indicated significant additive effects (P < 0.007) were present at all of the stages studied, and that negative heterosis (–4.6 %; P = 0.0091) was present at internal pipping. Myocardial Physiology at Head Tucking, and Internal and External Pipping
Cardiac Muscle Glycogen.
Embryos from E line dams had more (P < 0.03) cardiac muscle glycogen (CMG) than did the embryos from the RBC1 dams at internal and external pipping (Table 5). None of the sire line effects were significant at any of the 3 stages studied. A significant (P = 0.0475) sire x dam interaction for CMG levels was present at embryo tucking, indicating that the difference between the embryos from the 2 dam types (0.48 mg/g) was larger than the difference between the embryos from the 2 sire types (0.0 mg/g). Significant (P = 0.0075) additive effects were present at internal pipping for CMG for the E and RBC1 data set.
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). Significant (P < 0.04) additive genetic effects were present at internal pipping, external pipping, and at hatching for CMG. None of the sire x dam line interactions for CMG was significant at the first 3 stages studied, and none of the orthogonal contrasts for genetic effects was significant for the F and RBC2 data set at those 3 stages. At hatching, a sire x dam interaction was present (P = 0.0053) indicating that the difference between the CMG levels in the embryos from the F and RBC2 dams (0.43 mg/g) was smaller than was the difference (0.57 mg/g) observed between the levels in the embryos from the F and RBC2 sires. Significant (P = 0.0286) negative heterosis (–18.5%) was present for CMG at hatching.
Cardiac Muscle Lactate.
Most of the sire line, dam line, and sire line x dam line interaction effects for cardiac muscle lactate (CML) concentrations were not significant for the E and RBC1 data set (Table 6). The 2 exceptions were that a significantly lower level of CML was present in the embryos from the E line (0.38 mg/g) than from the RBC1 dams (0.48 mg/g) at external pipping, and that a significant sire line x dam line interaction was present at embryo tucking. In the latter case, the reciprocal crosses had consistently lower levels (0.28 mg/g) of CML than did the purelines (0.30 mg/g) at tucking. This resulted in a significant level of negative heterosis (–12.5%) being present for CML at tucking for the E and RBC1 data set.
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) for the F and RBC2 data set. Likewise, none of the tucking, internal pipping, or external pipping sire line x dam line interactions was significant for CML concentrations for those lines. The sire x dam line interaction for CML was, however, significant (P = 0.0071) at hatching, where the reciprocal crosses embryos had lower levels of CML (average = 0.665 mg/g) than did the pureline embryos (average = 0.765 mg/g). This resulted in a significant (P < 0.0274) level of negative heterosis (–13.5%) for the CML levels at hatching.
Ratio of Cardiac Muscle Glycogen to Cardiac Lactate Concentrations.
The ratios of cardiac muscle glycogen to cardiac muscle lactate (CMG:CML) are summarized for both data sets in Table 7. The dam line had a significant effect on the CMG:CML ratio both at internal (P = 0.0343) and at external (P = 0.0085) pipping for the E and RBC1 data set. At those stages, the CMG:CML ratio was higher in embryos from E line dams than in the embryos from the RBC1 dams. The orthogonal contrasts for the E and RBC1 data set indicated that significant (P = 0.0309) additive effects were present for the CMG:CML ratio at both internal and external pipping. There were no reciprocal or heterotic effects at any stage.
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). In all 3 cases, the embryos from the RBC2 line had higher CMG:CML ratios than did the embryos produced by the F line. None of the sire x dam interactions nor any of the orthogonal contrasts for the CMG:CML ratios for the F and RBC2 data set were significant. There were no reciprocal or heterotic effects at any stage. Hepatic Physiology at Head Tucking, and Internal and External Pipping
Hepatic Glycogen Concentrations.
Hepatic glycogen concentrations (HGC) were decreased (P < 0.0001) in embryos produced by E line dams in comparison with the levels observed in embryos produced by RBC1 dams at head tucking and internal pipping (Table 8). None of the sire effects for HGC or the interactions of sire line x dam line effects was significant for hepatic glycogen at tucking, internal pipping, external pipping, or hatching. The orthogonal contrasts indicated significant (P < 0.01) additive genetic effects for HGC were present at head tucking and internal pipping, but not at external pipping or hatching.
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). In that case, the embryos produced by the F line sires had lower HGC levels (7.8 mg/g) than did the embryos produced by the RBC2 sires (9.0 mg/g). None of the dam line effects were significant for HGC levels except at hatching. At that stage, the embryos produced by the F line dams had significantly (P = 0.0243) lower HGC levels (4.2 mg/g) than did the embryos produced by RBC2 dams. (4.9 mg/g). The sire line x dam line interaction for HGC in the F and RBC2 data set was also significant at hatching (P = 0.0051), indicating that the difference was larger between the embryos from the RBC2 and F line sires (0.9 mg/g) than it was between the embryos from the RBC2 and F line dams (0.7 mg/g), and that the pure line embryos had higher (average = 5.2 mg/g) hepatic glycogen levels than did the reciprocal cross embryos (average = 3.9 mg/g). None of the orthogonal contrasts were significant except at external pipping and at hatching, where significant (P < 0.04) additive effects were observed for the inheritance of embryo HGC in the F and RBC2 lines. Significant (P = 0.0294) negative heterosis (–25.0%) was also present for HGC at hatching for these lines.
Hepatic Lactate Concentrations.
Hepatic lactate concentrations (HLC) for the 2 data sets are summarized in Table 9. Sire effects were significant for HLC at internal (P = 0.0446) and external (P = 0.0099) pipping for the E and RBC1 data set. None of the dam line effects were significant at any of the ages for HLC in the E and RBC1 data set, except at hatching (P = 0.04), but they also approached significance (P < 0.07) at both the internal and external pipping stages. The orthogonal contrasts for HLC from the E and RBC1 data set indicated that significant (P = 0.0523) additive and significant (P = 0.0503) sex-linked or maternal effects were present at internal pipping, and that significant (P = 0.0292) sex-linked or maternal effects were present at external pipping. None of the genetic effects were significant for HLC levels at tucking or hatching for the E and RBC1 data set. There was no significant heterosis at any stage.
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). In both cases, the embryos from the RBC1 sires had slightly greater HLC levels than did those from the F line sires. None of the dam line or sire line x dam line interactions was significant at any of the 4 stages tested for HLC in the F and RBC2 data set. Likewise, none of the orthogonal contrasts for genetic effects were significant for HLC in that data set.
Ratio of Hepatic Glycogen to Hepatic Lactate Concentrations.
The ratios of hepatic glycogen to hepatic lactate (HGC:HLC) ratios are summarized for both data sets in Table 10. The dam line had a significant effect on HGC:HLC at both tucking (P = 0.0013) and at internal pipping (P < 0.001) for the E and RBC1 data set. In both cases the embryos from the RBC1 dams had higher HGC:HLC ratios than did the embryos from the E line dams. The RBC1 sires had significantly (P = 0.0191) higher HGC:HLC ratios at internal pipping than did the embryos from the E line sires. None of the sire line x dam line interactions for the HGC:HLC ratios was significant for the E and RBC1 data set, except at hatching (P = 0.0125) where the difference in the HGC:HLC ratio was larger between the embryos from the E and RBC1 sires (11) than it was between the embryos from the E and RBC2 dams (5), and the reciprocal crosses had higher ratios (average = 65) than did the 2 sets of pure line embryos (average = 49.5), resulting in significant (P 0.021) heterosis (31.3%). The orthogonal contrasts indicated that significant additive gene effects were present for the HGC:HLC ratio at both tucking (P = 0.0052) and at internal pipping (P<0.001).
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) were significant at external pipping (P = 0.0004) and at hatching (P = 0.0212). In both cases this ratio was considerably higher in the embryos from the RBC2 sires (142 and 46, respectively) than in the embryos from the F sires (79 and 34, respectively). None of these ratios were significant for the dam line comparisons except at hatching (P = 0.0301), where the embryos from the RBC2 dam also had higher ratios (46) than did the embryos from the F line dams (35). These differences led to significant additive genetic effects being present at external pipping (P = 0.0131) and hatching (P = 0.0057) for the F and RBC2 data set. Heterosis was inconsistent for the F and RBC2 data being positive (44.1%) and significant (P = 0.048) at external pipping, but negative (–25.8%) and significant (P = 0.040) at hatching. Plasma Creatine Kinase and Lactate Dehydrogenase Activities at Head Tucking, and Internal and External Pipping
Plasma Creatine Kinase.
The plasma creatine kinase (PCK) results for the 2 data sets are summarized in Table 11. None of the sire line effects was significant for PCK with the E and RBC1 lines, except at hatching where the embryos from the E line sires had significantly higher (P = 0.009) PCK activity than did the embryos from the RBC1 line sires. Embryos from E line dams had significantly (P = 0.0530) lower PCK activity at internal pipping (1,119 U/L) than did the embryos from the RBC1 dams (1,262 U/L). Orthogonal contrasts for the E and RBC1 data set showed the presence of heterosis (13.1%, P < 0.044) at external pipping, and the presence of maternal or sex-linked effects (P < 0.0064) at hatching, but there were no significant additive effects at any stage of development.
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. None of the sire line, dam line, or sire line x dam line interactions was significant for this data set. There was no significant additive variation. However, the orthogonal contrasts indicated the presence of significant (P = 0.0547) sex-linked or maternal effects at tucking, and significant negative heterosis (–26.2%, P = 0.0133) at internal pipping for PCK for the F and RBC2 lines.
Plasma Lactate Dehydrogenase Activity.
The plasma lactate dehydrogenase (PLDH) results for the 2 data sets are summarized in Table 12. The only effect that was significant for the PLDH levels for the E and RBC1 data set was the sire line effect at internal pipping. In that case, embryos from the E line sires had significantly (P = 0.0555) higher PLDH activity (446 U/L) than did the embryos from RBC1 sires (440 U/L). None of the orthogonal contrasts was significant for the E and RBC1 data set for PLDH activity.
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). Significant differences were also observed between the PLDH levels from embryos from the 2 dam lines at external pipping, where the poults from the F line dams had significantly (P = 0.0172) higher levels of PLDH at external pipping (583) than did the embryos from the RBC2 line dams (435 U/L). The orthogonal contrasts for the F and RBC2 data set indicated that a significant (P = 0.0020) level of heterosis (–16.4%) was present at external pipping and that significant (P = 0.0086) additive genetic effects were present at hatching. No other contrasts were significant. Plasma Glucose and Lactate Concentrations at Head Tucking, and Internal and External Pipping
Plasma Glucose.
The plasma glucose (PG) levels are summarized in Table 13, and the results were quite different between the 2 data sets. A significant difference observed in the PG levels of the embryos from the E and RBC1 data set was that the embryos from the RBC1 sires had significantly (P = 0.0225) higher (238 mg/dL) PG than did the embryos from the E line sires (230 mg/dL) at internal pipping. The cross line poults had a considerably higher level (average = 263.5 mg/dL) of PG than did the purelines (average = 251.5 mg/dL) at hatching (heterosis = 4.8%). The only other effect that was present for the E and RBC1 PG data was a significant (P = 0.0024) sire line x dam line effect at hatching. In that case, the difference between the PG levels of the embryos from the 2 types of sires (8 mg/dL) was significantly larger than the difference between the embryos from the 2 types of dams (2 mg/dL). None of the orthogonal contrasts for PG in the E and RBC1 data set were significant except for the presence of heterosis at hatching (4.8%, P = 0.0079).
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0.05) were present for the PG levels at internal and external pipping, but not at tucking or hatching (Table 13). In both cases, the embryos from the RBC2 sires had higher PG levels than did the embryos from the F sires. The embryos from the RBC2 dams had PG levels that either approached (P = 0.0684) or exceeded (P = 0.0081) accepted significance levels at tucking and internal pipping, but not at external pipping or hatch. A significant (P = 0.0007) sire line x dam line interaction was present at tucking, where the reciprocal crosses had significantly higher PG levels than either of the purelines. The orthogonal contrasts indicated the presence of significant (P < 0.05) additive effects for the F and RBC2 data set at tucking, internal pipping, and external pipping. A significant level (P = 0.007) of heterosis (7.8%) was also present at tucking.
Plasma Lactate (PL) Concentrations.
The plasma lactate (PL) concentrations are summarized for the 2 data sets in Table 14. The PL concentrations in the embryos from the E line sires (10.0 mg/dL) were significantly higher (P = 0.0005) than in the embryos from the RBC1 sires (8.9 mg/dL) at internal pipping, but not at any of the other stages studied. Embryos from the E line dams had significantly (P < 0.05) lower levels of PL at all of the stages studied. The orthogonal contrasts for the E and RBC1 data set indicated that significant (P = 0.0129) additive genetic effects were present at tucking and that significant maternal or sex-linked effects were present at internal pipping (P < 0.0001) and at hatching (P = 0.0503). A significant level (P = 0.0056) of heterosis (10.6%) was present at internal pipping in the E and RBC2 data set.
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). None of the other effects on PL was significant for the F and RBC2 data set, except that the PL concentration in the poults from the F line sires (16.5 mg/dL) was significantly (P = 0.036) larger at hatching than it was in the poults from the RBC2 sires (13.4 mg/dL) at hatching. Orthogonal contrasts indicated that both additive (P = 0.016) and sex-linked or maternal effects (P = 0.0515 were present at head tucking and that sex-linked or maternal effects were present (P = 0.0249) at hatching for the F and RBC2 data set. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Long-term selection by the turkey breeding industry has resulted in major growth and performance changes over the past 40 to 50 yr. The magnitude of some of those changes has recently been documented by Havenstein et al. (2007). Basically, their data indicated that the growth rate of domestic turkeys more than doubled during the 37 yr from 1966 to 2003, and as is well known, while those changes in growth were occurring negative concomitant changes in other performance factors such as egg production, egg size, embryo survival, and fertility were also taking place. Fortunately, scientists at the Ohio Agricultural Research and Development Center have maintained 2 long-term selected turkey lines and the randombred control lines from which they were derived, which allow the scientific community to study exactly what has changed in terms of the underlying physiology between the performance of the nonselected randombred control lines and their associated selected lines. One of the Ohio lines (E) has been long-term selected solely for increased egg production, and the other (F) has been selected for increased 16-wk BW. The 4 turkey lines maintained in Ohio provide a very unique opportunity for scientists to gain a better understanding of the concomitant changes that have taken place in nonselected traits as changes have taken place from selection for either egg production or for growth rate. By crossing the selected lines with their associated randombreds, one can utilize orthogonal contrasts of the pure and crossline performance to determine the general mode of inheritance (i.e., additive, heterosis, or combined maternal and sex-linked gene effects) that were involved with the specific trait performance changes. Unfortunately, the use of the 2 lines and their reciprocal crosses alone, as is done in this study, does not permit one to separate sex-linked gene effects from maternal effects. Much larger data sets involving back-crosses would have to be used to separate those effects.
Concomitant Changes in Fertility, Embryo Survival, and Heart Physiology Associated with Long-Term Selection for Increased Egg Production as Determined from the E and RBC1 Lines and Their Crosses
Embryo Survival. Overall survival rates of embryos from the matings of RBC1 line sires with E line dams was much higher than for embryos of either the E or RBC1 purelines or for embryos of the reciprocal cross (i.e., from matings of E line sires with RBC1 dams). This appears to be related to the RBC1 x E crossline embryo’s ability to survive at wk 4 of development, a time when oxygen becomes limiting and embryos must both grow and function (Dietz et al., 1998). Genetically, it appears that the combination of sex-linked and autosomal genes that are involved when the cross is made between the RBC1 line sires and the E line dams may cause the expression of some measurable cardiac traits that may be connected with survival. From this part of the study and from similar data on other traits that have been summarized in previous papers (Christensen et al., 2007a,b,c), it appears that long-term selection for egg production has reduced BW, heart weight, and liver weight, and has increased heart rate in the E line in comparison with what is seen in the RBC1 (i.e., in the randombred control from which the E line was established). These concomitant selection changes are, however, partially or totally overcome by crossing the 2 lines, and the changes in the performance of the cross lines have been shown by Christensen et al. (2007a, b) and herein to be due to a combination of additive, sex-linked, or maternal effects, and heterosis. The exact mode of inheritance appears to be dependent upon the trait being measured, and the exact embryonic stage at which it is measured.
Heart Weights and Rates. Some of the most obvious changes that have taken place in the E line due to its long-term selection for increased egg production include reduced BW (Christensen et al., 2007a), heart weight, and liver weight. The E line also has an increased heart rate in comparison to the RBC1 line. Another obvious change is in the energy balance and growth of the myocardium. Embryo heart rates are reduced by using RBC1 line sires, suggesting that reduced heart rates may be a key to the better survival noted in the embryos from that cross. Genetic selection for increased egg production has also decreased embryonic heart weight relative to BW, and that change has been mediated through the dam by maternal or sex-linked inheritance.
Cardiac Muscle Glycogen and Lactate Concentrations and Their Ratio. Increased myocardium glycogen is indicative of cardiomyopathy (Czarnecki et al., 1975; Czarnecki and Evanson, 1980; Mirsalimi et al., 1990). The E line embryos had more stored glycogen prior to hatching than did their randombred line. When mated to the same commercial dam line (Christensen et al., 2004), E sire line embryos exhibited greater rates of gluconeogenesis than did F sire line embryos. The E line embryonic myocardium glycogen in the current study was a function of the dam, but only additive effects were significant, suggesting that whereas the dam has the primary influence, the sire may also play a role.
Avian myocardium accrues and stores glycogen to a limited extent because the tissue expresses minimal lactate recycling (Pearce and Brown, 1971; Watford et al., 1981). Limited accrual may be because avian cardiac tissue possesses low glucose-6-phosphatase and phosphoenolpyruvate carboxykinase activities (Hazelwood, 1986). Both enzymes are essential for establishing and maintaining adequate myocardial glycogen levels during the plateau stage in oxygen consumption (Christensen et al., 1999). The avian liver and kidney recycle lactate and perform gluconeogenesis for the embryo (Fasenko, 1996). The preferable means for birds is to recycle lactate to glucose for the heart by the Cori cycle in hepatic tissue so glucose can be returned to the heart for cardiac energy (Watford et al., 1981; Liao et al., 1996). The kidney may also break down gluconeogenic amino acids (alanine) to create glucose, but this mechanism is not used to a great extent by birds (Hazelwood, 1986). Because of the vital importance of the liver in the process, the genetic basis for hepatic tissue growth and function was also examined.
Liver Weights. Neither dam line nor sire line in the E and RBC1 line comparisons affected liver weight. This observation suggests that selection for egg production and its consequent reduction in egg weight has not affected hepatic weight relative to the changes that have taken place in poult weight.
Hepatic Glycogen and Lactate Concentrations and Their Ratios. In cardiomyopathic poults, glycogen granules were observed in hepatic lysozomes (Staley et al., 1978), which were hypothesized to result from a block in the citric acid cycle. This prevents the complete breakdown of glycogen and results in altered hepatic metabolism, including decreased protein synthesis and increased metabolism of fat, possibly associated with liver damage. It was determined that glycogen branching levels were unaltered; therefore, the best explanation for the altered levels was a change in degradation of glycogen (Staley et al., 1978; Czarnecki and Evanson, 1980). Hepatic glycogen was reduced as a function of the dam in E line embryos compared with RBC1. The dam reduced glycogen at tucking and internal pipping but not at external pipping and hatching. These effects were additive in nature, indicating that both the sire and the dam from the selected line were responsible for the gene expression, but the increased hepatic lactate levels at both internal and external pipping were also influenced by significant reciprocal or sex-linked effects. Thus, the ratios of glycogen to lactate in livers of E and RBC1 line embryos were subject to both sire line and dam line effects, primarily at the plateau stage in oxygen uptake prior to the onset of convective respiration.
Plasma Creatine Kinase and Plasma Lactate Dehy-drogenase. A related line of research involves studying CK, LDH, and other enzymes involved in providing energy to the heart via the pathways of glycolysis and oxidative phosphorylation. The CK and LDH activity are indicative of myocardial damage (Czarnecki et al., 1978; Staley et al., 1978). Creatine phosphate carries a high-energy phosphate, which can be transferred to and from adenosine triphosphate (ATP) by CK in reversible reaction. It is a source of energy when other means of supplying ATP to cardiac muscle are inadequate (Liao et al., 1996). Simply stated, significant decreases in the speed at which the phosphorylation of ATP from phosphocreatine occurs in myocardium from cardiomyopathic turkeys were found. The amounts of both phos-phocreatine and ATP were decreased, whereas free carnitine levels were normal. The ATP synthesis via oxidative phosphorylation did not appear to be impaired when concentrations of major marker enzymes were measured. These observations were correlated with decreased contractility of isolated cardiac muscle fibers, indicating that there may be a strong association between decreased energy reserve and decreased contractility in the failing heart, although decreases in ATP may not occur until late in pathogenesis (Liao et al., 1996). The CK activity was decreased 40% in the myocardium of cardiomyopathic turkeys, the highest level for any of the energy related enzymes, including those involved in glycolysis (30% depression), the Kreb’s cycle (20% depression), and fatty acid oxidation (15% depression; Mirsalimi et al., 1990). The E sire x RBC1 dam embryos showed elevated CK activity at the plateau stage, whereas the embryos from the F sire by RBC2 dam showed the lowest CK activity and both traits showed some heterosis. Thus, selection for increased egg production appeared to have affected embryo CK divergently. Both E and F sires increased LDH activity in their offspring. The E sire increased embryo LDH activity only at the plateau stage. The LDH activity is indicative of the rate at which lactate is recycled to pyruvate in gluco-neogenesis. Thus, the increased LDH data support the results already mentioned for glycogen and lactate concentrations. It is suggested that the myocardium in E sire embryos is using energy faster than the physiological mechanisms can provide it, but once the shell is broken and access to oxygen is provided, the myocardium may function properly.
Plasma Glucose and Lactate Concentrations. Plasma concentrations of glucose and lactate may be indicative of the energy status of the neonate (Donaldson and Christensen, 1994). Glucose was affected divergently by dam and sire depending upon selection criteria. Heterosis was prominent in the reciprocal crosses between the E and RBC1 lines. Both crosses had elevated glucose in poults at hatching compared with either pure-line. The embryos from the E line sires had depressed glucose at internal pipping, whereas the embryos from the E line dams had depressed glucose at external pipping compared with embryos from the nonselected RBC1 dams and sires. Plasma lactate differences were especially evident at internal pipping or at the termination of the plateau in oxygen consumption, and those differences were due to the presence of heterosis and maternal or sex-linked gene expression.
Concomitant Changes in Fertility, Embryo Survival, and Heart Physiology Associated with Long-Term Selection for Increased Growth Rate as Determined from the F and RBC2 Lines and Their Crosses
Embryo Survival. Pure line F embryos hatched at a reduced rate compared with the pureline RBC2 and the reciprocal crosses, and most embryos that died did so at the plateau stage of development immediately before hatching. The F dam had a greater influence on embryo viability than did the sire in this data set, which differs from what was seen in the E and RBC1 data set. When the RBC2 sire was crossed on the F line dam, embryo livability improved modestly due to additive and reciprocal genetic effects.
Heart Weights and Rates. Selection of turkeys for heavier BW decreased heart weight relative to BW following the plateau stage. In contrast to the genetics of the E line, changes due to BW selection in the F line have been mediated by both maternal or sex-linked and additive effects. Thus, the inheritance of heart weight can be affected differently, depending upon the selection criteria of the parents.
Cardiac Muscle Glycogen and Lactate Concentrations and Their Ratio. Converse to what was observed with the E line embryos, in F line embryos, myocardial glycogen accrual was suppressed by the dam line before the plateau stage in oxygen consumption, but it was suppressed by the sire line following the plateau stage. Additive genetic effects on cardiac muscle glycogen concentrations as well as negative heterosis were present at hatching in the F and RBC2 data set. Thus, selection for increased BW resulted in a different expression in avian myocardial energy metabolism than did the selection for increased egg production.
Liver Weights. When liver weight changes were examined as a percentage of BW, the F line showed significant changes only following the plateau stage in oxygen consumption. Both the sire line and the dam line affected the reduction in embryonic liver relative to BW at external pipping and hatching for the F line data set through additive gene effects. Inheritance of liver weight was also affected by sex-linked or maternal effects, and by additive effects.
Hepatic Glycogen and Lactate Concentrations and Their Ratios. The F line embryos did not show reduced hepatic glycogen or increased lactate until after the plateau stage. Both F line dams and sires suppressed embryonic hepatic glycogen levels, and the F line sires increased hepatic lactate levels. The modes of inheritance showed significant additive effects and heterosis on the energy metabolism of the liver. The primary difference between the selected lines was the time at which each responded to the plateau stage. The E line responded before or during the plateau stage, whereas the F line responded at the end of the stage and thereafter.
Plasma Creatine Kinase and Plasma Lactate Dehy-drogenase. The E line sire increased embryo LDH activity only at the plateau stage of development, but the F line sire’s effect persisted through hatching and was mediated by additive gene effects. The LDH activity summarizes the rate at which lactate is recycled to pyruvate in gluconeogenesis. Thus, the increased LDH data support the results already mentioned for glycogen and lactate concentrations. The myocardium in F line poults appears to be using energy faster than the physiological mechanisms can provide it.
Plasma Glucose and Lactate Concentrations. Sire effects were nearly nonexistent in the E line, but when the F line was examined, the F line when used as the dam elevated plasma lactate at the plateau stage, but when that line was used as the sire plasma lactate increased it at hatching. These effects were caused by significant sex-linked or maternal effects. Long-term selection for increased BW has had some effects on embryo plasma glucose concentration, but the effects observed appeared to be dependent upon the stage of development. In contrast to what was observed following long-term selection for increased egg production, long-term selection for increased BW in the F line on embryonic plasma lactate has been primarily mediated by the sire.
In conclusion, long-term selection for increased egg production in the E line has reduced embryo heart weight and has altered the energy metabolism of the myocardium. The differences in energy metabolism may be due to the more rapid heart rates. Conversely, long-term selection for increased 16-wk BW has significantly decreased the heart rate of F line embryos and has not changed the weight of the heart relative to the BW until the embryo has passed through the plateau stage. The F line embryos show a different energy metabolism that relies much more on gluconeogenesis. Embryo deaths occur more frequently in turkey embryos when the energy metabolism of the myocardium shows elevated glycogen to lactate ratios as it did in the pure E and F lines.
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FOOTNOTES |
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Received for publication August 29, 2007. Accepted for publication January 23, 2008.
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