Results and Discussion
The LSN Trait
The LSN line has been maintained with the LSN condition being considered as a qualitative trait. Individuals with an exhaustion score of 6 or less were considered to be LSN. However, the exhaustion test scores of individuals used to reproduce the LSN line ranged from 0 to 6 and, as a result, the LSN condition was quantitative in nature, but was changed to a qualitative trait by the use of a range of values.
The LSN condition was first observed among F2 progeny in an out cross of chickens with hereditary MD to a commercial White Leghorn stock (L. J. Pierro and J. S. Haines, personal communication). Therefore, the question arises is the LSN muscle weakness a modified form of the MD condition. Birds with MD have an exhaustion score of 0 and an autosomal recessive gene (am) (Asmundson and Julian,1956) has a large influence on expression of MD but the trait is considered to be a polygenic trait (Holliday et al.,1968; Somes, 1990).
The biological properties of the pectoralis major muscles from LSN and MD birds differ. Avian MD is characterized by the continued expression of the neonatal isoform of myosin heavy chain in adult birds (Bandman, 1985). The LSN birds go through a normal myosin heavy chain isoform switching from the embryonic to the neonatal to the adult isoform (Velleman et al., 1993). Neonatal myosin is not present in the LSN adult bird. Adult MD birds also exhibit muscle fiber hypertrophy, but muscle fiber hypertrophy is not present in the LSN birds (Velleman et al., 1993). Dystrophic pectoral muscle is characterized by a lower proportion of glycolytic fibers (Type IIb) and a higher proportion of intermediate fibers (Type IIa) than LSN and normal muscle (Velleman et al, 1993). The distribution of muscle fibers did not differ between normal and LSN muscles. Studies on extracellular matrix expression have shown an increase in LSN decorin concentrations relative to normal muscle at day 20 of embryonic development (Velleman et al., 1996). Subsequent to the increase in decorin expression, collagen crosslinking, but not concentration is increased. The changes in decorin and collagen are not detected in pectoralis major muscles from MD birds. Based on the biological differences, it appears that the LSN and MD conditions are different traits.
Frequency of the LSN Condition in F1, F2, and Backcross Offspring
The results obtained in the F1 generation suggest that the LSN condition may be inherited as an autosomal dominant gene with incomplete penetrance (Table 1). Three normal individuals and 134 LSN individuals were obtained in the F1 of the LSN line males and the SPF White Leghorn females when an exhaustion score of 6 or less was used to identify the LSN condition. In the cross of the SPF White Leghorn males and LSN line females, 8 of 126 individuals were normal. The frequencies did not differ between sexes in either F1 cross, suggesting autosomal inheritance. The penetrance of the proposed dominant gene differed slightly in the two F1 crosses, 97.9% in the LSN line male X SPF White Leghorn female cross and 94.7% in the other F1 cross. The family averages of exhaustion scores did not differ between the F1 reciprocal crosses.
The results of the F2 generation, in general, suggest that the LSN trait is influenced by an autosomal dominant gene (Table 1). For the LSN line male X SPF White Leghorn female cross, the ratio of LSN to normal individuals did not deviate significantly from the expected 3:1 ratio in the sexes separately or when the sexes were combined. The ratio of LSN to normal for male offspring also did not deviate significantly from expected in the F2 of the cross of the SPF White Leghorn male and LSN line female. However, for female offspring and for both sexes combined, there was an excess of normal individuals resulting in the ratio deviating significantly from the expected 3 LSN to 1 normal (P < 0.01 for females; P < 0.05 for sexes combined). The significant deviation from expected in the female offspring remained even after adjustment for the possible reduced penetrance of the proposed dominant gene. The family averages of the exhaustion scores of the F2 individuals did not differ in the two crosses.
The ratio of LSN to normal individuals in the backcross of the F1 individuals to the SPF White Leghorns did not differ significantly from the expected ratio of 1 LSN:1 normal based on the assumption that the LSN trait is controlled by a dominant gene (Table 1). The ratio did not differ from expected in male and female offspring, suggesting an autosomal gene. The family averages of exhaustion scores did not differ between the reciprocal F1 crosses.
The results of the F1, F2, and backcross matings, in general, suggest that the LSN condition is controlled by a dominant gene with incomplete penetrance. Thus, the inheritance of the LSN condition is different from that of the MD trait that is primarily controlled by a recessive gene (Asmundson and Julian,1956).
Heritability of the LSN Trait Based on Offspring-Parent Regressions
The h2 estimates of unadjusted exhaustion scores were higher in the LSN male x SPF White Leghorn female cross (range = 0.520 to 1.107) than in the SPF White Leghorn male x LSN female cross (range = 0.161 to 0.621) when based on regression of F2 offspring on F1 mid-parent, sires, or dams for the sexes separately or combined (Table 2). The magnitude of the h2 estimates for the LSN male x SPF White Leghorn female cross suggests that the LSN trait is controlled by a small number of genes. However, the smaller h2 estimates for the reciprocal F1 cross suggest that a larger number of genes are involved in expression of the LSN condition.
Differences in performance of reciprocal crosses can be caused by maternal effects or sex linkage. The SPF White Leghorn line has a larger egg weight than the LSN line (56.2 vs. 49.2 g, unpublished data), therefore, maternal effects may have been expected, but there was no difference in exhaustion scores between the two F1 crosses (Table 1). The h2 estimates based on regression of offspring on dam were higher for male offspring than for female offspring for both F1 crosses (Table 2) suggesting the presence of sex linkage (Dickerson, 1959). The h2 estimates based on regression of offspring on sires did not differ between sexes.
Adjustment for hatch effects influenced the h2 estimates differently in the two F1 reciprocal crosses (Table 2). Adjustment of the data for hatch effects for the SPF White Leghorn male x LSN line female cross decreased the h2 estimates, but increased them for the reciprocal cross.
Data derived from RNA steady-state expression experiments suggests the involvement of more than one gene in the LSN condition. The TGF-b2 concentrations are increased in LSN birds at 20 d of incubation and at 1 day and 1 week posthatch compared to normals whereas TGF-b1 was increased only at 1 day posthatch in the pectoralis major muscle of LSN birds (Velleman and Coy, 1998). Steady-state transcripts for the extracellular matrix molecule, decorin, were modified during LSN pectoralis major muscle development (Velleman and Coy, 1997). At 20 days of incubation, LSN decorin mRNA concentrations were elevated compared to those detected for normal birds.
In summary, the frequency of the normal and LSN individuals in the F1, F2, and backcross populations suggest that the LSN condition is controlled primarily by an autosomal dominant gene with in complete penetrance. The h2 estimates based on offspring-parent regression suggest that several genes may be involved in the inheritance of the LSN condition, some of which are sex-linked. It was concluded that the expression of the LSN condition is primarily influence by an autosomal dominant gene, but several genes are involved in the inheritance of this trait. A similar situation is involved in the inheritance of MD except that the primary gene for MD is an autosomal recessive (Somes, 1990).