Introduction
An abnormal muscle weakness condition termed LSN was originally detected in 1977 at the University of Connecticut 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, Department of Animal Genetics, University of Connecticut, Storrs, CT 06268, personal communication). The MD population had been maintained at the University of Connecticut for several years prior to the out cross. The LSN nomenclature was selected to distinguish birds that showed an impaired ability to right themselves when repeatedly placed on their backs (exhaustion score test) from normal birds. At 2 to 3 months of age, normal birds have an exhaustion score of between 15 and 20 and MD birds are unable to right themselves. Birds that are classified as LSN are intermediate to these extremes. In 1997, the LSN line was transferred from the University of Connecticut to the Ohio Agricultural Research and Development Center (OARDC).
In addition to righting ability, LSN birds have reduced pectoral muscle mass. At 1 week posthatch, the weight of pectoral muscles expressed as a percentage of body weight (BW) was 1.9% in LSN and 3.2% in normal birds (Velleman et al., 1996). The LSN condition has proven to be useful in the study of muscle development due to changes in both extracellular matrix and sarcomere development (Velleman et al., 1993, 1996, 1997; Velleman and Coy, 1998; Velleman, 1999).
In studies on LSN muscle development, Velleman et al. (1993) observed that, at 6 weeks posthatch, pectoral muscle fibers of LSN birds did not vary significantly from normal controls in either size distribution or in the proportion of Type IIa and Type IIb fibers. However, the extracellular matrix proteoglycan, decorin, is expressed at elevated levels at 20 days of embryonic development in LSN birds (Velleman et al., 1996). Decorin is a multifunctional proteoglycan regulating collagen fibrillogenesis (Vogel et al., 1984; Vogel and Trotter, 1987; Weber et al., 1996), cell proliferation and differentiation (De Luca et al., 1996), and transforming growth factor-beta (TGF-b ; Yamaguchi et al., 1990). Fitting with the role of decorin as a regulator of collagen fibril formation, and cell proliferation and differentiation, LSN collagen crosslink levels are elevated nearly 200% by 6 weeks posthatch (Velleman et al., 1996) and LSN pectoral muscle satellite cells exhibit reduced proliferation and differentiation characteristics (Li et al., 1997).
Low score normal birds exhibit an alteration in sarcomere organization (Velleman et al., 1997) which implies that the LSN defect extends beyond changes in decorin expression and collagen fibril assembly. Recently, Velleman et al. (2000) have shown that integrin expression is downregulated in LSN pectoral muscle during proliferation and differentiation. In determining a common mechanism that could affect both decorin and integrin expression, TGF-b-1 and -2 have been shown to regulate the level and molecular size of decorin (Bassols and Massagué, 1988), and integrin expression and assembly (Heino et al., 1989; Kagami et al., 1996). During LSN pectoral muscle development, TGF-b-2 is upregulated from day 20 of embryonic development through 1 week posthatch (Velleman and Coy, 1998). This elevation in TGF-b concentration corresponds to the increase in decorin expression. Therefore, it is hypothesized that the LSN muscle weakness involves, in part, a misexpression in TGF-b that alters a signal transduction cascade regulating decorin and integrin expression. The outcome of this defect would impact muscle development, growth, and function. Therefore, understanding how the LSN muscle weakness is inherited is critical in determining the biological mechanism involved in producing the LSN phenotype.
The inheritance of the MD trait in chickens has been studied. Asmundson and Julian (1956) reported the MD trait was inherited as an autosomal recessive with the gene designated am. However, the heterozygotes have wider breast than normals but exhibit histological changes similar to homozygotes. Therefore, some researchers consider the trait to be co-dominant (Julian, 1979; Somes, 1990). Holliday et al. (1968) reported that it was possible to produce four separate dystrophic lines with different characteristics. Thus, the MD trait is probably a polygenic trait with the am gene having a major effect (Somes, 1990). To date, the mode of inheritance of the LSN condition has not been published. The purpose of the present study was to determine the mode of inheritance of the LSN muscle weakness condition in chickens.