Differential growth of skeletal muscle in mice selected divergently over 108 generations for low and high body weight

Experiments were performed on 180 mice from two lines dubbed light (L) and heavy (C), selected divergently for body weight over 108 generations. The main hypothesis was that the changes occurring in body weight and muscle weight as a result of directed divergent selection could be associated with changes in the transcription of some miogenic genes and/or with the level of proteins regulating myogenesis and with the composition of muscle fibres.Hind limb muscle masses from females and males of the two lines were weighed at 1 and 3 weeks and 3 month of age. Morphological analysis for histological cross-sections of the gastroenemius muscle was carried out in 3-week and 3-month-old mice. The percentage comparison of nuclei in muscle fibres were analysed, too. Levels of MYOD1, MYF-5 and myogenin at the same time points were determined using Western blotting. Microsatellite markers for MyoD1, Myf-5 and MYOG were used to compare allele frequencies of analysed genes in both lines. There were differences in muszle weight between the sexes at age of 3 months. Muscles of the hind limbs were heavier in males than in females by 23.7% in line C, and by 14% in line L. Significant differences in muscle mass were accompanied by changes in muscle fibre size. The number of large-diameter muscle fibres increased with animals’ age, and in females fibres of diameters of 60-80 μm accounted for 38% of the total in line C, as compared with 94% preponderance of smaller (20-60 μm) fibres in the muscle of line L females. The numbers of nuclei were clearly greater in line C than in line L individuals, as well as in 3-month-old animals as compared with those at 3 weeks of age.Selection have brought about change, not only in myogenesis, but also in the frequency of alleles of microsatellite markers MyoD1, Myf-5 and at the myogenin locus, thus suggesting that molecular differences between the lines have arisen. Differences in the levels of MYOD1, MYF5 and MYOG are evident between both sexes and the selected lines of mice.