Clonal origins of somites and their muscle derivatives: Evidence from allophenic mice

Clonal origins of somites and their muscle derivatives have been studied in allophenic mice with cellular genotypic mosaicism for two different homozygous allelic electrophoretic variants of the dimeric enzyme glucosephosphate isomerase (GPI). With new methods permitting GPI type(s) to be ascertained in extracts of single somites, individual somites collected from 8- to 9-day allophenic embryos were found to contain both electrophoretic types of GPI cell strains in most cases ( 30 38 somites). Therefore, a somite is not a clone; each somite must arise from at least two precursor cells. Although genotypic proportions varied among somites of an embryo, a sample of sequentially identified somites showed small groups of 2-3 neighboring ones with similar proportions, indicating that somites within each group may have developed from a smaller pool of at least two presomite-stage precursor cells. Individual eye muscles, each formed from myotome of a single somite, were obtained from postnatal allophenic mice. Each muscle tested contained both pure-strain GPI types; most ( 22 33 muscles) also had the heterodimeric type of GPI indicative of heterokaryon formation by fusion of uninucleated myoblasts of the two pure strains. Thus, the myotome component of a single somite is also not a clone and must have arisen from at least two precursor cells; the actual clonal number may be closer to 5 per myotome. Mosaicism without heterodimeric GPI in 33% of the eye muscles indicates fusions among cells of the same genotype and could be due chiefly to presence of very few clonal patches with few intergenotypic clonal interfaces. The variable mosaicism among allophenic eye muscles derived from the same myotome is a model for the phenotypic variability known to occur in muscles of women heterozygous for the X-linked Duchenne type of muscular dystrophy. The present data, together with results from earlier allophenic studies on other somite derivatives, are consistent with the possibility that the somite, at the time of its first appearance (late day 7), is already a collection of diverse determined clones which proliferated from clonal initiator cells genetically programmed at a presomite (day 5-7) period. This may have occurred in at least two stages, with determination of somites preceding specific and separate determination of their sclerotome, dermatome, and myotome components.