Cell lineage in the dorsal mesothoracic disc of Drosophila

Problems of cell lineage within the dorsal mesothorax of Drosophila are treated in a comprehensive way. The study utilizes both gynandromorphs and induced mosaics which have chaetae genetically “marked” at various developmental stages. It includes regions not previously studied due to lack of chaetae by inducing mosaics on a hairy background. It analyzes the distributions of spots by a combination of statistical techniques. Lineage relationships among the 11 mesonotal macrochaetae are traced by comparing correlation data from different samples. In general, the distances between pairs of macrochaetae are inversely proportional to phi correlation values. Related cells remain together and cell lineage pathways become gradually restricted during development. The dorsal suface of the mesonotum is divided into 63 different sections. Analysis of their interrelationships confirms results obtained by the study of macrochaetae, since distance between macrochaetae or sections does play a role in their lineage relationship and each section studied is significantly correlated with its surrounding and neighboring sections. Overall lineage relationships are defined by dividing the dorsal mesothorax into clusters of related sections by means of principal-component analysis. In some sections which are intimately related by cell lineage, the role of distance is secondary because of differential growth patterns. In addition, there are regionally variable sensitivities of developing cells for the induction of somatic crossing over, which may imply differential mitotic rates. Cell lineage of the mesonotum is nonrandom, in that the spacial relationship among cells developing from the same line is preserved throughout ontogeny, with modifications by differential growth rates.