This project will examine the genetic and epigenetic relationships involved in the gene expression, cell differentiation, and cell proliferation in the developing limbs of salamander species. Models for primary pattern formation in the limb predict different relationships between these processes in the "progress zone " and underlying cartilaginous condensations. The 'positional-valve' model predicts that the developmental transitions seen in these species likely result from the reduction in size of the progress zone or reduced transit times, i. e. increased cell proliferation, for cells in the progress zone. The epigenetic model for primary chondrogenic pattern in the limb predicts that these morphological differences would result from reduced cellular proliferation in the progress zone, with no changes to its size. Predictions of the two models will be emperically tested in salamander species differing in limb morphology. We will test predictions of a third hypothesis involving epigenetic inheritance which proposes that heritable changes in structural characteristics of the genome may have tissue-specific effects on cell cycle parameters, resulting in well integrated changes in developmental timing and morphology. In the Urodela, genome size increases are hypothesized to disproportionately increase S phase and total cell cycle durations in undifferentiated mesenchyme, resulting in the observed patterns of limb paedomorphism and genome evolution. In addition to measurements of in situ cell proliferation in the embryo, this study will estimate the "proliferative histories" of numerous, differentiated cell types in the salamander forelimb by measuring mean telomere lengths. This novel approach relies on the principle of telomere shortening (approximately 100 base pairs per division cycle) observed in almost all somatic cells. Quantitative fluorescence in situ hybridization (Q-FISH) will be used to make relative measures of past mitotic activity of cells expressing tissue-spesific genes and gene products. In summary, this project will provide an in depth comparison of the proliferation requirements of cell populations at various states of determination and differentiation in the developing limbs of species differing in limb morphology and, thereby, allow the testing of competing limb patterning models.