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Date of Award

Spring 2018

Degree Name

Bachelor of Science



First Advisor

Terri A. Williams

Second Advisor

Robert J. Fleming

Third Advisor

Daniel G. Blackburn


Segmentation is a key feature of arthropod diversity. Most arthropods add segments during development from a posterior region called “the growth zone”, which is the site of elongation and segment patterning. While segment patterning is studied in diverse taxa, the cell behaviors underlying elongation are less well known. The prediction for the growth zone is that there is a posterior region of undifferentiated cells dividing continually to provide the tissue required for new segments. We tested this model by examining cell division patterns in the posterior growth zone in a crustacean, Thamnocephalus platyurus, that adds segments after hatching. Our findings do not support this model of growth zone elongation. By looking at cells undergoing either DNA synthesis or mitosis, we found that the number of cells in mitosis in the growth zone is surprisingly low, and the cell cycle in the growth zone is slow but synchronized. Interestingly, our data show that DNA synthesis is spatially organized in the growth zone, with distinct anterior and posterior domains of cell cycling. Cells in the anterior growth zone undergo an apparent synchronization, resulting in all cells of the newly specified segment undergoing DNA synthesis. The domains of DNA synthesis suggest the cell cycle is compartmentalized within the growth zone. This idea is confirmed by discrete domains of expression of cell cycle regulators, string, cyclin A and cyclin E. The cell cycle domains in the growth zone are correlated with expression of the segment patterning genes, Wnt and caudal, suggesting that cell cycle and segment patterning are tightly integrated. When cells are blocked in DNA synthesis by hydroxyurea, segment addition is disrupted, confirming that cell cycle regulation is integrated with segment patterning in the growth zone. Overall in the growth zone, we find low numbers of cells in mitosis, and a slow but synchronized cell cycle that is tightly linked to segmental patterning.


Senior thesis completed at Trinity College, Hartford Connecticut for the degree of Bachelor of Science in Biology. Full text access is limited to the Trinity campus community.