Elongation during segmentation shows axial variability, low mitotic rates, and synchronized cell cycle domains in the crustacean, Thamnocephalus platyurus

Segmentation in arthropods typically occurs by sequential addition of segments from a posterior growth zone, but cell behaviors producing posterior elongation are not well known. Using precisely staged larvae of the crustacean, Thamnocephalus platyurus , we systematically examined cell division patterns and morphometric changes associated with posterior elongation during segmentation. We show that cell division is required for normal elongation but that cells in the growth zone need only divide ~1.5 times to meet that requirement; correspondingly, direct measures of cell division in the growth zone are low. Morphometric measurements of the growth zone and of newly formed segments suggest tagma-specific features of segment generation. Using methods for detecting two different phases in the cell cycle, we show distinct domains of synchronized cells in the posterior. Borders of cell cycle domains correlate with domains of segmental gene expression, suggesting an intimate link between segment generation and cell cycle regulation. Summary Statement Posterior growth zone has synchronized cell cycle domains but shows little cell division during segment addition in a crustacean. Dimensions of the shrinking posterior growth zone change at tagma boundaries.