Past Research

Does the posterior region in the arthropod embryo – commonly called a growth zone – actually grow while segments are added?

The most common means of developing segments is to add them one at a time from a posterior region of the animal, called the “growth zone”. This region has been the focus of intense study to discover the regulatory gene networks that pattern new segments. We focused on a less understood feature of the growth zone, the cell behaviors that underlie elongation and lead to the progressive budding of segments. Our international collaborative team (Williams at Trinity College, Nagy at University of Arizona, and Chipman at University of Jerusalem) examined three different species: the flour beetle Tribolium, the milkweed bug Oncopeltus, and the fairy shrimp Thamnocephalus, and used a parallel set of measures to describe growth and elongation from the posterior during segmentation in each species. Our description took into account changes in tissue dimensions over time, gene expression patterns and cellular behavior.

These results are the first consistent cross-species dataset of growth zone dynamics, and they have uncovered a number of previously unknown features. We found in two species that the growth zone is functionally divided into two parts: a posterior domain, where there is stable gene expression and relatively high rates of cell cycling, and an anterior domain, with dynamic gene expression and low rates of cell cycling. We found that cell behaviors underlying elongation are not constant over time, both cell division and cell movement vary during normal segment addition and, surprisingly, the rate of segment addition is variable in the flour beetle. We used a computational model of the beetle embryo to support our hypothesis that rapid segment addition can occur without significant addition of new cells. Finally, in the two species where segment addition spans distinct regions of the body axis, we find a clear correlation between axial position and growth zone cell behaviors. These data support a model of growth zone cell dynamics that displays surprising spatio-temporal variability and potential adaptability among arthropods.