How environment affects the mechanics of development in Dendraster
Organisms can develop normally and thrive in dynamic environments. The interplay between the environment and the biomechanics of morphogenesis can help us understand how development operates in a changing world. Effects of salinity on blastula expansion in the sand dollar Dendraster excentricus were used to investigate interactions among the environment, development, and mechanics. Salinity fluctuations, which are common in Dendraster's habitat, affect embryonic cell size. The hyaline layer, an extracellular matrix layer closely associated with the outside of the blastula, is hypothesized to resist blastula expansion. Four models describing how the hyaline layer responds to blastula expansion were compared. In three models the hyaline resists expansion as either an elastic, plastic, or viscous layer. In these models, salinity-driven changes in cell size were predicted to cause changes in the ratio of blastocoel volume to cell volume. In a fourth model, the hyaline is a perfectly accommodating layer that allows the embryo to expand freely as cells divide in a single layer. In this model, salinity changes were predicted not to affect the blastocoel-to-cell-volume ratio. To test these hypotheses, Dendraster embryos were placed in either 25‰ or 32‰ seawater wells and switched to wells of either the same or the other salinity. In a second experiment, Dendraster embryos were raised in 32‰ seawater until the 16-cell stage, at which point a subset of embryos were moved to 25‰ seawater for one cleavage stage and then moved back to 32‰ for the duration of the experiment. In both experiments blastocoel-to-cell-volume ratios were not affected by the salinity treatment. These results suggest that the hyaline layer does not resist blastula expansion in Dendraster, thereby maintaining a constant blastocoel-to-cell-volume ratio, despite salinity variation.