|Members of the genus Hydra|
Members of the genus Hydra are small, simple fresh-water animals. They are predatory, eating primarily small invertebrates like Daphnia and Cyclops. They are typically sedentary, but they can move around, especially when hunting, using a somersaulting motion (there are lots of videos of this on youtube).
They have a very simple body plan, composed of only two cell layers. They don't have muscles or brains, only a basic epidermis on the outside and a gastrodermis on the inside, which are separated by a gelatinous substance called mesoglia. Hydra usually reproduce asexually by budding (and live practically as immortal organisms) when environmental conditions are good, and sexually when conditions are harsher. In these organisms, the differentiation between body cells and gametes (sex cells) is mediated by interstitial stem cells (ISCs).
|Hydra species with tumors|
|Tumors in Hydra are the|
result of arrested gamete
When the authors looked at overall gene expression in both the normal and the tumor-affected Hydra, they found that the tumors have a very distinct gene expression profile that was quite different from the normal female Hydra cells - and that this difference in gene expression occurs very early on in egg development. In other words, while the process of egg and tumor development start out the same in Hydra, the differences in gene expression between the two cell types is what leads to a normal sexually reproducing Hydra, and a tumor-affected Hydra.