FIGURE 2.

A schematic overview of the Ronshaugen et al. (2002) experiments. Drosophila Ubx promotes abdominal identity and acts to repress limb development in the abdomen. Artemia Ubx is assumed (*) to promote abdominal identity but not repress leg development. Consistent with this, when full-length Artemia Ubx was expressed in Drosophila, it was capable of promoting abdominal identity but could not repress leg development. Deletion of a C-terminal region of the Artemia Ubx (dashed oval) conferred leg repression capacity. This led to the conclusion that evolution of the C-terminal domain was a critical aspect of the evolution of leg repression function in the Ubx lineage in arthropods. However, there are several considerations that should be kept in mind. First, additional studies suggest that there may be multiple reasons why Artemia Ubx does not repress leg development (Hsia et al., 2010). Second, the ideal test would be to determine whether the truncated Artemia Ubx could repress legs if placed back into the endogenous Artemia context. Without this experiment, it remains possible that the observed function is simply a product of the Drosophila genomic context, in which Ubx normally represses leg development. Given everything we know now, the most conservative interpretation is that clearly Artemia Ubx is not biochemically equivalent to Drosophila Ubx. These biochemical differences may have been critical for the evolution of limb repressing functions, but studies in Artemia itself, as well as other arthropods would be necessary to confirm this hypothesis.