Expression Pattern of the Hox Genes in Caprellids That Have Lost Some of Their Thoracic Legs and Abdominal Segments

The unique body plan of caprellids is likely to have derived from the gammarid body plan through the loss of abdominal segments and walking legs in the fourth and fifth thoracic segments. In this study, we investigated whether the caprellid body plan evolved through changes in Hox gene expression. We found that most caprellid Hox genes exhibit a comparable expression pattern with that of gammarids. Notably, <i>Ubx</i>, which plays a crucial role in the development of the morphology of the thoracic legs of gammarids, is expressed in the corresponding thoracic segments of caprellids. Therefore, it is likely that the evolutionary loss of the thoracic legs in caprellids occurred through the modification of a downstream target of <i>Ubx</i>. This expression pattern is consistent with a previous report that gammarid <i>Ubx</i> is required for gill development because caprellids develop gills in the thoracic segments where the legs were lost. We also found that caprellids possess <i>Abd</i><i>-</i><i>B</i>, the ortholog of which in gammarids is expressed in abdominal segments. Caprellid <i>Abd-B</i> is specifically expressed in the thoracic legs, and this neofunctionalization may have helped to retain this gene in the caprellid genome.