Apoptotic gene loss in Cnidaria is associated with transition to parasitism

The phylum Cnidaria consists of several morphologically diverse classes including Anthozoa, Cubozoa, Hydrozoa, Polypodiozoa, Scyphozoa, Staurozoa, and Myxozoa. Myxozoa comprises two subclasses of obligate parasites—Myxosporea and Malacosporea, which demonstrate various degrees of simplification. Myxosporea were previously reported to lack the majority of core protein domains of apoptotic proteins including caspases, Bcl-2, and APAF-1 homologs. Other sequenced Cnidaria, including the parasite Polypodium hydriforme from Polypodiozoa do not share this genetic feature. Whether this loss of core apoptotic proteins is unique to Myxosporea or also present in its sister subclass Malacosporea was not previously investigated. We show that the presence of core apoptotic proteins gradually diminishes from free-living Cnidaria to Polypodium to Malacosporea to Myxosporea. This observation does not favor the hypothesis of catastrophic simplification of Myxosporea at the genetic level, but rather supports a stepwise adaptation to parasitism that likely started from early parasitic ancestors that gave rise to Myxozoa.