Tudor genes of Holozoa: Early evolution and within Metazoa diversification of a multifaceted protein family

Early metazoan evolution was characterized by the expansion of many gene families involved in novel multicellularity-related functions, like the Tudor family. In eukaryotes, Tudor genes are numerous and heterogeneous, mostly associated with gene expression regulation. However, the family underwent a lineage-specific expansion in animals, with novel elements almost exclusively involved in the germline-specific regulation of retrotransposons through piRNAs (as spatiotemporal regulators of the key-element Piwi, another previously supposedly animal-specific gene). In the present analysis, we used online-available proteomes for a total of 25 major taxonomic groups to characterize the Tudor gene family at a holozoan-wide level, and we confirmed the apomorphic expansion of piRNA-related Tudor genes in animals. However, we could also interestingly observe the presence of elements of the piRNA pathway, both Tudor and Piwi genes, in some Ichthyosporea species, suggesting that some elements of the...