We have made a wide phylogenetic survey of Pix proteins, which are constituents of vertebrate centrioles in most eukaryotes.
We have also surveyed the presence and structure of flagella or cilia and centrioles in these organisms, as far as is possible from
published information. We find that Pix proteins are present in a vast range of eukaryotes, but not all. Where centrioles are
absent so are Pix proteins. If one considers the maintenance of Pix proteins over evolutionary time scales, our analysis would
suggest that their key function is to make cilia and flagella, and the same is true of centrioles. Moreover, this survey raises the
possibility that Pix proteins are only maintained to make cilia and flagella that undulate, and even then only when they are
constructed by transporting ciliary constituents up the cilium using the intraflagellar transport (IFT) system. We also find that
Pix proteins have become generally divergent within Ecdysozoa and between this group and other taxa. This correlates with a
simplification of centrioles within Ecdysozoa and a loss or divergence of cilia/flagella. Thus Pix proteins act as a weathervane to
indicate changes in centriole function, whose core activity is to make cilia and flagella
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