Why do tubulin gene families lack diversity in flagellate/ciliate protists?

Considerable amino acid sequence diversity is found among tubulin isotypes encoded by tubulin gene families in animal, higher plant, and fungal systems. In contrast, relatively little diversity is found among the isotypes produced by the gene families in a number of flagellate or ciliate protists. It is possible that proper assembly of the axoneme requires a homogeneous pool of tubulin subunits and that the axoneme thus provides a stringent selection against amino acid replacement substitutions among tubulin genes in these systems.