Cytasters (or cytoplasmic asters) can be found in the cortical cytoplasm of the bilaterian egg and zygote; they are centriole-based nucleation centers of microtubule polymerization. Cytasters play an important role in development, and are thought to be responsible for the organization of the ectoplasmic domain where networks of connected cytasters form the cortical microtubule cytoskeleton in the ectoplasm.
Salinas-Saavedra and Vargas propose a novel hypothesis in EvoDevo today regarding the conservation of cytasters as a homologous feature, found in Bilateria and already present the common ancestor to both protostomes and deuterostomes. They collate and compare published evidence from numerous studies, identifying an almost unanimous trend for the presence of cytasters in bilaterian species (with notable differences discussed in mice, rabbits and cows). The authors propose that the similarities in the development of cytasters seen across the majority of the studied species suggest that this trait is conserved and evolutionarily ancient, and thus support the hypothesis of a homologous rather than homoplasic origin of cytasters.
Cortical cytasters: A highly conserved developmental trait of Bilateria with similarities to Ctenophora
Miguel Salinas-Saavedra, Alexander O Vargas EvoDevo 2011, 2:23 (1 December 2011)
Abstract | Provisional PDF
Comparisons to the zygote cellular organization patterns seen in Cnidaria and Ctenophora are used to expand the hypothesis to suggest that the evolutionary origins to cytasters may be found in an ancestor with polyspermic fertilization. This leads to the suggestion that the phylogenetic tree of eumetazoans may be resolved by placing Ctenophora as a sister group to Bilateria, with a polyspermic common ancestor. They suggest that cnidarians may represent a primitive condition, with a single microtubule network and corresponding lack of differentiated cortical cytoplasm.
Further research is needed across a number of unrepresented phyla, to which this hypothesis was unable to draw conclusions regarding in identifying cytasters. The authors feel that there is strong support for homology of cytasters, citing the absence of these (as seen in cows) as a secondary loss. Further evidence is required in a number of phyla, for which there was no published research deciding on the presence of absence of cytasters, in order to explore this theory further.