Phylogenetic distribution of cytasters. The tree shows different lineages where the absence or presence of cytasters has been determined by appropriate visualization techniques. Cytasters are absent in Cnidaria and Ctenophora (blue font), but have been described in a wide sample of both deuterostome and protostome lineages (dark font). The available data supports the hypothesis that cytasters evolved only in the lineage leading to Bilateria (asterisk in figure) and were already present in the most recent common ancestor of protostomes and deuterostomes. While the Ctenophora do not have egg cytasters, after polyspermic fertilization the centrosomes of the sperm carry out a similar role in re-organizing cortical cytoplasm and establishing the oral-aboral axis. The position of Ctenophora as a sister group to Bilateria is supported by morphological evidence but is controversial according to molecular evidence, with other possibilities indicated by dashed lines. This phylogeny was adapted from Stach (2008)  for chordate relationships, Regier et al. (2010)  for arthropod relationships, from Zrzavý et al. (1998)  for Ctenophora, and from Hejnol et al. (2009) , and Philippe et al. (2009)  for all other relationships.