Pitx homeobox genes in<i>Ciona</i>and amphioxus show left–right asymmetry is a conserved chordate character and define the ascidian adenohypophysis

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Abstract

<jats:p><jats:bold>SUMMARY</jats:bold>All vertebrates have directional asymmetries in the organization of their internal organs. In jawed vertebrates, development of asymmetry is controlled by a conserved molecular pathway that includes<jats:italic>Pitx2</jats:italic>, which is expressed by lateral plate mesoderm cells on the left side of the embryo.<jats:italic>Pitx2</jats:italic>is a member of the Pitx homeobox gene family, the expression of which also marks stomodeal ectoderm and the adenohypophysis. Here we report the characterization of Pitx genes from<jats:italic>Branchiostoma floridae</jats:italic>(an amphioxus) and<jats:italic>Ciona intestinalis</jats:italic>(a urochordate), representatives of two basal chordate lineages and successively deeper outgroups to the vertebrates. Expression of<jats:italic>B. floridae</jats:italic>Pitx is similar to that reported from<jats:italic>B. belcheri</jats:italic>, a different amphioxus species. Expression of the<jats:italic>Ciona</jats:italic>Pitx ortholog in the embryonic primordial pharynx and adult neural complex leads us to propose the<jats:italic>Ciona</jats:italic>primordial pharynx and ciliated funnel are homologous to the adenohypophyseal placode and adenohypophysis, respectively. Additionally, in both species we identify asymmetrical left‐sided expression of Pitx genes during embryonic development. This shows that asymmetrical Pitx gene expression, and by inference directional asymmetry, evolved before the radiation of living chordates and should be considered a chordate character.</jats:p>

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