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Species-specific traits are thought to have been acquired by natural selection. Transcription factors play central roles in the evolution of species-specific traits. Hox genes encode a set of conserved transcription factors essential for establishing the anterior-posterior body axis of animals. Changes in the expression or function of Hox genes can lead to the diversification of animal-body plans. The tunicate ascidian Ciona intestinalis Type A has an orange-colored structure at the sperm duct terminus. This orange-pigmented organ (OPO) is the characteristic that can distinguish this ascidian from other closely related species. The OPO is formed by the accumulation of orange-pigmented cells (OPCs) that are present throughout the adult body. We show that Hox13 is essential for formation of the OPO. Hox13 is expressed in the epithelium of the sperm duct and neurons surrounding the terminal openings for sperm ejection, while OPCs themselves do not express this gene. OPCs are mobile cells that can move through the body vasculature by pseudopodia, suggesting that the OPO is formed by the accumulation of OPCs guided by Hox13-positive cells. Another ascidian species, Ciona savignyi, does not have an OPO. Like Hox13 of C. intestinalis, Hox13 of C. savignyi is expressed at the terminus of its sperm duct; however, its expression domain is limited to the circular area around the openings. The genetic changes responsible for the acquisition or loss of OPO are likely to occur in the expression pattern of Hox13.

Metamorphosis, a widespread life history strategy in metazoans, allows dispersal and use of different ecological niches through a dramatic body change from a larval stage [1, 2]. Despite its conservation and importance, the molecular mechanisms underlying its initiation and progression have been characterized in only a few animal models. In this study, through pharmacological and gene functional analyses, we identified neurotransmitters responsible for metamorphosis of the ascidian Ciona. Ciona metamorphosis converts swimming tadpole larvae into vase-like, sessile adults. Here, we show that the neurotransmitter GABA is a key regulator of metamorphosis. We found that gonadotropin-releasing hormone (GnRH) is a downstream neuropeptide of GABA. Although GABA is generally thought of as an inhibitory neurotransmitter, we found that it positively regulates secretion of GnRH through the metabotropic GABA receptor during Ciona metamorphosis. GnRH is necessary for reproductive maturation in vertebrates, and GABA is an important excitatory regulator of GnRH in the hypothalamus during puberty [3, 4]. Our findings reveal another role of the GABA-GnRH axis in the regulation of post-embryonic development in chordates.