Note to: Hox gene cluster of the ascidian, , reveals multiple ancient steps of cluster disintegration during ascidian evolution.

Background: In the previous paper published in 2017, we described the structure of Hox gene cluster of the ascidian, , and discussed the scenario for the disintegration of Hox gene clusters during evolution of ascidians. The description about the Hox gene cluster structure still represents the latest information, hence it has been left unchanged. In contrast, some points in Discussion, the description on the phylogenetic relationships among tunicates and the theoretical scenario for the disintegration of Hox gene cluster during evolution of ascidians, should be changed because the phylogenetic relationships among tunicates have recently been updated.

Hox gene cluster of the ascidian, , reveals multiple ancient steps of cluster disintegration during ascidian evolution.

Background: Hox gene clusters with at least 13 paralog group (PG) members are common in vertebrate genomes and in that of amphioxus. Ascidians, which belong to the subphylum Tunicata (Urochordata), are phylogenetically positioned between vertebrates and amphioxus, and traditionally divided into two groups: the Pleurogona and the Enterogona. An enterogonan ascidian, (), possesses nine Hox genes localized on two chromosomes; thus, the Hox gene cluster is disintegrated.