A model for the domestication and diversification processes of modern citrus cultivars in Japan

A diverse set of cultivars in the genus Citrus have been recognized as derived from the natural hybridization of several founder cultivars. However, knowledge regarding the process of domestication and diversification of modern citrus cultivars from these founders is quite limited. We recently identified the parent(s) of 67 citrus cultivars. Three unique aspects were revealed by the inferred parentages of these modern citrus cultivars: 1) only a few cultivars have served as parents of these modern cultivars; 2) repetitive hybridization events have occurred between the same cultivars; and 3) some offspring have been backcrossed with the parental cultivar. According to these observations, a few citrus cultivars, namely, 'Kishu', 'Kunenbo', 'Yuzu', 'Tachibana' and sweet orange, have been key to the development of modern citrus cultivars. In the present study, we further investigated the emergence of modern cultivars to understand the process of their diversification and domestication from a few cultivars. We developed the following hypothetical scenario to explain the domestication and diversification of different modern cultivars. First, a few pivotal citrus cultivars were cultivated together in the same region for an extended period. Next, seedlings with attractive characteristics arising by open pollination were consciously selected and maintained for production. Finally, some of the maintained seedlings occasionally backcrossed with a parental cultivar. Historical descriptions of these pivotal cultivars, which were widely grown throughout Japan, are consistent with our proposed model.