Intergeneric Allotetraploid xBrassicoraphanus Fruits Display a Compound Phenotype Derived from the Parents Brassica rapa and Raphanus sativus

Abstract

Hybridization and polyploidization occur in many flowering plants. In particular, hybridization between different species or genera may produce novel plant species with neomorphic phenotype. Although hybrids from interspecific or intergeneric crosses are generally hard to obtain. xBrassicoraphanus, also known as Baemoochae, was successfully produced as a newly synthesized intergeneric allotetraploid between Chinese cabbage (Brassica rapa L.) and radish (Raphanus sativus L.). One of the distinct characteristics is a compound morphology of the xBrassicoraphanus pistil and silique. The pistil and silique of xBrassicoraphanus display discrete but mixed features of both Chinese cabbage and radish. In this study, we examined the reproductive development of xBrassicoraphanus focusing on the pistil and silique formation and dehiscence. xBrassicoraphanus produces the flowers whose organ structures are similar in shape to the parents, but with a silique formation intermediate between Chinese cabbage and radish silique. For example, distal segment of the xBrassicoraphanus silique resembles that of radish. These segments are indehiscent after maturation. However, the shape of proximal segment of xBrassicaraphanus fruit coincides with that of Chinese cabbages silique. Proximal segment form dehiscence zone of valves through the replum, allowing the segments to separate. Interestingly, an unusual transverse cleft appears at the junction of proximal and distal segment, forming a joint region structure. To investigate duplicate gene alteration in fruit development, we isolated homologs of major genes in the fruit development genetic pathway of Arabidopsis. We explained fruit development by comparing gene expression levels between xBrassicoraphanus and its parents. This study is an example to show how altered expression of duplicate genes after genome hybridization makes neomorphic phenotype.