Subfunctionalization of the Ruby2-Ruby1 gene cluster during the domestication of citrus

Area of Science:

• Plant Biology
• Genetics
• Evolutionary Biology

Background:

• Anthocyanins contribute to plant coloration, but their synthesis is frequently lost in cultivated citrus.
• Primitive and wild citrus species can accumulate anthocyanins in leaves and fruits, unlike most cultivated varieties.

Purpose of the Study:

• To characterize the novel MYB regulatory gene, Ruby2, and its role in anthocyanin biosynthesis regulation in citrus.
• To elucidate the evolutionary history and regulatory mechanisms of the Ruby2-Ruby1 gene cluster in citrus.

Main Methods:

• Gene characterization of novel Ruby2 alleles (AbRuby2^Full and CgRuby2^Short).
• Analysis of gene interactions within the Ruby2-Ruby1 regulatory complex.
• Comparative investigation across primitive, wild, and cultivated citrus species.

Main Results:

• Different Ruby2 alleles exhibit opposing functions: AbRuby2^Full activates anthocyanin synthesis, while CgRuby2^Short represses it.
• CgRuby2^Short acts as a competitive repressor by interacting with CgbHLH1, a partner of CgRuby1.
• The Ruby2-Ruby1 cluster shows evidence of subfunctionalization across diverse citrus lineages.

Conclusions:

• The Ruby2-Ruby1 cluster plays a crucial, unique role in regulating anthocyanin biosynthesis and evolution in citrus.
• Subfunctionalization of the Ruby2-Ruby1 cluster explains the varying abilities of citrus species to synthesize anthocyanins.