Reproduction in woody perennial Citrus: an update on nucellar embryony and self-incompatibility

Citrus is one of the most important and widely grown fruit crops. It possesses several special reproductive characteristics, such as nucellar embryony and self-incompatibility. The special phenomenon of nucellar embryony in citrus, also known as the polyembryony, is a kind of sporophytic apomixis. During the past decade, the emergence of novel technologies and the construction of multiple citrus reference genomes have facilitated rapid advances to our understanding of nucellar embryony. Indeed, several research teams have preliminarily determined the genetic basis of citrus apomixis. On the other hand, the phenomenon of self-incompatibility that promotes genetic diversity by rejecting self-pollen and accepting non-self-pollen is difficult to study in citrus because the long juvenile period of citrus presents challenges to identifying candidate genes that control this phenomenon. In this review, we focus on advances to our understanding of reproduction in citrus from the last decade and discuss priorities for the coming decade.