Fertility relationships among Citrus and its relatives in the subfamily Aurantioideae

Data on fertility relationships between crop species and their relatives is of immediate practical value becausethey can be used to predict the ease of introgression of valuable traits into cultivars. Likewise, those dataare of evolutionary significance as evidence of phylogenetic relationships. Here, we review what is knownregarding the relative cross-compatibility of various Citrus taxa with each other as well as with membersof other genera in the Rutaceae subfamily Aurantioideae. For the most part these data were collected ascollateral information from breeding studies and therefore are uneven within the group. We compare howthe extant fertility relationship data support the various molecular phylogenies of the Aurantioideae. Weconclude with the identification of significant gaps in the data that could be filled with a series of strategic andfeasible crosses for the purposes of better defining the systematic relationships of the group as well as guidingfuture citrus improvement by breeding.