Rhobs el Arsa
The Rhobs al Arsa has long been viewed as a citron hybrid, though a different cultivars have been suggested for the parent, for example, Moroccan citron as a male parent and sour orange as the female parent, citron or lemon and a mandarin., or citron and the lime. Detailed genomic analysis has shown Rhobs al Arsa to be highly similar to several cultivars of "Citrus limetta", the limetta or Persian sweet lime, and likely shared a common origin with them, arising from a cross between citron ("Citrus medica") and sour orange ("Citrus" × "aurantium"). The common lemon arose from a distinct hybridization event involving the same two species.
Citrus taxonomy
The trifoliate orange is a cold hardy plant distinguishable by its compound leaves with three leaflets and its deciduous nature, but is close enough to the genus "Citrus" to be used as a rootstock. Swingle moved the trifoliate orange from "Citrus" to its own genus, "Poncirus". It was long thought to be the only member of this genus, until a second species, "Poncirus polyandra", was discovered in Yunnan (China) in the 1980s. Early phylogenetic analysis nested "Poncirus" within the citrus, suggesting they all belong to a single genus, but the genomic sequencing of Wu, "et al.", placed it outside the cluster representing "Citrus", and the authors retained a separate "Poncirus" genus.