Ponderosa lemon
The ponderosa lemon originated in roughly 1887, and is believed to come from a chance seedling grown in Hagerstown, Maryland. It was later named and introduced to the nursery trade in 1900. Though often referred to as a cross between the lemon and citron, a recent genomic analysis showed it to solely contain pomelo ("Citrus maxima") and citron ("Citrus medica") DNA, perhaps being an F2 hybrid. Thus they cannot derive from true lemons, which have a mandarin orange component.
Citrus taxonomy
Australian and New Guinean citrus species had been viewed as belonging to separate genera by Swingle, who placed in "Microcitrus" all but the desert lime, for which he used "Eremocitrus". However, genomic analysis shows that though they form a distinct clade from other citrus, this is nested within the citrus phylogenetic tree, most closely related to kumquats, suggesting that all these species should be included in the genus "Citrus". Wu, "et al.", found that several of the finger lime cultivars were actually hybrids with round lime, and concluded there were just three species among those tested, desert lime ("C. glauca"), round lime ("C. australis") and the finger lime ("C. australasica"), though their analysis did not include other types previously identified as distinct species. In more limited genomic analysis, the New Guinea wild lime, "Clymenia" and "Oxanthera" (false orange) all cluster with the Australian limes as members of "Citrus". The outback lime is a desert lime agriculturally-selected for more commercial traits, while Some commercial varieties of the Australian lime are hybrids with mandarins, lemons, and/or sweet oranges. "Clymenia", will hybridize with kumquats and some limes.