Relatively recently it was found through molecular phylogenetic relationships that Citron, Mandarin and Pummelo are the three original true citrus fruits from which all other citrus are descendants. Interestingly, the earliest workers also believed that there were only three or four valid species of citrus. (Linnaeus 1753 and Hooker, 1875. Recent molecular studies have supported this concept. - Millet

Interesting.

So, what were the necessary crosses to produce say,

Sour orange
Sweet orange
Key lime (Mexican or West Indies)
Lemons such as Eureka and Lisbon?

.....Alan.

The limes for example came from a fourth species you did not mention. The limes are a cross between the unmentioned Citrus micrantha and Citron.

And the endless inter-generic crosses between genus Citrus and genus Fortunella from which Calamondins, Mandarinquats, Limequats, Sunquats have all been derived, and some of these are major crops in certain countries. For example, Calamondins grown off-season can make somebody a millionaire.

Not to mention the Trifoliates and their hybrids which are one of the most popular rootstocks used the world all over.

And the latest craze about the relatively conferred genus called Microcitrus where the Australian finger limes and their related species belong. They are now being hybridized with citruses, producing exciting new fruit variations.

And lastly, the wild relatives of citruses that can confer some greening disease resistance, to which have never been before used as citrus parent materials of the cultivated western world citruses. That too will come in the future.

New genetic diagram should be shown to include the following in addition to the existing ones shown above:
Poncirus and their hybrids
Fortunellas and their Hybrids
Wild citrus relatives including the Papedas and their hybrids
Microcitruses and their hybrids (some of your names could be here)

I had to look up Citrus micrantha as in the photo it appears to be a Key lime, but apparently it's one of the progenitor species of it. Not sure yet what the other species in its make up are yet.

.....Alan.

Being relativey new to citrus, I had never run across the term papeda in Joe Real's thread, and so I began to look into it. I stumbled upon this list which looked (to me) to have relevance to this thread. I found it interesting, although I would be the last one to be able to determine if it is consistent with established theory of lineage. Barbara

http://www.everything2.org/index.pl?node_id=1315305

Citrus originated in the Malay Archipelago and in Southeast Asia. However, more recent evidence suggests that Yunnan province in south-central China may be the center of origin due to the diversity of species found, and the network of rivers in this area which could have provided "on route dispersal" to the south. - Millet