Another possibility, Yuzu and Ichang papeda may be more like siblings, with varying amounts of C. reticula in their ancestry, Yuzu having much more than Ichang papeda.
I think it very likely that both of them grew naturalized in the wild, before Yuzu came under human cultivation. But the original C. reticula probably escaped from human cultivation in the first place, I would think.

So there is this wild population of original papeda, and then its entire gene pool is subject to genetic incursion from mandarin that some humans decided to grow in their mountain village high up in the cold mountains. So a range of different fruits result (with different mix ratios) and these coalesce mainly into two main groups, Yuzu and Ichang papeda. But it's probably a primitive Yuzu, possibly not exactly the Yuzu we know today. Then the Yuzu is taken out of the wild and grown by humans, while Ichang papeda grows in place where the original papeda once grew. Maybe the hybridization ended up conferring a survival advantage. All this does not preclude the possibility Yuzu could have descended from Ichang papeda, though it really does not matter. The two should be seen as siblings, I think. (Not direct siblings, of course, but I just mean in the same family, containing different mix ratios of the same original two ancestors)

If anyone has trouble visualizing this, here are some more tangible albeit hypothetical examples:
C. reticula x papeda = yuzu
Then yuzu x papeda = Ichang papeda
or
(C. reticula x papeda) x papeda = Ichang papeda
Then Ichang papeda x C. reticula = yuzu

Of course the actual lineages could easily have been more complex than that, if these mixed populations were growing in the wild for a long time.
We might not know the lineages exactly but I think it is safe to say we have a good idea what their first ancestors were, and in roughly what ratios their ancestry is.

Louisport,  it might be fun to germinate and grow a tree from a  pummelo seed, but unfortunately the tree will not turn out to be a pummelo.  Pummelo seeds do not produce  true from seed. - Millwt

That's not true. While it is true that pomelo seeds do not turn out exactly true to type, the seeds are very likely to turn out similar to their parents, assuming the tree was not pollinated by some other citrus species in the vicinity. It's when the citrus variety is a hybrid that you are likely to have seeds that do not grow true to type, but in that case such a citrus is likely to be polyembryonic, meaning most of the seeds formed asexually and are exact genetic clones of the parent. Unless any seeds formed sexually, in which case those won't grow true to seed at all.

They seem to be the strongest fruit trees in the property.

It might have to do with the rootstock. Grapefruits are often grafted onto shaddock rootstock, and when Chandler is grafted onto shaddock, you're basically grafting pomelo onto pomelo, so it's going to be much more compatible and the tree is going to grow more vigorously. In any case, pomelos can get to be quite big trees. When grown own-root, they can get to be up to 50 feet high under optimal conditions, and about just as wide. Grapefruit can get up to 45 feet high. Well you can see why rootstock is used.

The primary reason however rootstock is used is to induce a degree of incompatibility to cause the tree to be more pernicious and start beginning fruit production earlier in its lifespan (so the tree's juvenile energy won't be diverted to growth).

The only problem, what's going to happen when their supply of liquid nitrogen gets cut off ?
Organizations don't last forever, no matter how big and permanent they might seem now. Things in storage will eventually fall into neglect, and there won't always be someone to step in and supply the liquid nitrogen. Or there could be supply problems. Doesn't matter how temporary, it only takes 1 week for those seeds to go without cryogenic freezing and then they're done for.
This is only a temporary solution.

I wonder how many ancient fruit varieties no longer survive today? Wars, government collapses, government departments being shut down, severe economic and financial difficulties...

My seedlings are not doing so well:



Although the small one grew a little taller:



These have been inside a humidity controlled enclosure under LED grow lights.

The two on the right (first picture) with the yellowing leaves, I brought outside three different times when it started to get warmer, for periods not exceeding one and a half hours. The smaller plant I used as a control and did not bring outside. Well obviously the mangosteen seedlings do not like being outside, even for very short periods of time. The air was fairly cool and humid so I'm pretty sure it was not because they dried out. Perhaps sunburn? The temperatures when I brought them outside ranged from 64 to 72 F. But even the seedling that did not go outside seemed to suffer some slight reddening of some of its leaves.

How cold tolerant are most of the pummelos at present time?

Not cold tolerant at all. Pomelos are the most cold vulnerable out of all the citrus, more cold vulnerable than orange and more cold vulnerable than grapefruit. That's probably one reason you don't see pomelo very often in the U.S. 

It's not really surprising. View oranges and grapefruits as being on a continuum between mandarins and pomelo. Mandarins are the more cold hardy of the two, so in general the more mandarin it has in its ancestry, the more cold hardy the citrus type will be.

That's part of what makes this challenging. It would be quite an accomplishment to have a pomelo variety that could grow in zone 9 (let alone even thinking about it surviving in zone 8 ). And I am talking specifically about pomelo, not grapefruit, because although the two are very similar there is a fundamental difference in flavor between these two groups. Grapefruits have more of a pungent, slightly putrid smell, and carry a small degree of tangy orange flavor. The aroma of pomelo is a little bit more floral and clean, and subtle with nuances, and usually have less bitterness than grapefruit.

Chandler is probably somewhat more cold hardy than other pomelos, but it's still a little more vulnerable to cold than the average grapefruit. What I mean specifically, most Thai pomelo varieties could be grown in zone 10 but they wouldn't grow that well.

People in the U.S. just don't know that much about pomelos. They are a big thing in Southeast Asia. And they were traditionally grown in Japan, although now grapefruits are much more prevalent in Japan than native pomelo varieties because of cheaper grapefruit imports from the U.S. and fruit being very expensive to grow in Japan because of the limited land and high cost of living and labor costs. Another problem is the language barrier. In Vietnamese the word used for grapefruits is the same as for pomelo, so this leads to much confusion. When Vietnamese talk in English they most often think the fruit they ate in Vietnam is called grapefruit in America, and Vietnamese are the main Southeast Asian group that has immigrated to America. Pomelos are also popular New Year festival fruits in the Canton province of China, where they are usually translated into English as "Chinese grapefruit". The early introduction of grapefruits into the American market place sort of superseded the awareness of pomelo, and so it has become the grapefruit that stuck and became the popular one. There are some interesting old stories from the 1930s about Americans encountering grapefruit for the first time and not knowing what to do with them. One housewife, when given a grapefruit, tried boiling it for two hours in water thinking it was like a cabbage.

So basically the existence of grapefruit has displaced pomelo from gaining popularity in the U.S., even though the two are not exactly the same. And all the exports of American grown grapefruit to other countries are displacing the sales and awareness of pomelo even there. Grapefruit is more practical for the U.S. citrus industry to grow than pomelo, because grapefruits have been bred to be more cold hardy than when they first appeared in Jamaica.

Another issue, most of the Asian pomelo varieties would require a lot of heat to fully ripen and, as hot as Southern California is, all the area within zone 10 is in proximity to the coast and still does not get quite enough heat throughout the year to become sweet enough. This has no doubt hindered the introduction of pomelo into California and explains why there is such little variety. All three of the pomelo varieties (2 of them actually being grapefruit-pomelo hybrids) commercially available in the state resulted from hybrids of 'Siamese Sweet', a very sweet but mostly flavorless pomelo variety, and were bred by the University of California research station in Riverside. Pomelos ripen well around Miami (Florida) though, and probably would in Brownsville (Texas) too, I would imagine.

>> One correction to add: I have heard of Chandler pomelos being grown in zone 9b but these are places that have a lot of heat (Texas, Florida)

According to one genetic study
Citrus sp. Bitrouni lime
C. sp. Hybride Fourny lemon
C. pyriformis Ponderrosa lemon
C. lumia sp. Jaffa lemon
appear to be all C. maxima and C. medica

I don't know about Ponderrosa, that's supposed to be a hybrid between citron and regular lemon (itself sour orange x citron). Maybe the specific markers they were looking for for mandarin got eliminated from the hybridization.

In addition, the Kulu lemon had very little C. reticula in its genome. It's worth pointing out that the Kulu lemon has been observed to be highly cold tolerant, during temperatures down to 18 F.

This is a list of different citrus in order of how much cold they can handle:

kumquat > mandarin > orange > grapefruit > pomelo

I might write:

orange > lemon > lime

I don't want to put them with grapefruit because there are different grapefruit varieties spanning the range from being as cold-hardy as orange to being as cold-sensitive as lime. Although kieffer limes are about as cold-hardy as orange (probably since they are believed to have some papeda in their ancestry).

Minneola tangelos are probably somewhere between mandarin and orange. Clementines and tangerines span the range between mandarin and orange.

Not only are pomelos the most vulnerable to cold, they also need a lot of heat to ripen. Despite being a pomelo-grapefruit hybrid, Oroblanco is probably one of the cold-hardier grapefruits, being close to orange in cold-hardiness. (Due to its comparatively smaller fruit size and sweetness it is also as easy to ripen as common grapefruits)


Among the extremely cold-hardy citrus varieties:

trifoliate > Ichang papeda > Thomasville citrangequat > common trifoliate hybrids (citrange) > yuzu > taiwanica lemon

There is in general a trade-off between cold-hardiness and edibility.

Thanks.

That site in your link says it's a cross between trifoliate x Temple tangerine, then crossed with Ichang papeda, then that was finally crossed with a Minneola tangelo.

They are most likely the same thing then, probably the source got confused about the order of its pedigree, and the fact there was trifoliate in it. They actually mentioned "Temple Orange" specifically (also saying it is a Tangor) so I'm pretty sure it must be the same thing.

Yes, you can, but I imagine it won't do too well.

Yuzu is not true from seed. It has been used to make many hybrids.

That's not what "true to seed" means.

When we say true to seed, we're not talking about if it gets pollenized by another citrus variety.

I also read citrus seeds take much longer to germinate if they have been previously dried out (5 weeks) and that the seeds are also slower to germinate, which makes the sprouts more vulnerable to mold. There was a comment by someone else that citrus seeds are only good for about 2 or 3 weeks if not refrigerated, but sometimes they can last up to 3 months.
I doubt most online sellers are aware of all this. Does anyone else have any other comments about this?