In this post I hope to help cast some light on the mystery of what Bloomsweet actually is, which lineages of citrus exactly it came from.
Firstly, it is known that Bloomsweet was probably brought to Texas by immigrant citrus orchard growers from Japan. The Japanese variety it corresponds to appears almost certainly to be a variety named Kinkoji.
(I wonder if this could have anything to do with how the extremely rare
Chorioactis geaster mushroom came to be naturalized in parts of Texas, but that's a very separate topic)
According to the following study, Kinkoji appears to be a direct hybrid between a Kunenbo and a pomelo-type citrus.
Hybrid Origins of Citrus Varieties Inferred from DNA Marker Analysis of Nuclear
and Organelle Genomes, Shizuoka, Japan
http://journals.plos.org/plosone/article/file?id=10.1371/journal.pone.0166969&type=printableThe obvious question arises as to why then the Bloomsweet (Kinkoji) is so cold-hardy. Neither Kunenbo nor Japanese pomelo-type citrus is extremely cold tolerant. What is more likely, I think, is that the pomelo-type parent may itself actually have been a pomelo-yuzu hybrid.
Yuzu carries genes from a papeda ancestor, but this papeda ancestor is not exactly from the same direct lineage as the Ichang papeda. Likely this old ancestor has been lost.
It has been speculated that the Bloomsweet is a hybrid between pomelo and Ichang papeda, but I do not believe this theory is actually correct, or historically likely.
There exist several traditional citrus varieties in Japan that had a Yuzu parent. One of these is Sudachi, which is used more often than Yuzu in the Tokushima region of Japan. Despite what some sources seem to suggest, Sudachi originated in Japan and was not directly descended from
C. Ichangsis (though it might not be entirely inaccurate to describe these fruits as having C. Ichangsis in their genetic lineage).
The reason Bloomsweet is so cold-hardy probably has to do with genes originating from papeda and passed down through yuzu, in combination with genes from mandarin (which does have more cold-hardiness than Orange, but still is not extremely cold-tolerant). The particular mandarin variety from which all Japanese mandarins descended is believed to be Kishu, which originally came from China.
The hypothesized yuzu-pomelo parent might have had cold-hardy yuzu genes but it also probably had genes from pomelo that made it vulnerable to cold. These genes would not get bred out until the next breeding cross. And a yuzu-pomelo hybrid would not have been very valuable for Japanese to propagate at that time. It would have been much less edible than a regular pomelo, but less useful than other yuzu-type citrus for flavoring food. It was probably just a novel citrus cross that was not preserved. Someone probably took seeds from it to see if anything more useful could come from it. And that was a cold-hardy pomelo-like fruit, the Kinkoji.
But here is something else I found. Hyuga-natsu is believed to be cross between pomelo and yuzu. (It's grown in the far South of Japan)
http://shun-gate.com/en/roots/roots_59.html It's possible Bloomsweet may have come from Hyuga-natsu.
Both Kinkoji and Hyuga-natsu are self-incompatible varieties.
Self- and Cross-incompatibility of Various Citrus Accessions, Kagoshima University
http://www.readcube.com/articles/10.2503/jjshs.75.372 That means that any seeds that were taken from a Hyuga-natsu would have been hybrids of some other citrus growing in the vicinity.
It also suggests anyone in a Northern climate trying to grow a Bloomsweet alone all by itself will have very poor to no fruit set if they don't also have another citrus variety. But I'm not too sure about this.
So here is my proposed ancestral lineage diagram for Bloomsweet:
...............papeda.............sour mandarin
...................l_______________l
................................l
..pomelo.................yuzu........sweet orange....kishu mandarin
.......l_____________l..................l___________l
..................l........................................l
..........Hyuganatsu (?)......................kunenbo
..................l_______________________l
......................................l
.................................Kinkoji
............................(Bloomsweet)
If this diagram is valid, that would put the ancestral composition of Bloomsweet at
43.75% mandarin, 12.5% sour mandarin
12.5% unknown papeda ancestor
and 31.25% pomelo ancestry, if we include the fact that sweet orange has 25% pomelo ancestry
I suspect that Bloomsweet's mitochondrial DNA (along the seed parent line) originated from pomelo.