Now that I think about it I always wondered if tiachang is truely a hybrid of ichang lemon and tiawanica lemon because the fruit is much smaller than both 'parent trees' and less hardy than either of them. I'm not sure how you could end up with a smaller fruit if they were bred together? Ichang lemons are the size of a medium sized grapefruit but kind of bell shaped. Tiawanica lemon (sour orange) are the size of a large flattened orange. Somehow I am more inclined to believe that tiachang is more closer to a hybrid of yuzu x some other small mandarin. I also think the seeds of tiachang look more like yuzu seeds.
When seedlings result from sexual recombination, it can be the luck of the draw for the genes it gets.
In this case it looks like this particular cross may not have gotten the best combination of genes from both parents.
Maybe someone should try making another hybrid between the two again.
I've been looking into the research for the origins of ichang lemon and taiwanica lemon, and I'm finding some conflicting information.
It's generally agreed that Ichang lemon resulted from a cross between C. maxima (pomelo) and C. ichangensis, but at least one source found more of gene marker correlation to Yuzu (C. junos) rather than C. ichangensis, which may or may not mean anything.
(Hybrid Origins of Citrus Varieties Inferred from DNA Marker Analysis of Nuclear and Organelle Genomes, Tokurou Shimizu, 2016, under 'Parentage analysis of parent–offspring dyads in the indigenous varieties', ninth paragraph down)
C. junos and C. ichangensis are closely related, if the one did not originate from hybridization/introgression of the other.
(not going to get into too much detail on this here)
Taiwanica lemon seems to have more obscure origins. Supposedly it originally came to Japan from Taiwan, and genetic marker studies seem to indicate it is an admixture of pomelo-type and mandarin-type (being equivalent to sour orange type). Apparently some cultivars of taiwanica lemon have fruits that can get the size of grapefruits, although the typical taiwanica lemon seen in the U.S. seems to be smaller. Apparently it is either endogenous to, or became naturalized in the wild in Taiwan, although it has since become endangered in the wild.
I suspect it may have come in part from a mandarin species closely related to C. tachibana which could have previously existed on the island (but this is speculation), due to the slightly elongated leaf shape and cold hardiness.
One genetic study found that C. depressa (Shikuwasa) and C. tachibana did not seem to have a direct relation, despite both being indigenous to Taiwan.
(Phylogenetic Relationships among Selected Citrus Germplasm among Selected Citrus Germplasm Accessions Revealed by Inter-simple Sequence Repeat (ISSR) Markers, Dequiu Fang, UC Riverside, 1998 )
However, Shikuwasa and Tachibana seemed to cluster closely together in another study.
(Hybrid Origins of Citrus Varieties Inferred from DNA Marker Analysis of Nuclear and Organelle Genomes, Tokurou Shimizu, 2016 )
The study (Shimizu 2016 ) also showed a high probability that Kishu (mandarin) was one of the parents of Natsudaidai (Tiwanica lemon), so one could infer that if there was any special hardiness passed down, it had to have come from the other unknown pomelo-type parent.
Also, Shikuwasa seemed to have citron/ichangensis ancestry (the two were difficult to differentiate in this particular analysis), whereas Natsudaidai (Tiwanica lemon) seemed to have pomelo ancestry instead. (bar graph of different varieties, Figure 4)
(of course they both have mandarin-type ancestry also)