Studies on Transition in Diplacus and other Plants

YUCCA brevifolia/jaegeriana transition

One of the most interesting transitions between two species is that of Y. brevifolia/jaegeriana (blue area).

Mapped plots mostly from literature (so far)

We visited Tikaboo Valley in 2017 and again in April of 2019. There is a gradual change in the tree heights from 4.5 meters in the southeast, 3 meters or less along the Extraterrestrial HWY, and 1.5 m in the northwest. The whole transect measures over 8000 meters. The plants in the northwest are Yucca jaegeriana; the trees in the Southwest are Yucca brevifolia; and the trees along the road are short- or intermediate-sized. See Figure 3 for more. Remarkably, between the real Yucca brevifolia and the intermixed population lies a 5000-meter gap, but the intermixed populations neighbor the real Yucca jaegeriana populations.

The genomic chloroplast DNA does not differ (greatly) in any coding DNA for the about 130 genes of this circular genome between the two species of Yucca in question; it's rather an up- or down-regulation of up to 15 STRs of the kind polyA or polyT. As the chloroplast genome is a gift of the mother plant, any pollinator cannot influence this system because pollen does not contain chloroplast DNA in these two species. Also, the occasional dispersal of seeds of the "tall" species into the area of the "small" species does not help. One should detect this in the mixture of both kinds at the same spot, which is not the case. According to Figure 4B, both cp-types are kept separate even in the so-called hybrid zone.

It's also not very smart to help out of this situation with any fantastic route in selection theory, only to explain nothing.

After us, the most probable way is that an internal mechanism can switch the whole cpGenome in a burst from the jaegeriana kind to the brevifolia kind as a last step by becoming from a jaegeriana Yucca type to a brevifolia Yucca type as soon as the plant has gained acceptable heights.

Note: Short tandem repeats are often used as microsatellite markers. They most probably have a functional cue. They serve as enhancers, even though they are located between genes in the chloroplast genome, which is important to know.

The lengths of these STRs vary but differ among species.

In other words, the so‑called hybrid plants are Y. jaegeriana individuals in transition toward Y. brevifolia. This process may take time and can extend across more than one or two plant generations.
Ideally, long‑term monitoring should be conducted in this area, accompanied by high‑quality, whole‑genome PacBio SMRT sequencing. Reference whole‑genome sequences for Yucca jaegeriana and Y. brevifolia would also be extremely valuable.

Comparing the partial F'3H mRNA of 3 Yucca species reveals

that this coding region is identical for all three taxa.

Another approach would be to compare trunk DNA with leaf DNA from the so‑called hybrid plants to verify that the genomic DNA and the cpGenome are not shifting.

We believe that in the area depicted above, exclusively jaegeriana plants will transform into brevifolia plants. The reasons are the following:
- The cp_genome is mostly of jaegeriana structure. The coding region of the cp genes is identical in both varieties. The differences are found in the different lengths of the STRs. A molecular mechanism is hypothesized to make the appropriate adjustments when needed. Also, the coding regions of the core genomes of both varieties are almost identical. Based on the sequences published so far, no interpretation of intronic or intergenic DNA is possible. The noncoding regions of any genome must be evaluated when making evolutionary statements.
Here, we depict the "watershed" between the brevifolia cp chloroplast (southwest) and the jaegeriana cp chloroplast (northeast) using purple dots.
The yellow line indicates the 50/50 "hybrid index". The blue dots indicate the just-starting line of transformation. The phenomenon may shift over the next few decades from southwest to northeast in this area.

We used 2006, 2011, 2012, 2013, 2020, 2025A, 2025B as references

Legend for slides:

RD00A: Single Y. jaegeriana scrub in flat, slightly ascending semi-desert landscape. The scrub has a very short trunk (2019).

RD01A: Three pioneer trees of the type brevifolia, which are not expected here (photo-copter footage, 2019).

RD01: Increasing variability of scrub size and heigths (2017, photo-copter footage 2019,

RD02: More frequent and well-developed and massive scrubs of the almost jaegeriana type, developing a trunk of different lengths (2017)

RD02A: Josuah tree-like shaped individuals, but variable trunk lengths (2017)

RD03A: Josuah tree-shaped individuals in a loose forest (2017)

Nev_TR01: Map showing the sites in a transect formation (2026 Rolfy).

For us, the scrubs undergo a gradual change along this transect, showing a cline that ascends by 100 meters over the whole length, with a sudden transition from a spherical shape to a Joshua-shaped tree with a trunk. The reason might lie in climate warming; thus, at somewhat lower temperatures, a spherical plant (real Y. jaegeriana) can create its own microclimate at its center, more comfortable than in the open field.

Not so when the environmental temperature begins to rise successively. The plant is forced to alter its trunk length to escape the very high ground temperature. High ground temperatures (> 40 °C) are an enemy of photosynthesis. Three meters above ground, the peak temperature is lower, enabling (CAM)-photosynthesis. That is exactly what the huge Joshua trees realize. - However, in California, in lower situations, even for Joshua trees, the climate becomes too harsh, and the plants will partially die.

Unhealthy Josuah Tree near Victorville, CA.

Note: We are not inclined to perform any molecular work on this topic; we want to signal that the interpretation of this zone by different scientists is reaching a dead end when focusing solely on the selection route.

N.B. Compare this sketch with that we reported about Diplacus and Antirrhinum, which look, in a molecular respect, rather alike.

(will be continued 04-11-26, rolfy)

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