Ambrosia x platyspina, a hybrid between Ambrosia dumosa and A. salsola
Important note! We have not yet verified that the single specimen discussed here is this hybrid, since we are waiting for it to bloom to nail it. But it seems highly likely that this single specimen is the hybrid, since we have no other possible determination for it. The hybrid is widespread through the rest of the desert in California, so if this is a hybrid, there ought to be more specimens not yet found.
A. dumosa | 
A. salsola | |
A. x platyspina | 
Leaf comparison | |
| Fig. 1. Top row and left pix in bottom row: comparison of the overall appearance for Ambrosa dumosa, A. salsola, and their hybrid A. x platyspina. Bottom right pix: a close-up of the leaves of these three taxa, a composite photo made from two individual photos in the field that have the same scale: one of the leaves of A. dumosa and the hybrid; and one of the leaves of A. dumosa and A. salsola.
Click on the pictures for larger versions. | ||
Ambrosia dumosa, burroweed, vies with Larrea tridentata, creosote, as the most abundant shrubs in the Borrego Desert nearly everywhere on the desert floor. A. salsola, cheesebush, is one of the most abundant shrubs in washes on the desert floor. If these two species could hybridize, you'd think hybrids would be abundant. But they aren't. As far as we know, there is only a single record of their hybrid ever found here, an iNaturalist observation by Daniel Donovan from 19 March 2024 (but see important note above). Fred Melgert and Carla Hoegen followed-up this discovery on 23 March 2024 and looked for more plants like this near it without success. I also followed-up this plant on 27 March 2024.
Prior to this discovery, there were no vouchers of this hybrid from here; no iNat observations from here; and neither I and my colleagues, nor Fred Melgert and Carla Hoegen, had ever found this hybrid, despite many hundreds of botanical trips by each group. I have in my notes that there was mention of such a hybrid from a CNPS San Diego Chapter field trip report from February 2010 or somewhat earlier, but I haven't been able to track down that report.
Vouchers of this hybrid are widespread in the other desert areas of California, with one record from Baja California. There are 46 vouchers, from at least 30 different locations. There are 10 iNat obs determined as this hybrid. Jim Morefeld commented in one of the iNat obs that A. dumosa x A. salsola shows up frequently in the southern White Mountains.
The first collection of this hybrid made no mention of it being a hybrid, but instead described it as a new species, Hymenoclea platyspina in 1975. The voucher is from 1972 from Baja California.
There was speculation that this taxon could be a hybrid earlier, but Baldwin et al showed definitively it was a hybrid in 1996 by showing the hybrid had the distinctive ribosomal DNA markers of the parent species. They also said that the hybrids show intermediacy or combinations of character-states of the suspected parental species in characters associated with foliage coloration, leaf shape, and numbers and shapes of phyllaries in pistillate heads. Fig. 1 shows the fairly-distinctive coloration of the hybrd, as well as the intermediacy of the leaf shape.
It is interesting that our plant looks like the type specimen, as well as two of the ten iNat observations: this one and this one, with narrow leaf lobes. The other eight of the iNat observations look similar to each other, with wider leaf lobes that look more like those of A. dumosa. It is possible that the difference in the two sets is which of the parents is the pollen donor.
One speculation was that this plant was a washed-down, very out of range Artemisia californica. I brought a fresh sample of A. californica with me when I saw this plant on 27 March 2024. The fragrances of the two taxa were strikingly different, and the shape of the leaf lobes were as well.
Search for Additional Specimens and Map of the Parent species Nearby
After I learned about this plant, on 27 March 2024 I did a search for additional examples of this taxon near the location of this specimen. While doing so, I mapped locations of the parents. My plan was to walk a spiral path starting at the location of this specimen.
In the field, I modified my plan slightly to "follow the plants". As I walked the spiral, if there were no target plants in the direction of the spiral path, but there were plants visible in the vicinity, I went to those plants to map them since my goal was to assemble as complete a map as I could of the target species. Fig. 2 shows the results of the mapping.
Fig. 2. Map of locations of A. dumosa (green filled circles); A. salsola (magenta filled circles); and the single hybrid location (large orange filled circle). My mapping path is shown in blue. The mapping is complete starting at the hybrid location through two rough circles. A few A. dumosa locations are also shown before I got to the hybrid. The arrows show the direction of my route. Deviations can be seen in my attempt to follow a spiral when no target species were seen in the direction of my spiral path. Note the significant number of gaps where none of the target species was seen in any direction from my path. The scale bar shows a distance of 0.5 mile (0.8 km)
I mapped two counter-clockwise paths, starting at the hybrid and ending at the A. salsola location at upper left. At that point, I had seen only two plants of A. salsola. The first was near the bottom of the map, and the second was the one at upper left. Both of those plants were in the only significant wash in the area, so I then decided to map down the wash to get a more complete map of A. salsola. I mapped A. salsola on the way down the wash, and A. dumosa on the way back up the wash.
I found no additional plants of the hybrid. Fred and Carla also looked for more hybrid plants using a similar strategy on 23 March 2024, but found none as well.
This single hybrid plant was easily visible at a distance of 100 feet (30 m) or so. The only plants I saw with a similar color were a few plants of Cryptantha ganderi.
As can be seen from Fig. 2, A. salsola is only found in that wash, where there was a total of just 11 plants. In comparison, I took 226 GPS points for A. dumosa. The number of plants of A. dumosa is much larger than the number of GPS points, probably a total of at least 1,000 plants.
Since A. dumosa outnumbers A. salsola here by a factor of ~100, this is a perfect situation for hybridization to occur on the A. salsola female flowers from the much-more abundant pollen of A. dumosa plants. If one of those plants set hybrid seeds, a dispersal event of a minimum of 0.15 miles (250 m) would be needed to produce the observed hybrid.
During my survey, I observed no Hyles lineata caterpillars on any of the Ambrosia plants I saw, despite the presence of thousands of the caterpillars munching on the annuals. Fred and Carla had observed two caterpillars on the probable hybrid on their visit.
It is interesting that this hybrid plant grows in A. dumosa territory, well away from the wash habitat of A. salsola.