Cuscuta psorothamnensis, Indigo Bush Dodder


Table of Contents

Introduction

Fascinating Facts about C. psorothamnensis et al

Geographic Distribution
    Distribution of C. psorothamnensis and its host plant from two detailed surveys
    An Area with no Cuscuta on Psorothamnus schottii?

How to Distinguish C. psorothamnensis from C. californica

How to Distinguish C. psorothamnensis from C. denticulata, C. nevadensis, and C. veatchii

C. psorothamnensis was "presumed extinct"!!

Taxonomic History


Introduction

We have a new species endemic to the Anza-Borrego Desert area! And even more amazing, it was declared extinct in the 2012 Second Edition Jepson Manual, even though it has been alive and well in our desert for a long period of time!

Thanks to wonderful analysis work done by Garcia et al published in 2018, building upon a lifetime of work on the Cuscuta genus by Mihai Costea, the fascinating story of Cuscuta psorothamnensis was finally revealed. Its true nature nicely accounts for why it had been misidentified for over a century.

Specimens corresponding to C. psorothamnensis have been variously identified in the past as C. denticulata, C. nevadensis, and C. veatchii. The brilliant analytic work by Garcia et al revealed that C. psorothamnensis is an ancient hybrid between C. denticulata and C. nevadensis, which is also the case for C. veatchii! Since morphological differences between all these species are, at best, subtle, it is no wonder there was confusion in the past about C. psorothamnensis.

The taxonomy of these species is now on a much firmer ground, and it turns out that all one had to do was to look at the chromosomes themselves and the host plants; no fancy DNA analysis was required. C. denticulata is a diploid species with 2n=30. C. nevadensis is a diploid species with 2n=30 as well, but its chromosomes are significantly larger than those of C. denticulata. C. psorothamnensis and C. veatchii are tetraploid species with 2n=60, with the 30 smaller chromosomes of C. denticulata and the 30 larger chromosomes of C. nevadensis. The work by Garcia et al even revealed that in both cases, C. denticulata was the maternal parent, and C. nevadensis the paternal parent (fancy DNA analysis was needed for that).

Since C. psorothamnensis and C. veatchii had the same original genes, it is essentially impossible to tell the difference between them with morphology or standard DNA analysis. But they are clearly different species now. They are geographically distinct, and parasitize different host plants. C. psorothamnensis is a younger taxon, since it still has maternal and paternal nrDNA types, whereas concerted evolution has completely homogenized the nrDNA types in C. veatchii.

What makes C. psorothamnensis easy to recognize in our area (at least the vast majority of the time; see below) is that it is the only one of these species that parasitizes Psorothamnus schottii. The only other Cuscuta species that grows on P. schottii is C. californica. These two species are easily distinguished by the presence or absence of corolla scales.

Fascinating Facts about C. psorothamnensis et al

It would be a fascinating experiment to see if C. veatchii could grow on Psorothamnus schottii, and if C. psorothamnensis could grow on Pachycormus discolor.

Who knew that Cuscutas would prove to be so interesting???!!!

Geographic Distribution

Fig. 1 shows a map of the distribution of C. psorothamnensis, as well as the distribution of C. californica growing on P. schottii, with all known occurrences as of 25 March 2020.

Fig. 1. The distribution of C. psorothamnensis (orange circles) and C. californica (yellow circles) growing on P. schottii as of 25 March 2020. See also map with a Google Earth satellite image background, and Fig. 5 for a detailed view of the northernmost area with C. psorothamnensis.

All locations showing C. californica come either from vouchers or from examination of the flowers by myself and/or Carla Hoegen and Fred Melgert to make sure they lack scales. A large number of the locations showing C. psorothamnensis come either from vouchers or from examination of the flowers by myself and/or Carla Hoegen and Fred Melgert to make sure they have scales. Other locations of C. psorothamnensis are from nearby plants with verified C. psorothamnensis growing on P. schottii. Previous versions of this map distinguished between the confirmed observations and the nearby plants, but with so many confirmed observations now, there is no doubt that the nearby plants also are C. psorothamnensis. See, for example, the map from 13 March 2020, showing unconfirmed locations as smaller circles.

Note that even though C. psorothamnensis is a newly-defined species, we have a excellent idea of its distribution since specimens corresponding to it have previously been determined as C. denticulata or C. nevadensis. Assuming Garcia et al 2018 are correct, specimens formerly determined as C. denticulata or C. nevadensis growing on P. schottii are now C. psorothamnensis.

Distribution of C. psorothamnensis and its host plant from two detailed surveys

I performed two detailed surveys where I recorded the location of every population of C. psorothamnensis and regularly recorded the location of its host plant, Psorothamnus schottii. The first survey was on 20 March 2020 along and south of Old Kane Spring Road east of the Harper Canyon Access Road, a loop trip of 6.3 miles. The second was on 24 March 2020 along and south of Old Kane Spring Road west of the Harper Canyon Access Road, a loop trip of 5.3 miles. The results are shown in Fig. 2.

Fig. 2. The distribution of C. psorothamnensis (orange circles) and its host plant P. schottii (small green dots) from two surveys. Click on the map for a larger version. Also see map with a topo map background.

The map shows that the host plant, P. schottii, is mainly confined to active washes and their immediate vicinity, and largely absent from older alluvial terrain.

The distribution of C. psorothamnensis is clumped on several scales shorter than one mile. If there is a host plant with C. psorothamnensis at a given location, it is more likely that there is another location within a mile that also has C. psorothamnensis. The likelihood goes up as the distance decreases, and becomes quite large on scales of hundreds of feet. Locations with C. psorothamnensis often have a number of host plants with C. psorothamnensis in close proximity.

The distribution could be explained by birds spreading the seed. Birds would most likely spread the seed to very nearby host plants, accounting for the clusters of host plants with C. psorothamnensis. Birds could occasionally transport the seed longer distances, as well, with decreasing probability of a successful transfer the farther away a host plant is.

An Area with no Cuscuta on Psorothamnus schottii?

Interestingly, there is a fairly large geographic region in the northeast corner of San Diego County where no Cuscuta of any species has ever been found on P. schottii; see Fig. 3. There is also a similar smaller area to the south.

Fig. 3. The distribution of Psorothamnus schottii (small dark green circles) in the Borrego Valley area, from iNat and my observations, and the distribution of all Cuscuta species from vouchers, iNat and my observations (all with yellow circles since I only wish to show where Cuscuta is found here regardless of species). Note the large area in the upper right corner showing abundant P. schottii with no Cuscuta observations. A similar area may occur in the lower left corner.

It is perhaps reasonable that C. californica is not found in the large area on the northeast, since it is primarily a coastal species, and that area is much more arid than the areas where C. californica is found. Aridity might also explain the absence of C. psorothamnensis there as well, since that species hugs the eastern end of the mountains to the south, which receive more rainfall.

How to Distinguish C. psorothamnensis from C. californica

Using Geography

The easiest way for most people is to use geography. There appears to be a very clear geographic demarcation between C. californica and C. psorothamnensis in most areas. There is only one small area where the species are in close proximity.

Remember, your Cuscuta plant has to be growing on Psorothamnus schottii so that there are only these two species in play.

Fig. 4 shows all iNat observations of all species of Cuscuta, on any host plant; my observations of all species of Cuscuta, on any host plant; voucher locations of C. californica, on any host plant; and all observations of C. psorothamnensis, which only grows on P. schottii.

Fig. 4. The distribution of all iNat and my observations of all species of Cuscuta (small white circles with black edges), along with voucher locations of C. californica (yellow circles) and confirmed locations of C. psorothamnensis (orange circles), as of 25 March 2020. The minimum range for both species is given.

Some of the iNat observations that fall within the range of C. psorothamnensis may also be C. psorothamnensis, but it wasn't clear in those observations what the host plant was.

C. subinclusa and C. denticulata also grow in the area shown on the map, but they apparently are never found on P. schottii, so aren't relevant to the discussion here.

See also map without the labeled ranges, and the same map with a Google Earth satellite image background, showing only plants growing on P. schottii.

The one location where C. californica and C. psorothamnensis are in close proximity is shown in a closer view in Fig. 5.

Fig. 5. The distribution of C. californica (yellow circles) and C. psorothamnensis (orange circles). All host plants for these observations are P. schottii, with the exception of the point labeled with a host of Ambrosia dumosa.

See also a detail map showing just plants growing on a P. schottii host, using a topo map background and using a Google Earth satellite map background.

In the Borrego Desert, considering only C. psorothamnensis and C. californica, only C. californica has been found north of Old Kane Spring Road (see labeled map). The iNat map of all Cuscuta species looks very similar to the voucher map of C. californica, and hence it is extremely likely that any Cuscuta found on P. schottii north of old Kane Spring Road is C. californica.

In the Borrego Desert, considering only C. psorothamnensis and C. californica, only C. psorothamnensis has been found at or south of Old Kane Spring Road. Hence it is extremely likely that any Cuscuta found on P. schottii in that area is C. psorothamnensis.

South of the Vallecito Mountains and east of the Tierra Blanca Mountains, considering only C. psorothamnensis and C. californica, only C. psorothamnensis is found. Hence it is extremely likely that any Cuscuta found there on P. schottii is C. psorothamnensis.

Using the Flowers

The gold standard for distinguishing C. californica and C. psorothamnensis is to look inside the flower for corolla scales. If it has scales, and it is growing on P. schottii, it is C. psorothamnensis. If it doesn't have scales, it is C. californica.

Most people will have a hard time finding the presence of absence of scales, since to do so, you need to cut the flower in half lengthwise, unfold it, and look under a microscope for the scales. If you want to try this, see an illustration from the Jepson eflora showing a flower of C. approximata with scales and a flower of C. californica without scales, to see what you are looking for. See photographs of the scales of C. psorothamnensis from Garcia et al 2018. The scale bar in each photograph is 1.0 mm.

It is also possible that other characteristics of the flowers might separate the two species, including how many flowers are in each cluster (1 to 5 for C. psorothamnensis; 3 to 20 for C. californica; the length of the flower (corolla tube 1 to 1.2 mm long for C. psorothamnensis; 1.6 to 2.4 mm for C. californica; styles 1/3 to 1/2 as long as the length of the ovary for C. psorothamnensis; styles longer than the length of the ovary for C. californica). But except possibly for the number of flowers in a cluster, it is probably harder to use these other characteristics than to simply check for scales.

One nice thing about the scales is that dead dried flowers on the plant in the field can still be used to look for the scales under a microscope. The easiest way to do that is to tease out the fruit capsule, and then make a hole in the brittle corolla to look for the scales inside the flower. It is best to look at several different flowers, since the scales are brittle in dried flowers, and could easily be removed while opening the flower and taking out the fruit capsule.

How to Distinguish C. psorothamnensis from C. denticulata, C. nevadensis, and C. veatchii

There is essentially no chance you'll be able to distinguish C. psorothamnensis from C. denticulata, C. nevadensis, and C. veatchii morphologically. Even the expert has had trouble with these very similar species, as shown by the simple observation that half of all the vouchers of C. psorothamnensis were previously determined as C. denticulata, and the other half were determined as C. nevadensis.

If you can count chromosomes, it is a piece of cake to distinguish C. psorothamnensis and C. veatchii, with 2n=60, from C. denticulata and C. nevadensis, with 2n=30.

Fortunately, C. psorothamnensis is apparently unique in parasitizing Psorothamnus schottii, and C. veatchii is apparently unique in parasitizing Pachycormus discolor, making it extraordinarily easy to distinguish those two species simply by looking at the host (and, in the case of C. psorothamnensis, also distinguishing it from C. californica). The geographic ranges of C. psorothamnensis and C. veatchii are 300 miles apart, with C. psorothamnensis living in a small area in se San Diego County and the farthest sw corner of Imperial County, and C. veatchii living in the middle of Baja California.

If C. psorothamnensis was truly confined to growing on P. schottii, the above would be sufficient to distinguish it from the other species. However, I now have strong circumstantial evidence that C. psorothamnensis can also grow on Chaenactis stevioides and Brassica tournefortii. It was previously known that C. denticulata can grow on those annuals. Hence if C. psorothamnensis can also grow on those annuals, and you are looking at a Cuscuta on those annuals, one cannot use the host to discriminate these two species.

The strong circumstantial evidence comes from observing four occurrences in which a Cuscuta was growing on those annuals, two each inside separate large patches of C. psorothamnensis growing on P. schottii, with no occurrences of C. denticulata anywhere close to those patches. See my iNat observations of those plants: 28 March 2020 plant 1; 28 March 2020 plant 2; 11 April 2020 plant 1, and 11 April 2020 plant 2.

If such occurrences on annuals happened inside only one of those patches, one could argue that C. denticulata somehow slipped inside one of those patches from some remote population of C. denticulata. But finding two such patches makes that a very unplausible argument, especially given the rarity of C. denticulata here.

Fig. 6 shows the known geographic distribution of C. denticulata and C. psorothamnensis as of 13 April 2020. There are only three known locations for C. denticulata, and two of those occurrences are on only a single plant. In addition, there is a voucher of C. denticulata with the vague location of "Borrego Valley, on Larrea", from 1940, but no one has ever found true C. denticulata in the Borrego Valley since then (remember, all vouchers formerly called C. denticulata are now C. psorothamnensis if they were on P. schottii).

Fig. 6. The known full geographic distribution of C. denticulata in the Anza-Borrego Desert area (red circles) and the partial distribution of C. psorothamnensis (orange circles) as of 13 April 2020. The two separate patches that contain C. psorothamnensis growing on annuals are marked with arrows. See Fig. 7 for an expanded view of the area marked with a rectangle in the middle of this map, where both species are found in close proximity.

Fig. 7 shows the single large patch of C. denticulata, with the nearby population of C. psorothamnensis.

Fig. 7. The known geographic distribution of C. denticulata (red circles) and C. psorothamnensis (orange circles) as of 13 April 2020. The arrow shows what is likely to be C. psorothamnensis growing on nearby plants of Chaenactis stevioides and Brassica tournefortii.

The C. denticulata population are all on a north-east-facing slope going down to the canyon bottom, whereas the C. psorothamnensis population is on the ridgetop along S2. The location with an arrow has Cuscuta growing on Chaenactis stevioides and Brassica tournefortii. I had formerly thought it must be C. denticulata, when I thought C. psorothamnensis was restricted to P. schottii. But after finding those two patches mentioned above of C. psorothamnensis growing on annuals, and seeing how close the plants marked with an arrow are to the C. psorothamnensis plants here, it seems very likely that this is another example of C. psorothamnensis growing on Chaenactis stevioides and Brassica tournefortii. Perhaps the Cuscuta experts can make a determination based on looking at chromosomes from the growing stem tips?

C. psorothamnensis was "presumed extinct"!!

In the 2012 Second Edition Jepson Manual, the entry for Cuscuta veatchii says (bold italicized text added by me):


Ecology: Desert, on Bursera, Schinus; Elevation: +- 400--1500 m. Bioregional Distribution: DSon (San Diego Co.; extirpated); Distribution Outside California: Baja California.

Since we now know that the actual species found in San Diego County was C. psorothamnensis, and that it looks almost exactly like C. veatchii and it lives only in this area, "extirpation" is equivalent to "extinction" for that species.

This note on extirpation was written by the world's foremost authorities on Cuscuta, Mihai Costea and Sasa Stefanovic, prior to their 2018 study that revealed the fascinating story of C. psorothamnensis and C. veatchii. Without their 2018 work, we would still be confused about the difference between those species.

So how was this conclusion reached?

The presence of C. veatchii in San Diego County was based on a collection by Brandegee in April 1889, a "fragment" determined as C. veatchii, stored in NY. This was the only specimen ever determined as C. veatchii from San Diego County. Interestingly, Brandegee also collected the type specimen for C. veatchii on 8 May 1889 from Baja California, just one month later.

If I recall correctly, the locations by Brandegee are not always correct. If so, it is possible that the "San Diego" collection might actually have been from Baja.

Unfortunately, the April 1889 voucher is not online, so I don't know if there is any information about the host plant in the voucher.

Shreve & Wiggins 1964 state that the host plants for C. veatchii are Bursera and Pachycormus veatchii. This might imply that the Brandegee San Diego County voucher was thought to be on Bursera. If so, that would account for why no other vouchers from San Diego County were ever determined as C. veatchii since C. psorothamnensis does not grow on Bursera. The lack of such vouchers led to the very reasonable conclusion by Costea and Stefanović that C. veatchii was now extirpated in San Diego County.

Taxonomic History

TBS

Adding to the confusion, one author considered C. veatchii as a subspecies of C. salina.


Voucher data provided by the participants of the Consortium of California Herbaria (ucjeps.berkeley.edu/consortium/).


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Updated 14 April 2020