Investigating species boundaries and taxonomy in the California pincushion plants
Faculty Mentor: Leigh Johnson, Department of Biology
Species in the genus Navarretia are annual wildflowers found almost exclusively in California. The Navarretia atractyloides group consists of just two species, N. atractyloides and N. hamata, that are more similar to each other morphologically than to any other Navarretia species. The most recent taxonomic investigation of these two species distinguishes between them based on bract shape, stamen attachment to the corolla, and whether the style is included and anthers included to slightly exserted from the corolla tube versus style exserted and anthers long exserted (Day, 1993). However, we have noticed that some specimens that seem to be clearly N. atractyloides based on bract features and overall appearance have uneven stamen insertion, like N. hamata, and somewhat more exerted stamens. There also seems to be more variation in flower coloration than previously documented. In this research, we looked at morphological characteristics in several specimens, both collected from the field and from herbarium mounts, to try and determine a clear pattern that distinguishes N. atractyloides from N. hamata. We also focused on some specimens of N. atractyloides that have white flowers instead of the typical purple ones to determine intra-species variation.
I organized and executed a week long field specimen collection trip to California. Most of my flower
dissections and DNA work were done using herbarium specimens or specimens collected previously by
Dr. Johnson. Using PCR, gel electrophoresis, and analysis programs like Sequencher, Aliview, and
PAUP, I performed comparative DNA sequencing of chloroplast and nuclear genes—specifically IDHB,
ITS, and cpDNA—from multiple populations of each morpho-type. I also completed morphometric
studies in which I dissected 26 different flowers from varying locations. Using photo microscopy and
measuring software, I examined stamen insertion, stamen length, corolla length, corolla lobe length and
width, corolla tube length, corolla throat length, and style length. I also performed a literature review in which I located and reviewed the original descriptions for all names (synonyms) associated with N. atractyloides and N. hamata using comprehensive online databases that exist for plants like the International Plant Names Index and Tropicos and regional floras such as Jepson’s California Flora.
The results of the flower dissections are not extremely conclusive. In general, N. hamata has greater corolla and style lengths, but corolla throat length, corolla tube length, corolla lobe length and width, stamen insertion distance, and stamen length all had overlap between the two species. It does appear that the stamen insertion length has the largest variation within one subspecies of N. hamata, which is consistent with the expected uneven stamen insertion of N. hamata. Also, from looking at the flower dissection data, it appears that the white flowered N. atractyloides make up a subset of the species, as the numbers from the white flowers tend to be more tightly grouped than the rest of N. atractyloides. From the DNA sequencing, we assembled phylogenetic trees. The IDHB tree separated N. hamata from N. atractyloides, but it did not show any subgroup of the white flowered N. atractyloides. The ITS and cpDNA trees showed the same relative pattern except with a very loose grouping of the white flowered N. atractyloides within the species. Also, as I reviewed the literature, I noted a lot of confusion and inconsistency. As I read various keys and their descriptions of the two species N. hamata and N. atractyloides, I realized that a clear definition between the two plants does not exist. While examining the type specimens, I realized that a clear understanding of the situation is not currently possible. From looking at photos of type specimens, I could only use visual characteristics like bract shape and orientation in order to determine which species was which, and these characteristics turned out to be insufficient.
There is still a lot of work to be done on this question. The data from the DNA sequencing quite distinctly shows that N. hamata and N. atractyloides are indeed to separate species, and there is some evidence in favor of a subset of white-flowered N. atractyloides. The flower dissection measurements do not support any conclusive characteristic that distinguishes N. hamata from N. atractyloides except for perhaps corolla length. Also, from my review of the literature, it is obvious that there is a lot of confusion around these two species. In order to continue this research, it would be necessary to visit the type locations and try and collect new specimens from which we could use DNA sequencing and phenotypic characteristics to learn more about the species. Unfortunately, the type specimens have only vague collection locations such as “California” and so revisiting the precise locations is almost impossible. Another potential way to continue answering this question would be to travel to the herbaria where the type specimens are deposited, and then receive permission to dissect a flower or two from the specimen.
Overall, this experience was wonderful for me. Despite the fact that we did not determine exactly what
the differentiating phenotypic characteristics are between Navarretia atractyloides and Navarretia hamata, we did make considerable progress. We also learned from the DNA and the flower measurements that there is potentially a small subgroup of white flowered N. atractyloides, though more work is still needed to determine if this feature has taxonomic merit. Most importantly, this was an invaluable learning experience for me. I learned an incredible amount about using DNA and morphology to determine relationships in plants. I learned all about the processes used to delineate and name species. Dr. Johnson and I will continue to work on this question and see what more we can figure out about Navarretia atractyloides and Navarretia hamata.
Day, A. 1993. New taxa and nomenclatural changes in Allophyllum, Gilia, and Navarretia
(Polemoniaceae). Novon 3: 331-340.