Me-Wuk Manzanita – A Sierran Shrub with a Complex History
By Steven Serkanic
The 2013 American Fire burned over 27,000 acres of terrain on Tahoe National Forest’s isolated American River Ranger District located between Foresthill and the Lake Tahoe basin. The real work began for the US Forest Service (USFS) after this fire was fully contained. Their post-fire response protocol aimed to ensure the safety of publicly accessible areas, protect property, secure biological and cultural resources, and address the integrity of clean water sources. Such projects also provide employment opportunities for young professionals looking to apply their skills under the guidance of experienced agency personnel. For two seasons, I had the good fortune to be hired as a seasonal field botanist helping advance these efforts. I found myself intrigued by local manzanita species.
The Rich World of Manzanitas
Among the many special-status plants our team targeted during survey efforts following the American Fire was True’s manzanita (Arctostaphylos mewukka subsp. truei). As many know, manzanita is the common name for a species-rich group of shrubs in the genus Arctostaphylos. They are sclerophyllous (having resilient evergreen leaves with thick cell walls) shrubs that are tightly linked to fire and are iconic elements of California’s chaparral. Few groups better symbolize the California flora. Of its over 60 species, most occur in the California Floristic Province (the Mediterranean-climate area of western North America, which includes most of California except the desert regions, plus small amounts of Oregon and Baja). For generations this group has triggered the imaginations of disputing botanists. Proof of this lies in the dizzying number of nomenclature changes — often name changes and modifications in taxonomic rank, such as elevating a subspecies to full species status, or vice versa — and hypotheses regarding the hybrid origins of various species. Arctostaphylos has been a source of both contention and curiosity for as long as botanists have been studying the California flora.
What Makes Me-Wuk Manzanita So Intriguing?
One such species, Me-Wuk manzanita (A. mewukka), has a colorful track record in the literature. It has long been the subject of speculation about potential hybrid origins. It is one of eight manzanita species known to occur in the Sierra Nevada mountain range. The Sierran cohort of manzanitas are morphologically divergent and have distributions that largely segregate across elevation, plant communities, and temperature regimes. Me-Wuk manzanita is particularly interesting because it exhibits morphological characteristics that are intermediate with respect to the lower elevation whiteleaf manzanita (A. viscida) and the montane greenleaf manzanita (A. patula). Me Wuk manzanita is also a tetraploid (four sets of chromosomes, two from each parent), whereas whiteleaf and greenleaf manzanitas are diploid (two sets of chromosomes, one from each parent). The nail in the coffin for the hypothesis about the hybrid origin of Me-Wuk manzanita is that fact that it has a distribution that overlaps greenleaf and whiteleaf manzanita, and prefers habitat where both these species grow.
So Much Variation
Our crew on the Tahoe knew that the special-status True’s manzanita is one of two subspecies of the Me-Wuk manzanita (A. mewukka). Some manzanitas have burl tissue (from which they can re-sprout after fire), and some do not. Based on the literature, we knew the type specimen (model example) of Me-Wuk manzanita had burl tissue (like many greenleaf manzanita specimens), and the type specimen of True’s manzanita lacked it (consistent with whiteleaf manzanita).
As field biologists, we crave these kinds of binary distinctions for the sake of streamlining a lengthy survey. This proved too rigid an interpretation in the case of Me-Wuk manzanita, however, as I later learned when revisiting this Sierran species complex for my Master’s thesis project at San Francisco State University. The American Fire burn scar is found across low and mid elevations of the Tahoe National Forest, overlapping much of the contact zone where whiteleaf and greenleaf manzanitas meet. This zone is where Me-Wuk manzanita occurs and where we focused the search for the special-status True’s manzanita. I was regularly drawn to habitat where Me-Wuk, whiteleaf, and greenleaf manzanitas occur together.
My imagination was in overdrive as I spent two seasons attempting to make sense of all the variation I observed at this mid-elevation zone. During this time I discovered the work of California botanists who had been inspired by similar observations and patterns associated with this species complex. I felt I had unlocked a paper trail leading to a rich story that lacked its final chapter.
Why Plant Species Produce Hybrids
Biologists nowadays have come to appreciate the importance of hybridization, particularly with regards to the transfer of advantageous genetic material among rapidly diversifying groups of species. Plant groups known for their potential to hybridize are often young, rapidly evolving, and have “porous” species boundaries. Unlike humans and most other animals, plants survive with redundant sets of chromosomes. Whereas you and I have two sets of chromosomes in each cell (one from mom and another from dad), which makes us diploids, plants have a fascinating ability to thrive with four, six, eight, twelve or many more sets of chromosomes.
Plants with redundant sets of chromosomes are known as polyploids. And when two diploid species hybridize and erroneously transfer unreduced gametes (each parent passes along diploid gametes), this can result in viable tetraploid hybrids. This type of hybridization occurs often in the plant kingdom (often with deleterious effects). When successful, it can result in a stable tetraploid hybrid entity that is referred to as an allopolyploid. These allopolyploids are largely reproductively isolated from their parents and can have strong species boundaries.
Me-Wuk Manzanita Long Suspected of Being Hybrid
Me-Wuk manzanita has long been suspected to be an allopolyploid resulting from hybridization between the diploid greenleaf and whiteleaf manzanitas. Kristina Schierenbeck addressed this question in the 1980’s under the guidance of the late G. Ledyard Stebbins.
Kristina collected data using morphological measurements and physical chromosome characteristics that supported the notion that Me-Wuk manzanita is the resulting allopolyploid between whiteleaf and greenleaf manzanita, albeit having formed at an earlier time and from evolutionarily younger forms of the respective parents. Tom Parker and I approached Kristina and suggested we revisit this classic study system with more modern tools.
Is Me-Wuk Manzanita Actually a Hybrid?
Research on the biology of allopolyploids suggests that these entities arise, not from a single hybridization event, but actually from untold numbers of hybridization events between respective progenitors. In the case of Me-Wuk manzanita, this would suggest that if whiteleaf and greenleaf manzanita are the progenitors, then Me-Wuk manzanita would have formed on many different occasions independently at separate locations where whiteleaf and greenleaf manzanita occur together. This arrangement has the potential to yield a species with a great amount of genetic diversity. It also offers a potential explanation for the impressive morphological variation of Me-Wuk manzanita seen across its distribution.
These hybridization events that lead to the formation of allopolyploids can exhibit reciprocal parentage. In this case, it would suggest that whiteleaf manzanita may be the mother at one location, and it may be the father at another. We used chloroplast sequence data (chloroplast is the maternally-inherited component cells in most groups of flowering plants) to test for parentage at separate locations throughout the distribution of MiWuk Manzanita. Our results lacked support for the hybridization hypothesis. This was largely because we were unable to detect maternally-inherited chloroplast DNA of either whiteleaf or greenleaf manzanita in any of the Me-Wuk manzanita samples collected from across its distribution.
Common manzanita (A. manzanita) growing in Sierran foothill oak woodland habitat
Insights About Me-Wuk Manzanita and Its Relatives
Instead, we found that Mi-Wuk manzanita shares a chloroplast with the common manzanita (A. manzanita), a widespread tetraploid that prefers lower-elevation foothill woodland and savanna habitat. This came as a complete surprise. What was the chloroplast of a divergent species doing in this Sierran tetraploid?
It turns out that shared chloroplasts among separate species occurs with some frequency in outcrossing perennial plants. This phenomenon, known as chloroplast capture, has been documented among groups such as the oaks (Quercus), alders (Alnus), beeches (Fagus), penstemons (Penstemon), heucheras (Heuchera), and others. Chloroplast capture is a signature of hybridization. The mechanisms of this usually involve an initial hybridization event, repeated backcrossing with one of the parents, and the reversion of the phenotype back to one of the parents through pollen-mediated gene flow. In this case, it suggests that at some point Me-Wuk manzanita may have picked up the chloroplast of common manzanita, or vice versa.
Details regarding the origin of the Me-Wuk manzanita remain uncertain. High-resolution tools that can unravel patterns occurring in the nuclear genome among members of this Sierran species complex offer the potential to deliver insight. What remains certain is that you never know where field travels will take you and what treasures they’ll reveal when accompanied with a sense of curiosity. In the words of CNPS San Luis Obispo chapter member Charlie Blair, “Keep going out.”
A model for chloroplast capture and interactions among Me-Wuk
and common manzanitas
Many thanks to Tom Parker, Kristina Schierenbeck, Mike Vasey, Bob Patterson, Greg Spicer, Felipe Zapata, Frank Cipriano, Scott Roy, Dave Graber, Kathy Van Zuuk, Bart O’Brien, and many others who were involved with this recent work. Their inspiration and enthusiasm reawakened the curiosity around this classic study system.
1 The following papers have contributed to various aspects of our understanding of this
important Sierran species complex:
Ball, C. T., Keeley, J., Mooney, H., Seemann, J., & Winner, W. 1983. Relationship between form,
function, and distribution of two Arctostaphylos species (Ericaceae) and their putative hybrid.
Acta Oecologica-Oecologia Plantarum.
Boykin, L. M., M. C. Vasey, V. T. Parker, and R. Patterson. 2005. Two Lineages of Arctostaphylos
(Ericaceae) Identified Using the Internal Transcribed Spacer (ITS) Region of the Nuclear
Genome. Madrono, 52: 139–147.
Dobzhansky, T. 1953. Natural hybrids of two species of Arctostaphylos in the Yosemite region of
California. Heredity, 7(1), 73–79.
Ellstrand, N. C., J. M. Lee, J. E. Keeley, and S. C. Keeley. 1987. Ecological isolation and
introgression: biochemical confirmation of introgression in an Arctostaphylos (Ericaceae)
population. Acta Oecologica-Oecologia Plantarum 8: 299-308.
Roof, J. B., 1967: Arctostaphylos mewukka, a hybrid. – Four Seasons 2: 16.
Schierenbeck, K. A., Stebbins, G. L., & Patterson, R. W. 1992. Morphological and cytological
evidence for polyphyletic allopolyploidy in Arctostaphylos mewukka (Ericaceae). Plant
Systematics and Evolution, 179(3–4), 187–205.
Stebbins, G. L. 1950. Variation and Evolution in Plants. New York: Columbia University Press.
Van Valen, L. 1976. Ecological species, multispecies, and oaks. Taxon 25: 233–239.
Wells, P. V. 1968. New taxa, combinations, and chromosome numbers in Arctostaphylos
(Ericaceae). Madroño 19: 193–210.