Tag Archives: Speciation

Intermountain Regional Herbarium Network

Manzanitas in Sympatry

Except for the North Rim of the Grand Canyon, Arizona has only two species of Manzanita, Arctostaphylos pungens and Arctostaphylos pringlei. According to recent molecular data, these two species are believed to be closely related (Wahlert, 2005)A. pringlei shares the same range with A. pungens occurring at a lower elevation. A. pringlei doesn’t have as an extensive a range as A. pungens and is limited to northern Baja and southern California in the Penninsular Ranges and an arc extending from the Hualapi Mts. near Kingman, Arizona to at least the Rincon Mts. near Tucson. In addition, A. pringlei has been found in extreme southwestern Utah and maybe southern Nevada. I can find no evidence of the species in New Mexico. Looking the map, notice that the species range divides into four distinct regions.

Intermountain Regional Herbarium Network

Intermountain Regional Herbarium Network

(IRHN, 2013)

A. pungens has nearly the same range but it continues down into central Mexico and sparingly across southern New Mexico and occurs in the Chisos Mts. in trans-Pecos Texas.

Intermountain Regional Herbarium Network

Intermountain Regional Herbarium Network

(IRHN, 2013)

Some questions:

How do the two species differ?

There are several visual and tactile clues to tell the two plants apart. One major character is the glandular hairs covering A. pringlei, especially on the flowers, the berries and the young leaves (Anderson, 2008). They are quite sticky. Glandular is a botanical term for hairs that secrete a liquid substance, in this case a sticky substance (Harris, Harris, 2001). This character occurs across other species and subspecies within the Arctostaphylos genus (Keeley, 1976), (Dobzhansky, 1953). A. pringlei leaves are a dull grey-green, glaucous (covered with a whitish waxy coating) and wider while A. pungens leaves are more narrow and a shiny bright green (Anderson, 2008). Generally,  A. pringlei is a larger plant. It must be remembered that all these characters have a spread of variability due to genetic and/or environmental factors. There are reproductive differences: A. pungens starts blooming in late January or early February after the winter storms and is one of the earliest blooming plants. A. pringlei blooms some three months later in April or May. Up until the 1980′s it was assumed that all Arctostaphylos species have the characteristic of nascent inflorences. The flower buds partially form before winter. It was “discovered” that A. pringlei doesn’t have this character (Vasey, 1999). Since then at least one other species of Arctostaphylos has been found without nascent inflorences (Vasey, Parker, 2008). This character is part of the reason that A. pungens is able to bloom so early as the flower buds are partially formed before hand. A final major character difference is the seed structure. The berry is called a drupe and has three main structures,  the skin or exocarp, the fleshy fruit or mesocarp and the stone or nut or endocarp. A. pungens has an endocarp that breaks apart into three to five nutlets while A. pringlei sometimes has a single nut or stone (Wells, 1968).

How do the two subspecies or varieties of A. pringlei differ?

There are two named subspecies or varieties of A. pringleiA. pringlei ssp. drupacea and A. pringlei ssp. pringlei. The species was first described by Cyrus G. Pringle from a sample he took from the Rincon Mts. east of Tucson, AZ in 1884 (Pringle, 1884).  In 1972, Philip V. Wells (Wells, 1972) described the two subspecies of A. pringleiA. pringlei ssp. drupacea occurs in southern California while he put A. pringlei ssp. pringlei in Baja California and Arizona. Below are the differences, obicular means circular in outline and subcordate means almost heart-shaped (Harris, Harris, 2001). I believe he is talking about the top and bottom part of the leaf. He also mentions the possibility of the single endocarp being an adaption to fire.

Are the two species sympatric?

Tom Chester (Chester, 2007) has shown that A. pungens and A. pringlei share most of their range in the San Jacinto Mts. of California with A. pungens dominating the lower elevations and A. pringlei the higher elevations where it intersects with forest. It must be noted that it may not be elevation in itself that is driving distribution but the effects of elevation on rainfall, temperature extremes and length of growing season.

Are the two species hybridizing?

I think I was rather sloppy in my comments in the last article so I want to first go through some definitions:

Species. What is and isn’t a species and the mechanisms of speciation is a major problem in biology. In 1942 Ernst Mayr proposed the Biological Species Concept (Mayr, 1942) which states that species can be typed by their reproductive isolation. Of course, this only applies to organisms that have sexual reproduction. Despite many other competing species concepts and many special cases, this remains the strongest definition of speciation. Species that exist in a situation where they can come into physical contact are called sympatric. Sometimes closely related sympatric species can produce hybrids and sometimes these hybrids are viable and can cross with one or both of their parent species, Hybrid species and segregates, species which are some mixture of two other species, are much rarer in nature although they do exist. Hybrids form when first generation or f1 crosses somehow become successful in a niche the two other plants cannot fill. Segragates, or species formed by back crosses can also do this. What can happen is that if a hybrid is fertile, back crosses can cause gene flow or introgression between two species. This was first suggested by Edgar Anderson in 1948 (Anderson, 1948). Introgression can move genes into both species or just one species and is also harder to show as it involves sections of chromosone or even single genes and not characters. The presence of hybrids and back crosses can verified with character data, but proving introgression requires allozyme or molecular data (Ellstrand, Lee, Keeley, Keeley, 1987).

Whether two species form viable or non-viable hybrids  and whether there is introgression or not is a complex process that is determined by the mechanisms of speciation. Usually there is some form or period of separation (Feder, 2005). During this period the sexual characters diverge as they tend to diverge faster. In plants, one cause is pollen – pistil incompatibility between two different species. Also, changes in environment can cause divergence. For instance, two plants may come into flower at different times so there is no chance of cross-pollenation. When the plants again become sympatric, the degree of mixture depends on the degree of sexual and environmental character divergence. Another important factor of incompatibility in plants especially is ploidy. Plants can double, triple or more the number of chromosomes and plants are usually incompatible if the number of chromosomes are different. All the Arctostaphylos hybrids mentioned in this article come from plants with single ploidy (Wells, 1968).

Research on Arctostaphylos hybrids

The study of Arctostaphylos hybrids has a long history in evolutionary science. One well-studied region is around and in Yosemite National Park (Ellstrand et al., 1987), (Dobzhansky, 1953), (Ball, Keeley, Mooney, Seemann, Winner, 1983), (Ellstrand et al., 1987). Theodosius Dobzhansky even wrote a paper about Arctostaphylos hybridization along this transect in 1953! [] The transect represents a change from a chaparral species to a montane species. Where the two species cross there are hybrids, called a hybrid swarm. The lower elevation species is A. viscida ssp. mariposa and the higher elevation species is A. patula. The character states of the two plants are almost the opposite of A. pungens and A. pringlei in terms of leaf shape, color, surface and amount of glandularity and size of the plant []. The only similarity is phenology, the chaparral plants both flower earlier than the montane. In 1987, introgression was shown within this hybrid swarm (Ellstrand et al., 1987). The study found 32% “pure” plants of both species with 5% hybrids, 4% segregates, 18% A. patula introgression and 9% A. viscida ssp. mariposa introgression. Notice the asymmetry in the introgressives, pretty typical. In California, various hybrid swarms with A. pungens and A. pringlei ssp. drupacea and other species have been studied. A. pungens and A. glauca (Keeley, 1976): A. pungens is the higher elevation species. A. pringlei ssp. drupacea and A. patula (Chester, 2008): in this study, A. pringlei ssp. drupacea is the lower elevation plant. Molecular data (Wahlert, 2005): I would call the molecular data preliminary as there is only one study and the spacer region studied only resolves half the genus but there are two interesting conclusions: 1) A. pringlei ssp. drupacea has suspected introgressed elements between A. glauca  and A. pringlei ssp. pringlei, and (2) A. pungens in Arizona, A. pringlei ssp. pringlei and A. austalis, a Baja species, form a clade separate from A. pungens in California.

Conclusions

Arctostaphylos  is a diverse genus of over 50 species and many subspecies, almost all of which occur in California. Based on the amount of hybrids mentioned in the literature, and I have cited only about 1/3, I would conclude that gene flow could have an important contribution to the diversity of the genus. Introgression has been show in only two cases and it is unknown just what is happening but the theory is that the mixing within the hybrid swarm provides a pool of variants that can come in handy during environmental fluctuations and challenges.What needs to be known is more about variation of characters within the pure stands. Phenotype diversity or plasticity within the species is another way plants can handle this problem (Chester, 2008), (West-Eberhard, 2003).

I haven’t looked very closely at all the different variations of Arctostaphylos, but the character states I have seen form a complex mosaic pattern which is likely environmental. Proof of this awaits a better philogeny. One character I am particularly interested in is the glandular trichomes. I wonder if there is a better way to quantify this in the field? These types of hairs have been shown to have many uses and the plant may utilize any number of them simultaneously. These include (Wagner, 1991): reflectance of light , the viscous liquid released can melt in the heat and flow down to coat the leaf , defense against herbivory, and by getting onto pollinators it can form a substrate that pollen can attach to (Moyano, Cocucci, Sérsic, 2003). In some plants, stickiness is an alternative phenotype. One such plant that has been studied is Datura wrightii. It has been shown that the sticky phenotype has a greater energetic cost to the plant as opposed to the glossy phenotype and the current research question is to why the sticky phenotype persists (Hare, Elle, 2004).

I will talk about what I have found in a future article but I think that even if A. pungens and A. pringlei were not hybridizing in Arizona that this would be important data.

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Wahlert, G.A., 2005. A Phylogeny of Arctostaphylos (Ericaceae) Inferred from Nuclear Ribosomal Its Sequence Data, San Francisco State University.
Anderson, J.L., 2008. Vascular Plants of Arizona: Ericaceae. CANOTIA, 4(2), pp.21 – 30. Available at: http://canotia.org/volumes/CANOTIA_2008_Vol4_2_Anderson_Ericaceae.pdf [Accessed July 16, 2013].
Harris, J.G. & Harris, M.W., 2001. Plant Identification Terminology: An Illustrated Guide 2nd ed., Spring Lake Publishing.
Keeley, J., 1976. Morpological evidence of hybridization between Arctostaphylos glauca and A. pungens (Ericaceae). Madroño, 23(8), pp.427–434.
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Moyano, F., Cocucci, A. & Sérsic, A., 2003. Accessory Pollen Adhesive from Glandular Trichomes on the Anthers of Leonurus sibiricus L. (Lamiaceae). Plant Biology, 5(4), pp.411–418. Available at: http://www.academia.edu/3576415/Accessory_Pollen_Adhesive_from_Glandular_Trichomes_on_the_Anthers_of_Leonurus_sibiricus_L._Lamiaceae [Accessed July 30, 2013].
Hare, J.D. & Elle, E., 2004. SURVIVAL AND SEED PRODUCTION OF STICKY AND VELVETY DATURA WRIGHTII IN THE FIELD: A FIVE-YEAR STUDY. Ecology, 85(3), pp.615–622. Available at: http://www.esajournals.org/doi/abs/10.1890/03-3069 [Accessed July 30, 2013].