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CLINAL DIFFERENTIATION AND PUTATIVE HYBRIDIZATION IN A CONTACT ZONE

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1052American Journal of Botany 88(6): 1052–1057. 2001.CLINAL DIFFERENTIATION AND PUTATIVEHYBRIDIZATION IN A CONTACT ZONE OFPINUS PONDEROSAANDP. ARIZONICA (PINACEAE)1BRYANK. EPPERSON,2,4FRANKW. TELEWSKI,3ANNEE. PLOVANICH-JONES,3ANDJILLE. GRIMES32Department of Forestry, Michigan State University, East Lansing, Michigan 48824 USA; and3W. J. Beal Botanical Garden, Department of Botany and Plant Pathology, Michigan State University, East Lansing, Michigan 48824 USAThe widely distributed Pinus subsection Ponderosae is a species complex that has a transition zone among taxa in the southwesternUnited States. In southern Arizona and New Mexico at least two recognized taxa, Pinus ponderosa var. scopulorum and Pinus arizonicaor P. ponderosa var. arizonica, are known to coexist in close proximity. In this study, we report the existence of populations wherethe taxa are sympatric. One of the key characteristics distinguishing taxa is the number of needles per fascicle; P. ponderosa typicallyhas three, P. arizonica has five. We examined the spatial distribution of needle-number types in a belt transect that covers a transitionzone from nearly pure three-needle types at the top of Mount Lemmon to five-needle types downslope, in the Santa Catalina Mountains,Arizona. The spatial distribution is inconsistent with there being both free interbreeding among types and selective neutrality of types.Trees with intermediate types, having combinations of three, four, and five needles and/or mean numbers of needles between 3.0 and5.0, are spatially concentrated in the middle of the transition zone. The spatial distribution supports the occurrence of hybridizationand introgression, and this is consistent with reported crossabilities of the types. The results suggest that selection is acting, either onneedle number per se or on other traits of the ecotype with which it may be in linkage disequilibrium, to maintain the observed steepclinal differentiation.Key words: clinal differentiation; contact zone; hybridization; Pinaceae; Pinus arizonica; Pinus ponderosa.Pinus ponderosa has a broad distribution across much ofthe montane western United States. It is considered to be ex-ceptionally widely adapted (e.g., Conkle and Critchfield,1988), and a number of regionally adapted ecotypes have beenidentified. These types have been variously classified as races,varieties, forms, or as separate species, but all are placed inthe subsection Ponderosae. Hybridization is widespreadamong the taxa (Conkle and Critchfield, 1988). In the south-western United States, populations exist in a transition zone.The distribution of Pinus ponderosa extends south along theRocky Mountains and to the broad plateau country in northernArizona and New Mexico, where it is generally considered toconsist of the taxon Pinus ponderosa var. scopulorum Engelm.In Mexico, several other taxa exist in the Sierra Madre, andsome of these extend northward up to the southern border ofArizona and New Mexico. Most notable among these is onetype originally identified as Pinus arizonica Engelm. or Pinusponderosa var. arizonica (Engelm.) Shaw. In the southernthird of Arizona and New Mexico, Ponderosae populationsare small and scattered, confined to mountaintops. Taxa in this1Manuscript received 13 June 2000; revision accepted 20 October 2000.The authors thank the USDA Forest Service, Santa Catalina Ranger Dis-trict, specifically Jennifer M. Ruyle, Steve Hensel, Steve Romero, and TerryAustin, for loaning the GPS, calibrating the data, and converting it into anArcView file; to Dean McAlister for providing access and permission to theforest for sample collection; Jason Kilgore, Department of Botany & PlantPathology, Michigan State University, Dr. Annita Harlan, Dept. of Ecology& Evolutionary Biology, and Thomas Harlan, Laboratory of Tree Ring Re-search, University of Arizona, for field assistance; Rosemarie Walter for com-ments on the manuscript; and the Steward Observatory, University of Arizona,especially Bob Peterson, for providing access to the Mt. Lemmon Observatorydormitory for base camp operations. This project was supported in part by agrant from the Plant Breeding and Genetics graduate program at MichiganState University.4Author for reprint requests (FAX: 517-432-1143, e-mail: [email protected]).area have been classified in various ways (see review by Reh-feldt, 1999), but there are at least two taxa, which differ mark-edly by a variety of morphological traits (Rehfeldt et al.,1996). Nonetheless, the taxa hybridize well in controlledcrosses (Conkle and Critchfield, 1988).Populations in the Santa Catalina Mountains, north of Tuc-son, Arizona, are of particular interest. Ponderosae are knownto have persisted there, and perhaps nowhere else in the re-gion, during the Wisconsin glacial era (Van Devender, 1990).The Santa Catalina Mountains contain at least two differenttaxa of ponderosa pine (Ponderosae), originally classified asthe Rocky Mountain variety, P. ponderosa var. scopularum,and the Arizona pine, P. arizonica or P. ponderosa var. ari-zonica (Kearney and Peebles, 1960). Rehfeldt (1999) presentsdetailed information on the distributions of different morpho-logical types in the southern regions of Arizona and New Mex-ico, including the Santa Catalina Mountains. The types differfor a variety of traits, including genetically based differencesin numbers of needles per fascicle and needle length, and var-ious seed cone and shoot elongation traits (Rehfeldt, 1999).Needle number per fascicle is a key characteristic with highheritability. The P. ponderosa var. scopularum type typicallyhas three needles per fascicle (although it often has two nee-dles in the Rocky Mountains), whereas arizonica typically hasfive needles.We have found a number of transitional populations in theSanta Catalina Mountains. Field observations on the distribu-tion of the Ponderosae based upon needle number per fasiclereport the existence of a relatively pure stand of the three-needled variety on the summit (2791 m) and north slope ofMt. Lemmon and a mix of three- and four-needle pines on thesummits of Mt. Kellogg (2605 m) and Mt. Bigelow (2605 m).The lower elevations, especially on the lower flanks of Mt.Lemmon and into the Wilderness of Rocks region, below 2438June 2001] 1053EPPERSON ET AL.—CLINAL VARIATION INPONDEROSAEFig. 1. Distribution of sampled trees, with their mean numbers of needlesper fascicle, in the belt transect. The percentage of fill in each pie diagramindicates the


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