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Pentanthera Webs: Interspecific and Interploid Hybridization Among Pentanthera Webs: Interspeci!c and Interploid Hybridization among Sympatric Azaleas in the Southern Appalachian Mountains Kimberly Shearer, !omas G. this bald suggests that the present azalea Ranney, Ron Miller, and Clarence species in the Southeast are likely to be re- Towe cent products of cycles of migration and Mountain Crop Improvement Lab, interaction attending the as many as 20 Department of Horticultural glaciations and warm periods since the be- Science, Mountain Horticultural ginning of the Pleistocene. Our contem- Crops Research and Extension porary list of recognized azalea species that Center, North Carolina State Kimberly Shearer Thomas Ranney seem so distinct and certain may simply be University, Mills River, North a momentary snapshot of a complex and Carolina rapidly changing evolutionary web that is the Pentanthera. ummary SModern evolutionary research sug- that famous swarm. !ere were tetraploids Introduction gests that new species often arise rapidly keying out to R. calendulaceum, some with Beginning with Charles Darwin (Darwin from hybridization and chromosome unusual variations (e.g., fragrance), within 1859), the evolutionary process typically doubling, augmenting the slow, divergent nearby woodlands. !e traits of several has been illustrated as a tree of life with processes originally detailed by Darwin. of the samples from Wayah/Wine Spring radiating limbs that branch and diverge Relationships between kindred species gave evidence for genetic exchanges be- but never reconnect. !is concept is are thus best represented not by a simple tween the diploid and tetraploid species; rooted in the notion of reproductively branching candelabra or tree, as pictured one triploid, such as might serve as a two- isolated populations accumulating genetic in our old biology texts, but by a complex way bridge between the azalea species, variation over time. Subject to natural web of exchanges and ploidy variations. was in fact found. In addition, triploids selection, these populations become Such complexities seem especially evi- had previously been discovered from R. increasingly distinctive as they gradually dent in the highly compatible and multi- calendulaceum-R. periclymenoides hybrids evolve along a one-way course without ploidal Southeastern deciduous azaleas, in northwestern South Carolina. !us intersections. Current research provides especially on those spectacular and much- mechanisms for gene exchange between ample evidence that this tree-of-life visited mountaintops called “balds.” We diploid and tetraploid species do indeed illustration is an oversimpli#cation at have conducted a survey of 92 samples exist. Examination of the complex dip- best (Arnold 2006; Arnold and Larson, taken from the azalea swarm on the bare loids of Gregory was still more suggestive. 2004). !e process of evolution is more top and adjacent woodlands of Gregory Flower colors, odors, and forms evince aptly depicted as a reticulate web of Bald within Great Smokies National Park. origins in R. cumberlandense and R. arbo- genetic exchange integrating periodic Flow cytometry was used to determine rescens and possibly R. viscosum. Diploid hybridization among nascent “species.” ploidy and published component traits of plants consistent with R. cumberlandense Interspeci#c hybridization and intro- recognized species were used to elucidate at this site could not, based on "ower sizes gression are common among plants and mating interactions at that highly diverse and forms, be distinguished from tetra- are a signi#cant evolutionary mechanism site. A similar study was undertaken with ploid R. calendulaceum. !e lack of dif- that can give rise to new evolutionary 12 samples from along the interfaces be- ferentiating morphological traits between lineages and species (Arnold 1997, 2006; tween the Rhododendron calendulaceum R. cum-berlandense and R. calendulaceum Barrier et al. 1999; Grant et al. 2005; (tetraploid) and R. arborescens (diploid) and similarity in average chromosome size Rieseberg and Carney 1998; Soltis et colonies on Wayah/Wine Spring Bald, to suggests that tetraploid R. calendulaceum al. 2009). By some estimations, at least the south of the Park. Cytometry revealed at this site is primarily derived from the 25% of plant species hybridize naturally no tetraploids within the open bald area of diploid R. cumberlandense with limited ge- (Mallet 2005) and as many as 50-70% Gregory, belying previous suggestions that netic infusions from other species. More- of all "owering plants are of hybrid origin R. calendulaceum is directly involved in over, the indeterminacy of species upon (Ellstrand et al. 1996; Rieseberg 1997). JOURNAL AMERICAN RHODODENDRON SOCIETY 187 !is process of introgressive hybridization and Vorsa 1993; Ramsey and Schemske Although these grassy balds have certainly can a$ord evolutionary bene#ts by 1998). !is reproductive barrier between been disturbed and impacted by human enhancing genomic diversity. !rough the cytotypes can foster sympatric divergence. beings (Lindsay 1976), they contain development and combination of novel Yet little is known about the potential hybrid zones of deciduous azaleas and adaptations, introgressive hybridization for in situ interploid gene "ow among provide model sites to study ongoing can enhance #tness, colonization success, coexisting taxa of di$erent cytotypes. genetic exchange. and adaptive radiation in new and Hybrid zones can provide an oppor- !e species found on these balds are changing environments (Arnold 2006; tunity to study the degree of reproductive characterized and may be di$erentiated Grant et al. 2005; Lewontin and Birch isolation and the presence of interspe- in the following ways (Galle 1987; Kron 1966; Lindqvist et al. 2003; Rieseberg and ci#c and interploid crossing within mixed 1993; Luteyn et al. 1996; Willingham, Jr. Carney 1998; Soltis et al. 2009). populations. Pentanthera azaleas native to 1976) (also see Table 1): Rhododendron species of the sub-sec- the Southern Appalachian Mountains of- tRhododondron arborescens is known tion Pentanthera are known for their wide- ten form hybrid zones or “swarms” with as the sweet azalea due to the sweet clove spread natural hybridization, including unusual phenotypic diversity indicative of or heliotrope-like fragrance. Flowers are over 18 di$erent combinations that some- natural hybridization (Galle 1987; Kron white, sometimes with a pink or reddish times involve multiple species (Millais 1993; Rehder 1921; Skinner 1955, 1961; blush, 4.0-5.2 cm across, with or without 1924; Skinner 1955, 1961; Galle 1968; Towe 2004). A number of studies have a yellowish blotch. !e transition from Leach 1958). Leach (1958) remarked that con#rmed interspeci#c hybridization and tube to corolla is gradual; sepals typically “hybridity is the most conspicuous single introgression among these azaleas, but have a fringed margin. Pistil and stamens feature of the entire Azalea population of these have been limited to diploid cyto- are characteristically reddish. our Southern mountains.” Skinner (1961) types (King 2000; Kron et al. 1993). tRhododendron calendulaceum, the also emphasized what he considered to be A well-known hybrid zone of "ame azalea, is named for its brilliant the reticulate nature of the evolutionary azaleas is located on Gregory Bald in "ower color which can range from yellow lineages within this section. the Great Smoky Mountains National to orange to red. Flowers are often larger !e development of polyploids is Park on the Tennessee-North Carolina than the "owers of other species, 4.0- also a fundamental evolutionary process border near Cades Cove. Based on 6.5 cm across, and have a blotch on the in "owering plants. Most angiosperms morphology and the overlapping ranges upper lobe. !ere is an abrupt transition are believed to have undergone whole of numerous species, this swarm has been from tube to corolla, no substantial genome duplication events, often followed thought to represent a complex hybrid fragrance, and typically a fringed sepal by rediploidization, throughout their zone among Rhododendron arborescens margin. Flowers (2n=4x=52) often bloom evolutionary history (Soltis et al. 2009). (Pursh), Rhododendron cumberlandense before leaf emergence (though highly Polyploids have numerous traits that can (E.L. Braun) Copeland, Rhododendron variable). Rhododendron calendulaceum contribute to successful speciation and calendulaceum (Michx.) Torr., and is the only tetraploid species of the four radiation, including a greater number Rhododendron viscosum (L.) Torrey (Hyatt taxa considered here (Jones et al. 2007; Li of potential alleles, greater potential 2001). Potential hybridization events in 1957; Sax 1930). heterozygosity, tolerance of deleterious this population are of particular interest tRhododendron cumberlandense (R. mutations, novel gene expression(s), on account of the complex genetic bakeri), the Cumberland azalea, closely and enzymatic multiplicity (Adams and interactions that may be occurring resembles R. calendulaceum but is diploid. Wendel 2005; Comai 2005; Hegarty between species that vary in ploidy level. !is species has been described as having and Hiscock 2008; Osborn et al. 2003; Of the four species found in that region, smaller "owers, 3.8-4.5 cm across, a bristly Soltis et al. 2003; Soltis and Soltis 1993). R. arborescens, R. cumberlandense, and sepal margin, and it typically blooms after !e formation of polyploids can also R. viscosum are diploid (2n = 2x = 26), leaves expand. !e pedicel is typically
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