Narrow hybrid zone between two subspecies of big sagebrush (Artemisia tridentata: Asteraceae)

Abstract

Several species of gall-forming insects specialize on big sagebrush (Artemisia tridentata), a species that shows much clinal and subspecific variation throughout its geographic range. Two of those subspecies, basin big sagebrush (A. t. ssp. tridentata) and mountain big sagebrush (A. t. ssp. vaseyana), form a narrow hybrid zone at Salt Creek, Utah. Reciprocal transplant experiments have shown that the hybrid big sagebrush at Salt Creek are more fit than either parental subspecies, but only in the hybrid zone. Do genotype and environment influence the density and distribution of galls on big sagebrush? We counted galls on parental and hybrid big sagebrush in three reciprocal transplant gardens at Salt Creek. Gardens were in each of the two parental zones and in the hybrid zone. Transplanted seedlings came from five source populations: two parental and three hybrid populations. We identified seven kinds of gall-forming flies (Rhopalomyia midges and Eutreta fruitflies) that produced identifiable galls. Densities of galls varied among the three gardens and five source populations, and there was also a significant garden by source interaction in gall density. In general, variation in gall density among gardens (i.e., environments) was much greater than the variation among source populations (i.e., genotypes). Nevertheless, significant genotype-environment interactions were observed for five of the seven kinds of galls. Overall density of galls, mostly due to Rhopalomyia ampullaria, was greatest in the high-elevation (mountain) garden and least in the low-elevation (basin) garden. To the best of our knowledge, this is the first reciprocal transplant experiment addressing herbivore richness in a hybrid zone.

Benefits of Conservation of Plant Genetic Diversity to Arthropod Diversity

“…], Floate et al 1996;eucalypts [Eucalyptus spp. ], Dungey et al 2000;and sagebrush [Artemisia], Graham et al 2001), and plant and animal interactions comprise a large proportion of biotic relationships. Thus, the consideration of habitat genetic diversity can be a general and efficient approach to conserving both process and diverse assemblages of both plants and animals.…”

“…Studies of arthropods on hybrids often consider only species‐specific population patterns and community density in determining patterns of resistance and do not consider community richness and composition (Aguilar & Boecklen 1992; Fritz et al 1994; Fritz et al 1998). Different species exhibit variable responses to parental and hybrid hosts (Fritz et al 1994; Graham et al 2001), leading to the expectation that community‐level patterns may be too messy to disentangle (Fritz et al 1999; Lawton 1999). Here, we examined community richness and composition and addressed the following four hypotheses: (1) natural cottonwood hybrids in common environments have the highest species richness and are compositionally dissimilar from the parental cross types; (2) arthropod composition is not different within a cross type between different environments, which addresses environment by cross‐type interactions; (3) natural cottonwood hybrid zones are centers of arthropod diversity because of higher cross‐type diversity; and (4) the patterns of diversity are stable across multiple scales including a common garden, sites within a river, and at a regional scale across oceanic drainage basins that involve different hybridizing species.…”

Trade-off between plant growth and defense? A comparison of sagebrush populations

“…to timberline. It can hybridize with basin big sagebrush to form hybrid zones at intermediate elevations (Graham et al 2001, and references therein). Wyoming big sagebrush, A. tridentata wyomingensis Beetle and Young, grows on shallow, fine-textured, poorly developed soils (Aridisols) between 1,500 and 2,000 m a.s.l.…”

“…Most colonizing insects were sagebrush specialists (see Messina et al 1996;Graham et al 2001 for references). The aphid Obtusicauda coweni (Hunter) was common during each of the first two seasons.…”

Resistance to pre-dispersal seed predators in a natural hybrid zone

“…By employing experimental crosses to produce hybrids of different genetic background and planting them into common gardens, one can separate genetic from environmental effects and compare different genetic classes of hybrids. Some recent studies have employed common gardens (Messina et al 1996;Fritz et al 1998;Graham et al 2001), but to our knowledge only one compared herbivores on F1 and F2 hybrids (Dungey et al 2000).…”