CHARACTERIZATION OF A NARROW HYBRID ZONE BETWEEN TWO SUBSPECIES OF BIG SAGEBRUSH (ARTEMISIA TRIDENTATA: ASTERACEAE)

Hybridization between Artemisia tridentata ssp. tridentata and A. t. ssp. vaseyana occurs in a narrow elevational zone along the west face and canyons of the Wasatch Mountains of Utah. Two sites in central Utah (Orem and Salt Creek) were examined. The parental taxa differ in anatomy, morphology, flavonoid and coumarin content, as well as various growth parameters. Hybrids are intermediate for the majority of traits examined, including flavonoid and coumarin composition. Multivariate statistics show that these taxa can be unambiguously identified using simple field measures of morphology. A hybrid index was constructed using principal component analyses on morphological variables.     

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

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.     

NARROW HYBRID ZONE BETWEEN TWO SUBSPECIES OF BIG SAGEBRUSH (ARTEMISIA TRIDENTATA: ASTERACEAE). IV. RECIPROCAL TRANSPLANT EXPERIMENTS

Does endogenous or exogenous selection stabilize the big sagebrush (Artemisia tridentata) hybrid zone? After two years of study, our reciprocal transplant experiments showed significant genotype by environment interactions for a number of fitness components, including germination, growth, and reproduction. Hybrids were the most fit within the hybrid garden. In the parental gardens, the native parental taxon was more fit than either the alien parental or hybrids. These results are consistent with the bounded hybrid superiority model, which assumes exogenous selection, but are clearly at odds with the dynamic equilibrium model, which assumes endogenous selection and universal hybrid unfitness.     

Variable responses of insects to hybrid versus parental sagebrush in common gardens

Both ecological and genetic mechanisms have been proposed to explain patterns of herbivore attack on interspecific plant hybrids, but distinguishing among them can be difficult in natural hybrid zones. We performed a common-garden experiment to evaluate four genetic hypotheses: dominance, additivity, elevated hybrid susceptibility, and elevated hybrid resistance. Censuses and cage experiments were used to compare insect responses to basin big sagebrush (Artemisia tridentata spp. tridentata), mountain big sagebrush (A. t. vaseyana), and their F₂ progeny. After two growing seasons, hybrid shrubs resembled mountain big sagenbrush in size, but were more similar to basin big sagebrush in flower production. Censuses of naturally colonizing insects (the gall midge Rhopalomyia obovata, the bagworm moth Apterona helix, and the aphid Obtusicauda coweni) tended to support the dominance hypothesis: if the insect clearly discriminated between the two parents, its frequency on hybrids closely resembled that on one of the parents. Moreover, colonization of hybrids in all three cases suggested a dominance deviation toward the susceptible parent rather than toward the resistant parent. In contrast to the censuses, cage experiments involving two insects supported the hybrid-susceptibility hypothesis; both survival and growth of the grasshopper Melanoplus sanguinipes and growth of the leaf beetle Trirhabda pilosa were higher on hybrid shrubs than on either parent. Because many secondary compounds have been determined to occur at intermediate concentrations in F₂ shrubs, dominance for susceptibility may indicate that insects respond to plant traits (e.g., oviposition stimulants and deterrents) in a threshold manner. Mechanisms underlying increased hybrid susceptibility are less clear, but our experimental design makes environmental explanations (e.g., the plant-stress hypothesis) unlikely. Although we eliminated several confounding factors, our results agree with the conclusion from natural hybrid zones that insect responses to hybrid plants are likely to be idiosyncratic; even congeneric species did not respond similarly to hybrid and parental plants.     

Topographic control of vegetation in a mountain big sagebrush steppe

Mountain big sagebrush steppes in Wyoming have strong spatial patterning associated with topography. We describe the spatial variability of vegetation in a sagebrush steppe, and test the relationship between topography and vegetation using canonical correlation. Results of the analysis suggest that the main control over vegetation distribution in this system is wind exposure. Exposed sites are characterized by cushion plant communities and Artemisia nova, and less exposed sites by the taller sagebrush species Artemisia tridentata ssp. vaseyana. Topographic depressions and leeward slopes are characterized by aspen stands and nivation hollows. Measurements of soil microclimate suggest that a major influence of topographic position on vegetation is snow redistribution and its effect on soil moisture and temperature.Abbreviations ARNO Artemisia nova - ARTRW Artemisia tridentata ssp. wyomingensis - ARTRV Artemisia tridentata ssp. vaseyana - PUTR Purshia tridentata - RIP riparian community - POTR Populus tremuloides - NIV nivation hollow community     

Sesquiterpene lactones of Artemisia: TLC analysis and taxonomic significance

Samples of Artemisia arbuscula ssp. arbuscula, A. tridentata ssp. tridentata, ssp. wyomingensis, ssp. vaseyana and ssp. vaseyana f. spiciformis were collected from various locations in Montana and analyzed by TLC for their sesquiterpene lactone content. Artemisia tridentata ssp. tridentata and ssp. wyomingensis are distinct morphologically and chemically, whereas ssp. vaseyana has three distinct chemical groups not yet separated morphologically. Artemisia arbuscula ssp. arbuscula and A. tridentata ssp. vaseyana f. spiciformis are easily separated by morphology but have identical TLC patterns. It has been further shown that the sesquiterpene lactones produced for a particular species or subspecies are the same regardless of the time of the year collected, although the quantity varies from winter to summer.     

Prescribed fires in Artemisia tridentata ssp. vaseyana steppe have minor and transient effects on vegetation cover and composition

Question: What is the impact of prescribed fires on the cover and composition of vegetation in Artemisia tridentata ssp. vaseyana steppe? Location: United States Department of Agriculture, Agricultural Research Service, United States Sheep Experiment Station, eastern Idaho (44°14′44′’ N, 112°12′47′’ W). Methods: Multiple prescribed fires were lit in 2002 and 2003 in an Artemisia tridentata ssp. vaseyana (mountain big sagebrush) steppe ecosystem that was relatively free of exotic plants. Measurements of cover components and plant species frequencies were taken pre‐ and for 2 to 3 years post‐fire. Results: Cover of forbs and grasses returned to pre‐fire levels after two years. Shrub cover declined from 36 to 6% in the first year post‐fire. Fire reduced the frequencies of three species, A. tridentata ssp. vaseyana, Festuca idahoensis, and Cordylanthus ramosus, of rangeland plants. Frequencies of four plant species, Hesperostipa comata, Polygonum douglasii, Chenopodium fremontii and Chenopodium leptophyllum increased, but only P. douglasii increased for more than a year. Conclusion: This study demonstrates that in an Artemisia tridentata ssp. vaseyana steppe ecosystem without significant non‐native species or anthropogenic disturbances vegetative cover and species composition of the herbaceous community are only minimally altered by fire. The herbaceous component returned to pre‐fire conditions within three years of a fire.     

Narrow hybrid zone between two subspecies of big sagebrush,Artemisia tridentata (Asteraceae). III. Developmental instability

Stable hybrid zones are believed to occur because gene dispersal is opposed by selection, but the nature of this selection remains unclear. The dynamic equilibrium model postulates that hybridization disrupts coadapted gene complexes, leading to alterations in development and subsequent hybrid unfitness. Alternatively, the bounded hybrid superiority model assumes that selection is due to exogenous factors. Here, we examine the developmental instability of sagebrush across a narrow hybrid zone. The results show no difference among the parental and hybrid taxa for the majority of comparisons examined. When significant differences in developmental instability were observed, one of the parental taxa (mountain big sagebrush) was typically the least developmentally stable. Hybrids were found to be the least developmentally stable for only two of the 28 comparisons studied. Interestingly, for some comparisons, hybrids were the most developmentally stable. These results contradict predictions of the dynamic equilibrium model. The sagebrush hybrid zone does not appear to be maintained by endogenous selection, as there is little indication that development is disrupted by hybridization.     

Inferences for life history strategies of Artemisia tridentata subspecies

Communities dominated by Artemisia tridentata ssp. tridentata, A. tridentata ssp. wyomingensis, and A. tridentata ssp. vaseyana, found in the Piceance Basin of western Colorado, were evaluated for life history strategy. Species cover data were analyzed using Grime's (1984) triangular model. Then a canonical analysis was conducted to obtain an ordination of relative species cover. Results from these two analyses were then used to infer strategy differences among life histories of subspecies, based on the respective plant community associates. These differences were found to be consistent with the divergent evolution assumed to have occurred in this species. The primary ordination axis was interpreted as an elevation-moisture gradient. Further analysis of soil data by factor analysis also separated the three Artemisia subspecies along a soil texture gradient.     

Plant hybrid zones as centers of biodiversity: the herbivore community of two endemic Tasmanian eucalypts

We found the hybrid zone between Eucalyptus amygdalina and Eucalyptus risdonii to be a center of insect and fungal species richness and abundance. Of 40 taxa examined, 73% were significantly more abundant in the hybrid zone than in pure zones, 25% showed on significant differences, and 2% were most abundant on a pure host species. The average hybrid tree supported 53% more insect and fungal species, and relative abundances were, on average, 4 times greater on hybrids than on either eucalypt species growing in pure stands. Hybrids may act as refugia for rare species: 5 of 40 species were largely restricted to the hybrid zone. Also, 50% of the species coexisted only in the hybrid zone, making for mique species assemblages. Although hybrids support more species and greater abundances, all hybrids are not equal: 68% of the 40 taxa examined were significantly more abundant on one hybrid phenotype than another. While herbivore concentrations on F1 type intermediates were rare, concentrations were common on phenotypes resembling backcrosses either to E. amygdalina or E. risdonii. For specialist herbivores, the hybrid phenotype most heavily utilized appears to be determined by its phenotypic affinity to its host species. Generalists exhibit an overall greater abundance on hybrids, but are less likely to utilize one hybrid phenotype over another. Mechanistic explanations for these distributions are numerous and probably species specific, but are likely to include: increased genetic susceptibility of hybrids due to hybrid breakdown; increased stress in the hybrid zone resulting in greater plant susceptibility; and a greater diversity of resources in the hybrid zone which could support more species. Seed capsule production by hybrids and their parental species is negatively correlated with herbivory. However, it is difficult to determine whether herbivores cause this pattern as hybrids may have inherently lower sexual reproduction. Laws enacted to protect rare and endangered species do not include hybrids. We argue that a re-examination of our current hybrid policy is warranted. Plant hybrid zones are centers of plant evolution and speciation, sources of economically important plants and potential biocontrol agents, and, as our study suggests, also provide essential habitats for phytophagous communities.     

Behavioral responses of hybrid lacewings (Neuroptera: Chrysopidae) to courtship songs

The study of hybrid courtship songs and the behavioral responses of hybrids and parental individuals to hybrid songs can be useful in understanding the origin of reproductive isolation among species that differ mainly in their courtship songs. Here we test the hypotheses (a) that hybrid lacewings prefer hybrid songs to either of the parental songs from a cross betweenChrysoperla plorabunda andC. johnsoni, and (b) that parental individuals prefer their own song over those of hybrids. Analysis of songs showed that most features of hybrid songs are intermediate between the two parents. Hybrids organize their songs with a series of simple volleys like those ofC. plorabunda. Female hybrids from two reciprocal crosses and females and males of the parental species were presented with choices of hybrid and parental songs. Hybrids responded more to recordings of hybrid songs than to recordings ofC. plorabunda but did not differ in the responses given toC. johnsoni and hybrid songs. In contrast, males and females of both parental lines preferred to duet with recordings of their own song types and did not respond to hybrid songs. Our results demonstrate that hybrids would be at a disadvantage in nature, because neitherC. plorabunda norC. johnsoni will respond to their songs.     

Genetic and morphological evidence of a geographically widespread hybrid zone between two crocodile species,Crocodylus acutus and Crocodylus moreletii

Hybrid zones represent natural laboratories to study gene flow, divergence and the nature of species boundaries between closely related taxa. We evaluated the level and extent of hybridization between Crocodylus moreletii and Crocodylus acutus using genetic and morphological data on 300 crocodiles from 65 localities. To our knowledge, this is the first genetic study that includes the entire historic range and sympatric zone of the two species. Contrary to expectations, Bayesian admixture proportions and maximum‐likelihood estimates of hybrid indexes revealed that most sampled crocodiles were admixed and that the hybrid zone is geographically extensive, extending well beyond their historical region of sympatry. We identified a few geographically isolated, nonadmixed populations of both parental species. Hybrids do not appear to be F₁s or recent backcrosses, but rather are more likely later‐generation hybrids, suggesting that hybridization has been going on for several to many generations and is mostly the result of natural processes. Crocodylus moreletii is not the sister species of C. acutus, suggesting that the hybrid zone formed from secondary contact rather than primary divergence. Nonadmixed individuals from the two species were distinguishable based on morphological characters, whereas hybrids had a complex mosaic of morphological characters that hinders identification in the wild. Very few nonadmixed C. acutus and C. moreletii populations exist in the wild. Consequently, the last nonadmixed C. moreletii populations have become critically endangered. Indeed, not only the parental species but also the naturally occurring hybrids should be considered for their potential conservation value.     

Deep sequencing of amplicons reveals widespread intraspecific hybridization and multiple origins of polyploidy in big sagebrush (Artemisia tridentata; Asteraceae)

• Premise of the study: Hybridization has played an important role in the evolution and ecological adaptation of diploid and polyploid plants. Artemisia tridentata (Asteraceae) tetraploids are extremely widespread and of great ecological importance. These tetraploids are often taxonomically identified as A. tridentata subsp. wyomingensis or as autotetraploids of diploid subspecies tridentata and vaseyana. Few details are available as to how these tetraploids are formed or how they are related to diploid subspecies. • Methods: We used amplicon sequencing to assess phylogenetic relationships among three recognized subspecies: tridentata, vaseyana, and wyomingensis. DNA sequence data from putative genes were pyrosequenced and assembled from 329 samples. Nucleotide diversity and putative haplotypes were estimated from the high-read coverage. Phylogenies were constructed from Bayesian coalescence and neighbor-net network analyses. • Key results: Analyses support distinct diploid subspecies of tridentata and vaseyana in spite of known hybridization in ecotones. Nucleotide diversity estimates of populations compared to the total diversity indicate the relationships are predominately driven by a small proportion of the amplicons. Tetraploids, including subspecies wyomingensis, are polyphyletic occurring within and between diploid subspecies groups. • Conclusions: Artemisia tridentata is a species comprising phylogenetically distinct diploid progenitors and a tetraploid complex with varying degrees of phylogenetic and morphological affinities to the diploid subspecies. These analyses suggest tetraploids are formed locally or regionally from diploid tridentata and vaseyana populations via autotetraploidy, followed by introgression between tetraploid groups. Understanding the phylogenetic vs. ecological relationships of A. tridentata subspecies will have bearing on how to restore these desert ecosystems.     

Hybrid songbirds employ intermediate routes in a migratory divide

Migratory divides are contact zones between populations that use different routes to navigate around unsuitable areas on seasonal migration. Hybrids in divides have been predicted to employ intermediate and potentially inferior routes. We provide the first direct test of this hypothesis, using light‐level geolocators to track birds breeding in a hybrid zone between Swainson's thrushes in western Canada. Compared to parental forms, hybrids exhibited increased variability in their migratory routes, with some using intermediate routes that crossed arid and mountainous regions, and some using the same routes as one parental group on fall migration and the other on spring migration. Hybrids also tended to use geographically intermediate wintering sites. Analysis of genetic variation across the hybrid zone suggests moderately strong selection against hybrids. These results indicate that seasonal migratory behaviour might be a source of selection against hybrids, supporting a possible role for migration in speciation.     

Sesquiterpene lactones of Artemisia arbuscula and A. tridentata

Chloroform extracts of Artemisia tridentata ssp. vaseyana f. spiciformis (Osterhout) Beetle and Artemisia arbuscula Nutt. ssp. arbuscula gave identical TLC patterns and contained the known germacranolides tatridin-A (I) and tatridin-B (II), and two new germacranolides deacetyllaurenobiolide (III) and spiciformin (IV). The structures of these lactones were shown through spectral studies and chemical transformations. Deacetyllaurenobiolide was converted to the known germacranolide laurenobiolide (IIIb) and spiciformin (IV) was synthesized from deacetyllaurenobiolide along with an isomeric epoxy lactone (V).     

Ecophysiology of first and second generation hybrids in a natural plant hybrid zone

Hybrids between related species vary widely in relative fitness, and that fitness can depend upon the environment. We investigated aspects of physiology that might influence fitness patterns in a plant hybrid zone. Seeds of Ipomopsis aggregata, I. tenuituba, F1 hybrids, F2 hybrids, and offspring of crosses between natural hybrids were planted into the relatively mesic site of origin for I. aggregata and the drier site for natural hybrids. We measured rates of photosynthesis (A _max), transpiration (E), instantaneous (A/E) and long-term (δ¹³C) indices of water use efficiency (WUE), and leaf nitrogen and carbon. We also examined correlations of these traits with plant size. Photosynthetic rate and A/E were higher in vegetative than flowering plants. WUE varied between sites and years, but differences among genotypic classes were spatially and temporally consistent. Instantaneous WUE was higher for F1 hybrids than for the average of the parental species, thereby showing heterosis. There was no evidence of hybrid breakdown, as WUE was no different in the F2 than the average across the F1 and parental species. Nor did WUE depend on cross direction in producing F1 progeny. Carbon isotope discrimination revealed higher long-term water use efficiency in I. tenuituba than I. aggregata. Leaf nitrogen was higher in I. tenuituba than I. aggregata, and higher in offspring of natural hybrids than in the F2. Results indicate heterosis for water use efficiency, with no hybrid breakdown. Heterosis in WUE may help to explain the relatively high survival of both reciprocal F1 hybrids in dry sites within the natural hybrid zone.     

Annual nitrous oxide flux and soil nitrogen characteristics in sagebrush steppe ecosystems

Soil nitrogen transformations and nitrous oxide fluxes were measured in a range of sagebrush steppe ecosystems in south-central Wyoming. Net nitrate production, measured in laboratory incubations, was highest in the ecosystem type dominated by Artemisia tridentata ssp. vaseyana, especially early in the growing season. Fluxes of nitrous oxide, measured in closed chambers and analyzed by gas chromatography, also tended to be higher in the same type, but only for short periods in the spring. Thereafter, all nitrous oxide fluxes were low and did not differ consistently among types. Estimated average annual fluxes for three Artemisia ecosystem types (dominated by Artemisia tridentata ssp. vaseyana, Artemisia tridentata ssp. wyomingensis, and Artemisia nova) were 0.32, 0.23 and 0.13 kg N₂O-N ha^–1 y^–1 repsectively. Average annual flux, weighted by the areal extent of these and other vegetation types in the region, was approximately 0.21 kg N₂O-N ha^–1y^–1. Assuming this landscape is representative of sagebrush steppe, we calculate a flux of 9.5 × 10⁹ g y^–1 of N₂O-N from U.S. sagebrush steppe, and a flux of 1.1 × 10¹¹ g y^–1 of N₂0-N from analogous desert and semi-desert shrublands of the world.     

Are plant hybrid zones centers of vertebrate biodiversity? A test in the Quercus grisea × Quercus gambelii species complex

The plant-hybrid-zones-as-centers-of-biodiversity (hereafter, PHZCB) hypothesis posits that plant hybrid zones represent areas of elevated biodiversity, and supports the inclusion of plant hybrid zones in conservation efforts. Support for the PHZCB hypothesis is limited to a handful of cases involving insect and fungal species and to two studies involving higher trophic levels. The PHZCB hypothesis requires further testing before plant hybrid zones can be established with any certainty as centers of biodiversity. We test whether the PHZCB hypothesis holds for higher trophic levels by examining the community structure of birds and reptiles associated with the Quercus grisea × Quercus gambelii species complex. Specifically, we compare patterns of species richness and abundance of these taxa in 10 hybrid zones and 11 contact zones (i.e., zones where both parental species occur but hybrids do not). In contrast to predictions of the PHZCB hypothesis, contact zones supported significantly more species of birds and reptiles than did hybrid zones. Species abundances exhibited idiosyncratic responses to zone type, but were higher generally in the contact zones.     

ELECTROPHORETIC CONFIRMATION OF INTERSPECIFIC HYBRIDIZATION IN AESCULUS (HIPPOCASTANACEAE) AND THE GENETIC STRUCTURE OF A BROAD HYBRID ZONE

Within a broad (>200 km wide) hybrid zone involving three parapatric species of Aesculus, we observed coincident clines in allele frequency for 6 of 14 electrophoretic loci. The cooccurrence of alleles characteristic of A. pavia, A. sylvatica, and A. flava was used to estimate genetic admixtures in 48 populations involving various hybrids between these taxa in the southeastern United States. High levels of allelic polymorphism (up to 40% greater than the parental taxa) were observed in hybrid populations and also in some populations bordering the hybrid zone. A detailed analysis of a portion of the hybrid zone involving A. pavia and A. sylvatica revealed a highly asymmetrical pattern of gene flow, predominantly from Coastal Plain populations of A. pavia into Piedmont populations of A. sylvatica. Computer simulations were used to generate expected genotypic arrays for parental, F₁; and backcross individuals, which were compared with natural populations using a character index scoring system. In these comparisons, hybrid individuals could be distinguished from either parent, but F₁ and backcross progeny could not be distinguished from each other. Most hybrid populations were found to include hybrids and one of the parental taxa, but never both parents. Three populations appeared to be predominantly hybrids with no identifiable parental individuals. Hybrids occurred commonly at least 150 km beyond the range of A. pavia, but usually not more than 25 km beyond the range of A. sylvatica. Introgression, suggested by genetically hybrid individuals and significant gene admixtures of two or more species in populations lacking morphological evidence of hybridization, may extend the hybrid zone further in both directions. The absence of one or both parental species from hybrid populations implies a selective disadvantage to parentals in the hybrid zone and/or that hybridization has occurred through long-distance gene flow via pollen, primarily from A. pavia into A. sylvatica. Long-distance pollen movement in plants may generate hybrid zones of qualitatively different structure than those observed in animals, where gene flow involves dispersal of individuals.     

Testing the hybrid superiority hypothesis in crested and marbled newts

We tested the hybrid superiority hypothesis in the zone of overlap and hybridization of the newts Triturus cristatus and T. marmoratus. To do so, we compared size, age, and growth-related parameters in F₁ hybrids and both parental species in Mayenne, France. We found significant differences in snout-vent length (SVL), body mass and average lifespan between the parental species and hybrids, increasing from T. cristatus – T. marmoratus – hybrids. The relation between age and SVL fitted von Bertalanffy's growth model and showed that SVL_max was significantly larger in hybrids than in the parental species, while the growth coefficient was lower in hybrids and T. marmoratus than in T. cristatus. Triturus cristatus appears to be the better competitor, since it attains sexual maturity faster and thus achieves more annual breeding opportunities. At the evolutionary level, the observed heterosis appears not to have further consequences as the hybrids are largely infertile. Our results support the hypothesis raised for the genus Triturus, that infertile hybrids allocate resources to growth.