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). 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.     

Narrow hybrid zone between two subspecies of big sagebrush (Artemisia tridentata: Asteraceae). II. Selection gradients and hybrid fitness

The dynamic equilibrium hypothesis proposes that hybrid zones are stabilized by a balance between dispersal and selection against hybrids. A key prediction of this hypothesis is that hybrids should have lower fitness than either parental taxon, regardless of habitat. Hybrid big sagebrush (Artemisia tridentata ssp. tridentata × ssp. vaseyana) in two narrow hybrid zones do show greatly decreased recruitment. Hybrids in one zone also show increased browsing by grasshoppers, while those in the other zone have increased numbers of aphids. Overall herbivore loads, however, are not greater on the hybrids than on the parental subspecies. Browsing by mule deer is greatest on ssp. vaseyana in both hybrid zones. Incidence of galls is also greatest on ssp. vaseyana in one hybrid zone. Moreover, browsing by Artemisia weevils is greatest on ssp. tridentata in one hybrid zone. Hybrids produce more flowers than either ssp. tridentata or ssp. vaseyana, while seed production rates of hybrids do not differ from those of the parental taxa. Finally, hybrid seeds germinate as well as those of ssp. tridentata and better than those of ssp. vaseyana. Thus, our data do not support the dynamic equilibrium hypothesis.     

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.     

Routes of origin of two recently evolved hybrid taxa: Senecio vulgaris var. hibernicus and York radiate groundsel (Asteraceae)

The possible pathways of origin of two recently arisen introgressant forms of Senecio vulgaris (i.e., var. hibernicus and York radiate groundsel) were investigated in experimental crosses between tetraploid S. vulgaris var. vulgaris and the normally diploid S. squalidus. Comparison of the morphology of synthesized hybrid progeny with established taxa, by discriminant function analysis, revealed that fertile hybrid offspring similar in morphology to S. vulgaris var. hibernicus and York radiate groundsel could be synthesized: (1) following formation of genomically stable diploid gametes by the triploid hybrid; (2) through the production of unreduced gametes by diploid S. squalidus; and (3) when a tetraploid form of S. squalidus acted as one of the parents. It was evident that hybrid offspring similar in morphology to the two introgressant taxa were more often produced in backcrosses to S. vulgaris than in segregating F2 or F3 generations (53% as opposed to 36%), and that fertile hybrid progeny were formed within two generations. Because hybridization between S. vulgaris and S. squalidus occurs regularly, although at very low frequency, in natural mixed populations in the British Isles, there is the potential for multiple origins to occur in the wild of both S. vulgaris var. hibernicus and York radiate groundsel.     

Ecological differentiation,lack of hybrids involving diploids,and asymmetric gene flow between polyploids in narrow contact zones of Senecio carniolicus (syn. Jacobaea carniolica,Asteraceae)

Areas of immediate contact of different cytotypes offer a unique opportunity to study evolutionary dynamics within heteroploid species and to assess isolation mechanisms governing coexistence of cytotypes of different ploidy. The degree of reproductive isolation of cytotypes, that is, the frequency of heteroploid crosses and subsequent formation of viable and (partly) fertile hybrids, plays a crucial role for the long‐term integrity of lineages in contact zones. Here, we assessed fine‐scale distribution, spatial clustering, and ecological niches as well as patterns of gene flow in parental and hybrid cytotypes in zones of immediate contact of di‐, tetra‐, and hexaploid Senecio carniolicus (Asteraceae) in the Eastern Alps. Cytotypes were spatially separated also at the investigated microscale; the strongest spatial separation was observed for the fully interfertile tetra‐ and hexaploids. The three main cytotypes showed highly significant niche differences, which were, however, weaker than across their entire distribution ranges in the Eastern Alps. Individuals with intermediate ploidy levels were found neither in the diploid/tetraploid nor in the diploid/hexaploid contact zones indicating strong reproductive barriers. In contrast, pentaploid individuals were frequent in the tetraploid/hexaploid contact zone, albeit limited to a narrow strip in the immediate contact zone of their parental cytotypes. AFLP fingerprinting data revealed introgressive gene flow mediated by pentaploid hybrids from tetra‐ to hexaploid individuals, but not vice versa. The ecological niche of pentaploids differed significantly from that of tetraploids but not from hexaploids.     

Nine-year reciprocal transplant experiment in the gardens of the basin and mountain big sagebrush (Artemisia tridentata: Asteraceae) hybrid zone of Salt Creek Canyon: the importance of multiple-year tracking of f itness

We have studied reciprocal transplant gardens involving the hybrid zone between basin and mountain big sagebrush ( Artemisia tridentata ) in Salt Creek Canyon, Utah, for 9 years. Previously, we showed that the parental taxa and hybrids had superior reproductive and vegetative performance in their native garden. These earlier data supported the Bounded Hybrid Superiority model. Now, after 9 years, we find that the mountain seed source plants have greater relative fitness than middle hybrid zone seed source plants in the middle hybrid zone garden. These results may be due to plant density and climatic factors more conducive to mountain seed source growth than that of either basin or middle hybrid zone seed source plants. On the other hand, these fitness estimates do not take into account the timing of reproduction, which together with the age-specific survival rate, can profoundly affect lifetime fitness. The intrinsic rate of increase ( r ) takes both of these factors into account, providing another estimate of fitness. Middle hybrid zone seed source plants had the greatest rate of increase in both the middle hybrid zone and mountain gardens and a greater rate of increase than either parent in the basin garden. This is most likely due to the greater reproductive performance of middle hybrid zone plants earlier in life than either parental taxon. These results partly support the Bounded Hybrid Superiority model and show the importance of long- term studies of hybrid fitness.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 86 , 213–225.     

Premating barriers to gene exchange and their implications for the structure of a mosaic hybrid zone between Chorthippus brunneus and C. jacobsi (Orthoptera: Acrididae)

Many hybrid zones contain a deficit of hybrid genotypes relative to expectations from tension zone models. This is often associated with separation of parental genotypes into distinct habitats (mosaicism), but sometimes parentals can be found co-occuring in the same local population (bimodality). In both cases, prezygotic isolation may play an important role in determining the genotypic composition of the zone. Chorthippus brunneus and C. jacobsi (Orthoptera: Acrididae) meet and form a complex hybrid zone in northern Spain. Analysis of stridulatory peg numbers reveals partial spatial and seasonal isolation in a 25 km2 area of the zone: C. jacobsi phenotypes predominate in June and July and are present in both valley and mountain habitats; C. brunneus phenotypes predominate in August and are restricted to valley habitats, always in sympatry with C. jacobsi. Strong assortative mating was observed in laboratory mating experiments. Spatial, seasonal and behavioural isolation combine to produce strong premating isolation in the study area. These results suggest a role for premating isolation in maintaining both the mosaic structure and bimodality of this hybrid zone.     

Introgressive hybridization between two species of waterstriders (Hemiptera: Gerridae: Limnoporus): geographical structure and temporal change of a hybrid zone

Where the distribution ranges of the waterstriders Limnoporus notabilis and L. dissortis meet in western Canada, extensive hybridization and introgression occurs. Multivariate ordination analyses of genetic and morphometric data by principal component analysis revealed that a single axis separating the two parent species could account for nearly all the variation in both data sets. Maps of principal component scores for both data sets revealed geographical patterns of variation reflecting specific topographic features in the region. Comparisons of morphometric data from some of the samples collected in the 1980s and from the same sites revisited in the 1990s revealed substantial changes. An ‘island’ of dissortis‐like populations inside the range of L. notabilis in interior British Columbia expanded, and a marked local protrusion of notabilis‐like phenotypes into the range of L. dissortis on the east slope of the Rocky Mountains diminished during the decade between collections. We conclude that introgressive hybridization between these two species of waterstriders is a spatially complex and highly dynamic process.     

Genetic structure of a hybrid zone between two violets,Viola rossii Hemsl. and V. bissetii Maxim.: dominance of F1 individuals in a narrow contact range

The genetic composition of a hybrid zone can provide insight into the evolution of diversification in plants. We carried out morphological and amplified fragment length polymorphism analyses to investigate the genetic composition of a hybrid zone between two violets, Viola bissetii Hemsl. and Viola rossii Maxim. Our aim was to clarify the formation and maintenance of hybrids between these Viola species. We found that most hybrid individuals (V. bissetii × V. rossii) were of the F₁ generation, with a few of the F₂ generation. We found no backcrosses. The scarcity of post‐F₁ hybrids indicates that a species barrier is established between the parental species. The F₁‐dominated hybrid zone occupied only a narrow, intermediate ecotone between the parental habitats, suggesting that selection by environmental factors against hybrids may help to maintain the current conditions in this hybrid zone.