Ecological genetics of seed germination regulation in Bromus tectorum L.

Regulation of seed germination phenology is an important aspect of the life history strategy of invading annual plant species. In the obligately selfing winter annual grass Bromus tectorum, seeds are at least conditionally dormant at dispersal in early summer and lose dormancy through dry-afterripening. Patterns of germination response at dispersal vary among populations and sometimes across years within populations. To assess the relative contribution of genotype and maturation environment to this variation, we grew progeny of ten parental lines from each of six contrasting populations in a common greenhouse environment. We then tested the germination responses of recently harvested seeds of the putative full-sib progeny at five incubation temperatures. Significant germination response differences among populations were observed in greenhouse cultivation, and major differences among full-sib families were evident for some populations and traits. Among-population variation accounted for over 90% of the variance in each trait, while within-family variance accounted for 1% or less. Germination responses of greenhouse-grown progeny were positively correlated with the responses of wild-collected seeds, but there was a tendency for lowered dormancy at higher incubation temperatures. This tendency was more marked in populations from cold desert, foothill, and plains habitats, suggesting a genotype-maturation environment interaction. Differences among populations in the amount of among-family variance were more evident at lower incubation temperatures, while among-family variance was more uniformly low at summer incubation temperatures. Populations from predictable extreme environments (subalpine meadow and warm desert margin) showed significantly less variation among families than populations from less predictable cold desert, foothill, and plains environments. Low among-family variance was not specifically associated with small population size or marginality of habitat, as small marginal populations from unpredictable environments showed variance as high as that of large populations. In populations with high among-family variance for germination traits, germination responses tended to be correlated across incubation temperatures, making it possible to characterize families in terms of their general dormancy status. The results indicate that seed germination regulation in this species is probably under strong genetic control, and that habitats with temporally varying selection are occupied by populations that tend to be more polymorphic in terms of their germination response patterns. Received: 19 May 1998 / Accepted: 27 January 1999     

The population biology of Bromus tectorum in forests: distinguishing the opportunity for dispersal from environmental restriction

Summary With increasing elevation and corresponding changes in the macroclimate, forest zones in the Intermountain Region of western North America are often dominated in turn by Pinus ponderosa, Pseudotsuga menziesii, Abies grandis, an Thuja plicata. Bromus tectorum (cheatgrass), and introduced annual grass now abundant in the Region's steppe, is uncommon in mature stands representative of these forest zones. In order to determine whether B. tectorum is largely excluded from these forests by insufficient seed dispersal or environmental restriction(s), the grass's demography was compared in each of four years among populations experimentally-introduced into mature forests. The number of recruits did not differ among the Pinus, Pseudotsuga, and Abies sites; recruitment was however significantly lower on the coolest site dominated by Thuja. Emergence in both the low elevation Pinus and Pseudotsuga sites was about the same in autumn, winter, and spring. In the cooler, moister Abies and Thuja sites, emergence was limited to autumn and early winter. Survival in these forest sites ranged between 0 and 87%. The percentage of the total population to survive until harvest was highest in the Pseudotsuga site, intermediate in the Pinus and Abies sites, and lowest in the Thuja site. Compared with B. tectorum in the steppe, the surviving plants were small, and few produced seeds. All parents were members of either the autumn or winter cohort, and most parents produced only one seed. No seeds were produced at the Thuja site. Although phenotypic plasticity apparently contributes to the wide ecological amplitude of this grass, its growing season on these sites in most years is too short for it to reproduce. Consequently, these forest zones broadly define the current environmental limits to the distribution of cheatgrass in this portion of its new geographic range.     

The effect of shading on photosynthesis,growth, and regrowth following defoliation for Bromus tectorum

Summary The effect of full sunlight, 60%, or 90% attenuated light on photosynthetic rate, growth, leaf morphology, dry weight allocation patterns, phenology, and tolerance to clipping was examined in the glasshouse for steppe populations of the introduced grass, Bromus tectorum. The net photosynthetic response to light for plants grown in shade was comparable to responses for plants grown in full sunlight. Plants grown in full sunlight produced more biomass, tillers and leaves, and allocated a larger proportion of their total production to roots than plants grown in shade. The accumulation of root and shoot biomass over the first two months of seedling growth was primarily responsible for the larger size at harvest of plants grown in full sunlight. Plants grown under 60% and 90% shade flowered an average of 2 and 6 weeks later, respectively, than plants grown in full sunlight. Regrowth after clipping was greater for plants grown in full sunlight compared to those grown in shade. Even a one-time clipping delayed flowering and seed maturation; the older the individual when leaf area was removed, the greater the delay in its phenology. Repeated removal of leaf area was more frequently fatal for plants in shade than in full sunlight. For plants originally grown in full sunlight, regrowth in the dark was greater than for shaded plants and was more closely correlated to non-flowering tiller number than to plant size. This correlation suggests that etiolated regrowth is more likely regulated by the number of functional meristems than by differences in the size of carbohydrate pools. Thus, shading reduces the rate of growth, number of tillers, and ability to replace leaf area lost to herbivory for B. tectorum. These responses, in turn, intensify the effect of competition and defoliation for this grass in forests. B. tectorum is largely restricted to forest gaps at least in part because of its inability to acclimate photosynthetically, the influence of shade on resource allocation, and the role of herbivory in exacerbating these effects.     

The population biology of Bromus tectorum in forests: effect of disturbance,grazing, and litter on seedling establishment and reproduction

Summary The effect of tree canopy, understory, herbivores, and litter depth on seedling establishment, survival, and reproduction of the alien grass, Bromus tectorum (cheatgrass), was examined in a series of experiments in four forest habitat types in western North America. Higher recruitment, survival, and reproduction on clearcuts, which would be expected if the overstory alone is limiting the distribution of cheatgrass in forests, were not observed. Removing the understory in an otherwise undisturbed Pinus ponderosa forest did, however, increase the emergence of B. tectorum, but plants in these experimentally-created openings were more vulnerable to grazing by small mammals. In contrast, removing the sparse understory in an Abies forest neither enhanced recruitment nor increased the incidence of grazing of B. tectorum seedlings. Regardless of the forest habitat, most grazed plants died before maturity; even fewer grazed plants produced seeds. Litter depth influenced both recruitment and biomass production: both the rate of germination and the size of resultant seedlings were lower on thick litter (6 cm) compared to results on thin litter (1.5 cm). In the more open Pinus ponderosa and Pseudotsuga menziesii forests, cheatgrass colonization may often occur in openings in the understory alone. Colonization in the more shady A. grandis and Thuja plicata forests is unlikely, however, unless the opening extends through both the understory and the overstory. As a result, cheatgrass is unlikely to increase in any of these forests unless the scale and incidence of disturbance increases substantially.     

Ecological genetics of vernalization response in Bromus tectorum L. (Poaceae)

BACKGROUND AND AIMS: Bromus tectorum (cheatgrass or downy brome) is an exotic annual grass that is dominant over large areas of former shrubland in western North America. To flower in time for seed production in early summer, B. tectorum plants generally require vernalization at winter temperatures, either as imbibed seeds or as established seedlings. METHODS: Variation in response to increasing periods of vernalization as seeds or seedlings for progeny of ten full-sib families from each of four B. tectorum populations from contrasting habitats was studied. KEY RESULTS: As vernalization was increased from 0 to 10 weeks, the proportion of plants flowering within 20 weeks increased, weeks to initiation of flowering decreased, and seed yield per plant increased, regardless of whether plants were vernalized as seeds or seedlings. Most of the variation was accounted for by differences among populations. Plants of the warm desert population flowered promptly even without vernalization, while those of the cold desert, foothill and montane populations showed incremental changes in response variables as a function of vernalization period. Populations differed in among-family variance, with the warm desert population generally showing the least variance and the cold desert population the most. Variation among populations and among families within populations decreased as vernalization period increased, whereas the non-genetic component of variance showed no such pattern. CONCLUSIONS: Variation in vernalization response was found to be adaptively significant and apparently represents the result of contrasting selection regimes on a range of founder genotypes.     

Population genetic analysis of Bromus tectorum (Poaceae) indicates recent range expansion may be facilitated by specialist genotypes

? Premise of the study: The mechanisms for range expansion in invasive species depend on how genetic variation is structured in the introduced range. This study examined neutral genetic variation in the invasive annual grass Bromus tectorum in the Intermountain Western United States. Patterns of microsatellite (SSR) genotype distribution in this highly inbreeding species were used to make inferences about the roles of adaptively significant genetic variation, broadly adapted generalist genotypes, and facultative outcrossing in the recent range expansion of B. tectorum in this region. ? Methods: We sampled 20 individuals from each of 96 B. tectorum populations from historically and recently invaded habitats throughout the region and used four polymorphic SSR markers to characterize each individual. ? Key results: We detected 131 four-locus SSR genotypes; however, the 14 most common genotypes collectively accounted for 79.2% of the individuals. Common SSR genotypes were not randomly distributed among habitats. Instead, characteristic genotypes sorted into specific recently invaded habitats, including xeric warm and salt desert as well as mesic high-elevation habitats. Other SSR genotypes were common across a range of historically invaded habitats. We observed very few heterozygous individuals (0.58%). ? Conclusions: Broadly adapted, generalist genotypes appear to dominate historically invaded environments, while recently invaded salt and warm desert habitats are dominated by distinctive SSR genotypes that contain novel alleles. These specialist genotypes are not likely to have resulted from recombination; they probably represent more recent introductions from unknown source populations. We found little evidence that outcrossing plays a role in range expansion.     

Life-history variation in Crepis tectorum (Asteraceae)

Summary Populations of the monocarpic plant Crepis tectorum were grown in a series of uniform environments to test the hypothesis that weedy populations are more r-selected than populations from a more natural habitat. Weedy populations exhibited a combination of r- and K-selected traits. The relatively rapid growth, the potential for a summer annual habit, and the relatively high fecundity that characterized at least one of the two weed populations studied were considered as r-selected traits favored in habitats of unpredictable duration. However, high levels of competition from other weedy species or from the crop in arable fields may explain at least some presumably K-selected traits observed in the weedy populations, e.g. relatively large seeds and late flowering in the summer. Results indicated that stress due to abiotic factors (strong winds, desiccation and nutrient deficiency) has been a more important selective factor than r- or K-selection, in non-weedy populations from calcareous grasslands (alvars) on the Baltic islands.     

Anabolic Rates Measured by Calorespirometry for Eleven Subpopulation of Bromus tectorum Match Temperature Profiles of Local Microclimates1

Bromus tectorum L. (cheatgrass) is a dominant weed that has increased the frequency of wildfire in western North America since being introduced about 120 years ago from Central Asia. Cheatgrass germinates in the fall, overwinters as young seedlings, grows rapidly in early spring, and completes the life cycle by early June. The dead grass then serves as fuel for wildfires. Cheatgrass seeds survive fire well while competing native perennials do not. Cheatgrass subpopulations have adapted to the thermal environments of different habitats in little more than a century. Calorespirometry was employed to study the effects of temperature (from 5 to 45°C) on growth rate, defined as net anabolic rate. Growth rate versus temperature curves tend to be bimodal for eleven subpopulations of cheatgrass. There are different growth rate responses to temperature among subpopulations, which appear to match the temperature profile of the native environment during the growth season and maximize growth. Both respiratory rates and efficiencies differ according to microclimate.     

Ecological genetics of seed germination regulation in Bromus tectorum L.

The probability that a seed will germinate depends on factors associated with genotype, maturation environment, post-maturation history, and germination environment. In this study, we examined the interaction among these sets of factors for 18 inbred lines from six populations of Bromus tectorum L., a winter annual grass that is an important weed in the semi-arid western United States. Seeds of this species are at least conditionally dormant at dispersal and become germinable through dry-afterripening under summer conditions. Populations and inbred lines of B. tectorum possess contrasting dormancy patterns. Seeds of each inbred line were produced in a greenhouse under one of three levels of maturation water stress, then subjected to immediate incubation under five incubation regimes or to dry storage at 20°C for 4 weeks, 12 weeks, or 1 year. Dry-stored seeds were subsequently placed in incubation at 20/30°C. Narrow-sense heritability estimates based on parent-offspring regressions for germination percentage of recently harvested seeds at each incubation temperature were high (0.518–0.993). Germination percentage increased with increasing water stress overall, but there were strong interactions with inbred line and incubation temperature. Inbred lines whose seeds were non-dormant over the full range of incubation temperatures when produced at low maturation water stress showed reaction norms characterized by little or no change as a function of increasing stress. For inbred lines whose dormancy status varied with incubation temperature, incubation treatments where seeds exhibited either very low or very high levels of dormancy showed the least change in response to maturation water stress. Inbred lines also varied in their pattern of dormancy loss during storage at 20°C, but maturation water stress had only a minor effect on this pattern. For fully afterripened seeds (1 year in storage at 20°C), inbred line and maturation water stress effects were no longer evident, indicating that differences in genotype and maturation environment function mainly to regulate dormancy and dormancy loss in B. tectorum, rather than to mediate response patterns of non-dormant seeds.     

Phenotypic plasticity inCrepis tectorum (Asteraceae)

Two experiments were carried out using two different approaches to compare populations ofCrepis tectorum (Asteraceae). One was based on a comparison of means of various vegetative and reproductive characters and another was based on a comparison of response patterns of the same characters in a series of environments. Population divergence within two earlier recognized form series, one from weed habitats and one from alvar habitats on Baltic islands, resulted in a partially overlapping pattern in cluster analyses based on character means. However, the pattern revealed by a comparison of the direction and amount of plastic response suggested that populations within the two form series had more similar response patterns than other combinations of populations. It was concluded that patterns of plasticity may provide useful additional information on the overall similarity among taxa. An hypothesis that plants in weed populations should exhibit a greater phenotypic response to the environments than plants in alvar populations was rejected.     

Ecological genetics of host plant exploitation in the green peach aphid, Myzus persicae

The ecological consequences of genetic variability in host plant exploitation of Myzus persicae (Hemiptera, Aphididae) were studied in several agro-ecosystems, differing in crop plant heterogeneity. An analysis of aphid populations sampled from sugar beet, potatoes, and lettuce revealed changes in the frequency distribution of aphid clones according to their respective host plant adaptations. The spatial unit of differentiation was below an average field size of 3–5 hectares and a differentiation could be found in the centre of potato and sugar beet fields but not at the edge of the fields. On a temporal scale, the differentiation of populations is transitory even on a large-scale basis of highly specialized cropping areas. The ecological parameters of differentiation are discussed with special reference to agro-ecosystems and integrated pest management. Increasing the genetic heterogeneity of crop fields may help to reduce population outbreaks.

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Zusammenfassung Die ökologischen Konsequenzen genetischer Variabilität in der Wirtspflanzennutzung von Myzus persicae (Hemiptera, Aphididae) wurden in mehreren Agro-Ökosystemen untersucht, die sich in der Kulturpflanzenheterogenität unterscheiden. Eine Analyse von auf Rüben, Kartoffeln und Salat siedelnden Populationen zeigte eine genetische Differenzierung in Wirtspflanzen-angepaßte Feld-populationen. Die räumliche Einheit solcher Differenzierungen liegt unterhalb einer Feldgröße von 3–5 ha. Eine Differenzierung konnte hier in der Mitte der Felder aber nicht am Rand festgestellt werden. Aus zeitlicher Sicht ist die Differenzierung selbst in großräumigen, spezialisierten Anbaugebieten nur vorübergehend feststellbar. Die ökologischen Parameter der Differenzierung werden besonders in Hinsicht auf die integrierte Schädlingsbekämpfung in Agro-ökosystemen diskutiert. Eine Erhöhung der genetischen Heterogenität von Kulturpflanzenbeständen kann möglicherweise zu einer Verringerung von Blattlaus-Massenvermehrungen beitragen.

     

Chloroplast genome organization of bromegrass,Bromus inermis Leyss

Summary A physical map of the Bromus inermis chloroplast genome was constructed using heterologous probes of barley and wheat chloroplast DNA (cpDNA) to locate restriction sites. The map was aligned from data obtained from filter hybridization experiments on single and double enzyme digests. Cleavage sites for the enzymes PstI, SalI, KpnI, XhoI and PvuII were mapped. The chloroplast genome of B. inermis is similar in physical organization to that of other grasses. The circular cpDNA molecule of B. inermis has the typical small (12.8 kbp) and large (81.3 kbp) single-copy regions separated by a pair of inverted repeat (21 kbp) regions. The cpDNA molecule of B. inermis is collinear in sequence to that of wheat, rye, barley and oats. No structural rearrangements or major deletions were observed, indicating that the cpDNA of Bromus is a useful tool in phylogenetic studies.     

Comparisons of Mycorrhizal Responsiveness with Field Soil and Commercial Inoculum for Six Native Montane Species and Bromus tectorum

Reestablishing native perennial plants and reducing invasive species are pivotal for many ecological restoration projects. The interactions among plant species, arbuscular mycorrhizal fungi, and soil P availability may be critical determinants of the success of native and non‐native plants in restoration and species invasions. Here we assessed mycorrhizal responsiveness for three late‐successional and three early‐successional plant species native to Rocky Mountain National Park and for the non‐native Downy brome, cheatgrass (Bromus tectorum L.) using field soil and commercial inoculum. Factorial greenhouse experiments were conducted to compare biomass of plant species with and without field soil and commercial inoculum treatments along a phosphorus (P) gradient, which ranged from ambient field levels to 12% of field levels, using dilutions of native soils. The two field soil inoculum treatments resulted in significant biomass differences for all species studied. Late‐successional species responded positively to field inoculum, whereas early‐successional species responded negatively. The two commercial inocula had low colonization rates (14 of 166 inoculated plants). The commercial inocula substrates had significant treatment effects on five of seven species included in the study in the apparent absence of mycorrhizal symbiosis. Soil P levels influenced mycorrhizal responsiveness in only one species, Smooth blue aster (Aster laevis L.). Our results show that, at least for the species studied here, locally collected field inoculum is the best choice for reestablishment of late‐successional native plant species.     

The evolution of self-fertility inCrepis tectorum (Asteraceae)

The extent of self-fertility was examined in 16 populations ofCrepis tectorum. A hypothesis that a weedy habit favours autogamy was only partly supported. Low levels of self-fertility characterized non-weedy populations from calcareous grasslands (alvars) on the Baltic island in Öland. By contrast, plants in nearly all weed populations studied were more or less self-fertile. However, the trend towards autogamy may have occurred independently of the trend towards a weedy habit, as shown by moderately to high levels of self-fertility in alvar populations from two other Baltic islands. In the weed group, there was a tendency for plants from two field populations to be more autogamous than plants from more ruderal habitats. There was an association between self-fertility and small, inconspicious heads in the alvar group but the association was weaker when weed populations were also considered. The relatively wide heads characterizing the ruderal weed populations may, at least partly, be an indirect effect of increases in overall plant size and/or in the size of the fruit associated with each flower.     

Geographical variation and genetic analysis of leaf shape inCrepis tectorum (Asteraceae)

Canonical variate analysis of plants raised in a uniform environment was used to study the pattern of geographical variation in leaf shape ofCrepis tectorum (Asteraceae). The diversity in leaf shape was much greater among populations confined to areas with exposed bedrock in the Baltic region than among weed populations scattered throughout Europe and Canada. A Ward's clustering linked outcrop populations from the archipelago of SW. Finland and the islands of Öland (Sweden) and Saaremaa (Estonia) due to the deeply lobed leaves characterizing these populations, while outcrop populations along the coast of E. Sweden were grouped due to their weakly lobed, narrow and dentate leaves. Most of the weed populations were grouped together but there was no tendency for the variation in this group to be related to habitat or geographical location. A mosaic of variation reflected in sharp (random) differentiation among local populations was superimposed on the large-scale ecogeographical pattern.—Crossing data indicated that most of the variation in degree of leaf dissection is governed by one major gene with deeply lobed leaves dominant over weakly lobed leaves. I suggest that the simple pattern of inheritance may have favoured rapid evolutionary changes in leaf shape, particularly in the Baltic area which emerged relatively late from the sea. Genetic correlations may have constrained the pattern of variation at higher taxonomic levels, since some of the trait associations detected in a segregating F₂ generation were also found at the among-population level.     

Soil water exploitation after fire: competition between Bromus tectorum (cheatgrass) and two native species

Summary Causes for the widespread abundance of the alien grass Bromus tectorum (cheatgrass) after fire in semiarid areas of western North America may include: (1) utilization of resources freed by the removal of fireintolerant plants; and (2) successful competition between B. tectorum and individual plants that survive fire. On a site in northwestern Nevada (USA), measurements of soil water content, plant water potential, aboveground biomass production, water use efficiency, and B. tectorum tiller density were used to determine if B. tectorum competes with either of two native species (Stipa comata and Chrysothamnus viscidiflorus) or simply uses unclaimed resources. Soil water content around native species occurring with B. tectorum was significantly lower (P<0.05) than around individuals without B. tectorum nearby. Native species had significantly more negative plant water potential when they occurred with B. tectorum. Aboveground biomass was significantly higher for native species without B. tectorum. However, the carbon isotope ratio of leaves for native species with B. tectorum was not significantly different from individuals without B. tectorum. Thus, B. tectorum competes with native species for soil water and negatively affects their wate status and productivity, but the competition for water does not affect water use efficiency of the native species. These adverse effects of B. tectorum competition on the productivity and water status of native species are also evident at 12 years after a fire. This competitive ability of B. tectorum greatly enhances its capability to exploit soil resources after fire and to enhance its status in the community.     

Morphometric differentiation,patterns of interfertility,and the genetic basis of character evolution inCrepis tectorum (Asteraceae)

Morphometric analyses of populations ofCrepis tectorum raised under uniform conditions support the earlier finding based on herbarium studies that extensive divergence has taken place in western Europe with a centre of diversity in the Baltic lowland area. A crossing experiment showed that subsp.pumila on the island of Öland has acquired a wide range of derived characters without the establishment of strong reproductive barriers, the only exception being a weak tendency for crosses with other populations on Öland to yield more fertile F₁ offspring than crosses with weedy and non-weedy populations in adjacent regions. Canadian weed populations showed a more uniform response in crosses with subsp.pumila than did morphologically similar weed populations scattered throughout Central Europe. While F₂ data suggest a monogenic basis of differences in the extent of leaf dissection, other distinctive traits appear to be governed by multiple genetic factors with individually slight effects.