Nebraska Populations Research Articles

Coexpression Clusters and Allele-Specific Expression in Metabolism-Based Herbicide Resistance.

In the last decade, Amaranthus tuberculatus has evolved resistance to 2,4-dichlorophenoxyacetic acid (2,4-D) and 4-hydroxyphenylpyruvate dioxygenase inhibitors in multiple states across the midwestern United States. Two populations resistant to both mode-of-action groups, one from Nebraska (NEB) and one from Illinois (CHR), were studied using an RNA-seq approach on F2 mapping populations to identify the genes responsible for resistance. Using both an A. tuberculatus transcriptome assembly and a high-quality grain amaranth (A. hypochondriacus) genome as references, differential transcript and gene expression analyses were conducted to identify genes that were significantly over- or underexpressed in resistant plants. When these differentially expressed genes (DEGs) were mapped on the A. hypochondriacus genome, physical clustering of the DEGs was apparent along several of the 16 A. hypochondriacus scaffolds. Furthermore, single-nucleotide polymorphism calling to look for resistant-specific (R) variants, and subsequent mapping of these variants, also found similar patterns of clustering. Specifically, regions biased toward R alleles overlapped with the DEG clusters. Within one of these clusters, allele-specific expression of cytochrome P450 81E8 was observed for 2,4-D resistance in both the CHR and NEB populations, and phylogenetic analysis indicated a common evolutionary origin of this R allele in the two populations.

Susceptibility of Nebraska Western Corn Rootworm (Coleoptera: Chrysomelidae) Populations to Bt Corn Events.

Transgenic plants have been widely adopted by growers to manage the western corn rootworm, Diabrotica virgifera virgifera LeConte, in field corn. Because of reduced efficacy in some Nebraska fields after repeated use of Cry3Bb1-expressing hybrids, single plant bioassays were conducted in 2012 and 2013 to characterize the susceptibility of western corn rootworm populations to the rootworm-active proteins Cry3Bb1, mCry3A, and Cry34/35Ab1. Results demonstrate that there are heritable differences in susceptibility of Nebraska western corn rootworm populations to rootworm-active Bt traits. Proportional survival and corrected survival data coupled with field histories collectively support the conclusion that a level of field resistance to Cry3Bb1 has evolved in some Nebraska populations in response to selection pressure and that cross-resistance exists between Cry3Bb1 and mCry3A. There was no apparent cross-resistance between Cry34/35Ab1 and either Cry3Bb1 or mCry3A. The potential implications of these results on current and future corn rootworm management strategies are discussed.

Frequency and Distribution of Extended Diapause in Nebraska Populations of <I>Diabrotica barberi</I> (Coleoptera: Chrysomelidae)

The frequency of extended diapause in populations of the northern corn rootworm, Diabrotica barberi Smith & Lawrence (Coleoptera: Chrysomelidae), was measured in eastern Nebraska. Adult collections were made during late summer in 2008 and 2009 from eight sites each year (seven sites were consistent over years). Eggs were obtained from 12 to 20 females per site and were held on moist soil under appropriate temperature profiles to facilitate egg survival, diapause development, and diapause termination. Percentage egg hatch was recorded after the first and second year for the 2008 and 2009 collections. Additional extended diapause expression was estimated for the 2008 and 2009 collections by counting remaining live eggs after the second-year egg hatch was completed. These data also were used to estimate each site's maximum egg viability. Results collectively indicate that the extended diapause trait was present in all eastern Nebraska populations; however, significant variation in the frequency of extended diapause was observed within and among D. barberi populations (first-year mean percentage egg hatch range: 2008 collections: 4.5-38.6%; 2009 collections: 10.7-42.5%). Geographically, the highest incidence of extended diapause was found along a north-south transect in eastern counties of Nebraska that parallels the Missouri River. In general, the frequency of extended diapause was lower in sites found west of the north-south transect. This study contributes to the knowledge base needed to develop appropriate D. barberi management strategies in Nebraska. These data can be used to pinpoint areas of Nebraska that may have the greatest risk of economic injury in first-year corn (Zea mays L.) when D. barberi densities are high.

Response of Nebraska Horseweed (Conyza canadensis) Populations to Dicamba

Dicamba-resistant soybeans are being developed to provide an additional herbicide mechanism-of-action for postemergence weed control in soybean. Numerous broadleaf species, including horseweed, have evolved resistance to glyphosate. It is anticipated that dicamba will be used by farmers as a primary tool to manage these weeds. Studying and understanding variability in horseweed response to dicamba will aid in developing appropriate risk management strategies to extend the utility of the dicamba-resistance technology. Horseweed plants from ten Nebraska populations were treated with one of nine doses of dicamba in greenhouse experiments. At 28 days after treatment (DAT) visual injury estimations were made and plants were harvested to determine dry weight. There was a three-fold difference in the I90 (90% visual injury estimate) between the least (638 g ha -1 ) and most (205 g ha -1 ) susceptible populations. Two plants from five populations were observed for an additional three months. No plants treated at doses above 280 g ha -1 survived to set seeds. These results suggest that maintaining use doses of 560 g ha -1 or greater may fully control horseweed populations from Nebraska and minimize the risk of

Genetic Differentiation of Western Corn Rootworm Populations (Coleoptera: Chrysomelidae) Relative to Insecticide Resistance

AbstractAs the single most important pest of field corn, Zea mays L., throughout most of the Corn Belt, the western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), has undergone repeated selection for resistance to a variety of insecticides that persist widely among Nebraska populations. In this study, we used 11 microsatellite markers to genotype two populations with high levels of resistance to methyl-parathion and aldrin (Polk and Stromsburg), two populations with low and intermediate levels of resistance (Mead and Clay Center) from Nebraska, and one population from outside the Corn Belt (Safford, AZ). The genetic diversity measured by observed heterozygosity (H0) was reduced 15–32% in the highly resistant populations compared with the more susceptible populations in Nebraska. Significant genetic differentiation was detected between the resistant and susceptible populations (Polk and Stromsburg versus Mead and Clay Center) in Nebraska (FST = 0.016) and between all the populations from Nebraska and Arizona (FST = 0.059). The average observed heterozygosities in the populations were positively correlated with insecticide susceptibility based on mortality at diagnostic concentrations of aldrin and methyl-parathion, respectively. These results indicate that the insecticide selection from exposure to aldrin and methyl-parathion may be a contributing factor in shaping the genetic structure of western corn rootworm populations in Nebraska. Factors including isolation by distance and a Wolbachia-induced breeding barrier may have contributed to differentiation of rootworm populations from Nebraska and Arizona.

Assessment of Allelic Variation Among Massasauga Rattlesnake Populations via Microsatellite Analysis

The decline in populations of the eastern massasauga rattlesnake (Sistrurus catenatus catenatus) across its range is of concern to conservation managers and the species is currently a candidate for federal listing. Because only four eastern massasauga populations are known to occur in Missouri, the species is on the Missouri Endangered Species List. We sought to produce a molecular genetic data set that could be used to inform conservation management decisions about three of these populations. Our method was to collect and analyze data from three microsatellites sites in an effort to provide insight into the genetic health of the populations and into relationships between them. Our method was controlled by inclusion of two populations of the western massasauga (Sistrurus catenatus tergeminus) from southeastern Nebraska. The final data set included 111 individuals from Missouri and Nebraska populations. Although a significant portion (92%) of alleles were shared between each population, low levels of heterozygosity were detected, indicating local inbreeding. Tests of population differentiation suggested that little to no gene flow between these populations has occurred, giving rise to unique allele frequencies and combinations which may be of high conservation value. Most notably, significant differentiation was observed between putative subpopulations at the Squaw Creek National Wildlife Refuge that are in close proximity to one another. Our results illustrate the value of molecular genetics analysis in conservation management and have important implications for the management of Missouri massasauga populations.

Monte Carlo analysis of parameter uncertainty in matrix models for the weed Cirsium vulgare

Summary Parameter uncertainty challenges the use of matrix models because it violates key assumptions underlying elasticity analyses. We have developed a matrix model to compare Monte Carlo methods with elasticity analyses for estimation of the relative importance of factors in the asymptotic population growth rate, λ, of Cirsium vulgare (spear thistle) in Nebraska, USA. We calculated λ for a base model using 11 parameter estimates available for Nebraska populations plus eight extracted from the literature, causing parameter uncertainty. We then calculated λ for 10 000 alternative models using Monte Carlo parameter estimation; parameters were drawn from the full range of each parameter in the literature and partial rank correlation analysis (PRCC) was used to order the parameters by the magnitude of their effect on λ. Monte Carlo analysis found that insect floral herbivory, affecting the regeneration transition, was the most important parameter affecting λ, whereas elasticity analyses suggested that the transition from small to medium size was the most significant. Statistical comparison, using PRCC vs. lower level elasticity (LLE), showed that the Monte Carlo analysis provided a more accurate assessment. As λ > 1 in 99% of the model runs even with significant floral herbivory, we added two parameters influenced by weed management (probability of large thistles dying without producing seed and proportion of seeds that failed to germinate). Simulations that included reductions in these parameters, along with floral herbivory, led to λ < 1 in 17% of the runs, suggesting these three factors interact to produce the low densities observed for this invasive thistle in our study area. Synthesis and applications. This study demonstrates the utility of the Monte Carlo approach for modelling weed dynamics with parameter uncertainty and multiple, potentially interacting, parameters. Invasive population growth by C. vulgare could be limited by a combination of weed management practices and the biotic resistance imposed by native floral herbivores.

Response of Common Lambsquarters (Chenopodium album) to Glyphosate as Affected by Growth Stage

Experiments were conducted to determine the efficacy of glyphosate on four common lambsquarters populations collected from Kansas, Nebraska, North Dakota, and Ohio. Glyphosate dose-response studies for common lambsquarters treated at 2.5-, 7.5-, and 15-cm heights showed that glyphosate at 1.1 kg ae ha−1caused more than 80% injury to 2.5-cm plants but less than 55% injury to 7.5- and 15-cm plants. All populations were susceptible to glyphosate at the 2.5-cm height. The glyphosate rate required to cause 50% injury (GR50) was 430, 500, 500, and 560 g ha−1for the Kansas, North Dakota, Ohio, and Nebraska populations, respectively. Differential response of common lambsquarters populations was evident with 15-cm plants where the GR50was glyphosate at 1,010, 1,230, 1,650, and 2,770 g ha−1for the Kansas, North Dakota, Nebraska, and Ohio populations, respectively. Reduced injury on 15-cm common lambsquarters plants by glyphosate may be partly attributed to reduced glyphosate accumulation per unit of plant tissues and enhanced calcium content in more-developed plants. All four common lambsquarters populations at the early seedling stage were susceptible to glyphosate, but tolerance increased as the plant developed and the extent of tolerance differed among populations.

Pollination biology of the endangered Blowout Penstemon (Penstemon haydenii S. Wats.: Scrophulariaceae) in Nebraska1

We studied the breeding system and pollinators of Nebraska populations of the endangered plant Blowout penstemon, Penstemon haydenii. This species appears to be one of the few known self-incompatible species of Penstemon few self-pollinated flowers set fruit and few of these produced seeds. We found no indication that sexual reproduction was limited by inadequate pollination or resources. Pollinators were several species of native bees which varied from population to population and from year to year; there did not appear to be a consistent P. haydenii pollinator fauna. Flowers were pollinated mainly by large apid bees in the genera Habropoda and Bombus, by (mostly) smaller halictid bees, and by intermediate sized megachilid bees, particularly in the genus Osmia. One frequent flower-visitor, the presumed Penstemon specialist, Hoplitis pilosifrons (Megachilidae), may be more of a pollen-parasite than a pollinator because it commonly collected pollen into the scopa but carried very little pollen on other body parts. Management plans for conservation of Penstemon haydenii need to recognize its need for pollinators, and that the health of pollinator populations may be site-specific.

VALIDITY OF PRAIRIE GROUSE HARVEST-AGE RATIOS AS PRODUCTION INDICES

The ratio of juveniles to adults in the fall harvest is a common index of production for Galliformes. The percentage of juvenile birds in the harvest has been shown to decline as the hunting season progressed for many galliforms, resulting in a biased index of production. Therefore, we used wing samples of plains sharp-tailed grouse (Tympanuchus phasianellus jamesi) and greater prairie-chickens (T. cupido pinnatus) from 4 public land areas in the Nebraska Sandhills, Nebraska, USA, to assess the potential for bias in harvest-age ratios across time. We hypothesized that the ratio of juveniles to adults in the harvest could change over time if susceptibility to harvest and/or fall survival were different between the juvenile and adult prairie grouse (Tympanuchus spp.) in the Nebraska populations. We found no change in the harvest-age ratio over time in either the sharp-tailed grouse or greater prairie-chicken data. Our findings were consistent with the published literature on harvest-age rates for sharp-tailed grouse but inconsistent with the greater prairie-chicken literature. Therefore, we maintain that analysis of the harvest data for bias due to a changing harvest-age ratio as the hunting season progresses is an essential adjunct to subsequent analysis or comparisons of production indices based on harvest-age ratios. In addition, limitations of harvest-age ratios must be known and care must be taken to minimize other potential biases before using harvest-age ratios as an index to production.

Carbaryl Susceptibility, Diagnostic Concentration Determination, and Synergism for U.S. Populations of Western Corn Rootworm (Coleoptera: Chrysomelidae)

Baseline carbaryl susceptibility and potential resistance mechanisms were identified in U.S. populations of western corn rootworm, Diabrotica virgifera virgifera LeConte, using lethal concentration, diagnostic concentration, and synergism bioassays. Twenty-two adult rootworm populations were examined from the states of Indiana, Illinois, Iowa, Kansas, Nebraska, Ohio, Pennsylvania, South Dakota, Texas, and Virginia. Lethal concentration bioassays, conducted within carbaryl-coated glass vials on 12 populations, identified resistance in only 2 Nebraska populations. A diagnostic concentration of carbaryl was subsequently identified which caused 100 and 40% mortality in standard susceptible and known resistant populations, respectively. In diagnostic concentration bioassays on all 22 populations, substantial decreases in mortality were observed only in 3 populations from outside of Nebraska. Synergism bioassays on standard susceptible and resistant Nebraska populations indicated significant reductions in resistance levels with inhibitors of both cytochrome P450 monooxygenases and esterases, suggesting an involvement in resistance by both these classes of detoxification enzymes. Results are discussed with respect to the validation of diagnostic concentration bioassays as a resistance monitoring tool for existing corn rootworm areawide management programs, and the potential roles of cytochromes P450 and esterases in carbaryl resistance.

Mitochondrial DNA polymorphism in a black fly, Simulium vittatum (Diptera: Simuliidae)

Mitochondrial DNA (mtDNA) was extracted from pooled field-collected samples representing six species of black flies (Cnephia dacotensis, Simulium bivittaum, S. johansenni, S. luggeri, S. piperi, S. vittatum) and compared by restriction fragment length polymorphism (RFLP) analysis. Morphospecies were molecularly distinct, with few shared restriction fragments. Eleven populations of S. vittatum were found that appeared to be homogeneous for a single mitochondrial haplotype. Ten other populations of S. vittatum showed extensive mitochondrial heterogeneity. In part, these samples contained mixtures of two cytologically recognized siblings: IIIL-1 and IS-7. About 70% of the mitochondrial genome of a population pure for sibling IIIL-1 was cloned as five HindIII fragments, which were used as hybridization probes to examine individual black flies. Thirteen mtDNA haplotypes involving permutations of 10 HindIII restriction sites were identified in individual black flies examined from 26 populations. DNA from 168 larvae cut with both EcoR1 and HindIII revealed five additional haplotypes. One HindIII haplotype was present in 84% of 390 larvae examined and predominated in every population examined from New York to California and in both the IIIL-1 and IS-7 siblings. Nebraska populations had individuals with nearly all known haplotypes. The most common haplotype was usually the only form present in warm, silty streams with organic enrichment. Rarer haplotypes were found in cool, spring-fed streams but without clear geographic or phylogenetic components.

Hatchling Size in the Turtle Chrysemys picta bellii from Western Nebraska: Relationships to Egg and Maternal Body Size

Eggs of the turtle Chrysemys picta bellii from four western Nebraska populations were incubated under laboratory conditions to compare egg size and hatchling size as measures of offspring size among populations and in the context of maternal body size. Generally, hatchling size (mass and carapace length) was linearly related to egg size (mass, width, and length) and did not vary between the two years of the study or among populations after adjusting for egg size. However, hatchling carapace length adjusted for egg width was greatest in a population with the most elongate eggs indicating that linear egg measurements may not be useful for interpopulation comparisons. The smaller range of variation, but greater variability in hatchling size relative to egg size and a narrow range of maternal body sizes (carapace length and mass), seemed to preclude a correlation between maternal body size and hatchling size within two populations. In two other populations, hatchling carapace length, egg width, and egg length increased with maternal carapace length by a common slope, although egg wet mass had a steeper slope than did hatchling wet mass. It is probable that under natural incubation conditions (e.g., warmer, drier, and more variable), hatchling mass may increase only slightly or not at all with maternal body size. My results suggest that both hatchling and egg size should be considered in turtle life history studies, particularly for models that predict delayed sexual maturation when offspring size and survival increase with maternal body size and age.