Tiina Ylioja

IMPACT OF MINIMUM WINTER TEMPERATURES ON THE POPULATION DYNAMICS OF DENDROCTONUS FRONTALIS

Predicting population dynamics is a fundamental problem in applied ecology. Temperature is a potential driver of short-term population dynamics, and temperature data are widely available, but we generally lack validated models to predict dynamics based upon temperatures. A generalized approach involves estimating the temperatures experienced by a population, characterizing the demographic consequences of physiological responses to temperature, and testing for predicted effects on abundance. We employed this approach to test whether minimum winter temperatures are a meaningful driver of pestilence from Dendroctonus frontalis (the southern pine beetle) across the southeastern United States. A distance-weighted interpolation model provided good, spatially explicit, predictions of minimum winter air temperatures (a putative driver of beetle survival). A Newtonian heat transfer model with empirical cooling constants indicated that beetles within host trees are buffered from the lowest air temperatures by approximately 1-4 degrees C (depending on tree diameter and duration of cold bout). The life stage structure of beetles in the most northerly outbreak in recent times (New Jersey) were dominated by prepupae, which were more cold tolerant (by >3 degrees C) than other life stages. Analyses of beetle abundance data from 1987 to 2005 showed that minimum winter air temperature only explained 1.5% of the variance in interannual growth rates of beetle populations, indicating that it is but a weak driver of population dynamics in the southeastern United States as a whole. However, average population growth rate matched theoretical predictions of a process-based model of winter mortality from low temperatures; apparently our knowledge of population effects from winter temperatures is satisfactory, and may help to predict dynamics of northern populations, even while adding little to population predictions in southern forests. Recent episodes of D. frontalis outbreaks in northern forests may have been allowed by a warming trend from 1960 to 2004 of 3.3 degrees C in minimum winter air temperatures in the southeastern United States. Studies that combine climatic analyses, physiological experiments, and spatially replicated time series of population abundance can improve population predictions, contribute to a synthesis of population and physiological ecology, and aid in assessing the ecological consequences of climatic trends.

Does lignin modification affect feeding preference or growth performance of insect herbivores in transgenic silver birch (Betula pendula Roth)

Transgenic silver birch (Betula pendula Roth) lines were produced in order to modify lignin biosynthesis. These lines carry COMT (caffeate/5-hydroxyferulate O-methyltransferase) gene from Populus tremuloides driven by constitutive promoter 35S CaMV (cauliflower mosaic virus) or UbB1 (ubiquitin promoter from sunflower). The decreased syringyl/guaiacyl (S/G) ratio was found in stem and leaf lignin of 35S CaMV-PtCOMT transgenic silver birch lines when compared to non-transformed control or UbB1–PtCOMT lines. In controlled feeding experiments the leaves of transgenic birch lines as well as controls were fed to insect herbivores common in boreal environment, i.e., larvae of Aethalura punctulata, Cleora cinctaria and Trichopteryx carpinata (Lepidoptera: Geometridae) as well as the adults of birch leaf-feeding beetles Agelastica alni (Coleoptera: Chrysomelidae) and Phyllobius spp. (Coleoptera: Curculionidae). The feeding preferences of these herbivores differed in some cases among the tested birch lines, but these differences could not be directly associated to lignin modification. They could as well be explained by other characteristics of leaves, either natural or caused by transgene site effects. Growth performance of lepidopteran larvae fed on transgenic or control leaves did not differ significantly.

Alternate attractors in the population dynamics of a tree-killing bark beetle

Among the most striking changes in ecosystems are those that happen abruptly and resist return to the original condition (i.e., regime shifts). This frequently involves conspicuous changes in the abundance of one species (e.g., an oubreaking pest or keystone species). Alternate attractors in population dynamics could explain switches between low and high levels of abundance, and could underlie some cases of regime shifts in ecosystems; this longstanding theoretical possibility has been difficult to test in nature. We compared the ability of an alternate attractors model versus two competing models to explain population fluctuations in the tree-killing bark beetle, Dendroctonus frontalis. Frequency distributions of abundance were distinctly bimodal, a prediction of the alternate attractors model, strongly indicating the lack of a single, noisy equilibrium. Time series abundance data refuted the existence of strong delayed density-dependence or nonlinearities, as required by the endogenous cycles model. The model of alternate attractors was further supported by the existence of positive density-dependence at intermediate beetle abundances. Experimental manipulations show that interactions with competitors and shared enemies could create a locally stable equilibrium in small populations of D. frontalis. High variation among regions and years in the abundance of predators and competitors could permit switches between alternate states. Dendroctonus frontalis now provides the strongest case that we know of for alternate attractors in natural population dynamics. The accompanying demographic instability appears to underlie spatially extensive outbreaks that have lasting impacts on forest ecosystems. Understanding feedbacks in populations with alternate attractors can help to identify thresholds underlying regime shifts, and potentially manage them to avoid undesirable impacts.

Field performance of chitinase transgenic silver birch (Betula pendula Roth): growth and adaptive traits

Fifteen transgenic birch lines carrying a chitinase IV gene from sugar beet and non-transgenic control plants were grown in a field trial, and traits connected to growth, quality and adaptation were monitored. Significant variation among the transgenic lines was observed in the growth parameters as well as parameters linked to stress status and leaf phenology of the trees. It was hypothesized that the differences among the lines could be explained by the differences in the level of transgene expression measured as sugar beet chitinase IV transcript accumulation. The level of the transgene expression was not detected to have influence on growth or leaf phenology, but instead it had influence on the parameters related to stress status of a tree. The increased levels of red colour and decreased general condition of the transgenic plants compared to the control plants may indicate physiological stress among the transgenic plants. The proportion of trees infested by Phytobia, larvae of which cause an easthetic defect to birch wood, was lower among many transgenic lines than in controls but the variation in Phytobia occurrence was explained by the differences in plant size only. Three lines out of fifteen were frequently different from the control plants in growth and leaf phenology, and these differences are suggested to be due to the position effect of the transgene. The observed changes in individual transgenic lines were not correlated with the sugar beet chitinase IV expression, and were more likely to impair than improve the traits that are usually considered important in adaptation and birch breeding.

No negative correlation between growth and resistance to multiple herbivory in a deciduous tree, Betula pendula

Abstract Plants are assumed to have a trade-off between growth and resistance. This trade-off has been demonstrated in herbaceous plants, but the information on woody plants is conflicting. In contrast to simple annual plant–herbivore systems, trees face highly variable herbivory. In this note, we measured clonal variation in the growth of Betula pendula and in its resistance to nine major herbivore species belonging to five different feeding guilds. There were significant differences in both the growth and the resistance of clones to all the herbivores studied. However, we found no negative correlation between growth and resistance. No clone was generally resistant or susceptible to most of the herbivores. This suggests that, if there is an ultimate trade-off in resource allocation of B. pendula between growth and resistance, it is masked by other factors, for example, by several opposite abiotic and biotic selection pressures.

Assessment of genetic and pheromonal diversity of the Cydia strobilella species complex (Lepidoptera: Tortricidae)

Combining pheromone trapping and genetic analyses can be useful when trying to resolve complexes of closely related insect taxa that are difficult to distinguish based on morphological characters. Nearctic and Palearctic populations of the spruce seed moth, Cydia strobilella L., have been considered taxonomically synonymous since 1983, but more recent work revealing distinct sex pheromones for Canadian and Swedish moths suggest that populations in the two regions belong to different species. In order to test this hypothesis, we performed field trapping using different pheromone lures at ten sites in North America, Europe and Asia, and reconstructed phylogenetic relationships among trapped moths using mitochondrial (cytochrome oxidase subunit I) and nuclear (elongation factor 1 alpha) DNA sequence data. Trapping data and tree topologies for both genes revealed distinct pherotypes in North America and Eurasia. A genetically distinct population from China was investigated further with respect to its sex pheromone. Electrophysiological data indicated that Chinese females produce a deviant ratio of the sex pheromone components (dienic acetates) compared to Swedish females. However, trapping experiments in both areas revealed a similar broad response profile in males to a wide range of acetate ratios, and these populations should be considered taxonomically synonymous. A previous suggestion of an agonistic effect on the attraction of C. strobilella males in Sweden when adding the corresponding alcohols to the binary acetate blend was also tested in Sweden as well as in China, with no observed effect on attraction of males. In conclusion, our study demonstrates the great potential of using pheromone trapping as a tool for identification and delimitation of taxa within cryptic species complexes. Based on our data, Nearctic and Palearctic populations of C. strobilella should be considered different species, and C. youngana Kearfott stat. rev. is resurrected here as valid name for North American populations, which was the case before the revision in 1983.

Oviposition and mining by Phytobia betulae (Diptera: Agromyzidae) in genotypes of European white birch (Betula pendula)

Abstract 1  The colonization success of herbivorous insects depends partly on the ability of females to choose suitable host plants. Phytobia betulae Kangas (Diptera: Agromyzidae), a miner of differentiating xylem, infests birch (Betula pendula Roth and Betula pubescens Ehrh.) trees.

Soil fertility alters susceptibility of young clonal plantlets of birch (Betula pendula) to a dipteran stem miner

Abstract Larvae of Phytobia betulae Kangas (Diptera: Agromyzidae) mine within the differentiating xylem tissue near the vascular cambium of birch trees. These larvae start in the canopy and move toward the base of the stem, leaving permanent brown tunnels in the wood. The aim of this study was to test whether young clones of European white birch (Betula pendula Roth) exhibit differences in susceptibility to Phytobia, whether altered growth media (soil fertility) and the subsequent growth reaction affect the susceptibility of birch to Phytobia, and whether there is genotype × environment interaction. Soil fertility levels of 2-year-old micropropagated plantlets of nine birch clones were manipulated in two experiments. The growth and size of the plantlets were measured and the density of Phytobia was estimated from larval tunnels (pith flecks). Phenotypic variation (nutrient effect) influenced the occurrence of Phytobia. Phytobia densities were significantly higher on plantlets grown at high soil fertility levels than on plantlets with the lowest soil fertility level. Clonal effects were detected in only one experiment. In general, Phytobia favoured birch clones with larger diameters.

Phenolic Compounds and Expression of 4CL Genes in Silver Birch Clones and Pt4CL1a Lines

A small multigene family encodes 4-coumarate:CoA ligases (4CLs) catalyzing the CoA ligation of hydroxycinnamic acids, a branch point step directing metabolites to a flavonoid or monolignol pathway. In the present study, we examined the effect of antisense Populus tremuloides 4CL (Pt4CL1) to the lignin and soluble phenolic compound composition of silver birch (Betula pendula) Pt4CL1a lines in comparison with non-transgenic silver birch clones. The endogenous expression of silver birch 4CL genes was recorded in the stems and leaves and also in leaves that were mechanically injured. In one of the transgenic Pt4CL1a lines, the ratio of syringyl (S) and guaiacyl (G) lignin units was increased. Moreover, the transcript levels of putative silver birch 4CL gene (Bp4CL1) were reduced and contents of cinnamic acid derivatives altered. In the other two Pt4CL1a lines changes were detected in the level of individual phenolic compounds. However, considerable variation was found in the transcript levels of silver birch 4CLs as well as in the concentration of phenolic compounds among the transgenic lines and non-transgenic clones. Wounding induced the expression of Bp4CL1 and Bp4CL2 in leaves in all clones and transgenic lines, whereas the transcript levels of Bp4CL3 and Bp4CL4 remained unchanged. Moreover, minor changes were detected in the concentrations of phenolic compounds caused by wounding. As an overall trend the wounding decreased the flavonoid content in silver birches and increased the content of soluble condensed tannins. The results indicate that by reducing the Bp4CL1 transcript levels lignin composition could be modified. However, the alterations found among the Pt4CL1a lines and the non-transgenic clones were within the natural variation of silver birches, as shown in the present study by the clonal differences in the transcripts levels of 4CL genes, soluble phenolic compounds and condensed tannins.

Environmental Aspects of Lignin Modified Trees

The increasing global need for food and fiber results in new demands for the efficiency of wood production and wood products (Fenning and Gershenzon 2002) which has to be attained on the basis of sustainable development. The majority of world wood products still comes from natural and seminatural forests, but there is a clear trend towards more efficient plantation forestry (Walter 2004). Development of vegetative propagation methods, including cutting technology, organogenesis and, in particular, somatic embryogenesis will yield additional profit for plantation forestry by the exploitation of non-additive genetic variation, by providing more homogenous material and by compensating potential shortage of improved seed stock. However, economically relevant clonal plantation forestry is currently a reality for only a few species, out of which Pseudotsuga menziesii Mirbel Franco, Pinus taeda L., Pinus radiata D.Don, Populus spp., Eucalyptus spp. and Picea spp. are prominent (Sutton 1999a,b; Pena and Seguin 2001; Cyr and Klimaszewska 2002). Conventional breeding of forest trees is in many cases hindered by long generation times and self-incompatibility mechanisms. Genetic transformation of forest trees has been considered the mechanism to achieve genetic gain when combined with conventional breeding and plantation forestry. There are several traits that show potential for a molecular breeding approach, e.g. reduction of generation time, production of sterile trees, pest or disease resistance, wood formation (including lignin and cellulose engineering), resistance to biodegradable herbicides, durability, phytoremediation of polluted sites and the production of novel chemicals and pharmaceuticals (reviewed by Pena and Seguin 2001; Fenning and Gershenzon 2002; Diouf 2003). Highly complex traits such as wood formation and the shortening of the juvenile phase have been considered the most important ones for achieving gain and for further domestication (Fenning and Gershenzon 2002). 5 Environmental Aspects of Lignin Modified Trees