Evolutionary relationships among the male and female mitochondrial DNA lineages in the Mytilus edulis species complex

A novel form of mitochondrial DNA (mtDNA) inheritance has previously been documented for the blue mussel (Mytilus edulis). Female mussels inherit their mtDNA solely from their mother while males inherit mtDNA from both their mother and their father. In males, the paternal mtDNA is preferentially amplified so that the male gonad is highly enriched for the paternal mtDNA that is then transmitted from fathers to sons. We demonstrate that this mode of mtDNA inheritance also operates in the closely related species M. galloprovincialis and M. trossulus. The evolutionary relationship between the male and female mtDNA lineages is estimated by phylogenetic analysis of 455 nucleotides from the large subunit ribosomal RNA gene. We have found that the male and female lineages are highly divergent; the divergence of these lineages began prior to the speciation of the three species of blue mussels. Further, the separation between the male and female lineages is estimated to have occurred between 5.3 and 5.7 MYA.      

Evolution of drought tolerance and defense: dependence of tradeoffs on mechanism, environment and defense switching

Plants evolve defenses against herbivores and pathogens in stressful environments; however, plants that evolve tolerances to other environmental stressors may have compromised defenses. Such tradeoffs involving defenses may depend on limited resources or otherwise stressful environments; however, the effect of stressful environments on defense expression might be different for different genotypes (G×E). To test these predictions, we studied genetic variation and co‐variation of drought stress tolerance and defenses at two levels of genetic variation: between and within closely related species. We did this across an experimental drought stress gradient in a growth room for species for which genetic variation in drought tolerance was likely. In apparent contrast to predictions, the species Boechera holboellii (Brassicaceae) from lower and dryer elevations had slower inherent growth rates and correspondingly higher total defensive glucosinolate concentrations than the closely related species B. stricta from higher elevations. Thus, B. holboellii was both drought tolerant and defended; however, optimality theory does predict tradeoffs between defense and growth. Differences between species in the direct effect of water deficiency on glucosinolate production did not obscure the grow‐or‐defend tradeoff. B. holboellii may also have been more resistant to the specialist herbivore Plutella xylostella; a trend that was less clear because it depended on plant development and water deficient conditions. At finer scales of genetic variation, there was significant variation among families and naturally occurring inbred lines of B. stricta in drought tolerance measured as inherent growth, the reaction norm of growth across drought treatments, shoot water potential, and transpiration rates. Evidence for tradeoffs was also found within B. stricta in genetic correlations between resistance and transpiration rates, or glucosinolates and growth rates. No G×E was detected at these finer scales of genetic variation, although sometimes the tradeoff was dependent on drought conditions. Direct effects of drought stress resulted in an apparent plastic switch between resistance and tolerance to damage, which might be a cost avoidance mechanism because tradeoffs never involved tolerance to damage. Thus, when drought tolerance is manifest as slow inherent growth rates, plants may also have relatively high defense levels, especially in stressful environments. Otherwise, defenses may be compromised by drought‐coping mechanisms, although plastic switches to less costly defenses may alleviate constraints in stressful environments.     

Transcriptome analysis of Arabidopsis clubroots indicate a key role for cytokinins in disease development

The clubroot disease of the family Brassicaceae is caused by the obligate biotrophic protist Plasmodiophora brassicae. Infected roots undergo a developmental switch that results in the formation of aberrant roots (clubs). To investigate host gene expression during the development of the disease, we have used the Arabidopsis ATH1 genome array. Two timepoints were chosen, an early timepoint at which the pathogen has colonized the root but has induced only very limited change of host cell and root morphology and a later timepoint at which more than 60% of the host root cells were colonized and root morphology was drastically altered. At both timepoints, more than 1,000 genes were differentially expressed in infected versus control roots. These included genes associated with growth and cell cycle, sugar phosphate metabolism, and defense. The involvement of plant hormones in club development was further supported; genes involved in auxin homeostasis, such as nitrilases and members of the GH3 family, were upregulated, whereas genes involved in cytokinin homeostasis (cytokinin synthases and cytokinin oxidases/dehydrogenases) were already strongly downregulated at the early timepoint. Cytokinin oxidase/dehydrogenase overexpressing lines were disease resistant, clearly indicating the importance of cytokinin as a key factor in clubroot disease development.     

Optimal defense in plants: assessment of resource allocation costs

Genetic and environmental variation of functional traits within populations might be maintained by natural selection when resource allocation costs (RACs) balance trait benefits. Despite the intuitive appeal of optimization models, empirical tests have failed to support the importance of RACs for plant traits that confer resistance against pests. To address this discrepancy, we modified an early model by allowing the cost function to vary across a resource gradient as predicted for RACs and by assuming that the benefits depend on variation in the pest population for susceptibility. Instead of the intermediate defense optimum of the original model, defenses were predicted to be either high or absent, depending on resource availability and history. This result is not supported by empirical tests for ecological or evolutionary outcomes, including our own examination of glucosinolate toxins from sib-families of Boechera stricta (Brassicaceae) grown across an NPK fertilizer gradient. Although we detected an apparent cost of defense in the absence of herbivores, the cost did not increase as resources became more limiting. Also defense production did not vary across the resource gradient as predicted by the modified model. Thus, a model based on explicit expectations of RACs produced predictions that are not supported. Instead, other kinds of costs, such as ecological (indirect) costs may be more important. Alternatively, general conflicts in gene expression and antagonistic crosstalk among signaling pathways may underlie apparent costs.     

Indole Glucosinolates and Camalexin do not Influence the Development of the Clubroot Disease in Arabidopsis thaliana

Root galls of Brassicaceae caused by Plasmodiophora brassicae are dependent on increased auxin and cytokinin formation. In this study we investigated whether indole glucosinolates are involved in indole‐3‐acetic acid (IAA) biosynthesis in root galls, by using a genetic approach. The cytochrome P450 enzymes, CYP79B2 and CYP79B3, convert tryptophan to indole‐3‐acetaldoxime (IAOx), which is a precursor for indole glucosinolates and the phytoalexin camalexin in Arabidopsis thaliana. Root galls of the Arabidopsis ecotypes Wassilewskija (WS) and Columbia (Col) accumulated camalexin, WS at levels up to 320 μg/g dry weight. By contrast, camalexin was absent in root galls of cyp79b2/b3 double mutants. Infection rate and disease index as a measure of club development in mutant and wild‐type plants of the two ecotypes were investigated and no differences were found in gall formation. This demonstrates that camalexin is an ineffective inhibitor of P. brassicae and indole glucosinolates are not the source of elevated levels of IAA in galls, because free IAA levels in mutant galls were comparable with those in wild type.     

Changes in root water flow properties of solution culture-grown trembling aspen (Populus tremuloides) seedlings under different intensities of water-deficit stress

To study the effects of water-deficit stress on root water flow properties in trembling aspen ( Populus tremuloides Michx.), seedlings were grown in solution culture and subjected to water-deficit stress by placing their roots in sealed high humidity chambers. After 17 h of stress treatment, seedlings showed mild stress (MS) symptoms with a decline in shoot water potentials. Within 20 h, shoot water potentials rapidly declined, and severe stress (SS) symptoms were present. Root hydraulic conductivity ( L _pr) increased more than two-fold and the relative concentration of apoplastic tracer dye trisodium 3-hydroxy-5, 8, 10-pyrenetrisulphonate (PTS₃) in xylem exudate decreased by 73.6% in MS seedlings. Conversely, L_pr decreased (55.3%) and PTS₃ increased (28.6%) in SS seedlings. Treatment of roots with 0.1 m M mercuric chloride decreased root volume flow density ( J _v) by about 29.0% in control and MS plants with no decrease measured in SS seedlings. Mercuric chloride also increased PTS₃ concentration in xylem exudate of control (59%) and MS (86%) seedlings with no change observed in SS plants. The results suggest that aquaporin-mediated transport is important in the regulation of root water flow under drought stress and that root water flow properties are strongly affected by the stress level. Regulation of root water flow may represent an important drought-stress resistance mechanism.     

Identification of genes encoding exported Mycobacterium tuberculosis proteins using a Tn552'phoA in vitro transposition system

Secreted and cell envelope-associated proteins are important to both Mycobacterium tuberculosis pathogenesis and the generation of protective immunity to M. tuberculosis. We used an in vitro Tn552'phoA transposition system to identify exported proteins of M. tuberculosis. The system is simple and efficient, and the transposon inserts randomly into target DNA. M. tuberculosis genomic libraries were targeted with Tn552'phoA transposons, and these libraries were screened in M. smegmatis for active PhoA translational fusions. Thirty-two different M. tuberculosis open reading frames were identified; eight contain standard signal peptides, six contain lipoprotein signal peptides, and seventeen contain one or more transmembrane domains. Four of these proteins had not yet been assigned as exported proteins in the M. tuberculosis databases. This collection of exported proteins includes factors that are known to participate in the immune response of M. tuberculosis and proteins with homologies, suggesting a role in pathogenesis. Nine of the proteins appear to be unique to mycobacteria and represent promising candidates for factors that participate in protective immunity and virulence. This technology of creating comprehensive fusion libraries should be applicable to other organisms.