Impact of eastern dwarf mistletoe (Arceuthobium pusillum) on host white spruce (Picea glauca) development, growth and performance across multiple scales.

Infection by eastern dwarf mistletoe (Arceuthobium pusillum) modifies needle and branch morphology and hastens white spruce (Picea glauca) mortality. We examined potential causal mechanisms and assessed the impacts of infection-induced alterations to host development and performance across scales ranging from needle hormone contents to bole expansion. Needles on infected branches (IBs) possessed higher total cytokinin (CK) and lower abscisic acid contents than needles on uninfected branches (UBs). IBs exhibited greater xylem growth than same-aged UBs, which is consistent with the promotive effect of CKs on vascular differentiation and organ sink strength. Elevated CK content may also explain the dense secondary and tertiary branching observed at the site of infection, i.e. the formation of 'witches' brooms' with significantly lower light capture efficiencies. Observed hormone perturbations were consistent with higher rates of transpiration, lower water use efficiencies (WUEs) and more negative needle carbon isotope ratios observed for IBs. Observed reductions in needle size allowed IBs to compensate for reduced hydraulic conductivity. Severe infections resulted in dramatically decreased diameter growth of the bole. It seems likely that the modifications to host hormone contents by eastern dwarf mistletoe infection led white spruce trees to dedicate a disproportionate fraction of their photoassimilate and other resources to self-shaded branches with low WUE. This would have decreased the potential for fixed carbon accumulation, generating a decline in the whole-tree resource pool. As mistletoe infections grew in size and the number of IBs increased, this burden was manifested as increasingly greater reductions in bole growth.

Genetic diversity in Chihuahuan Desert populations of creosotebush (Zygophyllaceae: Larrea tridentata).

We examined isozyme variation in the dominant Chihuahuan Desert shrub, Larrea tridentata (creosotebush), to determine the genetic variation within and among populations, the biogeographic relationships of populations, and the potential inbreeding in the species. We surveyed 17 populations consisting of 20 to 50 individuals per population along a 1600-km north-south transect across the Chihuahuan Desert. The southernmost population was near Villa Hidalgo, Mexico, and the northernmost near Isleta Pueblo, New Mexico. All 12 isozyme loci examined were polymorphic (H(t) = 0.416), with up to nine alleles per locus. Despite high levels of variation, we detected moderate inbreeding in L. tridentata populations. Most variation was found within rather than among populations (G(ST) = 0.118). Furthermore, recently established populations in the northern limits of the Chihuahuan Desert did not show decreased levels of genetic variation (H(o) = 0.336). A significant correlation was found between pairwise genetic and geographic distances (r = 0.305). Larrea tridentata showed and continues to show a massive range expansion into the arid and semi-arid regions of the American Southwest, but as shown by the high genetic variation, this expansion took place as a wave, rather than a series of founder events.

Genetic variation in piñon pine, Pinus edulis, associated with summer precipitation.

Three previous reports of microgeographical variation of glycerate dehydrogenase (Gly) frequencies in piñon, Pinus edulis, established the hypothesis that Gly frequencies contribute to adaptation to heterogeneous environments, specifically to variation in soil moisture. In each of these studies, the frequency of the Gly-3 allele or of Gly-33 homozygotes was higher on dry sites than on nearby moist sites. Here we attempt to extend these observations by testing the hypothesis that Gly frequencies respond to soil moisture variation on a range-wide scale. Gly frequencies were surveyed in 11 natural populations, and the frequency of the Gly-3 allele varied from 0.27 to 0.65 among the sample sites. Elevation varied from 1650 to 3100 m, and summer precipitation, defined as precipitation from April to August, varied from 13.7 to 26.4 cm. The soil types at the collection sites were schist, quaternary volcanic or a mixture of shale and sandstone. Logistic regression revealed that Gly frequencies did not respond to either elevation or soil type, but were related to summer precipitation (P < 0.01). The correlation between summer precipitation and the frequency of the Gly-3 allele was r = -0.92 (P < 0.001). Thus, the patterns of differentiation on microgeographical scales are consistent with greater differentiation on a range-wide scale.

Evolution of learning in three aplysiid species: differences in heterosynaptic plasticity contrast with conservation in serotonergic pathways.

We investigated the neurobiological basis of variation in sensitization between three aplysiid species: Aplysia californica, Phyllaplysia taylori and Dolabrifera dolabrifera. We tested two different forms of sensitization induced by a noxious tail shock: local sensitization, expressed near the site of shock, and general sensitization, tested at remote sites. Aplysia showed both local and general sensitization, whereas Phyllaplysia demonstrated only local sensitization, and Dolabrifera lacked both forms of learning. We then investigated a neurobiological correlate of sensitization, heterosynaptic modulation of sensory neuron excitability by tail-nerve stimulation. We found (1) an increase in sensory neuron (SN) excitability after both ipsilateral and contralateral nerve stimulation in Aplysia, (2) a smaller and shorter-lasting increase in Phyllaplysia, and (3) no effect in Dolabrifera. Because sensitization in Aplysia is strongly correlated with serotonergic (5-HT) neuromodulation, we hypothesized that the observed interspecific variation in sensitization and SN neuromodulation might be correlated with variation in the anatomy and/or functional response of the serotonergic system. However, using immunohistochemistry, we found that all three species showed a similar pattern of 5-HT innervation. Furthermore, they also showed comparable 5-HT release evoked by tail-nerve shock, as measured with chronoamperometry. These observations indicate that interspecific variation in learning is correlated with differences in SN heterosynaptic plasticity within a background of evolutionary conservation in the 5-HT neuromodulatory pathway. We thus hypothesize that evolutionary changes in learning phenotype do not involve modifications of the 5-HT pathway per se, but rather, changes in the response of SNs to the activation of this or other neuromodulatory pathways upon noxious stimulation.