Hydraulic Coupling of a Leafless Kauri Tree Remnant to Conspecific Hosts.

Trees are commonly regarded as distinct entities, but the roots of many species fuse to form natural root grafts allowing the exchange of water, carbon, mineral nutrients, and microorganisms between individuals. Exploiting the phenomenon of leafless (photosynthetically inactive) tree remnants being kept alive by conspecifics, we show tight physiological coupling of a living kauri (Agathis australis) stump to conspecific neighbors. The trunk remnant displayed greatly reduced, inverted daily sap flow patterns compared with intact kauri trees. Its stem water potential showed strong diel variation with minima during daytime and maxima at night, coinciding with peak and minimal sap flow rates in neighbors, respectively. Sudden atmospherically driven changes in water relations in adjacent kauri trees were very rapidly and inversely mirrored in the living stump's water status. Such intimate hydrological coupling suggests a "communal physiology" among (conspecific) trees with far-reaching implications for our understanding of forest functioning, particularly under water shortage.

Water relations of climbing ivy in a temperate forest.

Ivy (Hedera helix) is the most important liana in temperate European forests. We studied water relations of adult ivy in a natural, 35 m tall mixed deciduous forest in Switzerland using a construction crane to access the canopy. Predawn leaf water potential at the top of climbing ivy ranged from -0.4 to -0.6 MPa, daily minima ranged from -1.3 to -1.7 MPa. Leaf water potentials as well as relative sap flow were held surprisingly constant throughout different weather conditions, suggesting a tendency to isohydric behaviour. Maximum stomatal conductance was 200 mmol m⁻² s⁻¹. The use of a potometer experiment allowed us to measure absolute transpiration rates integrated over a whole plant of 0.23 mmol m⁻² s⁻¹. Nightly sap flow of ivy during warm, dry nights accounted for up to 20% of the seasonal maximum. Maximum sap flow rates were reached at ca. 0.5 kPa vpd. On the other hand, the host trees showed a less conservative stomatal regulation, maximum sap flow rates were reached at vpd values of ca. 1 kPa. Sap flow rates of ivy decreased by ca. 20% in spring after bud break of trees, suggesting that ivy profits strongly from warm sunny days in early spring before budbreak of the host trees and from mild winter days. This species may benefit from rising winter temperatures in Europe and thus become a stronger competitor against its host trees.

Equivalence of the fly orthodenticle gene and the human OTX genes in embryonic brain development of Drosophila.

Members of the orthodenticle gene family are essential for embryonic brain development in animals as diverse as insects and mammals. In Drosophila, mutational inactivation of the orthodenticle gene results in deletions in anterior parts of the embryonic brain and in defects in the ventral nerve cord. In the mouse, targeted elimination of the homologous Otx2 or Otx1 genes causes defects in forebrain and/or midbrain development. To determine the morphogenetic properties and the extent of evolutionary conservation of the orthodenticle gene family in embryonic brain development, genetic rescue experiments were carried out in Drosophila. Ubiquitous overexpression of the orthodenticle gene rescues both the brain defects and the ventral nerve cord defects in orthodenticle mutant embryos; morphology and nervous system-specific gene expression are restored. Two different time windows exist for the rescue of the brain versus the ventral nerve cord. Ubiquitous overexpression of the human OTX1 or OTX2 genes also rescues the brain and ventral nerve cord phenotypes in orthodenticle mutant embryos; in the brain, the efficiency of morphological rescue is lower than that obtained with overexpression of orthodenticle. Overexpression of either orthodenticle or the human OTX gene homologs in the wild-type embryo results in ectopic neural structures. The rescue of highly complex brain structures in Drosophila by either fly or human orthodenticle gene homologs indicates that these genes are interchangeable between vertebrates and invertebrates and provides further evidence for an evolutionarily conserved role of the orthodenticle gene family in brain development.

Developmental rescue of Drosophila cephalic defects by the human Otx genes.

The molecular mechanisms of head development are a central question in vertebrate and invertebrate developmental biology. The anteriorly expressed homeobox gene otd in Drosophila and its homolog Otx in mouse are required for the early development of the most anterior part of the body, suggesting that a fundamental genetic program of cephalic development might be conserved between vertebrates and invertebrates. We have examined this hypothesis by introducing the human Otx genes into flies. By inducing expression of the human Otx homologs with a heat shock promoter, we found that both Otx1 and Otx2 functionally complement the cephalic defects of a fly otd mutant through specific activation and inactivation of downstream genes. Combined with previous morphological studies, these results are consistent with the view that a common molecular ground plan of cephalization was invented before the diversification of the protostome and the deuterostome in the course of metazoan evolution.

Embryonic development of the Drosophila brain: formation of commissural and descending pathways.

The establishment of initial axonal pathways in the embryonic brain of Drosophila melanogaster was investigated at the cellular and molecular level using antibody probes, enhancer detector strains and axonal pathfinding mutants. During embryogenesis, two bilaterally symmetrical cephalic neurogenic regions form, which are initially separated from each other and from the ventral nerve cord. The brain commissure that interconnects the two brain hemispheres is pioneered by axons that project towards the midline in close association with an interhemispheric cellular bridge. The descending longitudinal pathways that interconnect the brain to the ventral nerve cord are prefigured by a chain of longitudinal glial cells and a cellular bridge between brain and subesophageal ganglion; pioneering descending and ascending neurons grow in close association with these structures. The formation of the embryonic commissural and longitudinal pathways is dependent on cells of the CNS midline. Mutations in the commissureless gene, which affects growth cone guidance towards the midline, result in a marked reduction of the brain commissure. Mutations in the single-minded gene and in other spitz group genes, which affect the differentiation of CNS midline cells, result in the absence or aberrant projection of longitudinal pathways. The analysis of axon pathway formation presented here reveals remarkable similarities as well as distinct differences in the embryonic development of the brain and the segmental ganglia, and forms the basis for a comprehensive genetic and molecular genetic dissection of axonal pathfinding processes in the developing brain.