Functions of passage cells in the endodermis and exodermis of roots

Passage cells frequently occur in the endodermis and exodermis but are not ubiquitous in either layer. Passage cells occur in the form of short cells in the dimorphic type of exodermis. In both layers, Casparian bands are formed in all cells, but the subsequent development of suberin lamellae and thick, cellulosic walls are delayed or absent in the passage cells. Available evidence suggests that passage cells of the endodermis are important for the transfer of calcium and magnesium into the stele and thus into the transpiration stream. They become the only cells which present a plasmalemma surface to the soil solution (and are thus capable of ion uptake) when the epidermis and central cortex die. This occurs naturally in some herbaceous and woody species and is known to be promoted by drought. Most evidence indicates that the development of suberin lamellae in both the endodermis and exodermis increases the resistance of the root to the radial flow of water. Passage cells thus provide areas of low resistance for the movement of water, and the position of these cells in the endodermis (i.e., in close proximity to the xylem) is explained in terms of function. Exodermal passage cells have a cytoplasmic structure suggesting an active role in ion uptake. This may be related to the tendency of the epidermis to die, leaving the passage cells as the only ones with their membranes exposed to the soil solution. Passage cells in the exodermis attract endomycorrhizal fungi while those in the endodermis do not. It is clear that passage cells of the endodermis and exodermis play a variety of roles in the plant root system.     

MORBILLIVIRUS INFECTION IN BOTTLENOSE DOLPHINS: EVIDENCE FOR RECURRENT EPIZOOTICS IN THE WESTERN ATLANTIC AND GULF OF MEXICO

Morbillivirus infection is widespread among odontocetes of the western Atlantic and Gulf of Mexico. Serologic evidence of infection in bottlenose dolphins, Tursiops truncatus , was first detected during an epizootic along the mid-Atlantic coast in 1987. Here, we report recurrent epizootics in the coastal dolphin population since at least the early 1980s based on serological surveys and regional stranding frequencies. The first observed epizootic of this series occurred in the Indian and Banana Rivers in 1982 and was followed by others on the mid-Atlantic coast in 1987–1988 and in the Gulf of Mexico between 1992 and 1994. This temporal pattern of infection is likely facilitated by the population size and its fragmentation into relatively discrete coastal communities. Introduction of morbillivirus into a community with a sufficient number of naive hosts may precipitate an epizootic, depending on the potential for transmission within the group. Propagation of an epizootic along the coast is probably determined by frequency of contact between adjacent communities and seasonal migrations.
Morbillivirus antibodies were also detected in serum from offshore bottlenose dolphins. The sero-prevalence in the latter may be higher than in coastal dolphins because of their close association with enzootically infected pilot whales ( Globicephala spp.). Occasional contact between offshore and coastal dolphins may provide an epizootiologic link between pilot whales and coastal dolphin communities.     

Projection pattern of sensory neurons in the central nervous system of a homeotic mutation of the moth Manduca sexta

Octopod (Octo) is a mutation of the moth Manduca sexta, which transforms the first abdominal segment (A1) in the anterior direction. Mutant animals are characterized by the appearance of homeotic thoracic-like legs on A1. We exploited this mutation to determine what rules might be used in specifying the fates of sensory neurons located on the body surface of larval Manduca. Mechanical stimulation of homeotic leg sensilla did not cause reflexive movements of the homeotic legs, but elicited responses similar to those observed following stimulation of ventral A1 body wall hairs. Intracellular recordings demonstrated that several of the motoneurons in the A1 ganglion received inputs from the homeotic sensory hairs. The responses of these motoneurons to stimulation of homeotic sensilla resembled their responses to stimulation of ventral body wall sensilla. Cobalt fills revealed that the mutation transformed the segmental projection pattern of only the sensory neurons located on the ventral surface of A1, resulting in a greater number with intersegmental projection patterns typical of sensory neurons found on the thoracic body wall. Many of the sensory neurons on the homeotic legs had intersegmental projection patterns typical of abdominal sensory neurons: an anteriorly directed projection terminating in the third thoracic ganglion (T3). Once this projection reached T3, however, it mimicked the projections of the thoracic leg sensory neurons. These results demonstrate that the same rules are not used in the establishment of the intersegmental and leg-specific projection patterns. Segmental identity influences the intersegmental projection pattern of the sensory neurons of Manduca, whereas the leg-specific projections are consistent with a role for positional information in determining their pattern. © 1995 John Wiley & Sons, Inc.     

Fly pollination of Linum lewish (Linaceae)

This study examines the reproductive biology of Linum lewisii Pursh. (Linaceae), a polyphilic species visited by small bees and generalist flies in montane Colorado. L. lewisii plants growing at different sites experience large temporal and spatial variations in pollinator visits. Their ability to attract both dipteran and hymenopteran pollinators allows pollination under varying conditions as pollinator pool composition changes. Although L. lewisii is self-compatible, hand-pollination studies indicate that insects are required for seed production. The relative effectiveness of fly and bee pollinators is assessed in terms of per-visit pollen deposition. Insect visitation patterns are combined with per-visit effectiveness data to evaluate the relative importance of different pollinator groups. Overall, bees tend to be more effective than flies in depositing pollen. However, in many instances flies appear to be responsible for more pollen deposition due to their higher visitation rates.     

The benefit to some minnows of spawning in the nests of other species

Synopsis Fishes that act as nest associates spawn simultaneously with nest-building hosts and then abandon their eggs. The proposed benefit for this behavior is increased brood survivorship, arising from the physical environment provided by the nest or the parental care provided by the host. Field and enclosure experiments indicated that associates benefit from the parental care provided by the host, and not from the physical environment provided by the nests of hosts. This information, along with the effect of nest association on host reproductive success, is necessary before the nature of this nesting symbiosis can be characterized.     

How does the G protein,Gi2, transduce mitogenic signals?

Serpentine receptors coupled to the heterotrimeric G protein, G_i2, are capable of stimulating DNA synthesis in a variety of cell types. A common feature of the G_i2-coupled stimulation of DNA synthesis is the activation of the mitogen-activated protein kinases (MAPKs). The regulation of MAPK activation by the G_i2-coupled thrombin and acetylcholine muscarinic M₂ receptors occurs by a sequential activation of a network of protein kinases. The MAPK kinase (MEK) which phosphorylates and activates MAPK is also activated by phosphorylation. MEK is phosphorylated and activated by either Raf or MEK kinase (MEKK). Thus, Raf and MEKK converge at MEK to regulate MAPK. G_i2-coupled receptors are capable of activating MEK and MAPK by Raf-dependent and Raf-independent mechanisms. Pertussis toxin catalyzed ADP-ribosylation of α_i2 inhibits both the Raf-dependent and-independent pathways activated by G_i2-coupled receptors. The Raf-dependent pathway involves Ras activation, while the Raf-independent activation of MEK and MAPK does not involve Ras. The Raf-independent activation of MEK and MAPK most likely involves the activation of MEKK. The vertebrate MEKK is homologous to the Ste11 and Byr2 protein kinases in the yeast Saccharomyces cerevisiae and Schizosaccharomyces pombe, respectively. The yeast Ste11 and Byr2 protein kinases are involved in signal transduction cascades initiated by pheromone receptors having a 7 membrane spanning serpentine structure coupled to G proteins. MEKK appears to be conserved in the regulation of G protein-coupled signal pathways in yeast and vertebrates. Raf represents a divergence in vertebrates from the yeast pheromone-responsive protein kinase system. Defining MEKK and Raf as a divergence in the MAPK regulatory network provides a mechanism for differential regulation of this system by G_i2-coupled receptors as well as other receptor systems, including the tyrosine kinases.     

Temperature and Antarctic plankton community respiration

Antarctic plankton community respiration rates were determinedfrom in vitro changes in dissolved oxygen. Oxygen consumptionrates, measured at in situ temperatures between 0 and 6°C,were found to lie in the range 0.3–3.7 µmol O₂ l^–1per 24 h. Water samples were collected between East FalklandIsland and South Georgia, South Atlantic Ocean, and incubatedshipboard in the dark at up to 36 temperatures between –2and 14–C. A respiration rate at each temperature was thendetermined and used to calculate the temperature coefficient(Q₁₀) of Antarctic planktonic community respiration from theArrhenius equation. Fourteen Q₀ values lay in the range 1–3,with four further values >5. This range of temperature coefficientvalues for community respiration is comparable to the publishedrange of values for plankton photosynthesis. Frequency distributionsof temperature coefficients for the two processes show similarmodal Q₁₀5 of 2–3. Thus, this study does not lend supportto the hypothesis of a differential response of photosynthesisand community respiration to low temperature.     

Measures of food quality as demographic predictors in freshwater copepods

This study examines the ability of a number of short-term measuresof algal food quality to predict longer term demographic parametersfor two species of freshwater calanoid copepods, Diaptomus minuiusand Epischura lacustris. Food quality of two species of algaethat are usually considered highly edible (Cryptomonas erosavar. reflexa and Chlamydomonas reinhardtii) are compared withrespect to: (i) the biochemical constituents of the algae (totalN, total C and protein); (ii) short-term foraging responsesby D.minutus to either algal species (e.g. clearance, ingestionand assimilation rates); (iii) longer term demographic responses(e.g. survivorship and reproduction) by both D.minutus and E.lacustrisfed a diet of either algal species. Demographic responses ofthe two copepod species indicate that C.erosa is a higher qualityfood. In fact, survival and reproduction of both copepod speciesfed C.reinhardtii were not different from starved treatments.Cryptomonas erosa treatments also had greater C, N and protein.However, D.minutus ingested five times more C.reinhardtii thanC.erosa, indicative of ‘compensatory feeding’ inthe presence of poor-quality food. Based upon these higher ingestionrates, individuals fed C.reinhardai actually ingested greateramounts of C, N and protein. Hence, ingestion rates taken aloneor coupled with biochemical parameters are not reliable predictorsof consumer demographic response. Assimilation rate, which waspositive for C.erosa and zero for C.reinhardtii, was the singlebest short-term predictor of food quality.