Oviposition Site Choice and Life History Evolution

SYNOPSIS. Studies of life history evolution, as well as muchof life history theory, have typically focused on "hard" componentsof life histories; phenotypic characteristics that can be readilyobserved, quantified, and ultimately, connected rather directlyto fitness. Typical of these are propagule size, propagule number,and age and size at maturity. What is largely missing from thestudy of life history evolution is consideration of the roleof behavior, principally female oviposition site choice, inthe evolution of life histories. For oviparous organisms, naturalselection cannot produce locally optimized "hard" componentsof life history phenotypes without a consistent environmentalcontext (whether invariant orvariable); in a variable environment,that consistent environmental context can be most effectivelyprovided by interactive oviposition site choice. I present amodel of selection on oviposition site choice in the contextof the evolution of "hard" components of life history phenotypes,along with some experimental data illustrating oviposition sitechoice in response to predators. The model and data are thenrelated to the overall question of the role of oviposition sitechoice in life history evolution. The conclusion is that ovipositionsite choice must be under equally strong selection with eggsize, egg number and the other hard components of life historiesin order to generate and optimize locally adapted or ecologicallyspecialized life history phenotypes, and must therefore, playa significant role in the evolution of life histories.     

The Importance of Body Stiffness in Undulatory Propulsion

During steady swimming in fish, the dynamic form taken by theaxial undulatory wave may depend on the bending stiffness ofthe body. Previous studies have suggested the hypothesis thatfish use their muscles to modulate body stiffness. In orderto expand the theoretical and experimental tools available fortesting this hypothesis, we explored the relationship betweenbody stiffness, muscle activity, and undulatory waveform inthe mechanical context of dynamically bending beams. We proposethat fish minimize the mechanical cost of bending by increasingtheir body stiffness, which would allow them to tune their body'snatural frequency to match the tailbeat frequency at a givenswimming speed. A review of the literature reveals that theform of the undulatory wave, as measured by propulsive wavelength,is highly variable within species, a result which calls intoquestion the use of propulsive wavelength as a species-specificindicator of swimming mode. At the same time, the smallest wavelengthwithin a species is inversely proportional to the number ofvertebrae across taxa (r² = 0.21). In order to determine ifintact fish bodies are capable of increasing bending stiffness,we introduce a method for stimulating muscle in the body ofa dead fish while it is being cyclically bent at physiologicalfrequencies. The bending moment (N m) and angular displacement(radians) are measured during dynamic bending with and withoutmuscle stimulation. Initial results from these whole body workloops demonstrate that largemouth bass possess the capabilityto increase body stiffness by using their muscles to generatenegative mechanical work.     

Effect of Water Potential and Temperature on the Germination of Four Species of African Savanna Trees

The germination responses of seeds from the African tree speciesColophospermum mopane, Combretum apiculatum, Acacia tortilisand Acacia karroo under varying regimes of temperature and waterstress (induced by incubation in PEG 8000) are reported Withthe exception of Combretum (at –0.14 and –0.29 MPa)and Colophospermum (at –0.29 MPa), incubation in PEG decreasedthe maximum achieved germination percentage (90–100% forall species), but did not extend the germination lag (exceptin Combretum) or affect the time required to reach maximum germinationCombretum and Colophospermum were found to germinate under thewidest range of temperatures and water potentials, for example,as strongly negative as –1.0 MPa at 20 and 30 °C,respectively These seeds also showed greater or equivalent hypocotylelongation in PEG solutions creating potentials of –0.14,–0.29 or –0.51 MPa when compared with seeds germinatedin water, indicating an additional stress adaptation Acaciaspecies showed progressive reduction in germination rates andradicle elongation in response to decreasing water potentialExperiments giving pre-imbibition treatments in water priorto transfer to PEG solutions showed that both Acacia speciesgerminated at approximately 90% if given such pre-treatmentand less than 10% if transferred directly to PEG It is concludedthat the most stress-adapted species studied are Colophospermummopane and Combretum apiculatum, a finding generally correlatedwith the growth habit of these trees Colophospermum mopane, Combretum apiculatum, Acacia tortilis, Acacia karroo, germination, water stress, Zimbabwe     

Proliferation of Maize (Zea mays L.) Roots in Response to Localized Supply of Nitrate

Maize (Zea mays L.) plants with two primary nodal root axeswere grown for 8 d in flowing nutrient culture with each axisindependently supplied with . Dry matter accumulation by roots was similar whether 1.0 mol m^–3 was supplied to on( or both axes. When was supplied to only one axis, however, accumulationof dry matter within the root system was significantly greaterin the axis supplied with . The increased dry matter accumulation by the +N-treated axis was attributableentirely to increased density and growth of lateral branchesand not to a difference in growth of the primary axis. Proliferation of lateral branches for the + N axis was associatedwith the capacity for in situ reduction and utilization of aportion of the absorbed , especially in the apical region where lateral primordia are initiated. Althoughreduced nitrogen was translocated to the –N axis, concentrationsin the –N axis remained significantly lower than in the+N axis. The concentratio of reduced nitrogen, as well as invitro reductase activity, was greater in apical than in more basal regions of the +N axis. The enhancedproliferation of lateral branches in the + N axis was accompaniedby an increase in total respiration rate of the axis. Part ofthe increased respiration was attributable to increased massof roots. The specific respiration rate (umol CO₂ exolved perhour per gram root dry weight) was also greater for the +N thanfor the –N axis. If respiration rate is taken as representativeof sink demand, stimulation of initiation and growth of lateralsby in situ utilization of a localized exogenous supply of establishes an increased sink demand through enhancedmetabolic activity and the increased partitioning of assimilatesto the + N axis responds to the difference in sink demand between+N and –N axes. Key words: NO₃^- reduction, NO₃^- uptake nitrogen partitioning, root respiration, sink demand     

Relative rates of shell dissolution and net sediment accumulation - a commentary: can shell beds form by the gradual accumulation of biogenic debris on the sea floor?

Rates of shell production rarely exceed 500 g CaCO₃.m^-2yr^-1 in clastic sediments. Loss of shell carbonate by dissolution greatly exceeds loss by bioerosion and abrasion in most habitats. Rates of shell dissolution in modern sediments, estimated from rates of organic carbon degradation or measured directly, usually exceed 1000 g CaCo₃.M^-2yr^-1. This taphonomic loss is concentrated at or just below the sediment-water interface in the taphonomically-active zone (TAZ). Consequently, except where rates of shell production are very high or rates of organic carbon degradation very low, shells cannot permanently accumulate on the sea floor. Preservation requires rapid burial, usually by physical 'event' processes, to slow down taphonomic loss. Only near the base of the TAZ does the long-term sedimentation rate become an effective mediator of shell preservation as sediment accumulation gradually removes buried shell material from the taphonomically-active zone.     

Carbon Dioxide and Water Vapour Exchange of Leaves on Field-Grown Citrus Trees

Carbon dioxide and water vapour exchange rates were measuredon attached leaves of field-grown citrus trees. The exchangerates were measured continuously during several weeks in thespring of two successive years. These data confirmed the ratherlow rates of maximum CO₂ exchange (6–11 µmol m^–2s^–1) by citrus leaves. However, the maximum rate was maintainedthrough the midday period on only about half the days. On theother days, characterized by high temperatures and high atmosphericwater vapour pressure deficits, pronounced midday depressionsin CO₂ exchange rates were observed. Since midday transpirationremained stable at a constant rate even with increasing vapourpressure deficit, these results indicate that stomatal closurewas occurring. In fact, the data suggest tfiat specific, maximumtranspiration rates were associated with differing rootstocks.Thus, the rate of water supply to the leaves may be an importantfactor in determining the maximum transpiration rate, and therebymediating control of stomatal conductance and the resultantmidday depression in CO² exchange rates.     

Voltage Changes Along Geranium Petioles after Leaf Blade Excision

Voltage changes were measured along petioles of geranium (Pelargoniumhortorum) plants after leaf blade excision. These voltages weredetected using a non-invasive, non-polarizable, Ag/AgCl measuringelectrode, which was located between 20 and 50 mm from the excisionsite. The initial onset of these voltage responses was so rapid thatit occurred while the excision was still in progress. Aftera few seconds, voltage changes typically attained maximum valuesranging from 1 to 20 mV, and then returned slowly toward zero.In most cases the predominant voltage change was negative, butin some cases the change was predominantly positive. In addition to the voltage response to the first cut, a voltagechange also occurred in the same petiole after a second cut.This second cut response was usually much smaller than the first Experiments using the Scholander pressure bomb indicated thatgeranium xylem vessel fluid is under a tensile stress rangingfrom 100 to 300 kPa. Thus, whenever a leaf was excised, the forces acting on thisxylem fluid were immediately unbalanced. This imbalance wouldhave caused the fluid to be accelerated away from the cut untila new balance was achieved. The resulting fluid motion appears to be responsible for thepetiole voltage change which occurred immediately after eachexcision.