ORGANIZATION OF PREDATOR-PREY COMMUNITIES AS AN EVOLUTIONARY GAME

We consider a simple predator-prey model of coevolution. By allowing coevolution both within and between trophic levels the model breaks the traditional dichotomy between coevolution among competitors and coevolution between a prey and its predator. By allowing the diversity of prey and predator species to emerge as a property of the evolutionarily stable strategies (ESS), the model breaks another constraint of most approaches to coevolution that consider as fixed the number of coevolving species. The number of species comprising the ESS is influenced by a parameter that determines the predator's niche breadth. Depending upon the parameter's value the ESS may contain: 1) one prey and one predator species, 2) two prey and one predator, 3) two prey and two predators, 4) three prey and two predators, 5) three prey and three predators, etc. Evolutionarily, these different ESSs all emerge from the same model. Ecologically, however, these ESSs result in very different patterns of community organization. In some communities the predator species are ecologically keystone in that their removal results in extinctions among the prey species. In others, the removal of a predator species has no significant impact on the prey community. These varied ecological roles for the predator species contrasts sharply with the essential evolutionary role of the predators in promoting prey species diversity. The ghost of predation past in which a predator's insignificant ecological role obscures its essential evolutionary role may be a frequent property of communities of predator and prey.     

Characterization of Mitochondrial Small-Subunit Ribosomal RNAs from Holoparasitic Plants

Mitochondrial small-subunit (19S) rDNA sequences were obtained from 10 angiosperms to further characterize sequence divergence levels and structural variation in this molecule. These sequences were derived from seven holoparasitic (nonphotosynthetic) angiosperms as well as three photosynthetic plants. 19S rRNA is composed of a conservative core region (ca. 1450 nucleotides) as well as two variable regions (V1 and V7). In pairwise comparisons of photosynthetic angiosperms to Glycine, the core 19S rDNA sequences differed by less than 1.4%, thus supporting the observation that variation in mitochondrial rDNA is 3–4 times lower than seen in protein coding and rDNA genes of other subcellular organelles. Sequences representing four distinct lineages of nonasterid holoparasites showed significantly increased numbers of substitutions in their core 19S rDNA sequences (2.3–7.6%), thus paralleling previous findings that showed accelerated rates in nuclear (18S) and plastid (16S) rDNA from the same plants. Relative rate tests confirmed the accelerated nucleotide substitution rates in the holoparasites whereas rates in nonparasitic plants were not significantly increased. Among comparisons of both parasitic and nonparasitic plants, transversions outnumbered transitions, in many cases more than two to one. The core 19S rRNA is conserved in sequence and structure among all nonparasitic angiosperms whereas 19S rRNA from members of holoparasitic Balanophoraceae have unique extensions to the V5 and V6 variable domains. Substitution and insertion/deletion mutations characterized the V1 and V7 regions of the nonasterid holoparasites. The V7 sequence of one holoparasite (Scybalium) contained repeat motifs. The cause of substitution rate increases in the holoparasites does not appear to be a result of RNA editing, hence the underlying molecular mechanism remains to be fully documented. Received: 18 May 1997 / Accepted: 11 July 1997     

Stability in an evolutionary game

A behavior or strategy which is evolutionarily stable must be both optimal and stable. The strategy must be optimal in that it maximizes the expected fitness of all the individuals using it. In addition, the strategy must be resistant to invasion by a mutant. The difference between the Nash solution of game theory and the ESS used in ecology is that the Nash solution only satisfies an optimality criterion and not an evolutionary stability criterion. We extend the ESS definition of Maynard Smith and Price so that it can be applied directly to two-strategy evolutionary games. The concept of a balanced game is introduced, and necessary conditions are derived which are similar to the Nash necessary conditions. The balanced game necessary conditions may be used for direct calculation of ESS candidates. These results are used to examine the optimal flowering time of an annual plant experiencing competition from neighboring plants. The plant competition model is general, and the results may be applied to a wide range of interference competition problems.     

Perilymphatic Fistula—A Definitive and Curable Cause of Vertigo Following Head Trauma

Vertigo or disequilibrium occurring in patients after concussive and nonconcussive head trauma may be due to a pathologic perilymphatic fistula from the inner ear through the oval and round window areas of the middle ear. Of 33 patients who had successful grafting of the fistulous area at middle ear exploration, 32 had resolution of vertigo, and 12 of 23 who had an associated hearing loss had improved hearing. Perilymphatic fistulas associated with vertigo and hearing loss after head trauma can be diagnosed with great certainty and are surgically curable in the great majority of cases. Patients with post-concussive syndrome, whose symptoms include vertigo or disequilibrium, should have a thorough otologic evaluation for the possibility of a perilymphatic fistula.     

Biomass production in mass cultures of marine phytoplankton at varying temperatures

Natural populations of marine phytoplankton obtained from a large outdoor pond were grown on waste water-sea water mixtures in laboratory continuous cultures in the temperature range 5–33 °C. Virtually all of the influent inorganic nitrogen (14.0 mg l^?1) was assimilated at every temperature tested. There was, however, a distinct change in dominant species with temperature: below 19.8 °C Phaeodactylum tricornutum was dominant, at 27 °C Nilzschia sp. was the main species, and as the temperature increased above 27 °C a blue-green alga, Oscillatoria sp., became increasingly dominant. There is some indication that the excellent growth of P. tricornutum below 10 °C was related to a dramatic increase in the nutrient content per cell as the temperature decreased. Thus at low temperatures reduced division rates are compensated for by increased nutrient uptake rates. It follows that there is a transfer of phytoplankton protein from numerous small cells at intermediate temperatures to large cells that are reduced in numbers at lower temperatures but which represent the same total organic matter. The effect of this phenomenon on annual food chain efficiencies in both controlled and natural marine ecosystems is unknown.     

Surface solar ultraviolet radiation for paleoatmospheric levels of oxygen and ozone

Many investigators have concluded that the level of solar ultraviolet radiation (200–300 nm) reaching the surface was a key parameter in the origin and evolution of life on Earth. The level of solar ultraviolet radiation between 200 and 300 nm is controlled primarily by molecular absorption by ozone, whose presence is trongly coupled to the level of molecular oxygen. In this paper, we present a series of calculations of the solar ultraviolet radiation reaching the surface for oxygen levels ranging from 10^–4 present atmospheric level to the present level. The solar spectrum between 200 and 300 mn has been divided into 34 spectral intervals. For each spectral interval, we have calculated the solar ultraviolet radiation reaching the Earth's surface by considering the attenuation of the incoming beam due to ozone and oxygen absorption. A one-dimensional photochemical model of the atmosphere was used for these calculations.     

Valine-sensitive acetohydroxy acid synthases in Escherichia coli K-12: unique regulation modulated by multiple genetic sites.

Summary Spontaneous mutants (146) of Escherichia coli K-12 were selected that were resistant to inhibition of growth by 1.2 mM L-valine (Val^r). The Val^r isolates, containing acetohydroxy acid synthase resistant to feedback inhibition by L-valine (AHAS^r), were classed according to cotransduction of the mutation with leu. Several mutations resulting in an AHAS^r phenotype were found to be cotransducible with glyA. However, no mutations causing a Val^r phenotype were linked to ilv. AHAS activity was more closely examined in representatives of three classes of mutants with Val^r linked to leu, labeled ilv-660, ilv-661, and ilv-662. The ilvE503 allele in E. coli K-12, known to cause a two- to three-fold derepression of AHAS, was found to affect regulation of synthesis of both valine-sensitive AHAS (AHAS^s) and AHAS^r in the mutants containing ilv-660 and ilv-661, whereas it affected repression of AHAS^s, only, in the mutant containing ilv-662. Further, both AHAS^s and AHAS^r in the ilv-661 mutant were repressed by valine, whereas valine did not repress AHAS^r synthesis in the strain carrying ilv-660 and only partially repressed AHAS^r in the strain carrying ilv-662. Unexpectedly, AHAS^r synthesis in strains carrying ilv-660 or ilv-662 was repressible by leucine. The ilv-660 locus appears to be similar in position to ilvH and encodes a product that confers valine-sensitivity upon AHAS activity in the wild-type E. coli K-12. The ilv-660 and ilv-662 loci may normally encode products that influence both the feedback sensitivity of AHAS and control of AHAS biosynthesis.     

Dissociation of cellular K+ accumulation from net Na+ transport by toad urinary bladder

Summary A number of published data suggest a variable stoichiometry between the rates of cellular potassium uptake and net sodium transport (J _Na) across the urinary bladder of the toad. This problem was examined by simultaneously studying the intracellular chemical activity of potassium (a _K) with open-tip K⁺-selective microelectrodes and micropipets, and monitoringJ _Na by measuring the short-circuit current (SCC). When bathed in the short-circuited state with solutions containing ana _K of 2.7mm, the mean ±sem values for intracellulara _K were 43±0.6mm.Ouabain, at a concentration of 10^–2 m, reduced intracellulara _K by 56–67% and SCC by 96–100%. At 5×10^–4 m, ouabain reversibly reduced intracellulara _K by 40–55%, and SCC by 63–68%; the inhibition of SCC was only partly reversible during the period of observation.Removal of external potassium reduced intracellulara _K by 69–80% and SCC by 51–76%. Restoration of external potassium entirely returned intracellulara _K to its control value, but only partially reversed the inhibition of SCC during the period of study. Furthermore, recovery ofa _K began 19–43 min before that of SCC; recovery ofa _K was 90–97% complete before any increase in SCC could be measured. Although other interpretations are possible, the simplest interpretation of the data is that the processes responsible for potassium accumulation and transepithelial sodium transport are not identical. We propose the existence of a separate transfer mechanism at the basolateral cell membrane, responsible for accumulating intracellular potassium, and not directly coupled to active sodium transport.