Spatial Separation of Ancestral Genomes in the Wild Grass Milium montianum Parl.

Previous work showed a strong tendency for genomes from twodifferent parents to be spatially separated in cell nuclei ofseveral man-made F₁ hybrids between grass species. An importantquestion therefore is whether similar nonrandom genome dispositionoccurs in wild species. Milium montianum Parl. (2n = 22) isa naturally occurring allopolyploid grass combining two geneticallydissimilar chromosome sets (V and M genomes), each originatingfrom a different ancestral species. These two ancestral genomeswere easily discriminated as all V genome chromosomes were largerthan all M genome chromosomes. In two-dimensional spread preparations,the V genome derived from M. vernale Bieb. (2n = 8), and theM genome (of different but uncertain origin) showed a highlysignificant tendency to lie apart. Generally, the V chromosomestended to surround the M chromosomes in both mitotic and meioticnuclei suggesting that this arrangement persists throughoutplant development. Such nuclear organization is probably undergenetic control and may facilitate some independent behaviourof ancestral genomes in allopolyploids. Indeed it may play asignificant role in plant evolution and speciation, especiallyif different intranuclear positions (e.g. central or peripheral)are correlated with preferential phenotypic expression of ancestralgenes. Milium montianum Parl., Gramineae, allopolyploid, spatial chromosome disposition, ancestral genome separation, plant speciation and evolution     

Studies in the Principles of Phytotoxicity: III. THE pH FACTOR AND THE TOXICITY OF 3:5-DINITRO-o-CRESOL, A WEAK ACID

It has previously been shown that the concentration of 3:5-dinitro-ocresolrequired in the external medium to halve (a) the growth rateof the mould Trichoderma viride or (b) the respiration rateof yeast increased same 75–100 times as the pH changedfrom 4 to 7. In the present investigation it has been foundthat when solutions or suspensions of 3:5-dinitro-o-cresol areapplied as sprays to seedlings of Brassica alba the concentrationneeded to kill half the plants changes by a factor of less than2 over the same pH range. On the other hand, when Lemma minor is grown in nutrient solutioncontaining the toxicant, the concentration required to halvethe rate of frond multiplication increases sharply as the pHis raised from 5.4 to 6.8. Moreover, when leaf disks of B. albaare vacuum infiltrated with buffered solutions of 3:5-dinitro-o-cresolthe concentration needed to halve the respiration rate rises50-fold between pH 4 and 8. It is considered that the essential condition which determinesthe similarity of the result for L. minor, the leaf disks, andmicro-organisms is that a relatively large volume of solutioncontaining the toxicant is in direct contact with the tissues.In contrast, under the conditions of spraying the volume ofspray droplets relative to that of plant tissue is small. Thusthe pH effect ie masked since the buffering capacity of thecells exceeds that of the droplets, with the result that 3:5-dinitro-o-cresollargely acts upon the cells at a pH determined by the tissues,a pH which may bear little relation to that of the originalsolution. Supporting evidence is produced that the pH factor operatesin the same way for other weak acids, such as the chlorophenoxyaceticacids.     

The hatching stimulus for eggs of Aedes punctor (Diptera: Culicidae)

ABSTRACT.

• 1 Eggs of the British woodland mosquito, Aedes punctor (Kirby), were subjected to a variety of media in an attempt to determine the hatching stimulus.
• 2 It was found that continuous immersion in distilled water evoked almost no hatching (0.2%); intermittent removal from the water gave low hatching (8.9%).
• 3 Slow deoxygenation of the medium by either bacterial growth or the introduction of nitrogen produced the highest level of hatching (26.0–90.0%). Eggs of A.punctor were shown to hatch within a 4 h period of the oxygen concentration in the medium reaching zero.

     

The effect of changes of environment on the electrical and ionic pattern of muscle

The resting and action potentials of sartorius muscles of the toad, Bufo marinus, have been measured under varying conditions of external environment. At the same time, analyses for Na(+) and K(+) content were carried out. There was a slight elevation of 2 mv. when the measurements were made in phosphate-Ringer instead of in bicarbonate-Ringer. The R.P. was independent of the hydrogen ion concentration between pH 6.5 and 8.5, although at these pH's there was marked alteration in the level of Na(+) and K(+) in the muscle. Alteration of the external K(+) level between 0 and 50 m.eq./liter has little influence on the internal K(+) concentration. When the log of the external K(+) concentration is plotted against the R.P. there is not a linear relationship until the external K(+) is raised above 12 m.eq./liter, at which point the cell is unexcitable. Above this value a straight line with a slope of 58 mv. per ten-fold change in concentration is obtained, but the absolute values at any point are about 35 per cent higher than those which would be given by the Nernst equation. Alteration of the external Na(+) level within a range of 45 to 650 m.eq./liter resulted in marked changes in the internal Na(+) content, without, however, having any effect on the ratio Na(+) (out)/Na(+) (in). This ratio has remained at about 3 in spite of marked fluctuations in the absolute value of the internal and external Na(+) levels. When the Na(+) level is lowered there is a decrease in the height of the action potential although there is no alteration in the ratio Na(+) (out)/Na(+) (in). As the Na(+) level is raised the height of the action potential is not affected even in the presence of a fivefold increase in Na(+) in the Ringer. The results do not support the conclusion that the bioelectric potentials can be calculated from the ionic ratios by means of simple physical chemical hypotheses such as the Nernst or Goldman equations. The maintenance of the normal K(+) content of the cell cannot be accounted for by a Donnan mechanism. No definite evidence has been produced to explain the mechanism of a Na(+) "pump." In other words, the concept of a Na(+) pump requires that there shall be a physico- or organochemical mechanism which will distinguish between Na(+) and K(+) (or other) ions. There is evidence that Na(+) can be extruded against a concentration gradient. On the other hand the cell is able to maintain a constant ratio of external to internal Na(+) even when the cell has been severely damaged by very high external Na(+) levels.     

Survey‐gap analysis in expeditionary research: where do we go from here?

Research expeditions into remote areas to collect biological specimens provide vital information for understanding biodiversity. However, major expeditions to little‐known areas are expensive and time consuming, time is short, and well‐trained people are difficult to find. In addition, processing the collections and obtaining accurate identifications takes time and money. In order to get the maximum return for the investment, we need to determine the location of the collecting expeditions carefully. In this study we used environmental variables and information on existing collecting localities to help determine the sites of future expeditions. Results from other studies were used to aid in the selection of the environmental variables, including variables relating to temperature, rainfall, lithology and distance between sites. A survey gap analysis tool based on ‘ED complementarity’ was employed to select the sites that would most likely contribute the most new taxa. The tool does not evaluate how well collected a previously visited site survey site might be; however, collecting effort was estimated based on species accumulation curves. We used the number of collections and/or number of species at each collecting site to eliminate those we deemed poorly collected. Plants, birds, and insects from Guyana were examined using the survey gap analysis tool, and sites for future collecting expeditions were determined. The south‐east section of Guyana had virtually no collecting information available. It has been inaccessible for many years for political reasons and as a result, eight of the first ten sites selected were in that area. In order to evaluate the remainder of the country, and because there are no immediate plans by the Government of Guyana to open that area to exploration, that section of the country was not included in the remainder of the study. The range of the ED complementarity values dropped sharply after the first ten sites were selected. For plants, the group for which we had the most records, areas selected included several localities in the Pakaraima Mountains, the border with the south‐east, and one site in the north‐west. For birds, a moderately collected group, the strongest need was in the north‐west followed by the east. Insects had the smallest data set and the largest range of ED complementarity values; the results gave strong emphasis to the southern parts of the country, but most of the locations appeared to be equidistant from one another, most likely because of insufficient data. Results demonstrate that the use of a survey gap analysis tool designed to solve a locational problem using continuous environmental data can help maximize our resources for gathering new information on biodiversity. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 85 , 549–567.