The mechanism of resistance to paraquat is strongly temperature dependent in resistant Hordeum leporinum Link and H. glaucum Steud.

Paraquat-resistant biotypes of the closely-related weed species Hordeum leporinum Link and H. glaucum Steud. are highly resistant to paraquat when grown during the normal winter growing season. However, when grown and treated with paraquat in summer, these biotypes are markedly less resistant to paraquat. This reduced resistance to paraquat in summer is primarily a result of increased temperature following herbicide treatment. The mechanism governing this decrease in resistance at high temperature was examined in H. leporinum. No differences were observed between susceptible and resistant biotypes in the interaction of paraquat with isolated thylakoids when assayed at 15, 25, or 35 °C. About 98 and 65% of applied paraquat was absorbed through the leaf cuticle of both biotypes at 15 and 30 °C, respectively. Following application to leaves, more herbicide was translocated in a basipetal direction in the susceptible biotype compared to the resistant biotype at 15 °C. However, at 30 °C more paraquat was translocated in a basipetal direction in the resistant biotype. Photosynthetic activity of young leaf tissue from within the leaf sheath which had not been directly exposed to paraquat was measured 24 h after treatment of plants with para. quat. This activity was inhibited in the susceptible biotype when plants were maintained at either 15 °C or 30 °C after treatment. In contrast, photosynthetic activity of such tissue of the resistant biotype was not inhibited when plants were maintained at 15 °C after treatment, but was inhibited at 30 °C. The mechanism of resistance in this biotype of H. leporinum correlates with decreased translocation of paraquat and decreased penetration to the active site. This mechanism is temperature sensitive and breaks down at higher temperatures.We are grateful to Zeneca Agrochemicals, Jealotts Hill, Berkshire, UK who provided [¹⁴C]paraquat. E.P. was supported through a Ph.D. scholarship from the Australian International Development Assistance Bureau and C.P. was the recipient of an Australian Research Council Postdoctoral Fellowship.     

Phenotypic and Genetic Analysis of ``chameleon,'''' a Paramecium Mutant with an Enhanced Sensitivity to Magnesium

Three mutant strains of Paramecium tetraurelia with an enhanced sensitivity to magnesium have been isolated. These new ``Chameleon' mutants result from partial- or codominant mutations at a single locus, Cha. Whereas the wild type responded to 5 mM Mg(2+) by swimming backward for 10-15 sec, Cha mutants responded with ~30 sec backward swimming. Electrophysiological analysis suggested that this behavior may be caused by slowing in the rate at which a Mg(2+)-specific ion conductance deactivates following membrane excitation. This would be consistent with an observed increase in the sensitivity of Cha mutants to nickel poisoning, since Ni(2+) is also able to enter the cell via this pathway. More extensive behavioral analysis showed that Cha cells also overresponded to Na(+), but there was no evidence for a defect in intracellular Ca(2+) homeostasis that might account for a simultaneous enhancement of both the Mg(2+) and Na(+) conductances. The possibility that the Cha locus may encode a specific regulator of the Mg(2+)- and Na(+)-permeabilities is considered.     

Extracellular GTP Causes Membrane-Potential Oscillations through the Parallel Activation of Mg2+ and Na+ Currents in Paramecium tetraurelia

Paramecium tetraurelia responds to extracellular GTP (≥ 10 nm) with repeated episodes of prolonged backward swimming. These backward swimming events cause repulsion from the stimulus and are the behavioral consequence of an oscillating membrane depolarization. Ion substitution experiments showed that either Mg²⁺ or Na⁺ could support these responses in wild-type cells, with increasing concentrations of either cation increasing the extent of backward swimming. Applying GTP to cells under voltage clamp elicited oscillating inward currents with a periodicity similar to that of the membrane-potential and behavioral responses. These currents were also Mg²⁺- and Na⁺-dependent, suggesting that GTP acts through Mg²⁺-specific (I _Mg) and Na⁺-specific (I _Na) conductances that have been described previously in Paramecium. This suggestion is strengthened by the finding that Mg²⁺ failed to support normal behavioral or electrophysiological responses to GTP in a mutant that specifically lacks I _Mg (``eccentric'), while Na<sup>+</sup> failed to support GTP responses in ``fast-2,' a mutant that specifically lacks I _Na. Both mutants responded normally to GTP if the alternative cation was provided. As I _Mg and I _Na are both Ca²⁺-dependent currents, the characteristic GTP behavior could result from oscillations in intracellular Ca²⁺ concentration. Indeed, applying GTP to cells in the absence of either Mg²⁺ or Na⁺ revealed a minor inward current with a periodicity similar to that of the depolarizations. This current persisted when known voltage-dependent Ca²⁺ currents were blocked pharmacologically or genetically, which implies that it may represent the activation of a novel purinergic-receptor–coupled Ca²⁺ conductance. Received: 28 October 1996/Revised: 24 December 1996     

Molecular phylogeny and divergence times of drosophilid species

The phylogenetic relationships and divergence times of 39 drosophilid species were studied by using the coding region of the Adh gene. Four genera--Scaptodrosophila, Zaprionus, Drosophila, and Scaptomyza (from Hawaii)--and three Drosophila subgenera--Drosophila, Engiscaptomyza, and Sophophora--were included. After conducting statistical analyses of the nucleotide sequences of the Adh, Adhr (Adh-related gene), and nuclear rRNA genes and a 905-bp segment of mitochondrial DNA, we used Scaptodrosophila as the outgroup. The phylogenetic tree obtained showed that the first major division of drosophilid species occurs between subgenus Sophophora (genus Drosophila) and the group including subgenera Drosophila and Engiscaptomyza plus the genera Zaprionus and Scaptomyza. Subgenus Sophophora is then divided into D. willistoni and the clade of D. obscura and D. melanogaster species groups. In the other major drosophilid group, Zaprionus first separates from the other species, and then D. immigrans leaves the remaining group of species. This remaining group then splits into the D. repleta group and the Hawaiian drosophilid cluster (Hawaiian Drosophila, Engiscaptomyza, and Scaptomyza). Engiscaptomyza and Scaptomyza are tightly clustered. Each of the D. repleta, D. obscura, and D. melanogaster groups is monophyletic. The splitting of subgenera Drosophila and Sophophora apparently occurred about 40 Mya, whereas the D. repleta group and the Hawaiian drosophilid cluster separated about 32 Mya. By contrast, the splitting of Engiscaptomyza and Scaptomyza occurred only about 11 Mya, suggesting that Scaptomyza experienced a rapid morphological evolution. The D. obscura and D. melanogaster groups apparently diverged about 25 Mya. Many of the D. repleta group species studied here have two functional Adh genes (Adh-1 and Adh-2), and these duplicated genes can be explained by two duplication events.      

The case for multinet growth in growing walls of plant cells

The basis of multinet gwoth, the multinet growth hypothesis, is examined in view of recent criticisms. It is shown that the strain across a growing wall may be calculated by simple means and the expected reorientations are deduced (a) for a wall in which the microfibrils of the innermost wall lamella always lie helically with the same pitch and (b) in which the microfibrils lie at random. Calculations are presented both for cells increasing in length only and for cells also increasing in breadth. Both the strains and the reorientations are smaller than commonly implied and are too small to be reliably detectable in wall sections. Observations on wall sections cannot therefore be accepted as proof that microfibril reorientation has not occured and it is concluded that the multinet growth hypothesis still stands as applying both to parenchyma and to collenchyma cells. In view of the wide dispersity in the structure of the walls of growing cells, it is recommended that the qualifying multinet should be dropped and replaced by passive reorientation.Abbreviation MGH multinet growth hypothesis     

Pleiotropic Effects on Fitness of Mutations Affecting Viability in DROSOPHILA MELANOGASTER

The relative viabilities and fitnesses of wild-type second chromosomes in heterozygous condition were determined. Joint analysis of these permitted an estimation of a parameter that relates the viability effect of a mutation to its effect on fitness as a whole. For newly arisen mutations, the estimate was slightly greater than one, indicating that the reductions in viability caused by these mutations are associated with reductions in other components of fitness. For mutations from an equilibrium population, the estimate of the parameter was near zero, implying that the deleterious viability effects of these mutations are compensated by improvements in other aspects of fitness.     

Components of Hybrid Dysgenesis in a Wild Population of DROSOPHILA MELANOGASTER

Hybrid dysgenesis is a condition found in certain interstrain hybrids of Drosophila melanogaster caused by the interaction of chromosomal and cytoplasmic factors. Germ-line abnormalities, including sterility, high mutability and male recombination, appear in the affected individuals. There are at least two distinct systems of hybrid dysgenesis. We examined a Wisconsin wild population in two consecutive years to determine the distribution of the chromosomal P factor and the extrachromosomal M cytotype that together cause one kind of hybrid dysgenic sterility. The P factor was found to be very common in the population, with all three major chromosomes being polymorphic for it. This polymorphism was strongly correlated with variability for male recombination elements, suggesting that these two traits are part of the same system of hybrid dysgenesis. There was a slight tendency for the P factor to be lost in lines taken from this population and inbred in the laboratory for many generations. A large-scale search for the M cytotype, which causes susceptibility to the P factor, showed that it is present in the population at only very low frequencies. Further evidence that the population is mostly immune to the action of the P factor was our finding of a general lack of dysgenic sterility in the wild flies themselves. However, we were able to isolate several wild strains that consistently showed the M cytotype. In some cases, the frequency of the M cytotype could be maintained in these lines, but it could not usually be increased by artificial selection. Some possible consequences of hybrid dysgenesis for the evolutionary biology of Drosophila are suggested.     

Specificity of two isolated wheat carboxypeptidases

The substrate specificity of two purified carboxypeptidases from germinated wheat has been examined. Both enzymes were active on a wide variety of carbobenzoxy substituted peptides but inactive with unsubstituted dipeptides. Neither enzyme was active upon endoprotease or amidase substrates and only low levels of esterase activity were evident. In time course studies, both enzymes gave rapid non-specific sequential release of amino acids, including proline, from the carboxyterminal of proteins and polypeptides of known amino acid sequence.