The dynamics of the cassava green mite Mononychellus tanajoa in a seasonally dry area in Kenya

The population dynamics of the cassava green mite Mononychellus tanajoa was studied on cassava during 35 weeks (early March to first of November 1989) in an experimental field near Lake Victoria in Western Kenya. The mite population peaked at the onset of the long dry season with 1,100 mites/leaf, declined sharply to a level of about 300 individuals/leaf, not to increase again until the next rainy season commenced. An indigenous phytoseiid predator Iphiseius degenerans was abundant during the dry spell with a maximum about 9 predators/leaf.A nonlinear regression analysis revealed that food depletion in combination with I. degenerans predation limited the population growth of the mites, whereas rain intensity had no effect. The predator exhibited no aggregative response to high densities of M. tanajoa and stayed mainly in the lower part of the canopy while the spider mites preferred the top, indicating that I. degenerans is a generalist predator without capacity to control M. tanajoa alone. However, in combination with another density dependent factor, such as food depletion, the predator may have prevented the spider mites from causing complete defoliation during the dry season.     

The effect of the cassava green mite Mononychellus tanajoa on the growth and yield of cassava Manihot esculenta in a seasonally dry area in Kenya

The effect of the cassava green mite Mononychellus tanajoa on the growth and yield of cassava Manihot esculenta was studied over a 10-month period in two field trials near Lake Victoria in Kenya. One plot was maintained free of mites by means of acaricide, while the other was artificially infested.The highest population density of M. tanajoa occurred during the dry season. A maximum leaf area index (LAI) of about 2 was reached at the onset of the dry season. The total leaf area of mite infested plants was reduced compared with uninfested plants during the dry spell. During the following rainy season infested plants recovered and attained the same leaf area as uninfested plants. A multiple regression model predicting the leaf area showed that 58% of the seasonal variation could be explained by plant age, soil water, and leaf injury.The net growth rate of infested plants was lower than that of uninfested plants. Maximum values of 21 (infested plants) and 49 (uninfested plants) g m^-2 week^-1 were attained at the onset of the second rainy season. No difference was found between uninfested and infested plants with respect to net assimilation rates per unit leaf area during the dry season. The net assimilation rates reached a maximum almost at the same time as the growth rates, but the infested plants peaked slightly earlier and at a lower level than the uninfested plants. M. tanajoa did not affect the relative allocation of dry matter into stems and storage roots, but the absolute allocation of dry matter declined with increasing mite injury. Thus, after 10 months the dry matter of infested plants was reduced by 29% and 21% for storage roots and stems, respectively, compared with the uninfested plants.     

Isolation, identification and synthesis of locustamyoinhibiting peptide (LOM-MIP), a novel biologically active neuropeptide from Locusta migratoria.

A novel peptide termed locustamyoinhibiting peptide (LOM-MIP) was isolated from brain-corpora cardiaca-corpora allata-suboesophageal ganglion extracts of the locust, Locusta migratoria. The primary structure of this nonapeptide has been determined Ala-Trp-Gln-Asp-Leu-Asn-Ala-Gly-Trp-NH2. LOM-MIP suppresses the spontaneous contractions of the hindgut and oviduct of Locusta migratoria and of the hindgut of Leucophaea maderae. This novel peptide is, however, structurally different from leucomyosuppressin, a hindgut suppressing peptide isolated from Leucophaea maderae heads. LOM-MIP has a Gly-TrpNH2 carboxy-terminal in common with APGWamide, a penis retractor muscle inhibiting peptide isolated from the snail, Lymnea stagnalis. In addition, it shows carboxy-terminal sequence similarities with locust AKH II which ends in AGWamide. No sequence similarities were found with other vertebrate or invertebrate peptides. Synthetic LOM-MIP showed biological as well as chemical characteristics indistinguishable from those of native LOM-MIP.     

A simulation study of how simple mark-recapture methods can be combined with destructive subsampling to facilitate surveys of flying insects

Mark-recapture techniques are used for studies of animal populations. With only three sampling occasions, both Bailey's triple-catch (BTC) and Jolly-Seber's (J-S) stochastic method can be applied. As marking and handling of fragile organisms may harm them, and thereby affect their chances of being recaptured, handling should be minimized. This can be achieved by taking a subsample before the main sample at the second sampling occasion. Individuals in the main sample are marked and released, whereas those in the subsample are only used for identifying recaptures. Monte-Carlo simulation was used to compare the subsampling method with the ordinary mark-recapture methods. Model-generated populations were sampled with and without subsampling to provide estimates of population size, loss, and dilution rates. The estimated parameters were compared with their true values to identify biases associated with the sampling methods, using 81 different combinations of population size, dilution rate, loss rate, and sampling effort. Each combination was replicated 1,000 times. In no cases did subsampling perform more poorly than the ordinary methods. J-S was slightly more accurate than BTC to estimate the population size, but only when sampling effort was high. The relative biases associated with estimates of dilution and loss rates were substantial, but declined with increasing population size and sampling effort. Confidence limits for the population parameters generally were reliable and tended to be conservative. We therefore conclude that ordinary mark-recapture methods can be supplemented with subsampling without sacrificing accuracy. Subsampling is especially advantageous in cases where marks are difficult to observe under field conditions.     

Temperature-dependent development and reproductive traits of <Emphasis Type="Italic">Tetranychus macfarlanei</Emphasis> (Acari: Tetranychidae)

Development and reproductive traits of Tetranychus macfarlanei Baker & Pritchard (Acari: Tetranychidae) were investigated on kidney bean, Phaseolus vulgaris L., at eleven constant temperatures. Tetranychus macfarlanei was able to develop and complete its life cycle at temperatures ranging from 17.5 to 37.5°C. At 15 and 40°C, a few eggs (2–4%) hatched but further development was arrested. Development from egg to adult was slowest at 17.5°C and fastest at 35°C for both females and males. Using Ikemoto and Takai’s linear model, the estimated lower developmental thresholds for egg-to-female adult, egg-to-male adult and egg-to-egg development were 12.9–13.0°C. The thermal constants for the respective stages were 110.85, 115.99 and 125.32 degree-days (DD). The intrinsic optimum temperatures (T _Φ) calculated by non-linear SSI model were determined as 24.4, 24.4 and 24.2°C for egg-to-female adult, egg-to-male adult and egg-to-egg development, respectively. The net reproductive rate (R ₀) was highest at 25°C (167.4 females per female) and lowest at 17.5°C (42.6 females per female). The intrinsic rate of natural increase, r _m, increased linearly with the rising of temperature from 0.102 at 17.5°C to 0.441 day⁻¹ at 35°C. These values suggested that T. macfarlanei could be growing quickly in response to increasing temperatures from 17.5 to 35°C and provide a basis for predicting its potential geographical range.     

Recombination rate variation and speciation: theoretical predictions and empirical results from rabbits and mice

Recently diverged taxa may continue to exchange genes. A number of models of speciation with gene flow propose that the frequency of gene exchange will be lower in genomic regions of low recombination and that these regions will therefore be more differentiated. However, several population-genetic models that focus on selection at linked sites also predict greater differentiation in regions of low recombination simply as a result of faster sorting of ancestral alleles even in the absence of gene flow. Moreover, identifying the actual amount of gene flow from patterns of genetic variation is tricky, because both ancestral polymorphism and migration lead to shared variation between recently diverged taxa. New analytic methods have been developed to help distinguish ancestral polymorphism from migration. Along with a growing number of datasets of multi-locus DNA sequence variation, these methods have spawned a renewed interest in speciation models with gene flow. Here, we review both speciation and population-genetic models that make explicit predictions about how the rate of recombination influences patterns of genetic variation within and between species. We then compare those predictions with empirical data of DNA sequence variation in rabbits and mice. We find strong support for the prediction that genomic regions experiencing low levels of recombination are more differentiated. In most cases, reduced gene flow appears to contribute to the pattern, although disentangling the relative contribution of reduced gene flow and selection at linked sites remains a challenge. We suggest fruitful areas of research that might help distinguish between different models.     

Adaptive evolution and effective population size in wild house mice

Estimates of the proportion of amino acid substitutions that have been fixed by selection (α) vary widely among taxa, ranging from zero in humans to over 50% in Drosophila. This wide range may reflect differences in the efficacy of selection due to differences in the effective population size (N(e)). However, most comparisons have been made among distantly related organisms that differ not only in N(e) but also in many other aspects of their biology. Here, we estimate α in three closely related lineages of house mice that have a similar ecology but differ widely in N(e): Mus musculus musculus (N(e) ～ 25,000-120,000), M. m. domesticus (N(e) ～ 58,000-200,000), and M. m. castaneus (N(e) ～ 200,000-733,000). Mice were genotyped using a high-density single nucleotide polymorphism array, and the proportions of replacement and silent mutations within subspecies were compared with those fixed between each subspecies and an outgroup, Mus spretus. There was significant evidence of positive selection in M. m. castaneus, the lineage with the largest N(e), with α estimated to be approximately 40%. In contrast, estimates of α for M. m. domesticus (α = 13%) and for M. m. musculus (α = 12 %) were much smaller. Interestingly, the higher estimate of α for M. m. castaneus appears to reflect not only more adaptive fixations but also more effective purifying selection. These results support the hypothesis that differences in N(e) contribute to differences among species in the efficacy of selection.     

Cryopreservation of Grapevine (Vitis spp.) Embryogenic Cell Suspensions by Encapsulation–Vitrification

Embryogenic cell suspensions of two grapevine rootstocks: 110 Ritcher (V. berlandieri × V. rupestris), 41B (V. vinifera × V. berlandieri) and several table grape and wine cultivars (Vitis vinifera) were successfully cryopreserved by the encapsulation–vitrification method. Embryogenic cell suspensions were precultured for 3 days in liquid MGN medium supplemented with daily increasing sucrose concentrations of 0.25, 0.5, 0.75 M. Precultured cells were encapsulated and directly dehydrated with a highly concentrated vitrification solution prior to immersion in liquid nitrogen for 1 h. After rewarming at 40 °C for 3 min, cryopreserved cells were post-cultured on solid MGN medium supplemented with 2.5 g l^–1 activated charcoal. Surviving cells were transferred to solid MGN medium for regrowth or solid MG medium for embryo development and then to solid WPM for plant regeneration. Optimal viability was 42–76% of cryopreserved cells when cell suspensions were precultured with a final sucrose concentration of 0.75 M and dehydrated with PVS2 at 0 °C for 270 min. Biochemical analysis showed that sucrose preculture caused changes in levels of total soluble protein and sugars in cell suspensions. Although the increase in fresh weight was significantly lower in cryopreserved cells than in control cells, the growth pattern of the cryopreserved cells and control cells was the same after two subcultures, following re-establishment in cell suspensions. Protocol developed in this study suggests a universal and highly efficient cryopreservation system suitable for several genetically diversed Vitis species.     

Are environmental factors responsible for geographic variation in the sex ratio of the Greenlandic seed‐bug Nysius groenlandicus?

Until recently nothing indicated an unequal sex ratio in the widespread Greenland seed‐bug Nysius groenlandicus (Zetterstedt) (Heteroptera: Lygaeidae). However, recently populations more or less devoid of males were discovered in high arctic Northeast Greenland. This initiated an inspection of the entire material of the species collected in Greenland and now preserved at the Zoological Museum, University of Copenhagen. It was found that the sex ratio varied significantly among different locations. In most cases females were most abundant, but males were either scarce or absent only in samples from Northeast Greenland, indicating that here the species reproduces asexually. This paper demonstrates that the differing sex distributions can be explained by climatic factors (temperature, precipitation) and that the degree of continentality (distance from the open sea) promotes female‐biased sex ratios.