Reduced Fitness of Virulent Aphis glycines (Hemiptera: Aphididae) Biotypes May Influence the Longevity of Resistance Genes in Soybean

Sustainable use of insect resistance in crops require insect resistance management plans that may include a refuge to limit the spread of virulence to this resistance. However, without a loss of fitness associated with virulence, a refuge may not prevent virulence from becoming fixed within a population of parthenogenetically reproducing insects like aphids. Aphid-resistance in soybeans (i.e., Rag genes) prevent outbreaks of soybean aphid (Aphis glycines), yet four biotypes defined by their capacity to survive on aphid-resistant soybeans (e.g., biotype-2 survives on Rag1 soybean) are found in North America. Although fitness costs are reported for biotype-3 on aphid susceptible and Rag1 soybean, it is not clear if virulence to aphid resistance in general is associated with a decrease in fitness on aphid susceptible soybeans. In laboratory assays, we measured fitness costs for biotype 2, 3 and 4 on an aphid-susceptible soybean cultivar. In addition, we also observed negative cross-resistance for biotype-2 on Rag3, and biotype-3 on Rag1 soybean. We utilized a simple deterministic, single-locus, four compartment genetic model to account for the impact of these findings on the frequency of virulence alleles. When a refuge of aphid susceptible was included within this model, fitness costs and negative cross-resistance delayed the increase of virulence alleles when virulence was inherited recessively or additively. If virulence were inherited additively, fitness costs decreased the frequency of virulence. Combined, these results suggest that a refuge may prevent virulent A. glycines biotypes from overcoming Rag genes if this aphid-resistance were used commercially in North America.     

Polar secretion of endothelin-1 by cultured endothelial cells.

The aim of this study was to determine the permeability of endothelial monolayers for endothelin-1 and a possible directionality of the endothelin-1 secretion process. Human umbilical vein endothelial cells were cultured on acellular amniotic membranes, dividing the tissue culture wells into an apical (luminal) and a basolateral (abluminal) compartment. Whereas in the absence of endothelial monolayers 44.9 +/- 2.3 and 43.5 +/- 2.0% of the unilaterally added endothelin-1 permeated from the apical to the basolateral side and from the basolateral to the apical side, respectively, only 6.5 +/- 0.6 and 6.6 +/- 0.4% diffused in the presence of endothelial cells. Analyzing endothelin-1 secretion, approximately 80% of the total amount of synthesized endothelin-1 was found in the basolateral compartment; thrombin (10 units/ml) stimulated the production of endothelin-1 approximately 2-fold, but did not change the relative distribution of endothelin-1 between the apical and basolateral compartments. In the presence of dexamethasone (10(-7) M), a decrease in the level of endothelin-1 was found in the apical compartment, whereas the total amount of endothelin-1 produced was not affected. Dexamethasone did not influence the permeability of human umbilical vein endothelial cell monolayers for endothelin-1. These results strongly support the hypothesis that endothelin-1 is a local paracrine regulator of vasotone.     

Conditions requisite for stability of polymorphic balance in Philaenus spumarius (L.) (Homoptera)

Frequencies of colour morphs were recorded in 1969–74 in 5 island populations of Philaenus spumarius. The polymorphic balance was accurately maintained in three of them, but less constantly in the other two. One of the latter populations experienced a great fluctuation in size, and this was accompanied by a profound alteration in morph frequencies.One of the five populations consists of two subpopulations about 5 m apart. Although differing in the same island-specific respects from the populations of the other islands, the subpopulations were significantly unlike in the detailed distribution of the morph frequencies. These differences were probably due to the dissimilar distribution of the subniches available to the subpopulations. The dissimilar, island-specific patterns of morph frequences recorded in the other four populations may also be due to dissimilar subniche distribution. Migration between islands occurs at too slight a rate to cause an appreciable dysmetric load in the populations. In general, the rate of migration in Philaenus appears to be governed by the need for a balance between effective ability to colonize and effective subniche-specific protection of polymorphism.The study has been aided by grants from the University of Helsinki and from the National Research Council for Sciences, and the N.R.C. for Agriculture and Forestry (Academy of Finland).This paper is report no. 528 from Tvärminne Zoological station.