Bernard Delobel

Melon resistance to the aphid Aphis gossypii: behavioural analysis and chemical correlations with nitrogenous compounds

In the melon, the Vat (monogenic, dominant) resistance gene governs both an antixenotic reaction to the melon aphid Aphis gossypii Glover (Homoptera, Aphididae) and a resistance to non‐persistent virus transmission, restricted to this vector species. We investigated the behavioural features and tissue localisation of the antixenosis resistance by the electrical penetration graph technique (EPG, DC system). We also compared the chemical composition in amino compounds and proteins of the phloem sap collected from two isogenic lines of melon (Cucumis melo L.), carrying the Vat gene or not. All behavioural and chemical data indicated that this resistance is constitutive. EPG analysis clearly showed that access to phloem, although delayed by alterations in pathway activities, was not impaired in terms of frequency of access or initiation of feeding. The most striking feature was, however, a very reduced duration of ingestion from phloem of resistant plants, making this compartment one of the tissues where the effects of the Vat gene are unambiguously expressed. This was confirmed by clear differential activity of phloem extracts in artificial no‐choice bioassays. Chemical analyses have shown that phloem saps from the two isogenic lines were extremely similar in profiles of ninhydrin positive compounds, and contained a low total amount of free amino acids (less than 10 mM). Out of more than 40 distinguishable peaks in the chromatograms (protein and non‐protein amino acids, as well as small peptides), only five differentiated the two genotypes. Two of them were increased in the resistant genotype: glutamic acid and a major unknown peak, probably a non‐protein amino acid (different from β pyrazolyl‐alanine, a Cucumis‐specific amino acid). The three others were depressed in resistant plants, and included the sulphur amino acid cystine and a peptide peak partly composed of the cysteine‐containing peptide glutathione (reduced form). Sap collection also showed that phloem exudation rates, as well as total protein and glutathione levels, were depressed in phloem sap from resistant plants. Such data are all indicative of a modified phloem‐sealing physiology, linked to sulfhydryl oxidation processes, in plants carrying the Vat gene. The originality of the mechanism of Vat resistance to aphids is discussed.

Physiological considerations of importance to the success of in vitro culture: An overview

Abstract The implications of the main physiological host-parasitoid relationships on artificial diet rearing are considered. Based on their own experience and some literature, the authors have emphasized some points such as larval growth and development with hormonal implications, osmotic pressure, respiration, nutrition and toxicity. Many parameters control each physiological function and its was shown that all the parameters act simultaneously. To take into account the multiple interactions, use of multifactorial analysis and design is proposed.

Influence of secondary compounds in the phloem sap of cassava on expression of antibiosis towards the mealybug Phenacoccus manihoti

Identification and assay of cyanogenic and phenolic compounds in phloem sap of cassava (Manihot esculenta Crantz, Euphorbiaceae) and in honeydew of the cassava mealybug Phenacoccus manihoti Matt. Ferr. (Homoptera, Pseudococcidae) were realised.

Aphid-specific triglycerides in symbiotic and aposymbiotic Acyrthosiphon pisum

The influence of Acyrthosiphon pisums bacterial endosymbiosis on the biogenesis of the aphid peculiar triglycerides (TG) was investigated. We present a convenient way to obtain aposymbiotic aphids, extensive NMR data on the four main aphid-specific TG (1,3-di-myristoyl 2-hexanoyl, octanoyl, sorboyl or octatrienoyl glycerides) and an HPLC method to analyse these TG. The main results are: (1) aphid symbiotes are not responsible for the biosynthesis of the unusual sorbic and octatrienoic acid containing TG; (2) diet-reared aphids, ruthough much fatter than plant-reared individuals, are not altered in their triglyceride profile, ruling out plant-specific dietary requirements for the synthesis of the odd TG; and (3) the sorbic acid containing TG, already described in cornicular secretions of aphids, was found to be strongly tissue-specific between normal flat-body and mature cornicular fat-cells ( 35% in the latter).

Dietary specialization in European species groups of seed beetles (Coleoptera: Bruchidae: Bruchinae)

Because of their particular biology, seed beetles exhibit a strong relationship with their larval host plants. In Europe, however, field data have long been scarce and unreliable. The results of Legume seed collections of nearly 1,000 samples belonging to 292 species from various locations in Europe are summarized. The status of current Bruchidius species groups is amended on morphological and phylogenetic bases. Recent advances in the knowledge of phylogenetic structures of both Fabaceae and Bruchinae provide a new picture of Bruchinae–Fabaceae interactions. It reveals a certain level of host conservatism. The hypothesis of radiative adaptation seems the most compatible with observed data.

Biological and chemical characteristics of a genetic resistance of melon to the melon aphid

Host plant resistance to pests is an important component of integrated control strategies. Due to the specialised phloem-feeding of aphids and many other hemipteran insect pests, which necessitates specific chemical treatments and control of virus transmission, this genetic strategy seems particularly attractive for this insect group. In the melon Cucumis melo L., a gene labelled vat, for virus aphid transmission (Pitrat & Lecoq, 1982), brings both a resistance to the melon aphid Aphis gossypii Glover and a resistance to the transmission by this vector of many non-persistent viruses. This gene induces both an antixenotic response to adults and an apparent antibiosis towards A. gossypii larvae. The purpose of the experiments reported here was to investigate the mechanisms of this resistance (mechanical, chemical, tissue localisation...) and to characterize any chemical difference between genotypes differing by the presence/absence of the vat gene.

Acyrthosiphon pisum performance in response to the sugar and amino acid composition of artificial diets, and its relation to lucerne varietal resistance

In order to understand the mechanisms of resistance of two lucerne cultivars (susceptible ‘Resistador’ and resistant ‘Lahontan’ clones) to a French biotype of Acyrthosiphon pisum Harris, several biological parameters of this aphid were investigated on sap‐copied holidic diets differing either by their amino acid or sucrose concentrations or by the relative proportions of some amino acids.

Effects of pyrazole compounds from melon on the melon aphid Aphis gossypii

Abstract Two secondary compounds of the genus Cucumis , pyrazole and β (pyrazol-1-yl) l -alanine ( β PA), were found and measured in the phloem sap of melon. The effects of these compounds on attractivity, growth inhibition and mortality were tested on aphids from cucurbit and noncucurbit hosts. β PA had no noticeable effect on the aphids tested. In contrast, pyrazole had highly deleterious effects on Acyrthosiphon pisum , which does not feed on melon. In addition, the strain of Aphis gossypii which is adapted to melon appeared to be much more tolerant to pyrazole than the cotton strain of the same species. Both strains of A. gossypii were stimulated to feed by moderate concentrations of pyrazole, whereas A. pisum was not. The resistance to A. gossypii due to the Vat gene in melon cannot be explained by the distribution of pyrazoles in the two susceptible/resistant lines tested: β PA was present in these two lines at comparable levels and pyrazole at levels too low to mediate any behavioural effect.

Composition corporelle en acides amines du parasitoide Phryxe caudata (Diptera) au cours de sa croissance larvaire

Abstract The analysis of amino acids in acid hydrolysates of whole larvae of the parasitoid Diptera, Phryxe caudata, has permitted us to establish a relationship showing the quantity Q of each of the seventeen aminoacids measured, as a function of the fresh weight P of the larva. We have considered two kinds of relationships: allometric (Q = cPd) and polynomial ones (Q = eP2 + gP), these adapt most precisely to the whole field of study. Those relations show that the incorporation of the aminoacids in the living material is not exactly proportional to the weight of growing larva. Tyrosine, β alanine, phenylalanine, methionine, and histidine accumulate, whereas the ratios of the other aminoacids decrease during growth. Thus the relative needs of the larva in tyrosine are ten times greater at the end of growth than at the beginning. It has been also shown that, because of the very fast larval growth, the quantitative needs in aminoacids become considerable. The consequences of these facts are discussed in view of realisation of in vitro satisfactory nutritional conditions. The total protids, which are constant during growth (10% of the fresh weight) represent a half of the dry weight at the beginning of larval growth, but, because of the accumulation of non protidic reserves, (probably lipids) represent only one third, at the end of development.

Storage proteins in coleoptera: A new class of tyrosine-rich proteins from the pupae of two weevils, Sitophilus oryzae and Rhynchophorus palmarum (Coleoptera: Curculionidae)

Abstract The occurrence of tyrosine-rich storage proteins was investigated in two weevils (Coleoptera: Curculionidae). Three monomers (two major, 44 and 31 kDa, and one minor, 51 kDa) were purified from pupae of Sitophilus oryzae , and one major monomer (65 kDa) from Rhynchophorus palmarum . These related proteins belong to a new class of insect storage proteins, different from either arylphorin or other identified storage proteins. They occur as insoluble granules in the pupal fat body, and are only extracted by denaturing conditions such as 1% SDS or 8 M urea. A convenient tyrosine-specific stain was adapted to track them easily during the purification process, which involved preparative SDS-PAGE. The monomers contained no lipids, no phosphate groups and their glycoprotein nature is uncertain. According to their amino acid compositions, these proteins may be divided into two subclasses: all of them yield three predominant amino acids amounting to more than 15% of the total amino acids each (tyrosine, serine and glycine), but the smaller So31 polypeptide exhibits and exceptionally high tyrosine content (more than 27%). The presence of arylphorin in Curculionid hemolymph was investigated, and the comparative features of these two classes of tyrosine-rich proteins are discussed.