No abstract

Multivariate techniques, principally the method of canonical variates, were used to investigate morphological variation within and between populations of the group of Myzus persicae (Sulzer) from North and South America, Europe, Africa and Asia. The scores on the first canonical variate of samples from tobacco in North America, the Mediterranean region, the Middle East, Africa and Sri Lanka all grouped consistently when compared with samples from other host-plants, even after aphids from tobacco had been reared for up to seven years on a non-tobacco host. Thus there is a widely-distributed tobacco-adapted form, closely related to M. persicae but with its own characteristic morphology. Morphological discriminants are given for the recognition of apterous and alate viviparae of this tobacco form, which is given the name M. nicotianae sp. n. Both M. persicae and M. nicotianae have In = 12, and both are frequently heterozygous for apparently the same autosomal translocation, which they must have acquired independently. M. nicotianae is presumably isolated from M. persicae by being permanently parthenogenetic. In Japan and Central Asia, however, aphids of the M. persicae group on tobacco can produce sexual morphs; the taxonomic status of these latter populations is still unclear. Multivariate comparison of European and North American populations of dark green aphids of the M. persicae group with 13 or 14 chromosomes in somatic cell nuclei instead of the normal 12, led to the conclusion that these all belong to one morphologically variable taxon, M. antirrhinii (Macchiati). Keys are provided to the apterous and alate virginoparae of the species of the M. persicae group in America.

The peach^potato aphid Myzus persicae (Sulzer) can resist a wide range of insecticides, but until recently (1990) the only mechanism identi¢ed was the increased production of carboxylesterases (E4 or FE4), which cause enhanced degradation and sequestration of insecticidal esters. We have now identi¢ed two forms of target-site resistance involving changes in the acetylcholinesterase (AChE) and sodium channel (kdr) genes. Biochemical and DNA diagnostic methods can be used to identify all three mechanisms in individual aphids, and thereby establish their spatial distributions and temporal dynamics. Ampli¢ed genes underlie the increased production of esterases, but their expression is modulated by DNA methylation. Ampli¢cation of the E4 gene is in strong linkage disequilibrium with the kdr mechanism. This may re£ect strong insecticidal selection favouring aphids with multiple mechanisms, tight chromosomal linkage and/or the prominence of parthenogenesis in many M. persicae populations. The decreased ¢tness of resistant aphids under winter conditions may be a consequence of the altered sodium-channel gene a¡ecting behaviour and/or the perception of external stimuli.

Diploid female chromosome numbers are listed for 180 aphid species not previously karyotyped. The list includes the first chromosome records for several aphid tribes (Tramini, Greenideini, Anomalaphidini, Nipponaphidini). Variation in chromosome number at different systematic levels is discussed. Usually the karyotype is particularly stable within a genus, but there are notable exceptions (e.g. Arnphorophoru) where considerable evolutionary increase in chromosome number has occurred by autosome dissociation with little accompanying morphological change. In several genera differences in gross chromosome morphology can be useful to the taxonomist. Within-species karyotype variation is relatively common in aphids, and instances of structural heterozygosity are particularly numerous in species and groups which have partially or completely abandoned the sexual phase of the life cycle in favour of permanent thelytoky .

To study life cycle variability in Rhopalosiphum padi (Linnaeus), the responses to short days (L:D 10:14) of 70 clones were tested at two temperatures (10°C and 15°C) in the laboratory. Three clones originating from spring populations on the primary host, bird cherry (Prunus padus), were holocyclic, producing gynoparae and then males in the second generation under experimental conditions. Of the other clones, all originating from Gramineae, 78.5% were anholocyclic and 21.5% androcyclic (male-producing) at 15°C, but at 10°C only 12.7% were anholocyclic, 82.6% were androcyclic, and 4.7% showed an 'intermediate' response (i.e., some of the alate females produced both sexual and parthenogenetic offspring). There was considerable interclonal and inter-individual variation in the numbers of males produced by androcyclic clones. These results are discussed in relation to field studies on R. padi in Britanny, which indicate that 1. primary hosts are rather rare, but nevertheless commonly develop spring infestations; 2. anholocyclic genotypes are regularly decimated every few years by severe winters. It appears that the preponderance of androcyclic clones is important for the maintenance of life cycle variability.