Roger L. Blackman

Local variability in the life cycle of the bird cherry-oat aphid, Rhopalosiphum padi (Homoptera: Aphididae) in western France

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 parthe-nogenetic 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.

Chromosomal location of the amplified esterase genes conferring resistance to insecticides in Myzus persicae (Homoptera: Aphididae)

A genomic probe encompassing most of an esterase gene (E4) that is amplified in insecticide-resistant Myzus persicae was hybridized in situ to mitotic and meiotic chromosome preparations of aphid clones of known esterase type and resistance level. Binding, which was detected using the biotin-avidin system located both known types of amplified esterase sequences (E4 and FE4). All except one of the E4-producing clones had a single amplified site, on autosome 3 near the breakpoint of an autosomal 1,3 translocation which previous work had shown to be genetically linked to insecticide resistance. The exceptional clone had two other E4-encoding sites. The most resistant FE4-producing clone (800F) had amplified sequences at five sites (three loci: two homozygous and one heterozygous). Altogether, amplified E4 and/or FE4 sequences were found on four of the five autosome pairs of M. persicae. Possible origins of these multiple loci are discussed.

Microsatellite variation in cyclically parthenogenetic populations of Myzus persicae in south-eastern Australia

We examined the population structure of the introduced aphid, Myzus persicae collected mainly from its primary host, Prunus persica, in south-east Australia. Myzus persicae has been present in Australia since at least 1893. Samples were collected in the spring of 1998 from two mainland and three Tasmanian localities and isofemale lines were established in the laboratory. The reproductive mode (life cycle), karyotype and 17-locus microsatellite genotype of each clone were determined. All populations showed significant population differentiation (FST 0.058–0.202) even over small geographic distances (<50 km). All clones were karyotypically normal except for a subset of clones from one site that was exposed to the carbamate insecticide, Pirimor, the week prior to sampling. Those clones were heterozygous for an autosomal 1,3 translocation frequently associated in M. persicae with insecticide resistance. In contrast to other loci and despite being on different chromosomes, loci myz2A and M55A showed general and significant linkage disequilibrium. These loci may be affected by epistatic selection. We discuss the observed high clonal diversity, moderate but significant population differentiation, general conformance to Hardy-Weinberg equilibria and low linkage disequilibria with particular focus on the global population biology of M. persicae.

Ribosomal DNA is frequently concentrated on only one X chromosome in permanently apomictic aphids, but this does not inhibit male determination.

The ribosomal DNA arrays in the nucleolar organizer regions (NORs) of aphids are generally located in a telomeric or subtelomeric position on the X chromosomes. In aphid populations or species that have lost the sexual part of their life cycle and become permanent apomicts, multiple rDNA copies are often concentrated on only one of the original X chromosomes. This situation has been found in apomictic members of three aphid subfamilies, although not as yet in the tribe Aphidini, which includes several permanent apomicts that are important pests. Some clonal cultures ofAcyrthosiphon pisum andMyzus persicae reared for many years in conditions that prolong apomixis and inhibit sexual reproduction also have their rDNA arrays concentrated on one X chromosome. A 50-year-old clone ofAphis fabae, however, still retains a pair of similar-sized rDNA arrays on its X chromosomes. Although pairing of the X chromosomes by their NORs during prophase of the maturation division seems to be required for determination of XO males in aphids, a clone ofA. pisum with one rDNA array was nevertheless able to produce males when subjected to appropriate environmental conditions.

Host-correlated morphological variation of Myzus persicae (Hemiptera: Aphididae) populations in Greece.

Morphological variation in nine characters of 157 clones of Myzus persicae (Sulzer) was examined by multivariate analysis. The clones were collected from peach, Prunus persica, the primary host and the secondary hosts tobacco, Nicotiana tabacum, cabbage, Brassica oleracea, sugarbeet, Beta vulgaris and pepper Capsicum annuum. The 156 clones originated from various regions of Greece, both in the north, where a large part of the population has an annual bisexual generation on peach, and in more southerly regions, where populations are predominantly unisexual. One clone was collected from tobacco in Caserta, Italy. All clones were laboratory-reared on potato. Canonical variate analysis, hierarchical cluster analysis and a non-parametric classification tree method both revealed morphological differences associated with the host-plant on which they were collected. The scores of the first two canonical variates separated the tobacco-feeding clones from those originating from other secondary host-plants. However, in tobacco-growing areas the tobacco-feeding form predominated in spring populations on peach, and was sometimes found on other secondary hosts. In addition, using cluster analysis, the clones from tobacco which were sampled in the most southeasterly region showed a relatively large phenotypic distance from those collected further north and west. Moreover, clonal phenotypes were affected both by host plant and by long-term parthenogenetic rearing. However, in spite of these effects, the tobacco form was generally distinguishable from aphids originating from other hosts, indicating that the difference must have a genetic basis. In separate analyses of the clones originating from secondary hosts no association was found between morphology and either life cycle category or colour. Discriminant analysis showed that 89% of 1723 specimens could be correctly classified into the two groups.

High diversity of structurally heterozygous karyotypes and rDNA arrays in parthenogenetic aphids of the genus Trama (Aphididae: Lachninae)

Karyotypes of permanently parthenogenetic aphids of three species of the genus Trama show great diversity, particularly in the number and distribution of chromosomal elements containing highly repetitive sequences. Sampling at only a few sites in southern England, chromosome number varied from 14 to 23 in T. troglodytes, 9–12 in T. caudata and 10–14 in T. maritima, with some colonies having individuals of more than one karyotype. This variation was paralleled by differences in the number and distribution of rDNA arrays revealed by in situ hybridization. This high intraspecific karyotype diversity contrasts with very low genetic diversity in the same populations, suggesting rapid karyotype evolution. Although T. troglodytes feeds on many species of composite plants there was no evidence of any karyotype-associated host race formation.

Life cycle variation of Myzus persicae (Hemiptera: Aphididae) in Greece.

During the years 1995-1999 the life cycle category of 2797 clones of Myzus persicae (Sulzer) was examined. The clones originated from primary and secondary hosts from different localities of North and Central Greece and the island of Crete in the south. Four different overwintering life cycle strategies were found that have also been described for M. persicae and other heteroecious species previously. A geographical variation was found in the proportion of holocyclic clones from tobacco and other secondary hosts associated with the abundance of the primary host in the sampling regions. In Central Macedonia, around the main peach-growing regions, the proportion of holocyclic clones was mostly above 50% and in some cases reached 100%. In localities of East Macedonia, holocyclic clones were also frequent. On the other hand, further south or in north-eastern Greece, where peach is not common, the proportion of holocyclic clones varied between 0 and 33%. Fifty seven percent of examined anholocyclic clones produced males under short day conditions, suggesting that androcyclic clones in Greece represent an important factor of genetic variability. Intermediate clones were sampled from all host-plants but at low frequencies (3.6% of total examined clones and 6.9% of non-holocyclic ones). Moreover, a regional variation was found in different colour forms feeding on tobacco plants. Red clones were predominant in regions where aphids overwinter parthenogenetically on weeds or winter crops. However, almost all clones from the primary host were green. The ecological aspects of life cycle variation are discussed.

Electrophoretic distinction between the peach-potato aphid, Myzus persicae, and the tobacco aphid, M. nicotianae (Homoptera: Aphididae).

The electrophoretic mobility of the enzyme glutamate oxaloacetate transaminase (GOT) on cellulose acetate plates was compared among sibling species of the Myzus persicae (Sulzer) group ( M. persicae, M. nicotianae Blackman, M. antirrhinii (Macchiati)). M. persicae itself is monomorphic for GOT-1 (genotype ff ), whereas European populations of M. nicotianae are polymorphic for this enzyme, with two forms of slightly different mobility (alleles s, f ). In the samples of M. nicotianae examined, M. persicae-like ff genotypes were rare and heterozygotes ( sf ) were in large excess, even in samples from Greece where M. nicotianae has a regular holocycle (i.e., annual sexual reproduction). In North America, where M. nicotianae is probably entirely anholocyclic, samples of both red and green colour morphs of this species were found to be heterozygous for GOT-1. The enzyme difference can thus provide a means of distinguishing most individual specimens, including trapped alatae, of M. persicae and M. nicotianae . The anholocyclic taxon M. antirrhinii appears to be a fixed heterozygote for GOT-1 and thus resembles most M. nicotianae , but can be distinguished electrophoretically from both M. persicae and M. nicotianae by its distinctive pattern of esterases.

Vertical Food Web Interactions: Evolutionary Patterns and Driving Forces

edited by K. Dettner, G. Bauer and W. Völkl, Ecological Studies Vol. 130, Springer-Verlag, 1997. pound76.00 (xxi+390 pages) ISBN 3 540 62561 5.

The karyotype of Bemisia tabaci (Hemiptera: Aleyrodidae)

Whiteflies of the Bemisia tabaci (Gennadius) group are a major and increasing problem for agriculture throughout the world. In particular, the appearance and rapid spread of a highly virulent new form (silver-leaf whitefly, or B-biotype; Brown et ah, 1995), and recognition that the problem involves a complex of sibling species, host races and/or biotypes has stimulated intensive multidisciplinary research on this insect in recent years (Gerling & Mayer, 1996).