Many species of insects display dispersing and nondispersing morphs. Among these, aphids are one of the best examples of taxa that have evolved specialized morphs for dispersal versus reproduction. The dispersing morphs typically possess a full set of wings as well as a sensory and reproductive physiology that is adapted to flight and reproducing in a new location. In contrast, the nondispersing morphs are wingless and show adaptations to maximize fecundity. In this review, we provide an overview of the major features of the aphid wing dimorphism. We first provide a description of the dimorphism and an overview of its phylogenetic distribution. We then review what is known about the mechanisms underlying the dimorphism and end by discussing its evolutionary aspects.

Aphids display an abundance of adaptations that are not easily studied in existing model systems. Here we review the biology of a new genomic model system, the pea aphid, Acyrthosiphon pisum. We then discuss several phenomena that are particularly accessible to study in the pea aphid: the developmental genetic basis of polyphenisms, aphid-bacterial symbioses, the genetics of adaptation and mechanisms of virus transmission. The pea aphid can be maintained in the laboratory and natural populations can be studied in the field. These properties allow controlled experiments to be performed on problems of direct relevance to natural aphid populations. Combined with new genomic approaches, the pea aphid is poised to become an important model system for understanding the molecular and developmental basis of many ecologically and evolutionarily relevant problems.

Identifying the genes underlying genetically complex traits is of fundamental importance for medicine, agriculture, and evolutionary biology. However, the level of resolution offered by traditional quantitative trait locus (QTL) mapping is usually coarse. We analyze here a trait closely related to fitness, ovariole number. Our initial interspecific mapping between Drosophila sechellia (8 ovarioles/ovary) and D. simulans (15 ovarioles/ovary) identified a major QTL on chromosome 3 and a minor QTL on chromosome 2. To refine the position of the major QTL, we selected 1038 additional recombinants in the region of interest using flanking morphological markers (selective phenotyping). This effort generated approximately one recombination event per gene and increased the mapping resolution by approximately seven times. Our study thus shows that using visible markers to select for recombinants can efficiently increase the resolution of QTL mapping. We resolved the major QTL into two epistatic QTL, QTL3a and QTL3b. QTL3a shows sign epistasis: it has opposite effects in two different genetic backgrounds, the presence vs. the absence of the QTL3b D. sechellia allele. This property of QTL3a allows us to reconstruct the probable order of fixation of the QTL alleles during evolution.

BACKGROUND: The best studied insect-symbiont system is that of aphids and their primary bacterial endosymbiont Buchnera aphidicola. Buchnera inhabits specialized host cells called bacteriocytes, provides nutrients to the aphid and has co-speciated with its aphid hosts for the past 150 million years. We have used a single microarray to examine gene expression in the pea aphid, Acyrthosiphon pisum, and its resident Buchnera. Very little is known of gene expression in aphids, few studies have examined gene expression in Buchnera, and no study has examined simultaneously the expression profiles of a host and its symbiont. Expression profiling of aphids, in studies such as this, will be critical for assigning newly discovered A. pisum genes to functional roles. In particular, because aphids possess many genes that are absent from Drosophila and other holometabolous insect taxa, aphid genome annotation efforts cannot rely entirely on homology to the best-studied insect systems. Development of this dual-genome array represents a first attempt to characterize gene expression in this emerging model system.

RESULTS: We chose to examine heat shock response because it has been well characterized both in Buchnera and in other insect species. Our results from the Buchnera of A. pisum show responses for the same gene set as an earlier study of heat shock response in Buchnera for the host aphid Schizaphis graminum. Additionally, analyses of aphid transcripts showed the expected response for homologs of known heat shock genes as well as responses for several genes with unknown functional roles.

CONCLUSION: We examined gene expression under heat shock of an insect and its bacterial symbiont in a single assay using a dual-genome microarray. Further, our results indicate that microarrays are a useful tool for inferring functional roles of genes in A. pisum and other insects and suggest that the pea aphid genome may contain many gene paralogs that are differentially regulated.

Aphids are the leading pests in agricultural crops. A large-scale sequencing of 40,904 ESTs from the pea aphid Acyrthosiphon pisum was carried out to define a catalog of 12,082 unique transcripts. A strong AT bias was found, indicating a compositional shift between Drosophila melanogaster and A. pisum. An in silico profiling analysis characterized 135 transcripts specific to pea-aphid tissues (relating to bacteriocytes and parthenogenetic embryos). This project is the first to address the genetics of the Hemiptera and of a hemimetabolous insect.