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.

How do evolved genetic changes alter the nervous system to produce different patterns of behavior? We address this question using Drosophila male courtship behavior, which is innate, stereotyped, and evolves rapidly between species. D. melanogaster male courtship requires the male-specific isoforms of two transcription factors, fruitless and doublesex. These genes underlie genetic switches between female and male behaviors, making them excellent candidate genes for courtship behavior evolution. We tested their role in courtship evolution by transferring the entire locus for each gene from divergent species to D. melanogaster. We found that despite differences in Fru+ and Dsx+ cell numbers in wild-type species, cross-species transgenes rescued D. melanogaster courtship behavior and no species-specific behaviors were conferred. Therefore, fru and dsx are not a significant source of evolutionary variation in courtship behavior.

In animals, Hox transcription factors define regional identity in distinct anatomical domains. How Hox genes encode this specificity is a paradox, because different Hox proteins bind with high affinity in vitro to similar DNA sequences. Here, we demonstrate that the Hox protein Ultrabithorax (Ubx) in complex with its cofactor Extradenticle (Exd) bound specifically to clusters of very low affinity sites in enhancers of the shavenbaby gene of Drosophila. These low affinity sites conferred specificity for Ubx binding in vivo, but multiple clustered sites were required for robust expression when embryos developed in variable environments. Although most individual Ubx binding sites are not evolutionarily conserved, the overall enhancer architecture-clusters of low affinity binding sites-is maintained and required for enhancer function. Natural selection therefore works at the level of the enhancer, requiring a particular density of low affinity Ubx sites to confer both specific and robust expression.

During a four month study of male territoriality males of the euglossine bee Eulaema meriana exhibited the two alternative behavior patterns of territoriality and transiency. Territorial males patrolled an area adjacent to a tree upon which they perched. Territorial males utilized the same territory for up to 49 days, though often not on consecutive days, and appeared to non-violently relinquish territories to new males. Transients did not defend territories but flew from one territory to another and flew with the territorial male around the territory, rarely bumping, and never grappling. Transient males left the territory soon after the territorial male flew back and forth in front of the perch tree in a zig-zag flight. The alternative behaviors were correlated with wing wear such that males with little wing wear defended territories and males with considerable wing wear pursued a transient strategy. Behavior patterns were not correlated with head width. Comparison of territory trees with the territory trees of a closely related species indicate that each species utilized trees of a certain diameter class for perching. In addition, analysis of hemispherical canopy photographs indicates that males appeared to prefer territories that received a maximum of diffuse sunlight but a minimum of direct sunlight. Both territorial and transient males consistently returned to specific territories over their lifetime but appeared to travel long distances to forage for fragrances. Territorial and transient males visited fragrance baits with equal frequency suggesting that non-territorial, as well as territorial, males required fragrances.

Michael Akam has been awarded the 2007 Kowalevsky medal for his many research accomplishments in the area of evolutionary developmental biology. We highlight three tributaries of Michaels contribution to evolutionary developmental biology. First, he has made major contributions to our understanding of development of the fruit fly, Drosophila melanogaster. Second, he has maintained a consistent focus on several key problems in evolutionary developmental biology, including the evolving role of Hox genes in arthropods and, more recently, the evolution of segmentation mechanisms. Third, Michael has written a series of influential reviews that have integrated progress in developmental biology into an evolutionary perspective. Michael has also made a large impact on the field through his effective mentorship style, his selfless promotion of younger colleagues, and his leadership of the University Museum of Zoology at Cambridge and the European community of evolutionary developmental biologists.

Phenotypic plasticity allows organisms to quickly adapt in response to changing environments. Little is known of the genetic, environmental and epigenetic contribution to the expression of alternative adaptive developmental outcomes. We study aphid polyphenisms, which offer a unique, compelling opportunity to study multiple levels of biological organization, especially insect epigenetics. The pea aphid, Acyrthosiphon pisum, exhibits an adaptive reproductive polyphenism whereby genetically identical individuals reproduce either sexually (meiosis) or asexually (parthenogenesis) depending on environmental conditions during maternal development (short or long photoperiod, respectively). To understand how facultative asexuality evolved in aphids, we first determined meiosis gene activity in sexuals and asexuals. I determined that the pea aphid genome encodes single copies of homologs for the majority of the core meiotic machinery, suggesting that meiotic plasticity is not due simply to gene loss or expansion. Next, we determined if these core meiosis genes are expressed using PCR spanning across at least one intron from cDNA isolated from asexual and sexual ovaries. Surprisingly, meiosis specific genes (e.g., Spo11, Msh4, Msh5, Hop2 and Mnd1) are expressed in not only in asexual ovaries but also in somatic tissue and an obligately asexual aphid strain. Interestingly, the Spo11 PCR product contained intronic sequence, thus representing unspliced mRNA. Germline expression of Spo11, Mnd1 and Hop2 was confirmed by in situ analysis. Preliminary results identified candidate methylation sites in the Spo11 locus, indicating an epigenetic basis for this expression difference. Further characterization will help us better understand the molecular and epigenetic mechanisms underlying this adaptive facultative plasticity.

Meiosis is a highly conserved process in which a diploid genome is recombined and assorted into haploid gametes. Remarkably, the pea aphid Acyrthosiphon pisum exhibits a reproductive polyphenism whereby environmental signals trigger a switch between apomixis (parthenogenetic reproduction) and meiosis (sexual reproduction). Aphid apomixis results in daughter embryo clones with 2n genome content without male contribution or recombination. This important adaptation allows aphid populations to not only rapidly expand upon abundant resources during summer but also survive winter. How aphids have evolved this ability to switch between parthenogenesis and sexual meiosis is unknown. To arrive at a mechanistic explanation for this developmental plasticity, I determined meiosis gene activity in sexuals and asexuals. I first identified homologs of a core set of meiosis genes from the pea aphid genome. Next, I tested the expression of these core meiosis genes by PCR spanning across at least one intron from cDNA isolated from asexual and sexual ovaries. Surprisingly, meiosis specific genes (e.g., Spo11, Msh4, Msh5, Hop2 and Mnd1) are expressed in asexual ovaries. Additionally, the Spo11 PCR product contained intronic sequence, thus representing unspliced mRNA. Future experiments looking at the quantities and localizations of mRNA and protein will help to distinguish among several possible explanations for these results. Further molecular characterization of this phenotypic plasticity will be helpful in understanding how multiple interacting pathways can evolve to create alternate developmental phenotypes.

Mutualism with ants is suspected to be a highly labile trait within homopteran evolution. We used molecular phylogenetic evidence to test whether the mutualism has multiple origins within a single aphid genus. We constructed a molecular phylogeny of 15 Chaitophorus Koch (Hemiptera: Aphidoidea) species, using mitochondrial cytochrome oxidase I and II sequences. Ant tending evolved, or was lost, at least five times during Chaitophorus evolution. Parametric bootstrapping rejected the hypothesis of a single origin of ant tending in this genus. Further, the Chaitophorus made at least two host genus switches from poplars (Populus) to willow (Salix), and four switches in feeding position, from leaf feeding to stem feeding or vice versa. This is the first phylogenetic confirmation that ant tending is an evolutionarily labile trait in aphids.

Morphology evolves often through changes in developmental genes, but the causal mutations, and their effects, remain largely unknown. The evolution of naked cuticle on larvae of Drosophila sechellia resulted from changes in five transcriptional enhancers of shavenbaby (svb), a transcript of the ovo locus that encodes a transcription factor that governs morphogenesis of microtrichiae, hereafter called ’trichomes’. Here we show that the function of one of these enhancers evolved through multiple single-nucleotide substitutions that altered both the timing and level of svb expression. The consequences of these nucleotide substitutions on larval morphology were quantified with a novel functional assay. We found that each substitution had a relatively small phenotypic effect, and that many nucleotide changes account for this large morphological difference. In addition, we observed that the substitutions had non-additive effects. These data provide unprecedented resolution of the phenotypic effects of substitutions and show how individual nucleotide changes in a transcriptional enhancer have caused morphological evolution.