Aphids exhibit divergent modes of embryogenesis during the sexual and asexual phases of the life cycle. To explore how a single genome can give rise to these alternative developmental modes, we have initiated embryological studies of the pea aphid, Acyrthosiphon pisum. Here we present a detailed description of parthenogenetic, viviparous embryonic development in the pea aphid. We compare and contrast development of the parthenogenetic embryo with that of the embryo resulting from sexual reproduction. The primary difference between the embryos is the scale on which development occurs: early parthenogenetic development occurs in a volume approximately three orders of magnitude smaller than the sexual egg, largely because of the apparent absence of yolk in the parthenogenetic egg. This results in a drastically different duration of syncytial energid cleavage and, presumably, patterning processes in the two embryos must act at scales that differ by orders of magnitude. The eggs also develop on time scales that differ approximately by an order of magnitude and the timing of the embryonic movements, collectively called blastokinesis, have temporally shifted relative to growth of the embryo. In addition, the endosymbiotic bacteria are transferred from mother to embryo in different ways in the two embryos. Finally, the function of the serosa has diverged greatly in the two embryos: in the sexual egg the serosa deposits a thick cuticle that protects the egg, whereas the serosa of the parthenogenetic embryo is greatly reduced and its function is unclear. The pea aphid is a useful model system for examining how a single genome has evolved to allow divergent modes of development.

Symbiotic relationships between bacteria and insect hosts are common. Although the bacterial endosymbionts have been subjected to intense investigation, little is known of the host cells in which they reside, the bacteriocytes. We have studied the development and evolution of aphid bacteriocytes, the host cells that contain the endosymbiotic bacteria Buchnera aphidicola. We show that bacteriocytes of Acyrthosiphon pisum express several gene products (or their paralogues): Distal-less, Ultrabithorax/Abdominal-A, and Engrailed. Using these markers, we find that a subpopulation of the bacteriocytes is specified prior to the transmission of maternal bacteria to the embryo. In addition, we discovered that a second population of cells is recruited to the bacteriocyte fate later in development. We experimentally demonstrate that bacteriocyte induction and proliferation occur independently of B. aphidicola. Major features of bacteriocyte development, including the two-step recruitment of bacteriocytes, have been conserved in aphids for 80–150 million years. Furthermore, we have investigated two cases of evolutionary loss of bacterial symbionts: in one case, where novel extracellular, eukaryotic symbionts replaced the bacteria, the bacteriocyte is maintained; in another case, where symbionts are absent, the bacteriocytes are initiated but not maintained. The bacteriocyte represents an evolutionarily novel cell fate, which is developmentally determined independently of the bacteria. Three of five transcription factors we examined show novel expression patterns in bacteriocytes, suggesting that bacteriocytes may have evolved to express many additional transcription factors. The evolutionary transition to a symbiosis in which bacteria and an aphid cell form a functional unit, similar to the origin of plastids, has apparently involved extensive molecular adaptations on the part of the host cell.

Nitrogen acquisition and assimilation is a primary concern of insects feeding on diets largely composed of plant material. Reclaiming nitrogen from waste products provides a rich reserve for this limited resource, provided that recycling mechanisms are in place. Cockroaches, unlike most terrestrial insects, excrete waste nitrogen within their fat bodies as uric acids, postulated to be a supplement when dietary nitrogen is limited. The fat bodies of most cockroaches are inhabited by Blattabacterium, which are vertically transmitted, Gram-negative bacteria that have been hypothesized to participate in uric acid degradation, nitrogen assimilation, and nutrient provisioning. We have sequenced completely the Blattabacterium genome from Periplaneta americana. Genomic analysis confirms that Blattabacterium is a member of the Flavobacteriales (Bacteroidetes), with its closest known relative being the endosymbiont Sulcia muelleri, which is found in many sap-feeding insects. Metabolic reconstruction indicates that it lacks recognizable uricolytic enzymes, but it can recycle nitrogen from urea and ammonia, which are uric acid degradation products, into glutamate, using urease and glutamate dehydrogenase. Subsequently, Blattabacterium can produce all of the essential amino acids, various vitamins, and other required compounds from a limited palette of metabolic substrates. The ancient association with Blattabacterium has allowed cockroaches to subsist successfully on nitrogen-poor diets and to exploit nitrogenous wastes, capabilities that are critical to the ecological range and global distribution of cockroach species.Flavobacteria ͉ insect endosymbiont ͉ nitrogen conservation ͉ uricolysis

Insect mitochondrial genome (mtDNA) analysis is a powerful tool for the study of population genetics and phylogenetics. In the past few years primer sequences for the PCR amplification of various insect mtDNA genes have been published. The objectives of this study were (1) present new primer sequences for six insect mitochondrial genes and (2) test primers designed in our laboratory and some previously published primers on a wide range of insects to determine if amplification of the target fragment could be obtained. The primers for the amplification of the two ribosomal RNA gene (16S and 12S rRNA) fragments are universal for insects and related groups; the primers for NADH5 and NADH4 dehydrogenase gene fragments and cytochrome c oxidase I gene fragment are applicable broadly.

Cockroaches are among the most ancient winged insects, the earliest fossils dating back to about 400 million years. Several conflicting phylogenies for cockroach families, subfamilies, and genera have been proposed in the past. In addition, the relationship of Cryptocercidae to other cockroach families and the relationship between the cockroach, Cryptocercus punctulatus, and the termite, Mastotermes darwiniensis, have generated debate. In this paper, a phylogeny for cockroaches, mantids, and termites based on DNA sequence of the mitochondrial ribosomal RNA genes is presented. The results indicated that cockroaches are a monophyletic group, whose sister group is Mantoidea. The inferred relationship among cockroach families was in agreement with the presently accepted phylogeny. However, there was only partial congruence at the subfamil and the generic levels. The phylogeny inferred here does not support a close relationship between C. punctulatus and M. darwiniensis. The apparent synapomorphies of these two species are likely a manifestation of convergent evolution because there are similarities in biology and habitat.Cockroaches (order: Dictyoptera; suborder: Blattaria) are among the oldest winged insects known, dating back to the Carboniferous (1). About 4000 species of cockroaches have been described (2). A number of conflicting classifications exist for cockroaches, the most widely accepted ofwhich is that of McKittrick (1), based on morphological characters. She considered the order Dictyoptera to include cockroaches, mantids, and termites, each with its own suborder. She divided the suborder Blattaria into two superfamilies, Blaberoidea and Blattoidea, and five families, Polyphagidae, Blattellidae, and Blaberidae (all Blaberoidea), and Blattidae and Cryptocercidae (both Blattoidea). Three other major cockroach classifications, based on morphological characters, have been published during the past four decades (3-5).In addition to the overall phylogenetic relationships among cockroaches, two other issues have generated debate. The first is the relationship of Cryptocercidae to other cockroach families. Cryptocercidae consists of one genus (Cryptocercus) and three species (6) and is generally considered a sister group of Blattidae (1). However, it was recently proposed that Cryptocercidae be merged with Polyphagidae (7). The second issue concerns the relationship among cockroaches, mantids, and termites. Three major schemes have been proposed: Blattaria and Mantoidea are sister groups and Isoptera is a sister group of the Blattaria-Mantoidea complex (8), cockroaches and termites belong to the order Blattodea and mantids are a sister group to that order (9), and all three groups belong to Dictyoptera (1, 10). Of particular interest is the presumed close phylogenetic relationship between Cryptocercus and the termite, Mastotermes darwiniensis. M. darwiniensis has been considered the most archaic living termite species and the "missing link" between cockroaches and termites (11). It is the sole living rep...