Development of Tribolium castaneum

Abstract

With the publication of the Tribolium castaneum genome (Richards et al. 2008), carefully annotated insects genomes are now available for three insect orders: the flies (Diptera: 12 Drosophila species and two mosquito species: Anopheles gambiae, Aedes aegypti), the bees, wasps, and ants (Hymenoptera: Apis mellifera), and the beetles (Coleoptera) to which Tribolium belongs. Furthermore, sequencing has been (partially) finished and annotation is in progress for several other insect genomes: the mosquito (Diptera) Culex pipiens, the parasitic wasp (Hymenoptera) Nasonia vitripennis, and two related Nasonia species, the silk worm (Lepidoptera) Bombyx mori and three hemimetabolous insects species: the blood-sucking bug (Hemiptera) Rhodnius prolixus, the pea aphid (Hemiptera) Acyrthosiphon pisum, and the human louse (Phthiraptera) Pediculus humanus (for review on recent advances in insects genomics see Grimmelikhuijzen et al. 2007). Together, these data provide an impressive basis for comparative insect genomics. They will allow us to reconstruct the common features of the genomes of holometabolous insects and they will reveal features of the basal genomes present in the ancestor of hemiand holometabolous insects. They also will help to resolve phylogenetic questions. One fascinating outcome from recent phylogenomic studies is a new tree of holometabolous insects, which locates the bees and wasps (Hymenoptera) at the base of the radiation of holometabolous insects (Savard et al. 2006; Zdobnov and Bork 2007; see also Misof et al. 2000 for a more conventional approach). Classical phylogeny had given the beetles a basal position within the tree (Grimaldi and Engels 2005). The new phylogeny has important implications for the evolutionary analysis of developmental processes. Beetles are the only holometabolous insect order with definite short-germ-band development (Sander 1976; Schwalm 1988; Davis and Patel 2002). Short-germ-band embryos harbor anlagen only of anterior segments (usually head and thorax) prior to gastrulation while the posterior (abdominal) segments form from a growth zone. This mode of the development is common to all hemimetabolous insects while all holometabolous insects, with the exception of the beetles, show long-germ-band development. In longgerm-band embryos, all segment anlagen are present prior to gastrulation and all segments form either simultaneously or sequentially within a narrow time window. In addition to the mode of segmentation, the organization of extraembryonic membranes of beetles (size and position of serosal anlagen, early amnion formation) is more similar to that of hemimetabolous insects (Schwalm 1988; van der Zee et al. 2005; Panfilio 2008). Since the holometabolous insects clearly are monophyletic, the new phylogeny offers two alternative interpretations. First, it is possible that the common ancestor of all holometabolous insects had already acquired long-germ development. According to this assumption, short-germ development in beetles would be a secondary character. This might explain why long-germ development can be observed among beetles (Patel et al. 1994). It also would fit the observation that the early fate map of the DV axis in Tribolium is very similar to that of long-germ embryos (Chen et al. 2000). A second interpretation of the new phylogeny is that short germ development Dev Genes Evol (2008) 218:115–118 DOI 10.1007/s00427-008-0215-2