Functional genomics opens doors to understanding metamorphosis in nonmodel invertebrate organisms.

Metamorphosis can be described as the period in the life cycle of an animal in which an immature individual undergoes drastic anatomical and physiological changes to develop into an adult. Usually these transitions are accompanied by a change of habitat or behaviour. Because it is present in a large number of animal phyla, we can infer that metamorphosis is an important evolutionary and developmental transition. Metamorphosis is a remarkable example of modularity in life cycles, in that a single genome can produce highly distinctive phenotypes occupying different niches. For instance, many marine invertebrates are characterized by a distinct larval stage, often pelagic, that metamorphoses into a benthic, often sessile, adult. Although metamorphosis is known to have evolved multiple times independently throughout the animal tree, there are a few similarities across phyla in how this developmental transition takes place. One striking similarity in marine invertebrates is the concept of metamorphic competence. Metamorphic competence refers to the ability of a developing larva to complete metamorphosis into the adult benthic stage upon induction by external settlement cues. These cues are represented by a broad range of stimuli on the benthos such as the presence of bacterial biofilms, diverse algal species, or animal prey among others. In most cases, however, each species has only one or few corresponding specific cues that are highly effective in inducing settlement. In this issue, Williams et al. present findings from a molluscan species on how the transcriptome varies throughout metamorphosis after induction of settlement. Studying the molecular underpinnings of how metamorphosis takes place in diverse species has been revamped by insight from genomic data. While overall gene expression profiles have been assessed during life cycle of insects (Arbeitman et al. 2002) and vertebrates (Baldessari et al. 2005), little information is available for the diverse lophotrochozoan phyla (Heyland & Moroz 2006). In their manuscript, Williams et al. (2009) address this gap by describing the changes in transcriptional profiles in the tropical abalone Haliotis asinina (Mollusca: Gastropoda) from mid-late larval development through to metamorphosis following induction of settlement (Fig. 1). Once abalone larvae are exposed to a coralline alga known to effectively induce metamorphosis in this gastropod, the transcriptome deviates significantly from that of unexposed larvae. At least five distinct temporal gene expression profiles can be observed throughout the metamorphic transition. Although it was previously thought that competent larvae exhibit reduced transcriptional activity (Hadfield et al. 2001), this new study …