Fatal attraction

L ook carefully at a solitary animal and you will find that it is not so alone after all. Animals play host to entire ecosystems that teem with diverse life. Some of the microbes that live on (or in) animals are beneficial to their host. However, these microbes' more sinister brethren, parasites and pathogens, cause damage and disease. Between these two extremes is a class of organisms that, it seems, do not harm or benefit their hosts: instead, these organisms reap their reward when the host animal dies of other causes. This lifestyle is termed 'necromeny' (Sudhaus and Schulte, 1989) and has been considered an evolutionary intermediate to full-blown para-sitism (Sudhaus, 2008). Now, in eLife, Ray Hong of California State University (CalState) and co-workers, who include Jessica Cinkornpumin and Dona Wisidagama as joint first authors, report the discovery of a molecular mechanism used by a nematode worm called Pristionchus pacificus to find its insect host, the oriental beetle. This necromenic nematode lives inside the beetle and waits for the beetle to die, so that it can feed off the bacteria that grow on the decomposing carcass. Working with colleagues from two Max Planck Institutes—the MPI for Biology of Ageing and the MPI for Developmental Biology—the CalState researchers have identified a new molecular player in detection of chemical signals. They have also revealed the dual nature of the chemical cue that lures these nematodes to the beetles and, at the same time, arrests their development (Cinkornpumin et al., 2014). To understand how P. pacificus detects host odours, the researchers performed a genetic screen for mutant nematodes that were no longer attracted to a beetle pheromone called ZTDO. First discovered as a beetle sex pheromone, this chemical was subsequently identified as an odour that attracts P. pacificus (Herrmann et al., 2007). The screen identified worms with mutations in a gene called obi-1. This gene encodes a protein from a family of proteins that are released by diverse nematode species and bind to fatty molecules (lipids). Cinkornpumin, Wisidagama et al. speculate that the OBI-1 protein might function as a part of an extracellular clearance mechanism for lipid odorants. Such a mechanism might be required to detect small changes in odorant concentration and navigate towards the source of the pheromone. Alternately, OBI-1 might function as part of a receptor mechanism in which a chemical cue is first bound to a protein, which then carries …