Genetics of entomopathogenic nematodes

Entomopathogenic Nematodes (EPNs) are classified into Steinernematidae and Heterorhabditidae families and have great potential as biological control agents against insect pests. These nematodes are soil organisms, which symbiotically associate with the bacterium Xenorhabdus and Photorhabdus, respectively. A recent genetic-based classification places the Steinernematidae within the suborder Tylenchina, which also includes insect parasitic allantonematids and neotylenchids. The Heterorhabditidae were positioned within the suborder Rhabditina which includes, among others, free-living Rhabditidae and animal parasitic Strongylidae. Despite the unknown systematic status for the majority of nematode and bacterial taxa, nematode-bacterium entomopathogens are some of the best-studied members of these highly diverse groups of organisms. Genome sizes are estimated at 2.3 × 10⁸ bp in S. carpocapsae and 3.9 × 10⁷ bp in H. bacteriophora. Repetitive DNA content represents 39% and 51% of these respective genomes. Compared with animal and plant nematodes, EPN genomics is running slowly. In June 2005, it was announced the targeting of H. bacteriophora for high quality coverage, but until now the sequencing project is still in process. S. carpocapsae genome sequencing project was recently put up by Rafael Montiel and Nelson Simoes, and financed by a Mexican agency. However, functional gene libraries construction and annotation is a helpful platform to provide a basis for genomic studies, accelerating research towards a better understanding of the events that occur in the parasitic process of EPNs, including those mechanisms involved in symbiosis with bacteria. cDNA and SSH libraries have been done for both S. carpocapsae and H. bacteriophora, and used for gene cloning and screens to isolate new gene activities. Serine proteases, chitinases, metalloproteases, protease inhibitors and other molecules have been described. All this information is enhancing the use of ENPs as model organisms, as part of a tripartite model, to study both symbiosis and parasitism.