In the past few years, an increasing number of draft genome sequences of multiple free-living and parasitic nematodes have been published. Although nematode genomes vary in size within an order of magnitude, compared with mammalian genomes, they are all very small. Nevertheless, nematodes possess only marginally fewer genes than mammals do. Nematode genomes are very compact and therefore form a highly attractive system for comparative studies of genome structure and evolution. Strikingly, approximately one-third of the genes in every sequenced nematode genome has no recognisable homologues outside their genus. One observes high rates of gene losses and gains, among them numerous examples of gene acquisition by horizontal gene transfer. Not only does the ‘gene for parasitism’ not exist, but also there appear to be no common genomic characteristics of parasitic nematode genomes which would distinguish them from genomes of free-living nematodes.

Key Concepts:

• Nematode genomes tend to be compact.
  
• Nematode genomes vary in their gene composition due to extensive gene gain and loss.
  
• Genes are lost through gene deletion or rapid evolutionary change beyond the point where they can be recognised as homologous to a gene in another species.
  
• Genes are acquired through gene duplication, de novo formation and horizontal gene transfer.
  
• Horizontal gene transfer allows nematode species to acquire new physiological properties.
  
• All nematode genomes sequenced so far contain operons, multigene transcription units giving rise to a single pre-mRNA, which is broken up into single protein coding mRNAs by trans-splicing and polyadenylation.
  
• Within the nematodes parasitism has arisen multiple times independently and a ‘gene for parasitism’ or unifying parasite genomic features were not identified.