Operons are found across multiple kingdoms and phyla, from prokaryotes to 
chordates. In the nematode Caenorhabditis elegans, the genome contains >1000 
operons that compose approximately 15% of the protein-coding genes. However, 
determination of the force(s) promoting the origin and maintenance of operons in 
C. elegans has proved elusive. Compared to bacterial operons, genes within a C. 
elegans operon often show poor coexpression and only sometimes encode proteins 
with related functions. Using analysis of microarray and large-scale in situ 
hybridization data, we demonstrate that almost all operon-encoded genes are 
expressed in germline tissue. However, genes expressed during spermatogenesis 
are excluded from operons. Operons group together along chromosomes in local 
clusters that also contain monocistronic germline-expressed genes. Additionally, 
germline expression of genes in operons is largely independent of the molecular 
function of the encoded proteins. These analyses demonstrate that mechanisms 
governing germline gene expression influence operon origination and/or 
maintenance. Thus, gene expression in a specific tissue can have profound 
effects on the evolution of genome organization.