Comparison of a vitellogenin gene between two distantly related rhabditid nematode species

Three vitellogenin genes from the free-living nematode Caenorhabditis elegans have previously been characterized at the molecular level. In order to study evolutionary relationships within this poorly understood taxon, we have cloned a vitellogenin gene, CEW1-vit-6, from a distantly related species belonging to the same family as C. elegans. Screening of a genomic library with a probe to total poly(A+) RNA yielded three clones that hybridized more intensely than all others, and all three corresponded to a single gene homologous to C. elegans vit-6. Comparison of CEW1-vit-6 with Ce-vit-6 reveals both strong similarities and surprising differences. Life Ce-vit-6, the gene is about 5 kb long and contains four unusually small introns (38-41 nt), but only one interrupts the gene at the same location as a Ce-vit-6 intron. The promoter region contains five matches to Vitellogenin Promoter Element 1 (VPE1) and no matches to VPE2, both previously shown to be required for vit gene transcription in C. elegans. Codon usage is in general similar to that of the Ce-vit genes, but a few codon biases are quite different. Alignment of the CEW1-vit-6 protein with Ce-vit-6 and Ce-vit-2 products suggests the existence of two domains which have evolved at different rates. Sequence comparison shows that nematode vitellogenins are much more closely related to vertebrate than to insect vitellogenins.