Presence or absence? Primary structure, regioselectivity and evolution of Δ12/ω3 fatty acid desaturases in nematodes

For vertebrates, the adequate supply of polyunsaturated fatty acids (PUFA) by the diet, in particular omega 3 long-chain PUFA, is considered essential for neural development, growth and reproduction. In contrast to aquatic ecosystems, omega 3 long-chain PUFA apparently are not widely available in the terrestrial food chain. Their de novo synthesis requires the presence of Delta 12 and omega 3 fatty acid desaturase enzymes, which are absent in vertebrates but present, for example, in the nematode Caenorhabditis elegans (FAT-2 and FAT-1). This raises the question if soil-dwelling nematodes offer substantial supply of these valuable nutritional compounds in terrestrial food webs. BLAST searches in available nematode genomes revealed the existence of fat-2 like genes in almost all Glade III-V species, but failed to identify orthologs in Glade I-II nematodes. An additional RT-PCR screen across soil-dwelling nematode species identified six novel fat-2 like genes. Hints for the genetic basis of a omega 3 (fat-1) desaturase activity was found only in selected Glade IV-V species, but not in clades I to III nematodes. Fatty acid pattern analyses following a PUFA-free cultivation and enzymatic characterization of six selected fat-2 or fat-1 like desaturases in yeast confirmed the findings from the genetic approaches. Thus, in similar soil habitats, taxa exist that can synthesize omega 3 long-chain PUFA (as Panagrolaimus, Mesorhabditis and Caenorhabditis) whereas others are unable to do so (Acrobeloides, Cephalobus and Oscheius). While these nematodes do not differ in trophic position or major diet, distinction in reproduction mode may have led to the observed variations in desaturase genes.