Evolution of ‘front-end’ desaturases in Echium (Boraginaceae)

‘Front-end’ desaturases of higher plants catalyse the desaturation of either fatty acids attached to phospholipids (Δ6-desaturases), or the long-chain base of sphingolipids (Δ8-desaturases). In a few plant families like the Boraginaceae, a Δ6-desaturase activity is responsible for the synthesis of unusual fatty acids like the γ-linolenic acid (18:3n − 6) or the octadecatetraenoic acid (18:4n − 3). Due to their sporadic appearance in nature, the biological role of these fatty acids has been questioned, and remains unknown. In this paper we describe a Δ6-desaturase related gene (D6DESΨ) from Echium (Boraginaceae), which likely represents a pseudogene recently originated in this evolutionary lineage. This finding allowed us to check whether selective pressure is acting on the gene encoding the functional desaturase (D6DES) that is responsible for the synthesis of GLA/OTA. Determination of the substitution rates for the D6DES/D6DESΨ pairs from three Echium species shows that they are considerably higher for D6DESΨ genes than for their functional counterparts. This provides further support for a possible biological role of the products of the active paralogues.