Is retinoic acid a signal for nerve regeneration in insects

Until quite recently the retinoic acid （RA） signaling pathway was believed to occur only in vertebrates, but in the last decade, several ground-breaking studies have shown that it also exists in invertebrates. These include the discovery of endogenous retinoids （Nowickyj et al., 2008）, of retinoid receptor orthologs in insects （Hayward et al., 1999） and the very specific effect of RA on axonal guidance in the fresh water snail Lymnaea stagnalis （Dmetrichuk et al., 2008）. Despite some earlier indications of RA-dependent gene activation in insects, these papers really started retinoid research in invertebrates.