Lancelets, which are primitive chordates, possess only one homologue of "Drosophila" "Hh" (figure 7). Vertebrates, on the other hand, have several Hedgehog ligands that fall within three subgroups – "Desert", "Indian" and "Sonic", each represented by a single mammalian gene. This is a consequence of the two rounds of whole genome duplication that occurred early in the vertebrate evolutionary history. Two such events would have produced four homologous genes, one of which must have been lost. Desert Hedgehogs are the most closely related to "Drosophila" "Hh". Additional gene duplications occurred within some species such as the zebrafish "Danio rerio", which has an additional "tiggywinkle hedgehog" gene in the "sonic" group. Various vertebrate lineages have adapted hedgehogs to unique developmental processes. For example, a homologue of the "X.laevis" "banded hedgehog" is involved in regeneration of the salamander limb.