Editing Graptolite (section)

==Graptolites in evolutionary development==
[[File:Pterobranch gonal asymmetry.png|thumb|300px|left|Left and right gonads (g) in ''[[Rhabdopleura compacta]]''.]]

In recent years, living graptolites have been used as a hemichordate model for [[Evo-Devo]] studies, as have their sister group, the [[acorn worms]]. For example, graptolites are used to study asymmetry in hemichordates, especially because their gonads tend to be located randomly on one side. In ''[[Rhabdopleura normani]]'', the [[testicle]] is located asymmetrically, and possibly other structures such as the oral lamella and the [[gonopore]].<ref name= Sato8>Sato, A. & Holland, P. (2008). Asymmetry in a Pterobranch Hemichordate and the Evolution of Left-Right Patterning. ''Developmental Dynamics'', 237:3634 –3639)</ref> The significance of these discoveries is to understand the early vertebrate left-right [[asymmetry]] due to [[chordates]] being a sister group of hemichordates, and therefore, the asymmetry might be a feature that developed early in [[deuterostomes]]. Since the location of the structures is not strictly established, also in some [[enteropneusta|enteropneusts]], it is likely that asymmetrical states in hemichordates are not under a strong developmental or evolutionary constraint. The origin of this asymmetry, at least for the gonads, is possibly influenced by the direction of the basal coiling in the tubarium, by some intrinsic biological mechanisms in pterobranchs, or solely by environmental factors.<ref name="Sato8" />

''[[Hedgehog signaling pathway|Hedgehog]]'' (hh), a highly conserved gene implicated in neural developmental patterning, was analyzed in Hemichordates, taking ''Rhabdopleura'' as a pterobranch representative. It was found that hedgehog gene in pterobranchs is expressed in a different pattern compared to other hemichordates as the [[enteropneust]] ''[[Saccoglossus kowalevskii]]''. An important conserved glycine–cysteine–phenylalanine (GCF) motif at the site of autocatalytic cleavage in hh genes, is altered in ''R. compacta'' by an insertion of the [[amino acid]] [[threonine]] (T) in the N-terminal, and in ''S. kowalesvskii'' there is a replacement of [[serine]] (S) for [[glycine]] (G). This mutation decreases the efficiency of the autoproteolytic cleavage and therefore, the signalling function of the protein. It is not clear how this unique mechanism occurred in evolution and the effects it has in the group, but, if it has persisted over millions of years, it implies a functional and genetic advantage.<ref name= Sato2009>Sato, A., White-Cooper, H., Doggett, K. & Holland, P. 2009. Degenerate evolution of the hedgehog gene in a hemichordate lineage. ''Proceedings of the National Academy of Sciences'', 106(18):7491-7494.</ref>