Gene dis­cov­ery may re­veal how scaly di­nosaurs be­came feath­ery birds

The Guardian Australia - - Science - Dr Dave Hone

It is fi­nally be­com­ing com­mon knowl­edge to the gen­eral pub­lic that birds really are di­nosaurs. What’s more, an ever-in­creas­ing num­ber of dis­cov­er­ies gives us in­cred­i­ble in­sight into the form and di­ver­sity of feath­ers in var­i­ous non-avian di­nosaurs and early birds. We have a grow­ing un­der­stand­ing of how feath­ers spread and changed in var­i­ous lin­eages, their func­tions, and why they might have evolved in the first place but a fun­da­men­tal gap re­mains in our un­der­stand­ing – how did they evolve?

Feath­ers are com­posed of ker­atin, which also makes up scales (and for that mat­ter claws and parts of skin – and, in­deed, our own hair) and they are both growth of the skin, so pre­sum­ably they have some kind of shared evo­lu­tion­ary his­tory – but that is about as far as re­searchers have got. There are a num­ber of sug­gested path­ways to get from scale to feather, but while some are well thought of, none are es­pe­cially well thought of. Com­pli­cat­ing the process are the odd pat­terns that evo­lu­tion has thrown up from time to time.

Al­though mod­ern birds have scaly feet, th­ese are, in fact, very heav­ily mod­i­fied feath­ers and not some hold-over from their di­nosaurian an­ces­try. Var­i­ous di­nosaurs and early birds show that they were com­pletely cov­ered in feath­ers – even down to their toes – and had pre­sum­ably ex­changed scales for feath­ers at this point. Only later did “scales” reap­pear on the feet of birds, so sadly their own feath­ers can­not im­me­di­ately tell us about how scales might have trans­formed.

In 2014, a new and very un­usual di­nosaur called Kulin­dadromeus was un­earthed in Siberia, which ap­peared to show not just feather-like fil­a­ments, but that some of th­ese were grow­ing out of scales. Al­though this lit­tle an­i­mal is from a very dif­fer­ent lin­eage that that which gave rise to birds, it sug­gests at least that some feather-like struc­tures might have evolved di­rectly from highly mod­i­fied scales.

En­ter a new study pub­lished last week (Wu et al., 2017), which looks at how scales and feath­ers form. The re­searchers took genes thought to be im­por­tant in feather de­vel­op­ment and had them ex­pressed in em­bryos of al­li­ga­tors and chick­ens dur­ing the de­vel­op­ment of scales and feath­ers re­spec­tively. They also iden­ti­fied some new genes that reg­u­late the genes for de­vel­op­ment and changed their de­grees of ac­tiv­ity to fur­ther al­ter th­ese changes.

In mod­i­fy­ing th­ese var­i­ous genes, they were able to pro­duce new kinds of mod­i­fied scales. True, th­ese were not feath­ers from al­li­ga­tors, but they do show that rel­a­tively sim­ple changes to a few genes can cause the early de­vel­op­ment of scales even in mod­ern al­li­ga­tors to pro­duce things with much in com­mon with the an­ces­tral feath­ers that we see in early non-avian di­nosaurs. In short, it’s not a mas­sive step from th­ese to some­thing like a true early feather, not least when you con­sider that the al­li­ga­tor lin­eage split from that of birds over 250m years ago. Cou­ple that with the fact than any early “proto-feath­ers” that con­ferred a ben­e­fit on their own­ers would be un­der nat­u­ral se­lec­tion to re­tain and re­fine them, and it’s not a big jump to sug­gest that feath­ers may have formed rel­a­tively eas­ily.

Mod­i­fi­ca­tion of the genes in the chick­ens pro­duced a va­ri­ety of feather forms, in­clud­ing those seen in var­i­ous di­nosaurs, so the gap from scale to feather is at least partly bridged from the other side by this study. Some of th­ese too match the the­o­ret­i­cal steps be­tween feath­ers and scales that have been pro­posed, and this sug­gests that we have been on the right track for a while, if lack­ing in some sup­port­ing ev­i­dence which is now be­ing dis­cov­ered.

Clearly there is some way to go, but the iden­ti­fi­ca­tion of only a few genes, still present in mod­ern crocodil­ians such as al­li­ga­tors, that can be changed quite eas­ily into rather sim­ple feather-like fil­a­ments is most in­trigu­ing. There is a huge di­ver­sity in the feath­ers seen in the di­nosaurs on the run-up to birds, as well as those in other di­nosaur lin­eages and their rel­a­tives the fly­ing pterosaurs, and this shows the pos­si­bil­i­ties when it comes to fil­a­ments. A few more tweaks to th­ese genes might show us the com­plete path­ways from scale to feather – and then the real ques­tion of how birds be­came birds and first grew their feath­ers can, per­haps, be an­swered.

Ref­er­ences:

Wu, P., Yan, J., Lai, Y.C., Ng, C.S., Li, A., Jiang, X., Elsey, R., Widelitz, R., Ba­j­pai, R., Li, W.H. and Chuong, C.M., 2017. Mul­ti­ple reg­u­la­tory mod­ules are re­quired for scale-to-feather con­ver­sion. Molec­u­lar Bi­ol­ogy and Evo­lu­tion.

Feath­ers are syn­ony­mous with birds, but ul­ti­mately de­rive from the scales of their di­nosaurian an­ces­tors. Pho­to­graph: Tahir Ab­bas/Getty Images/iS­tock­photo

Var­i­ous feather types are seen in numer­ous fos­sils such as th­ese on the arm of the di­nosaur An­chior­nis Pho­to­graph: David Hone

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