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Little genomes for big dinosaurs

March 7, 2007
Courtesy Harvard University
and World Science staff

They might be gi­ants, but many di­no­saurs ap­par­ent­ly had ge­nomes no larg­er than that of a mod­ern hum­ming­bird. So say bio­lo­g­ists who’ve gauged the ge­nome sizes of 31 spe­cies of ex­tinct di­no­saurs and birds, their de­scen­dants.

Al­losaurus, a typ­i­cal large the­ro­pod di­no­saur. The­ro­pods also in­clude T. rex, as well as birds, the pre­sumed des­cen­dants of some of these di­no­saurs. (Cour­te­sy Utah Pub­lic Pi­o­neer)
This sug­gests a stripped-down ge­nome may have been one fea­ture that helped birds take flight, by sav­ing them en­er­gy, ac­cord­ing to the re­search­ers.

They estimated ge­nome sizes us­ing a pre­v­i­ous­ly not­ed re­la­tion­ship be­t­ween those, and the size of bone cells.

“We see dis­tinct dif­fer­ences be­t­ween two ma­jor lin­e­ages of di­no­saurs,” said Chris Or­gan of Har­vard Uni­ver­si­ty in Cam­b­ridge, Mass., one of the sci­en­tists, who re­por­t­ed the find­ings in the March 8 is­sue of the re­search jour­nal Na­ture.

“The­ro­pod­s—car­nivores such as Ty­ran­no­saur­us rex and Ve­loci­rap­tor—have very small ge­nomes, in the range of mod­ern birds. Or­nithis­chi­ans—which in­clude Steg­o­saur­us and Tricer­atops—had more mod­er­ate­ly sized ge­nomes, akin to those of liv­ing lizards and crocodil­ians. We aren’t sure about the ge­nomes of the long-necked sauropods yet.”

Re­search­ers had pre­vi­ously reported that the sizes of var­i­ous cell types tend to re­flect an or­gan­is­m’s ge­nome size. Or­gan and Ed­wards had found this al­so ap­plies to bone cells called os­teo­cytes. Since these lie in small pock­ets in bone, the size of the pock­ets be­trays that of the cell­s—let­ting the in­ves­ti­ga­tors take a meas­ure of ex­tinct spe­cies’ ge­nomes by stu­dying their fos­sils.

 Or­gan said the clear di­chot­o­my in di­no­saur ge­nome sizes is like­ly due to dif­fer­ent amounts of re­pet­i­tive gene se­quences, or se­quen­ces of a type some­times dubbed “junk DNA” be­cause they seem to be in­ac­tive. Those two fac­tors large­ly ac­count for var­i­a­tion in ge­nome size across an­i­mal spe­cies, he not­ed.

The findings indicate that the spare ge­net­ic make­up of birds—which have re­mark­ably small ge­nomes—evolved in di­no­saurs some 230 to 250 mil­lion years ago, rath­er than when mod­ern birds emerged just 110 mil­lion years ago, Or­gan and col­leagues said. They sug­gest­ed that af­ter this shrink­age, the­ro­pod ge­nome size sta­bi­lized for hun­dreds of mil­lions of years, con­tin­u­ing to the pre­s­ent in birds. 

The work re­futes a the­o­ry that birds’ di­min­u­tive ge­nomes “co-evolved with flight,” said Har­vard’s Scott Ed­wards, who with Or­gan led the stu­dy. “These stream­lined ge­nomes arose long be­fore the first birds and flight, and can be added to the list of di­no­saur traits pre­vi­ously thought to be found on­ly in mod­ern birds, in­clud­ing feath­ers, pul­mo­nary in­no­va­tions, and pa­ren­tal care and nest­ing.”

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They might be giants, but many dinosaurs apparently had genomes no larger than that of a modern hummingbird. So say scientists who’ve gauged the genome sizes of 31 species of extinct dinosaurs and birds, descendants of dinosaurs. This suggests a stripped-down genome may have been one feature that helped birds take flight, by conserving energy, according to the researchers, who gauged genome sizes using a previously noted relationship between the size of a genome and of that bone cells. “We see distinct differences between two major lineages of dinosaurs,” said biologist Chris Organ of Harvard University in Cambridge, Mass., one of the scientists, who report the findings in the March 8 issue of the research journal Nature. “The theropods—carnivores such as Tyrannosaurus rex and Velociraptor—had very small genomes, in the range of modern birds. Ornithischians—which include Stegosaurus and Triceratops—had more moderately sized genomes, akin to those of living lizards and crocodilians. We aren’t sure about the genomes of the long-necked sauropods yet.” Organ said the clear-cut dichotomy in dinosaur genomes is likely due to different amounts in each of gene sequences that are repetitive, or of a type sometimes dubbed “junk DNA” because they seem to be inactive. Those two factors are largely responsible for variation in genome size across animal species, he noted. His team estimated that active, repetitive DNA might have comprised an average 12 percent of the ornithischian genome but just 8.4 percent in theropods. The work, they added, indicates the spare genetic makeup of birds—which have remarkably small genomes—evolved in dinosaurs some 230 to 250 million years ago, rather than when modern birds emerged just 110 million years ago. Organ’s team suggest after this shrinking, theropod genomes then stabilized in size for hundreds of millions of years, continuing to the present in birds. The work refutes a theory that birds’ diminutive genomes “co-evolved with flight,” said Harvard’s Scott Edwards, who with Organ led the study. “These streamlined genomes arose long before the first birds and flight, and can be added to the list of dinosaur traits previously thought to be found only in modern birds, including feathers, pulmonary innovations, and parental care and nesting.” Researchers had previously noted that the sizes of various cell types tend to reflect the size of an organism’s genome. Organ and Edwards had found this also applies to bone cells called osteocytes. Since these lie in small pockets in bone, the size of the pockets betrays that of the cells—allowing the invest igators to take a measure of extinct species’ genomes by studying their fossils.