13 novembre 2003
Comment procède l’évolution ?

Référence :
Nature 426, 176 - 178 (13 novembre 2003)
Allele substitution at a flower colour locus produces a pollinator shift in monkeyflowers
H. D. Bradshaw Jr, Douglas W. Schemske

Résumé :
Le rôle des mutations dans l’évolution est débattu depuis plus d’un siècle. On considère en général que les mutations adaptatives sont en nombre quasi-infini avec des effets phénotypiques infinitésimaux. Les auteurs montrent que d’autres processus sont envisageables. Ils ont ainsi examiné le phénomène de pollinisation de deux Scrofulariacées, M. lewisii et M. cardinalis. Le gène YUP contrôle le dépôt de pigments caroiténoïdes qui donnent une couleur rose à la première, habituellement pollinisée par des bourdons, et rouge à la seconde, pollinisée par des oiseaux-mouches. Or, la mutation dirigée du gène YUP aboutit à un changement de coloris suivie d’un changement de pollinisateurs. Au cours de l’évolution, si les deux plantes ont été présentes dans des environnements où les deux espèces pollinisatrices étaient en répartition inégale, une mutation unique a pu avoir des effets importants. Il se peut donc que des mutations majeures aient un effet évolutif quasi-immédiat, plutôt que des seules micro-mutations aux effets très graduels.

Abstract :
The role of major mutations in adaptive evolution has been debated for more than a century. The classical view is that adaptive mutations are nearly infinite in number with infinitesimally small phenotypic effect, but recent theory suggests otherwise. To provide empirical estimates of the magnitude of adaptive mutations in wild plants, we conducted field studies to determine the adaptive value of alternative alleles at a single locus, YELLOW UPPER (YUP). YUP controls the presence or absence of yellow carotenoid pigments in the petals of pink-flowered Mimulus lewisii, which is pollinated by bumblebees, and its red-flowered sister species M. cardinalis, which is pollinated by hummingbirds. We bred near-isogenic lines (NILs) in which the YUP allele from each species was substituted into the other. M. cardinalis NILs with the M. lewisii YUP allele had dark pink flowers and received 74-fold more bee visits than the wild type, whereas M. lewisii NILs with the M. cardinalis yup allele had yellow-orange flowers and received 68-fold more hummingbird visits than the wild type. These results indicate that an adaptive shift in pollinator preference may be initiated by a single major mutation.

© Nature

Commentaire Eurekalert (www.eurekalert.org)

Research sheds new light on evolution

For more than a century, scientists have concluded that a species evolves or adapts by going through an infinite number of small genetic changes over a long period of time.
However, a team of researchers, including a Michigan State University plant biologist, has provided some new evidence that an alternate theory is actually at work, one in which the process begins with several large mutations before settling down into a series of smaller ones.
The research is published in the Nov. 13 issue of the journal Nature.
"The question is asked, 'If a population finds itself in some maladaptive state, due perhaps to a change in climate, how will it adapt?'" said Douglas Schemske, MSU Hannah Professor of Plant Biology and a member of the research team. "The evidence that has come to light recently ñ both in plants and other organisms ñ is that the initial changes are bigger than we might have expected."
To study the question, Schemske and his colleagues used a common plant called the monkeyflower, changing its genetic make up in a rather dramatic way to see if it would attract new pollinators - hummingbirds instead of bees or vice versa.
By moving a small piece of the genome between two different species of the plants - the pink-flowered M. lewisii and the red-flowered M. cardinalis - the researchers created different colored flowers that attracted new pollinators.

"We discovered that moving this single genetic region caused a dramatic increase in visitation by a 'new' pollinator," Schemske said. "Specifically, the orange flowers produced on the previously pink flowered and bee-pollinated M. lewisii were regularly visited by hummingbirds but shunned by bees.
"Also, the pink flowers of the previously hummingbird-pollinated M. cardinalis were attractive to both bees and hummingbirds," he said.
Schemske and H.D. "Toby" Bradshaw, a professor of biology at the University of Washington and the lead author of the paper that appeared in Nature, said altering the genetic region responsible for the flowers' color is much like what could happen during a naturally occurring mutation.
"Perhaps a single mutation having to do with color changed the pollinator milieu back when there was only a single species," Bradshaw said. "That one big evolutionary step may then have been followed by many smaller steps triggered by pollinator preferences that led ultimately to different species."
Schemske compared the process to the repairing of a finely tuned watch.
"In our model, the first adaptive adjustments might require big changes, similar to banging the broken watch a few times before making the final small tweaks to restore its optimal performance," he said.
The plants used in the work were produced in a campus greenhouse and then transported to an area near the Yosemite National Park where natural populations of both species occur.
"This was a rather unique aspect of the work," Schemske said, "in that it combined molecular genetic techniques and ecological observations to elucidate the process of adaptation in natural populations."