A 'Big Bang' of plant evolution
Dec 07,2007 00:00 by Bend_Weekly_News_Sources

Sci­en­tists are shed­ding light on what Charles Dar­win called an “abom­inable mys­tery”: how and when flow­ers evolved.

In two pa­pers to be pub­lished next week, re­search­ers re­port that two of the larg­est groups of flow­er­ing plants are more closely re­lat­ed to each oth­er - than any of the oth­er ma­jor lin­eages are. These are the mono­cots, which in­clude grasses and their rel­a­tives, and the eu­di­cots, which in­clude sunflow­ers and toma­toes.

New studies indicate the two larg­est groups of flow­er­ing plants are more closely re­lat­ed to each oth­er than any of the oth­er ma­jor lin­eages. These are the mono­cots, which in­clude grasses and their rel­a­tives, and the eu­di­cots, which in­clude sun­flow­ers (above) and toma­toes. (Pho­to courtesy U.S. Nat'l Park Ser­vice)

The sci­en­tists al­so found that a stun­ning di­ver­sifica­t­ion of flow­er­ing plants they call the “Big Bang” hap­pened in the rel­a­tively short span of un­der five mil­lion years—and led to all five ma­jor lin­eages of flow­er­ing plants alive.

“Flow­er­ing plants to­day com­prise around 400,000 spe­cies,” said Pam Soltis Un­ivers­ity of Flor­i­da, a mem­ber of one of the re­search teams. “So to think that the burst that give rise to al­most all of these plants oc­curred in less than five mil­lion years is pret­ty amaz­ing—esp­e­cially when you con­sid­er that flow­er­ing plants as a group have been around for at least 130 mil­lion years.”

Rob­ert Jan­sen, a bi­ol­o­gist the Un­ivers­ity of Tex­as at Aus­tin and member of the second research group, said the two pa­pers set the stage for all fu­ture com­par­a­tive stud­ies of flow­er­ing plants. The new work is to ap­pear in the re­search jour­nal Pro­ceed­ings of the Na­tio­n­al Aca­de­my of Sci­en­ces.

“If you are in­ter­est­ed in un­der­stand­ing the ev­o­lu­tion of flow­er­ing plants, you can’t do that un­less you un­der­stand their rela­t­ion­ships,” he said.

Botanists pre­dat­ing Dar­win no­ticed that flow­er­ing plants, which com­prise at least 60 per­cent of all green plant spe­cies, di­ver­si­fied ab­ruptly shortly af­ter they ap­peared. The de­tails, and es­pe­cially the cause, of this di­ver­sifica­t­ion—Dar­win’s “abom­inable mys­tery”—have been a hot top­ic in botany ev­er since. The speed of the di­ver­sifica­t­ion is “one of the rea­sons why it’s been hard to un­der­stand ev­o­lu­tion­ary rela­t­ion­ships among the ma­jor groups of flow­er­ing plants,” Jan­sen said.

Re­search­ers with the two un­ivers­i­ties an­a­lyzed DNA from plant chloro­plasts, the cel­lu­lar com­part­ments re­spon­si­ble for plants’ abil­ity to con­vert sun­light in­to sug­ar. Jan­sen and col­leagues at his un­ivers­ity an­a­lyzed DNA se­quences of 81 genes from chloro­plasts of 64 plant spe­cies; Un­ivers­ity of Flor­i­da re­search­ers an­a­lyzed 61 genes from 45 spe­cies. The two groups al­so per­formed a com­bined anal­y­sis.

Through this pro­cess they grad­u­ally built a kind of family tree for plants, a di­a­gram of rela­t­ion­ships among lin­eages show­ing di­ver­sifica­t­ion over the eons. Based on known rates of ge­net­ic change checked against fos­sils of known ages, they es­tab­lished a time scale that showed es­ti­mat­ed dates of ma­jor ev­o­lu­tion­ary branch­ing events that pro­duced new spe­cies.

Pre­vi­ous re­search had found that flow­er­ing plants split in­to three branches shortly af­ter ap­pear­ing. That pro­cess was grad­u­al, at least com­pared with the rap­id radia­t­ion, or branch­ing out, that fol­lowed. The de­tails of that radia­t­ion have been murky. The lat­est stud­ies clar­i­fy the pic­ture by show­ing that all plants fall in­to five ma­jor lin­eages that de­vel­oped over five mil­lion years or less, re­search­ers said.

The di­ver­sifica­t­ion’s cause re­mains mys­te­ri­ous, said the Un­ivers­ity of Flor­i­da’s Pam and Doug Soltis. It could have been spurred by some ma­jor cli­mat­ic event, they said. It’s al­so pos­si­ble, they added, that a new ev­o­lu­tion­ary trait—a wa­ter-conducting cell that trans­fers wa­ter up plant stems—proved so ef­fec­tive that it trig­gered a flour­ish­ing of new spe­cies.

Courtesy University of Florida and World Science staff