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A “Big Bang” of plant evolution

Nov. 26, 2007
Courtesy University of Florida
and World Science staff

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 the two 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.

* * *

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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 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 and tomatoes. 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, said the two papers, to appear in the research journal pnas, set the stage for all future comparative studies of flowering plants. “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 the researchers 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 they appeared about 130 million years ago. 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 that a new evolutionary trait—a water-conducting cell that transfers water up plant stems—proved so effective that it spurred a flourishing of new species. The cell is either lacking or poorly developed in the first three flowering plant lineages, Doug Soltis said. The earliest lineages, added Pam Soltis, also did not have a completely fused ovary, which in later flowering plants may better protect the seeds.