Hail To The Hornworts

By Aussiegirl

The hornwort, pictured above, is an unassuming plant that might easily escape notice. Yet, according to this interesting article, it may help resolve long-debated questions about the origin and evolution of land plants.

Hail To The Hornworts

TerraDaily Newsletter
Hail To The Hornworts
by Staff Writers
Ann Arbor MI (SPX) Oct 16, 2006

In the history of life on earth, one intriguing mystery is how plants made the transition from water to land and then went on to diversify into the array of vegetation we see today, from simple mosses and liverworts to towering redwoods.
A research team led by University of Michigan evolutionary biologist Yin-Long Qiu has new findings that help resolve long-debated questions about the origin and evolution of land plants. The work will be published online in the Proceedings of the National Academy of Sciences. [....]

The second event was a key change in plant life cycles. Plants exhibit a phenomenon known as alternation of generations, in which two alternating forms with different amounts of DNA make up a complete life cycle. One form, known as a sporophyte, produces spores, which grow into individuals of the other form, called gametophytes. Gametophytes produce gametes, eggs and sperm which unite to form a fertilized egg capable of becoming a new sporophyte, thus completing a life cycle. While all plants exhibit alternation of generations, some spend most of their life cycle as sporophytes, and others spend more time in the gametophyte phase.

"Early in the history of plant evolution, a shift occurred," said Qiu, an assistant professor of ecology and evolutionary biology. "If you look at the so-called 'lower' plants such as algae, liverworts and mosses, they spend most of their life cycle as gametophytes. But if you look at plants like ferns, pines and flowering plants, they spend most of their time as sporophytes. Geneticists, developmental biologists and evolutionists have been wondering how the switch happened and have put forth two competing hypotheses." [....]

"Basically we captured a few major events that happened in the first few tens of millions of years of land plant evolution," Qiu said. The results make sense in light of the plants' life cycle patterns. Charophyte algae, liverworts and mosses spend most of the cycle in a free-living gametophyte phase; the sporophyte is a small, short-lived organism that lives on the gametophyte.

Vascular plants, on the other hand, spend most of their time as free-living sporophytes, with small, short-lived, gametophytes that often live on the sporophytes. Hornworts may hold a clue to understanding how this shift happened, as they spend most of their life cycle in the gametophyte phase, but their sporophytes---unlike those of liverworts and mosses -- show a tendency to become free-living. [....]