An intriguing mystery for the evolutionary biology is how aquatic algae made the transition from water onto land to diversify after that into a broad array of vegetation we see today, from mosses and liverworts to over one hundred meters tall tree. "Two major steps kicked off the chain of events that helped land plants prosper, forming the basis for modern land-based ecosystems and fundamentally altering the course of evolution of life on earth," said Yin-Long Qiu, assistant professor of ecology and evolutionary biology at University of Michigan.
First, the land was colonized by descendants of a group of aquatic algae known as charophyte (living species are stoneworts, brittleworts or skunkweed). The plants entered a new world where the sun's light was undiminished by passage through water and where carbon dioxide, an essential nutrient for photosynthetic plants, was abundant.
The second event was a crucial change on the plant life cycle. Green algae from which land plants evolved exhibit alternation of generations, in which two alternating forms with different amounts of DNA make up a complete life cycle. The sporophyte produces
spores and is diploid (has a double quantity of DNA because it has a double number of chromosomes so every gene has a pair). Spores grow into gametophytes which are haploids (have a single set of chromosomes, that means half of the DNA quantity in sporophytes). Gametophytes produce gametes (female: eggs and male: sperm) which through fecundation form a fertilized egg capable of becoming a new sporophyte, thus completing a life cycle.
Some types of plants 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.
"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."
Qiu's team compared three complementary sets of genetic data used, involving more than 700 gene sequences, of the four major lineages of land plants: liverworts ( formerly classified as Hepaticae mosses), mosses, hornworts (photo)(primitive plants formerly classified amongst mosses) and vascular plants (which include ferns, gymnosperms and flowering plants).
The analysis proved that liverworts, tiny green, plate-like plants often found in wet places along river banks, represent the first lineage that diverged from other land plants when ancient charophyte first emerged onto land, and the obscure group of the hornworts, often found in abandoned corn fields, represents the ancestors of the vascular plants. "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 plants' life cycle patterns plead with the genetic results. Charophyte, liverworts and mosses are predominantly gametophytes (spend most of the cycle in a free-living gametophyte phase); the sporophyte is a small, short-lived organism that lives on the gametophyte. But vascular plants are predominantly sporophytes, with small, short-lived, gametophytes that often live on the sporophytes.
The sporophyte, being diploid, has a far greater adapting power because more genes means more variants to the challenges of the environment.
That's the way vascular plants dominate land flora. Hornworts are predominantly gametophytes, but their sporophytes present a tendency to become free-living. This offers a clue on how the shift happened more than 400 millions ago during the Paleozoic era.
"Understanding evolutionary relationships among plant groups is crucial to understanding their biology, just as understanding relationships among primates advances our knowledge of human behavior, anatomy and physiology," Qiu said.
"Understanding evolutionary history really is the foundation of biology, and with today's emphasis on biofuels and medically important plants, it should be clear how important it is to learn the evolutionary history of all the organisms on our planet."
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