Evolution of Plants that Conquered the Land
News comes this morning of another discovery in evolutionary history, and it's a big one. In essence, the genes that control the development of root hairs in plants have been found in mosses. In the mosses, the genes serve a different purpose. Far back in history, these genes were recruited into service to develop the first true roots, possibly at the same time that plants made the switch from a haploid condition (one set of genes per cell) to diploid (two sets of genes per cell). Suffice to say that the world would be a very different place today had this change never occurred.
Moss Genes Provided Fuse For Plant Life Explosion
Scientists from the John Innes Centre have identified the genes that control the development of root hairs on plants. Published in the journal Science, Professor Liam Dolan reports that these genes are also found in moss, a finding that changes our understanding of how the plants we see today evolved over 400 million years ago...
Not all plants have roots. Evolutionarily ancient plants like mosses instead grow cells called caulonema and rhizoids. Caulonemal cells increase the surface area for nutrient absorption, and rhizoids provide anchorage. The scientists found that the genes that control root hair growth are very similar to the genes that regulate the development of caulonema and rhizoids in the moss Physcomitrella patens.
In fact, they were able to replace the genes they turned off in plants with the equivalent genes from moss, and produce hairy roots...
The number of chromosomes represents one of the major differences between mosses and other land plants. Moss exists with one chromosome for the majority of its lifecycle; only during its reproductive stage does it have two copies of its chromosomes. The plants that evolved from these organisms have pairs of chromosomes...
With this change in the dominant part of the life cycle came an enormous increase in the size and diversity of plants known as the Devonian explosion, which started around 400 million years ago. The great variety of plant life that we see today evolved during this period of time. The invasion of the land by these plants fundamentally changed the existing ecosystems, and brought about pronounced climate change...
Reference: An Ancient Mechanism Controls the Development of Cells with a Rooting Function in Land Plants. Benoît Menand, Keke Yi, Stefan Jouannic, Laurent Hoffmann, Eoin Ryan, Paul Linstead, Didier G. Schaefer, Liam Dolan. Science 8 June 2007: Vol. 316. no. 5830, pp. 1477 – 1480 DOI: 10.1126/science.1142618
Discoveries like this one are near and dear to my heart. One of the things I've often wondered about is the common ancestor of the Bryophytes (especially the Hepatophyta), fungi, and syphonous marine algae such as Acetabularia. There are some striking morphological similarities between the three groups that make them seem to me like a good paraphyletic grouping in which to look for some deep common progenitor. The close-in ancestors of the syphonous algae, for example, are documented from the fossil record some 570 mya, practically back to the Precambrian, thanks to their habit of building calcareous support structures. We're thus talking about some of life's deepest homologies, and that automatically makes it all incredibly fascinating — and probably one more of those multitudes of things I'll never have time to explore for myself!
