Ancestors of land plants revealed
New research published April 17, 2011 at the open access journal BMC Evolutionary Biology shows that the closest relatives to land plants are conjugating green algae such as Spirogyra. That’s in contrast to the previous idea that land plants evolved from stonewort-like algae.
Ancestors of green plants began to colonize the land about 500 million years ago and it is generally accepted that they evolved from streptophyte algae (a group of green freshwater algae). But this group of algae is very diverse and currently ranges from simple one cell flagellates to more complex branching algae such as stoneworts (Chara).
It was thought that Charales were the closest relatives to land plants because they share (among other characteristics) a similar method of fertilization – oogamy, with a large egg and small swimming sperm. For flowering plants this sperm is contained within pollen grains. In contrast, another type of streptophytes, the Zygnematales, use conjugation, a method of reproduction where the gametes are of equal size (isogamy), and one or both crawl, amoeba-like, into a fertilization tube where they meet and fuse.
Some phylogenetic analysis had been done previously on a smaller number of genes, which seemed to support the Charales theory. However, a multinational team, involving researchers in Germany and Canada, analyzed genetic divergence in 129 genes from 40 different green plant taxa. This data showed that, despite the differences in reproductive strategy, the closest living relatives to land plants are in fact the Zygnematales. A co-author on the new study, Dr. Burkhard Becker, explained:
It seems that Zygnematales have lost oogamy and their ability to produce sperm and egg cells, and instead, possibly due to selection pressure in the absence of free water, use conjugation for reproduction. Investigation of such a large number of genes has shown that, despite their apparent simplicity, Zygnematales have genetic traces of other complex traits also associated with green land plants. Consequently Zygnematales’ true place as the closest living relative to land plants has been revealed.
Bottom line: Land plants were thought to have evolved from stonewort-like algae. A study published April 17, 2011 in BMC Evolutionary Biology shows that the closest relatives to land plants are conjugating green algae such as Spirogyra.