The researchers discovered how a gene called DUO1 known to control sperm production inside pollen grains of flowering plants, is also used by primitive land plants to produce free-swimming sperm. They found that the gene originated in the stoneworts, an ancient group of aquatic algae that diverged from land plants over 700 million years ago.
The paper suggests that it was a simple change in the DUO1 gene sequence that allowed the algal ancestors of land plants to produce small swimming sperm to increase the chances of fertilization in an aquatic environment.
The DNA sequences of DUO1 were compared in primitive land plants such as liverworts and mosses, with those in freshwater algae. This uncovered quite small genetic changes that allowed the ancestral algal DUO1 protein to recognize a new DNA sequence. This change enabled DUO1 to control a gene network needed to make swimming sperm. The researchers went on to show that liverworts with a mutated DUO1 gene were infertile because they were unable to make active sperm.
Interestingly, the role of DUO1 in plant sperm formation has adapted to the demands of life on land. For example, DUO1 is needed to produce the whiplash flagella used for propulsion of liverwort sperm, but this is not the case in the flowering plant thale cress (Arabidopsis thaliana). Instead, the sperm of flowering plants do not swim and are transported to the egg by a tube that grows out from each pollen grain. In these sperm, DUO1 has taken control of the specialized network of genes needed for the union of sperm and egg.