One reason it has been so challenging to accurately classify the protists is because many share similar characteristics despite not sharing a recent common ancestor. Many also share characteristics with fungi, plants and animals. Protists that have morphological features similar to other organisms may have evolved these structures because of similar selective pressures—rather than because of recent common ancestry. This phenomenon is called convergent evolution. Convergent evolution is a process whereby distantly related organisms independently evolve similar traits to adapt to similar environmental pressures.
For example, flight has evolved in bats, birds and insects, and all three have structures we refer to as wings, which are adaptations to flight. However, bat, bird and insect wings have evolved from very different original structures. The trait in the different organisms came to be similar in structure and have the same function, flying, but did so separately from each other.
Amongst the protists, we see many examples of convergent evolution. One striking example involves the apparent similarity between the fungi and a group of protists known as the oomycetes.
The oomycetes include organisms known as water molds and downy mildews. Most are heterotrophic decomposers that feed on dead and decaying organic matter in aquatic and terrestrial environments, but some attack living plants and animals. The name "oomycete" means "egg fungi," which is a reference to the reproductive structures of sexually reproducing oomycetes.
Although oomycetes are not closely related to fungi, the two groups have some similarities. In particular, both groups are heterotrophs that break down food externally and then absorb nutrients from their surroundings (we will learn more about the fungi later in the course). Additionally, many of the multicellular oomycetes form hyphae, which are very similar to fungal hyphae though the structures are not identical.
However, there are more differences than similarities. For example, the cell walls of oomycetes are composed of cellulose, not chitin (the compound found in the cell walls of fungi). Unlike true fungi, which are haploid in the feeding stage (and the majority of their life cycle), the hyphae of oomycetes are composed of diploid cells.
The Ecological and Economic Importance of the Oomycetes
Oomycetes have been responsible for a number of catastrophic historic events, including the outbreak of downy mildew that nearly wiped out the French wine industry in the late nineteenth century. Plasmopara , a native of the New World, was inadvertently brought to France from the United States in 1870 in a shipment of American grape root stock. It quickly became a devastating problem. A mixture of lime and copper sulfate, the first chemicals used to combat a plant pathogen, successfully treated grapes affected by this organism.
The most devastating event linked to an oomycete (by an organism know as Phytophthora infestans , or potato blight) is the Great Potato Famine, which killed nearly one-million people in Ireland in the late 1800s and drove one million more out of the country. Phytophthora secretes enzymes that break down leaf and stem tissue which kills plants rapidly. Tubers can become infested, turning soft and black, seemingly overnight. Potato fields in Ireland became infested with Phytophthora , which thrived in the cool damp climate of Ireland and wiped out nearly the entire country's crop in one week. In nineteenth-century Ireland, potatoes were the primary food source of the poorest classes, who often grew nothing other than potatoes. The loss of this staple crop led to mass starvation.
Phytopthera infestans is still an important plant disease that impacts both potato and tomato crops. The Great Potato Famine shows the dangers of relying on a single main food source.