THE LIVING WORLD

Unit Four. The Evolution and Diversity of Life

17. Protists: Advent of the Eukaryotes

17.5. The Base of the Protist Tree

Among the 17 phyla of living protists, two unicellular groups seem more closely linked to early eukaryotes, at the very base of the protist phylogenetic tree. Both groups possess flagella, but lack mitochondria. Because mitochondrial genes are found in their nuclei, it seems that these two groups lost their mitochondria, rather than never having had them.

Diplomonads Have Two Nuclei

Diplomonads propel themselves through the water with flagella, and are unusual in that each single-celled individual has two nuclei. The diplomonad Giardia intestinalis, a common parasite, passes from one human to another via feces-contaminated water, causing diarrhea.

Parabasalids Swim with Undulating Membranes

Parabasalids propel themselves through the water with undulating membranes as well as flagella. The Trichomonas vaginalis seen in this photograph is a parasite that causes vaginitis, a sexually transmitted disease in humans. Other species of parabasalids play key roles in forest ecosystems. They live in the guts of termites and digest cellulose, something the termites themselves cannot do; this symbiosis aids forests in recycling the carbon tied up in fallen trees.

Key Learning Outcome 17.5. Molecular evidence suggests that diplomonads and parabasalids are the closest living protists to now-extinct early eukaryotes.