Supergroup Excavata – Background

Many of the protist species classified into the supergroup Excavata are asymmetrical, single-celled organisms with a feeding groove “excavated” from one side. This supergroup includes heterotrophic predators, photosynthetic species, and parasites. Among its subgroups are the metamonads, including diplomonads, parabasalids, and oxymonads, and the euglenozoans, including euglenids and kinetoplastids. In this lab, we will look at representatives from each of these groups.

Diplomonads

Diplomonads are named for their unusual trait of having two nuclei. Until recently, these heterotrophic protists were believed to lack mitochondria. Mitochondrial remnant organelles, called mitosomes, have since been identified in diplomonads, but these mitosomes are essentially nonfunctional. Diplomonads exist in anaerobic environments and use alternative pathways, such as glycolysis, to generate energy. They generally use several flagella for locomotion.

Parabasalids

Parabasalids are named for their parabasal body, which is a large modified Golgi apparatus. Like diplomonads, these heterotrophs lack functional mitochondria. Instead, they have hydrogenosomes, highly modified mitochondria that perform anaerobic respiration. Many parabasalid species are symbionts. Some live in the guts of insects, such as termites or cockroaches, and help their host break down cellulose. Others are parasites, such as Trichomonas vaginalis, which is a common cause of vaginitis.

Oxymonads

Oxymonads are flagellated heterotrophs that live in the guts of termites and other wood-eating insects. It is presumed that they also play a role in cellulose digestion, although this has not been directly demonstrated. They are often covered in spirochete bacteria.

Kinetoplastids

The kinetoplastid subgroup is named after the kinetoplast, a DNA mass carried within the single, oversized mitochondrion possessed by each of these cells. This subgroup includes several parasites, collectively called trypanosomes, which cause devastating human diseases and infect an insect species during a portion of their life cycle. Diseases caused by trypanosomes include sleeping sickness, leishmaniasis, and Chagas disease.

Euglenids

Euglenids move through their aquatic habitats using two long flagella that guide them toward light sources sensed by a primitive ocular organ called an eyespot. The genus Euglena encompasses some mixotrophic species that display a photosynthetic capability only when light is present. In the dark, the chloroplasts of Euglena shrink up and temporarily cease functioning, and the cells instead take up organic nutrients from their environment.