Question

Monocercomonoides are a genus of single-celled organisms that live as parasites in the guts of small animals. They lack mitochondria, but other evidence shows them to be most closely related to members of the excavates. Looking at the tree for eukaryotes, what can you conclude about the Monocercomonoides?

A. Since excavates are eukaryotes, it is extremely unusual for one of them to lack mitochondria.
B. Excavates branched off before the evolution of mitochondria, so the absence is not surprising.
C. Since they are parasites of animals, they should be placed within the opisthokont branch of the tree.
D. A phylogenetic tree reflects evolutionary relationships, so they should be placed within the excavates.
E. Their lineage likely had mitochondria but lost them at some point.

Answer 1

Monocercomonoides is most closely related to excavates and likely had mitochondria in the past, but lost them, making option E the correct conclusion.

Step-by-step explanation:

Monocercomonoides, being a genus of single-celled organisms that do not possess mitochondria yet are closely related to members of the excavates, can give us insight into the evolutionary history of eukaryotes. The presence of linear chromosomes wrapped around histones within the nucleus suggests that Monocercomonoides is indeed a eukaryote; however, the lack of detectable mitochondrial genes is notable. Eukaryotic members of Excavata such as diplomonads and parabasalids often have modified mitochondria-like organelles (mitosomes and hydrogenosomes, respectively) indicating they once had but then lost mitochondria. Therefore, the correct conclusion about Monocercomonoides is that their lineage likely had mitochondria but lost them at some point.

Answer 2

Answer: A. Excavates branched off before the evolution of mitochondria, so the absence is not surprising.

C. Since they are parasites of animals, they should be placed within the opisthokont branch of the tree.

E. Their lineage likely had mitochondria but lost them at some point.

Step-by-step explanation:

Based on the information given, the conclusion that can be made about the monocercomonoides include:

• Excavates branched off before the evolution of mitochondria, so the absence is not surprising.

• Since they are parasites of animals, they should be placed within the opisthokont branch of the tree.

• Their lineage likely had mitochondria but lost them at some point.