Two Pond Cells and a Misplaced Kingdom
An intern at a water-treatment plant filters 500 mL of pond water and finds two kinds of cells. Organism A is 40 µm across, moves with two flagella, contains green chloroplasts, and has a rigid cell wall made of cellulose. Organism B is 1.5 µm across, has no nucleus or membrane-bound organelles, has a cell wall containing peptidoglycan, and carries a single circular chromosome plus several small circular plasmids. The intern sequences the small-subunit ribosomal RNA gene of each. Organism A's sequence differs by 2.8% from a described species in the genus Chlamydomonas. Organism B's sequence matches a laboratory strain of Escherichia coli at 99.9%. The intern proposes placing A and B in the same kingdom because both are single-celled and both came from the same pond.
1. Classify organisms A and B into domain and kingdom, and justify each placement with at least three independent lines of evidence.
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Organism A belongs to domain Eukarya, in the green algal/plant lineage; in a course that uses kingdom Protista for algae, it is Protista, and in a plant-focused scheme it is Plantae. The evidence is independent and agrees: a 40 µm cell with a nucleus and membrane-bound organelles (including chloroplasts), a cellulose cell wall, two flagella, and an rRNA sequence only 2.8% different from Chlamydomonas, a green alga.
Organism B belongs to domain Bacteria. It is 1.5 µm, lacks a nucleus and organelles, has peptidoglycan in the wall, carries a circular chromosome plus plasmids, and its rRNA matches E. coli at 99.9%. Cell structure, cell-wall chemistry, genome form, and sequence data all point the same way, which is why the placements are robust.
2. Evaluate the intern's proposal. Explain why "single-celled" is not a valid basis for grouping these two organisms.
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The proposal fails because being single-celled is a shared ancestral condition, not a shared derived character. A and B are unicellular because their common ancestor was unicellular, not because they are close relatives; that same ancestor is also ancestral to plants, animals, and fungi. Grouping on that trait creates a paraphyletic group that excludes multicellular descendants.
The differences are fundamental: nucleus versus no nucleus, cellulose versus peptidoglycan, organelles versus none. The sequence data reinforce this: 2.8% divergence versus 0.1% divergence places them in different domains. Classification should reflect evolutionary history, not habitat or body plan.
3. What does the 2.8% rRNA difference tell you about organism A's species status, and what are the limits of using a single gene for this decision?
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Ribosomal RNA genes are used because they occur in all organisms, are function-critical and therefore slowly evolving, and contain both conserved and variable regions. A 2.8% difference in small-subunit rRNA is large for prokaryotes but falls within the range seen among green algal species and genera. It supports calling A a distinct species, possibly a new genus, relative to the named Chlamydomonas species.
But a single gene cannot establish species status by itself. Species designation also needs morphology, mating compatibility, ecology, and genome-wide data. The sequence establishes relatedness, not reproductive isolation, so it can place the organism but not name it.