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Healthy individuals of Paramecium bursaria contain photosynthetic algal endosymbionts of the genus Chlorella. When within their hosts, the algae are referred to as zoochlorellae. In aquaria with light coming from only one side, P. bursaria gather at the well-lit side, whereas other species of Paramecium gather at the opposite side. The zoochlorellae provide their hosts with glucose and oxygen, and P. bursaria provides its zoochlorellae with protection and motility. P. bursaria can lose its zoochlorellae in two ways: (1) if kept in darkness, the algae will die; and (2) if prey items (mostly bacteria) are absent from its habitat, P. bursaria will digest its zoochlorellae.
Which of the following experimental results would suggest that the zoochlorellae and P. bursaria are mutualists?
A) The reproductive rate of P. bursaria is higher with zoochlorellae than without.
B) The mortality rate of P. bursaria is higher with zoochlorellae than without.
C) Zoochlorellae reproduce more slowly than free-living Chlorella.
D) The swimming speed of P. bursaria is higher with zoochlorellae than without.

Question

Quizplus · Accepted Answer

If the reproductive rate of P. bursaria is higher with zoochlorellae than without, it suggests that the algae are providing a benefit to the host. This would indicate mutualism, as both the host and the endosymbiont are benefitting from the association.

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