When the adults have reach a certain degree of weakness, the process of binary fission is replaced by conjugation in

Analyze the question: It asks which organism normally reproduces by binary fission but switches to conjugation when the adult population becomes weak or stressed (for example, after many generations of asexual division).

Explain the concept:

• Binary fission is asexual reproduction in which one parent cell splits into two identical daughter cells.
• Conjugation is a sexual process in which two individuals temporarily join and exchange genetic material (micronuclei). It restores vigor and genetic variation.

Go through the options:

• A. Paramecium: It reproduces mainly by binary fission. After many generations of fission, the culture shows aging and loss of vigor. Conjugation then occurs, with an exchange of micronuclei that rejuvenates the cells. This matches the question.
• B. Euglena: It reproduces by longitudinal binary fission. Conjugation is not a normal feature of its life cycle.
• C. Amoeba: It reproduces by binary fission (and multiple fission when forming cysts). Conjugation is not known in Amoeba.
• D. Plasmodium: It reproduces by schizogony (multiple fission) in humans and by sporogony in mosquitoes. It has gamete fusion, but this is not conjugation replacing binary fission.

Answer: Paramecium is the ciliate classically described as switching from binary fission to conjugation when its population weakens.

Exam tip: Remember "Paramecium = Conjugation." Whenever a JAMB question mentions conjugation in a protozoan, think of the ciliates (Paramecium) first. Also remember that Spirogyra, a green alga, conjugates too, so check whether the options include it.