Protists reproduce both asexually and sexually, although sex occurs independently of reproduction in some species. Some multicellular protists exhibit alternation of generations, alternating between multicellular haploid and diploid life stages.
learning outcomes
You should be able to:
Distinguish between sex (in the sense of exchange of genes leading to genetic recombination) and reproduction.
Predict the evolutionary consequences of reproduction without sex for long periods of time.
Apply the definition of “alternation of generations” to determine whether a species is undergoing the process.
Why is conjugation between paramecia considered a sexual process but not a reproductive process?
Two paramecia exchange genetic material during conjugation, but there is no increase in the number of individuals. As a result, there is sex (genetic recombination) but no reproduction.
Why do you think paramecia that are not allowed to conjugate begin to die out after about 350 rounds of asexual reproduction?
The sex that occurs in conjugation allows the removal of deleterious mutations through genetic recombination. Without conjugation, paramecia can only reproduce clonally, which is not viable over long periods of time.
Although most diploid animals have haploid stages (e.g., eggs and sperm), their life cycles are not considered an example of alternation of generations. Why not?
Alternation of generations involves alternation of multicellular, independent haploid and diploid stages. Sperm and eggs are unicellular and are not independent organisms.
Given the diversity of protists and of the environments in which they live, it is not surprising that they influence their environments in numerous ways.