Although most protists engage in both asexual and sexual reproduction, sexual reproduction has yet to be observed in some groups. In some protists, as in all prokaryotes, the acts of sex and reproduction are not directly linked.
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Ciliate conjugation is a sexual process but is not a reproductive process.
Organisms that engage in alternation of generations have distinct multicellular haploid and diploid phases.
Several asexual reproductive processes have been observed among the protists:
The equal splitting of one cell into two by mitosis followed by cytokinesis
The splitting of one cell into multiple (i.e., more than two) cells
The outgrowth of a new cell from the surface of an old one (known as budding)
The formation of specialized cells (spores) that are capable of developing into new individuals (know as sporulation)
Asexual reproduction results in offspring that are genetically nearly identical to their parents (they differ only by new mutations that may arise during DNA replication). Such asexually reproduced groups of nearly identical organisms are known as clonal lineages.
Sexual reproduction among the protists takes various forms. In some protists, as in animals, the gametes are the only haploid cells. In others, the zygote is the only diploid cell. In still others, both diploid and haploid cells undergo mitosis, giving rise to alternating multicellular diploid and haploid life stages.