In fungi, the sexual fusion of haploid cells is called karyogamy. The result of karyogamy is the formation of a diploid cell called the zygote or zygospore. This cell may then enter meiosis or mitosis depending on the life cycle of the species.
In plants, the zygote may be polyploid if fertilization occurs between meiotically unreduced gametes.
In the amoeba, reproduction occurs by cell division of the parent cell: first the nucleus of the parent divides into two and then the cell membrane also cleaves, becoming two "daughter" Amoebae.