Final answer:
Fungi have a haploid-dominant life cycle where mitosis occurs in the haploid stage forming the mycelium and meiosis occurs after fertilization to create haploid spores. The diploid stage is transient, only represented by the zygote before meiosis. Sexual reproduction involves the alternate stages of haploid and diploid cells.
Step-by-step explanation:
In the sexual life cycle of fungi, there are stages that are haploid and others that are diploid. The haploid-dominant life cycle is characteristic of most fungi, where the ecologically significant multicellular form is haploid. Haploid cells undergo mitosis to form the tissues of the fungus, primarily the mycelium. When it comes to sexual reproduction, specialized haploid cells, termed (+) and (-) mating types to designate their compatibility, fuse during a process called plasmogamy to create a diploid zygote.
Following fertilization, the diploid zygote immediately undergoes meiosis to form four haploid spores. These haploid spores can remain dormant but eventually germinate to form a new multicellular haploid structure when conditions are right. Unlike in diploid-dominant life cycles, where the multicellular diploid stage is prominent, in fungi, this diploid phase is short-lived and only seen during the zygotic stage. Sexual and asexual reproduction in fungi, hence, alternate between haploid and diploid stages, with meiosis and mitosis occurring during specific parts of their life cycle.