How does sexual reproduction occur in Ascomycota and Basidiomycota?

ASCOSPORES
Fruiting bodies ( ascus ) are first formed when two mycelia of different mating strains ( A , a ) come together. The fertilisation is initiated when a cell of opposite mating type contacts a part of the trichogyne. This is followed by the fusion of the two haploid cells.

First the ascus initial cell is formed, by the fusion of nuclei of the fertilising cell and a nucleus from the ascogonium
Then the fusion of nuclei in the fused haploid cells takes place and a diploid nucleus is formed.

The formation of this diploid nucleus is immediately followed by sequential meiotic cell division resulting in four haploid cells, two of each strain A and a.
One further mitotic division leads to four each nuclei of both mating strains . Each ascus thus contains eight ascospores.

BASIDIOSPORES
sexual reproduction in Basidiomycota takes place in the specialised fruiting body called basidia.

The basidia is formed by plasmogamy between mycelia from two spores from different strains. This results in a binucleate hypha i.e. hyphae with two types of nuclei, one from each parent. In the gills of the fruiting body some cells undergo fusion of these two nuclei. These cells form the basidia.

Soon after fusion meiosis takes place resulting in four haploid nuclei. The nuclei then migrate to the terminus of the basidium and form four individual projections. These projections are then separated by cell walls to become spores.