The aggregation of two or more protoplasts is not enough to start a fusion. Protoplast surfaces bear strong negative charges. In contrast to animal cells is the surface charge not caused by sialic acid residues but by phosphate groups. Intact protoplasts in suspension do thus repel each other. They can be very impressively linked and fused by the addition of calcium ions or polyethylenglycol.
Electric fields are an alternative.
Sexual hybridization occurs when haploid cells generated in a previous meiosis fuse. The fusion of somatic diploid cells should generate a tetraploid fusion product provided that the nuclei fuse, too. If this is the case, it is spoken of a synkaryon . A fusion product where the nuclei stay separate is called a heterokaryon . For conditions comparable to sexual hybridization is haploid starting material needed.