The gene encoding Arabidopsis HAP2 is allelic with GCS1 (Generative cell-specific protein 1). HAP2 is expressed only in the haploid sperm and is required for efficient guidance of the pollen tube to the ovules. In Arabidopsis the protein is a predicted membrane protein with an N-terminal secretion signal, a single transmembrane domain and a C-terminal histidine-rich domain. HAP2-GCS1 is found from plants to lower eukaryotes and is necessary for the fusion of the gametes in fertilisation. Studies in the green alga Chlamydomonas and the malaria organism Plasmodium showed that it is involved in a novel mechanism for gamete fusion where a first species-specific protein binds male and female gamete membranes together after which a second, broadly conserved protein, either directly or indirectly, causes fusion of the two membranes together. The broadly conserved protein is represented by this HAP2-GCS1 domain, conserved from plants to lower eukaryotes. In Plasmodium berghei the protein is expressed only in male gametocytes and gametes, having a male-specific function during the interaction with female gametes, and being indispensable for parasite fertilisation. The gene in plants and eukaryotes might well have originated from acquisition of plastids from red algae.