HAP2/GCS1 is involved in the sexual reproduction system of the marine macroalga Ulva compressa (Ulvales, Chlorophyta)

Abstract

Gamete fusion or fertilization is one of the most important steps in sexual reproduction. HAP2/GCS1 is a potential gamete membrane fusion component. We analyzed whole genome sequences of two strains with opposite mating types (mt- and mt+) in the marine macroalga Ulva compressa and found that identical genomic regions homologous with HAP2/GCS1 existed in their draft genome sequences. Although an RNA-Seq analysis revealed that a variety of transcripts was expressed from the two regions containing HAP2/GCS1 in both the gamete and gametophyte of the mt- and mt+ genotypes, a putative transcript coding a protein containing a HAP2-GCS1 domain was identified only in the mt- gamete. Immunofluorescence observations using an antibody against the putative HAP2/GCS1 demonstrated that the HAP2/GCS1 translational products localized to the cellular surface between the two flagella at a potential site for the mt- mating structure. The antibody inhibited mating of the mt- gamete with the mt+ gamete in U. compressa. Our data suggest that HAP2/GCS1 functions in the mt- gamete and plays a role in gamete fusion. Moreover, the specific function of HAP2/GCS1 in the mt- mating type may be regulated by an alternative splicing mechanism in U. compressa.