title: Function of the male-gamete-specific fusion protein HAP2 in a seven-sexed ciliate.
abstract: HAP2, a male-gamete-specific protein conserved across vast evolutionary distances, has garnered considerable attention as a potential membrane fusogen required for fertilization in taxa ranging from protozoa and green algae to flowering plants and invertebrate animals [1-6]. However, its presence in Tetrahymena thermophila, a ciliated protozoan with seven sexes or mating types that bypasses the production of male gametes, raises interesting questions regarding the evolutionary origins of gamete-specific functions in sexually dimorphic species. Here we show that HAP2 is expressed in all seven mating types of T. thermophila and that fertility is only blocked when the gene is deleted from both cells of a mating pair. HAP2 deletion strains of complementary mating types can recognize one another and form pairs; however, pair stability is compromised and membrane pore formation at the nuclear exchange junction is blocked. The absence of pore formation is consistent with previous studies suggesting a role for HAP2 in gamete fusion in other systems. We propose a model in which each of the several hundred membrane pores established at the conjugation junction of mating Tetrahymena represents the equivalent of a male/female interface, and that pore formation is driven on both sides of the junction by the presence of HAP2. Such a model supports the idea that many of the disparate functions of sperm and egg were shared by the "isogametes" of early eukaryotes and became partitioned to either male or female sex cells later in evolution.
authors.affiliation: Biology Department, St. Olaf College, Northfield, MN 55057, USA. Electronic address: colee@stolaf.edu.
authors.forename: Eric S
authors.initials: ES
authors.lastname: Cole
authors.affiliation: Department of Microbiology and Immunology, Cornell University, Ithaca, NY 14853, USA.
authors.forename: Donna
authors.initials: D
authors.lastname: Cassidy-Hanley
authors.affiliation: Department of Microbiology and Immunology, Cornell University, Ithaca, NY 14853, USA.
authors.forename: Jennifer
authors.initials: J
authors.lastname: Fricke Pinello
authors.affiliation: Department of Microbiology and Immunology, Cornell University, Ithaca, NY 14853, USA; College of Life Science, Fujian Normal University, Fuzhou 350108, China.
authors.forename: Hong
authors.initials: H
authors.lastname: Zeng
authors.affiliation: Department of Microbiology and Immunology, Cornell University, Ithaca, NY 14853, USA.
authors.forename: Marion
authors.initials: M
authors.lastname: Hsueh
authors.affiliation: Department of Microbiology and Immunology, Cornell University, Ithaca, NY 14853, USA.
authors.forename: Daniel
authors.initials: D
authors.lastname: Kolbin
authors.affiliation: MCD Biology, University of Colorado, Boulder, CO 80309, USA.
authors.forename: Courtney
authors.initials: C
authors.lastname: Ozzello
authors.affiliation: MCD Biology, University of Colorado, Boulder, CO 80309, USA.
authors.forename: Thomas
authors.initials: T
authors.lastname: Giddings
authors.affiliation: MCD Biology, University of Colorado, Boulder, CO 80309, USA.
authors.forename: Mark
authors.initials: M
authors.lastname: Winey
authors.affiliation: Department of Microbiology and Immunology, Cornell University, Ithaca, NY 14853, USA. Electronic address: tgc3@cornell.edu.
authors.forename: Theodore G
authors.initials: TG
authors.lastname: Clark
mesh_terms: D005075:Biological Evolution; D017353:Gene Deletion; D005854:Germ Cells / Q000502:physiology*; D008954:Models, Biological; D008969:Molecular Sequence Data; D015800:Protozoan Proteins / Q000235:genetics* / Q000378:metabolism; D012098:Reproduction; D017422:Sequence Analysis, DNA; D016808:Tetrahymena thermophila / Q000235:genetics / Q000502:physiology*
chemical_list: D015800:Protozoan Proteins