Ploidy and sex expression in monoecious hop (Humulus lupulus)

Humulus lupulus L. is a dioecious perennial plant, cultivated for its female inflorescences. Spontaneously arising monoecious hop plants, carrying male and female flower types on a specific plant, occasionally occur in the progeny of certain hop crosses. We assessed the ploidy of 58 monoecious plants, progenies of various crosses of diploid parents, to provide additional data on hop monoeciousy. Flow cytometric analysis revealed that a high percentage (41.4%) were triploid. An inheritance analysis of parental alleles using six codominant SSR markers demonstrated a primarily paternal origin (84.2%) of unreduced gametes as the cause of triploidy in monoecious plants. All triploids had predominantly male flowers with a few female cones; this phenotype was found only within this ploidy group. All other monoecious plants were diploid, except for one genotype with the highest nuclear DNA content, which showed an aneuploidic number of chromosomes (21). Male hops showed the lowest nuclear DNA amount, as measured by DAPI and propidium iodide fluorochromes. The estimated AT frequency placed hop among species with a high AT content, which was slightly higher in male than in female plants (63.0% vs. 62.5%). The results are discussed in relation to the occurance of ploidy and sex expression in monoecious hop. Key words: hop, flow cytometry, nuclear DNA content, SSR markers, unreduced gametes. On cultive l'Humulus lupulus L., une plante dioique perenne, pour ses inflorescences femelles. Des plants de houblon monoiques se forment spontanement, portant les types de fleurs males et femelles sur une plante specifique, apparaissant occasionnellement dans la progeniture de certains croisements de houblon. Les auteurs ont evalue la ploidie de 58 plantes monoiques, provenant de divers croisements entre parents diploides, afin d obtenir des donnees additionnelles sur la monoecie du houblon. L'analyse cytometrique en flux revele un fort pourcentage (41,4%) d'individus triploides. Une analyse d'heredite des alleles parentaux en utilisant six marqueurs SSR codominants demontre surtout une origine paternelle des gametes non reduits comme cause de la triploidie des plantes monoiques. Tous les triploides possedent surtout des fleurs males avec quelques cones femelles; on retrouve ce phenotype seulement dans ce groupe de ploidie. Toutes les autres plantes monoiques sont diploides, sauf pour un genotype possedant le contenu le plus important en ADN nucleique, indiquant un nombre aneuploidique de chromosomes (21). Les houblons males montrent la plus faible quantite d ADN nucleique, tel que mesure par la coloration DAPI et les fluorochromes de l'iodure de propidium. Les frequences AT estimees placent le houblon parmi les especes a contenu AT eleve legerement plus important chez les plants males que chez les femelles (63,0% vs. 62,5%). Les auteurs discutent les resultats en relation avec l'incidence de la ploidie et l'expression du sexe chez le houblon monoique. Mots-cles : houblon, cytometrie en flux, teneur en ADN nucleique, marqueurs SSR, gametes non reduits. [Traduit par la Redaction]
Introduction Most flowering plants are hermaphroditic, with only about 6% of angiosperms being dioecious (Renner and Ricklefs 1995). Dioecious plants have evolved quite recently from ancestors with hermaphroditic flowers (Westergaard [...]