Sexual development and sex chromosomes in hop

The stages of floral development in staminate lend pistillate plants of hop (Humulus lupulus) were defined using scanning electron microscopy and ligh t microscopy. Vegetative meristems of male and female plants are morphologi cally indistinguishable. On transition to the reproductive phase, infloresc ence spices reduce greatly in size and striking developmental sex differenc es become apparent. The first ses-specific differences occur extremely earl y in floral ontogeny. Both male and female plants initiate inflorescence me ristems at each leaf node, each meristems being enclosed within a bract. Ma le secondary inflorescence meristems give rise to clusters of asynchronousl y developing flowers. Female inflorescence meristems produce flowers arrang ed in 'cones'. Each male floral meristem initiates a whorl of five sepal pr imordia, followed by an inner whorl of five stamen primordia. There is no s ign of carpel development at ally stage. In females, two carpel primordia a re initiated, surrounded at thier base by a vestigial perianth whorl. No st amen development is observed. Several monoecious lines carry bisexual flowe rs, either within cymose panicles or within the basal bracts of terminal fe male inflorescences. Bisexual flowers usually possess perianth, stamen and carpel whorls. The central whorls are often highly variable, and range from a pair of stigmas fused to a thin central filament to a well developed gyn oecium. Chimaeric central whorls consisting of fused staminoid-carpelloid s tructures also occur. Sex differences in unisexual hop flowers are determin ed at an extremely early stage in ontogeny. The inappropriate set of sex or gans is suppressed before it becomes visible or, more probably, it is not i nitiated at all. Genes directing the development of sex are likely to act a t an extremely early stage, well in advance of floral organogenesis. The se s chromosomes of dioecious hop plants are described, as well as the chromos ome constitutions of monoccious plants and those,se carrying bisexual flowe rs.