Sexual development and sex chromosomes in hop

Abstract

The stages of floral development in staminate and pistillate plants of hop (Humulus lupulus) were defined using scanning electron microscopy and light microscopy. Vegetative meristems of male and female plants are morphologically indistinguishable. On transition to the reproductive phase, inflorescence apices reduce greatly in size and striking developmental sex differences become apparent. The first sex-specific differences occur extremely early in floral ontogeny. Both male and female plants initiate inflorescence meristems at each leaf node, each meristem being enclosed within a bract. Male secondary inflorescence meristems give rise to clusters of asynchronously developing flowers. Female inflorescence meristems produce flowers arranged in ' cones '. Each male floral meristem initiates a whorl of five sepal primordia, followed by an inner whorl of five stamen primordia. There is no sign of carpel development at any stage. In females, two carpel primordia are initiated, surrounded at their base by a vestigial perianth whorl. No stamen development is observed. Several monoecious lines carry bisexual flowers, either within cymose panicles or within the basal bracts of terminal female 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 gynoecium. Chimaeric central whorls consisting of fused staminoid-carpelloid structures also occur. Sex differences in unisexual hop flowers are determined at an extremely early stage in ontogeny. The inappropriate set of sex organs is suppressed before it becomes visible or, more probably, it is not initiated at all. Genes directing the development of sex are likely to act at an extremely early stage, well in advance of floral organogenesis. The sex chromosomes of dioecious hop plants are described, as well as the chromosome constitutions of monoecious plants and those carrying bisexual flowers.

Genome size in Humulus lupulus L. and H. japonicus Siebold and Zucc. (Cannabaceae)

“…(Table 2), is the only known exception to the rule. Our measurements of the mitotic chromosomes in males revealed X:Y size ratio approximated to 1.5, that is in agreement with the previous data estimated for X and Y chromosomes in male meiosis, reported by Shepard et al (2000). The length difference between male and female chromosome complement in this species was only 1.62%.…”

“…and H. yunnanensis (Small 1978). Both H. lupulus (European common hop) and H. japonicus (Japanese ornamental hop) are dioecious, climbing plants with sex chromosomes (Winge 1929) and sex determination system based on X/autosome balance (Parker and Clark 1991;Shepard et al 2000). There are no reports on chromosome number and sex determination in H. yunnanensis.…”

“…lupulus, a perennial species important in the brewing industry is usually diploid with 2n=20. The simplest sex chromosome system described for hop plants of European origin is XX/XY, where males are heterogametic with the Y smaller than the X chromosome (Winge 1929;Parker 1990;Parker and Clark 1991;Shepard et al 2000). In common hop, X/autosome ratio of 0.5 or less gives male phenotype, while a ratio of 1.0 gives rise to female plants.…”

“…Triploids (2n=3x=27+XXY, X/A=0.67) are generally of male phenotype with only a few terminal female Vol. 75, (Parker and Clark 1991;Shepard et al 1999Shepard et al , 2000. Occasionally, on such monoecious triploid plants bisexual flowers are formed (Shepard et al 2000).…”

“…75, (Parker and Clark 1991;Shepard et al 1999Shepard et al , 2000. Occasionally, on such monoecious triploid plants bisexual flowers are formed (Shepard et al 2000). Small (1978) (Parker and Clark 1991;Shepard et al 1999).…”