Dosage sex-chromosome systems in plants

Abstract

Dosage sex-chromosome systems have been detected in two genera of flowering plants. Rumex and Humulus. In Rumex subgenus Acetosa dioecious species have a multiple sex chromosome system with two X chromosomes in females and an X and two Y chromosomes in males. The Y chromosomes are facultatively heterochromatic appearing as chromocentres in somatic interphase nuclei, but undergo RNA transcription, show DNase-I sensitivity, and are genomically-similar to the rest of the complement. Sex determination is dependent on X/autosome balance, and androecium development is not dependent on the presence of Y chromosomes. Intermediate X/autosome ratios between 0.5 and 1.00 give hermaphrodite individuals. The autosomes also affect sex expression, with some enhancing male and others female development. Disomy for one quarter of an X-chromosome is sufficient for functional female development. Y-chromosomes act sporophytically and are required for meiotic success, although a complete Y-complement is not a haploid requirement for pollen maturation or fertilisation. The American annual species R. hastatulus is polytypic for sex-chromosomes and has features both of X/autosome balance and active-Y systems. Humulus lupulus (hop) is highly polytypic with at least six sex-chromosome types. The dosage system is similar to that of Rumex although Y-chromosomes are required for successful pollen maturation. In Humulus dioecy is less strict than in Rumex and a simple synthetic hormone treatment is able to induce male flowers on a female cultivar.