Function ofY chromosomes inRumex thyrsijlorus.

1. Numerous polyploid and diploid plants ofR. thyrsiflorus with various numbers ofX andY chromoso mes were obtained. Analysis of these plants confirmed earlier conclusions (Żuk, 1963) thatY chromosomes carry no sex-determining genes. 2. Investigation of pollen fertility of male plants withoutY chromosomes, and with one or moreY chromosomes, seems to indicate that in theY chromosomes ofR. thyrsiflorus are located some factors reponsible for the fertility of male plants. This result is supported by cytological data (Żuk, 1969) indicating thatY chromosomes ofR. thyrsiflorus are active in premeiotic stages of microsporogenesis. Meiosis in plants withoutY chromosomes and with oneY chromosome was also studied. In all cases conjugation of chromosomes and meiotic divisions in PMC were normal. These data suggest that there are no genes located inY chromosomes responsible for synapsis or the course of meiotic division in PMC ofR. thyrsiflorus. 3. In a natural population ofR. thyrsiflorus the male individuals are less numerous than the female ones. This is due to nonrandom fertilization. The gametesYY + 6A = 8 fertilize less frequently than do gametesX + 6 A = 7. In crosses involving a male plant with chromosome constitutionXYYYY + 12A = 17, which produces more uniform gametes as regardsY chromosomes, there were more male individuals than normally. This is interpreted as evidence that the preponderance of females inR. thyrsiflorus is connected withY chromosomes.