Sex ratios in sexually dimorphic umbelliferae

Abstract

Sex ratios in 35 inflorescence and plant counts of perennial, sexually dimorphic Umbelliferae vary from 0.96 to 87.33 times as many males as females. The ranges of ratios are similar in dioecious and gynodioecious populations. In 10 populations in which both the inflorescences and plants were counted, the male/female ratios are approximately one in populations in which the plants produce only one inflorescence per year and increase as the average number of inflorescences increases. The interpretation offered is that the sex ratio is approximately one until reproduction begins, and in subsequent years increasingly malebiased ratios develop because sexual reproduction utilises more of the available resources of females than of males. Following reproduction, male plants survive longer and grow more and so become predominant. In dioecious Angiosperms generally, male-biased ratios are characteristic of long-lived repeatedly flowering species and may be partly due to differential post-reproductive growth. It is postulated that male preponderance is not directly selected for, but is a secondary consequence of separate competition among males and among fema'es during sexual reproduction. The seed set and fitness of total populations may actually decrease with the development of marked male preponderance. © 1973 The Genetical Society of Great Britain.