Sex ratios in sexually dimorphic umbelliferae

Abstract

SUMMARYSex ratios in 35 inflorescence and plant counts ofperenthal, sexually dimorphic Umbelliferae vary from 096 to 8733 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.

SEX RATIO OFRUMEX HASTATULUS:THE EFFECT OF ENVIRONMENTAL FACTORS AND CERTATION

“…Alternatively, Lewis (1942), Mulcahy (1967) and Kaplan (1972) have suggested thatfemalebiased sex ratios are selected to provide for greater seed production by the population. Lloyd (1974) shows that while seed production can be maximized when the sex ratio is female-biased, it is difficult to explain how group selection could maintain sex ratios producing maximum seed set when individual selection favors equality of expenditure on progeny of each sex. A third alternative is that the sex ratio, rather than being directly selected, is a secondary consequence of differential gamete or embryo success caused by the differentiation of X and V chromosomes (Lloyd, 1974).…”

The evolution and maintenance of gynodioecy in sexually and vegetatively reproducing plants

“…Kay and Stevens (1986) discussed a number of dioecious and subdioecious species in the flora of the British Isles that rely heavily on vegetative reproduction, and several examples of vegetatively reproducing gynodioecious species are given in table 2. Lloyd (1973) Origanum vulgare rhizomes Lewis and Crowe (1956), Kheyr-Pour (1980) Saxifraga granulata bulbils Stevens and Richards (1985) Silene vulgaris stolons Dulberger and Horovitz (1984) Succisa pratensis lateral buds Kay (1982) Vegetative reproduction per se has several important consequences for gynodioecy. One consequence concerns the nature of fitness differences between hermaphrodites and females.…”

“…Several authors have compared sexual fitness components in sex types from gynodioecious populations (Connor, 1965;Assouad et al, 1978;Philipp, 1980;Webb, 1981;Van Damme and Van Delden, 1984), and a few comparisons of vegetative fitness components have also been made (Lloyd, 1973;Webb and Lloyd, 1980;Stevens, in press). The three latter studies confirm that sex types in natural gynodioecious populations may indeed differ in vegetative reproduction.…”

Behaviour of a sex-differential fertility gene in hermaphrodite populations

“…Since that time, however, a considerable amount of evidence linking the two breeding systems has been obtained, e.g. in several Hawaiian genera (Cariquist, 1966), in Pimelea (Burrows, 1960(Burrows, , 1962Ross, 1970), in New Zealand Umbelliferae (Lloyd, 1973) and in Mexican species of Fuc/lsia (Kahn Arroyo and Raven, 1975). Some of these groups contain closely related dioecious and gynodioecious species, and it appears that dioecy has evolved from gynodioecy by gradual reduction of seed set on hermaphrodites.…”