Seed production and sex ratio in anemophilous plants

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

“…If a female-biased sex ratio has been selected in R. hastatulus, it follows that males must be more expensive to produce than females. 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.…”

Sex ratios in sexually dimorphic umbelliferae

“…Most group selection hypotheses argue that the sex ratios are optimised at equality of the sexes (Kalmus and Smith, 1960) or when females predominate (e.g. Mulcahy, 1967;Kaplan, 1972) and hence do not apply to the present situation. Godley (1964) has suggested that" an excess of males may have arisen by selection to compensate for inefficiencies in pollination, in situations where seed production is limited by the availability of pollen and not by the number of ovules ".…”

“…But when the males outnumber the females by several times, as in some of the Umbelliferae populations and in a proportion of the counts listed by Godley, the total seed capacity of the population is severely curtailed. Theoretical studies of the effect of the sex ratio on seed production in dioecious populations predict that the frequency of males will not exceed that of females, even when pollination is sparse (Kaplan, 1972;Lloyd, unpublished). Faegri and van der Pijl (1966) suggested that the preponderance of males in Silent otites was an indication of transition from insect to wind pollination, but in view of the wide occurrence of male excesses in insect-pollinated plants (Godley, bc. cit., and the Umbelliferae considered here) this explanation is not generally tenable.…”

“…Kalmus and Smith (1960) proposed that both the opportunities for mating and the genetical variation available for natural selection are maximised when the sexes are equally common. Lewis (1942), Mulcahy (1967) and Kaplan (1972) have suggested that the sex ratio selected in plants maximises the total seed production of the population; the predicted sex ratio varies from equality to a marked excess of females, depending on the level of pollination. Wildish (1971) proposed that the sex ratio in Orehestia (Crustacea) populations regulates the reproductive potential of a population, and thus moves the density towards the optimal level.…”

Translocation heterozygosity and sex ratio in Viscum fischeri

“…These species have virtually no interspecific competition, high host specificity, high seed and seedling mortality, and generally highly unpredictable reproductive success. Several workers have, in fact, suggested that the adaptive significance of female-predominant sex ratios in angiosperms is that it maximises seed production (Lewis, 1942;Mulcahy, 1967;Kaplan, 1972). However,Lloyd (1 974a,b) has questioned the idea.…”