Sex allocation in plants and the evolution of monoecy

Question: Which ecological factors favour the transition from perfect flowers to separate male and female flowers on the same individual?Mathematical methods: ESS computation in sex allocation models.Key assumptions: Within a flower, the costs of attraction, pollen production, style/ovary and seed-bearing fruit are assumed to be fixed. Often costs of seed-bearing fruit outweigh other costs in the same flower. Perfect flowers are more economic because the costs of non-sexual parts of the flower are shared by male and female function. Female flowers produce more seeds than hermaphrodite flowers due to less pollen-stigma interference.Conclusions: When sex allocation at the flower level is strongly female-biased, plants respond by producing either male flowers or flowers without fruit. Hermaphroditism evolves to andromonoecy (male and hermaphrodite flowers on the same plant) and then to monoecy. The presence of male flowers facilitates the production of female flowers. The alternative route via gynomonoecy (female and hermaphrodite flowers on the same plant) is improbable, since it requires unrealistically high levels of seed production in female flowers. High costs of attraction always favour producing perfect flowers. Monoecious species are likely to have: (i) small, inexpensive flowers; (ii) large, costly fruits and seeds; and (iii) high fertilization rates.