When Two Plants Are Better Than One: The Evolution and Ecology of Dioecy in Plants
Female flowers of Maclura pomifera (Moraceae), Osage orange
People are often surprised that plants do not usually have a gender in the same way as animals. Most plant species are hermaphrodite, with flowers that are both male and female. However, there are some plants (about 1 in 20) that do have separate male and female individuals. These are termed 'dioecious'.
One of the fundamental questions in biology is how the enormous variety of different organisms evolved and how this diversity is maintained today. Dioecious organisms present a paradox, because compared to hermaphrodites only half of the reproductively mature individuals (females) produce seeds; the other half (males) produce pollen. Therefore, if hermaphrodite populations produce double the number of seeds as dioecious populations, they should grow faster, outcompeting the dioecious population, which would go extinct. That there are so many dioecious plant species means that they must overcome this handicap. Dioecious females could invest resources, not put into pollen, into producing larger seeds, so that the newly-germinated seedlings grow faster or survive better than seedlings of hermaphrodites. Alternatively, each female could manufacture more seeds, producing an equivalent number to two hermaphrodites. A problem with this is that a greater concentration of seeds around parent individuals may attract more pests and pathogens, leading to greater offspring mortality. One way of counteracting this is to disperse seeds further from the parent.
Male flowers of Maclura pomifera (Moraceae), Osage orange
Tropical forests are the ideal place to investigate these processes, because they harbour a large number of species with different breeding systems. These species encompass a wide variety of growth forms and life history strategies. However, when answering the questions posed above, it is also important to take into account the similarities between species, in terms of their evolutionary history -- closely related species are much more likely to have similar characteristics because they share a common ancestor. Overall, there has been little study of the interaction between breeding systems and other ecological traits in plants, and none linking these to population dynamics. It is vital that we understand these processes, given the importance of pollination and seed dispersal for natural and agricultural systems.
The two-fold cost of sex. If each individual contributes to two offspring, the dioecious sexual population (a) remains the same size while the asexual/hermaphrodite population (b) doubles in size each generation.