Experimental tests of sex-allocation theory in plants.

A general explanation for diversity in plant breeding systems is offered by sex-allocation theory. This theory assumes a trade-off between allocation of resources to the two sexual functions. It explains the high frequency of hermaphroditism in angiosperms by diminishing fitness returns on investment of more resources in a single function. Recent experimental studies provide tests of this theory by measuring male and female fitness gains, and examining the trade-off assumption. These studies show how fitness responds to shifts in allocation. Allocation traits often show heritable variation, but support for a trade-off remains weak.