Most creatures on Earth have one sex that fertilizes and one that gets fertilized. Not so with olive trees. Last week scientists described how these trees evolved a system of males and a third sex which can go both ways.
The sexual system these trees have is called androdioecy: It includes males and a third "hermaphrodite" sex. A group of French researchers last week explained how such a setup could evolve from a pure hermaphrodite system. Initially, the trees were probably all able to pollinate or be pollinated. But over time, some of the trees mutated and lost their female functionality. Now, a very sizable male population exists among the olive trees.
But how? You'd think that males, who can only reproduce by pollinating, would have a strong disadvantage in a system where their competitors can reproduce either by pollinating or being pollinated.
However, among the olive trees these scientists studied, the androdioecy had reached a stable state. That's because the males could pollinate all the third sex trees. But the third sex trees could only pollinate about half of their cohorts. So in the end, the males and third sex trees had equal opportunities to pass along their genetic material.
According to the researchers:
In [olive tree] Phillyrea angustifolia L., we found that high male frequencies can be maintained in natural populations because hermaphrodites belong exclusively to one of two self-incompatibility groups, and thus, each can fertilize only half of all pollen recipients. In contrast, males can pollinate all hermaphrodites. Thus, in this species, the reproductive disadvantage that males face due to the loss of female function is offset by the fact that all males are fully compatible with all pollen recipients.
Other trees have evolved out of a pure third sex scenario into a male/female one, or into one with females and third sexes. What's cool about these specific olive trees is that they reached their stable androdioecy when the third sex lost its female functionality. In other trees, the same situation usually occurs when females gain male parts instead.
I feel like I've just read an Octavia Butler novel in the pages of Science magazine.
Image via Nature