The inconstant sex of Mercurialis annua

Many plants have flowers that are hermaphroditic with male and female parts. Some are dioecious, with male or female flowers. Mercurialis annua keeps its options open. If you think the name is familiar, it’s because we covered some other work on polyploidy connected to it recently. That was on how the flowers were held by the plant. This is paper takes a closer look at the flowers themselves and their sex.

Plants have different sexes, but usually, this is in the gametophyte generation of the plant. Pollen is not plant sperm. It’s a tiny male plant. The female equivalent sits in the ovule of a flower, awaiting the pollen to deliver the sperm to fertilise it.

However, some plants have flowers that produce gametophytes of only one sex. They’re effectively male or female flowers. M. annua is one of these plants that appears to have sexes. A recent article by Cossard and Pannell shows that in reality, it’s not that simple. M. annua‘s sexes are ‘leaky’.

While M. annua might have a habit of returning to hermaphroditism (having both sexes), it’s not clear how. Cossard and Pannell wanted to see if there was a pattern in how the sex expression broke down.

The results of their experiments were a little unexpected. Usually, when plants become hermaphroditic, it’s the males that break down. However, for M. annua, it was the female plants that were more likely to have some male flowers. In contrast, they found that when male plants did start showing female flowers, the proportion of flowers much greater than the male flowers on female plants.

So why does it happen?

Cossard and Pannell note that when you have a lot of plants, then only having flowers of one sex is a good strategy. You’re guaranteeing outcrossing of genetic material with another partner. But if you’re a pioneer plant, with few partners around, then having a single sex limits reproductive success. Here, self-pollination is a helpful strategy, even if just for a short while.

The study shows that when sex expression breaks down for M. annua, it isn’t haphazard, but part of strategy to give M. annua reproductive success, wherever it might find itself.