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Regulation of sex ratios in plants

“Under drought conditions,� says Bänziger, CIMMYT’s director for corn research, “the maize plant puts more resources into pollen formation and less into seeds.� From the plant’s point of view this makes sense. Pollen is much cheaper energy-wise for the plant to make, and, if the pollen manages to fertilize another plant’s seed, the drought-afflicted parent will still contribute 50% of its genes to the offspring. But this is of little help to farmers, who sell kernels, not pollen." -- Nature 452:273

Maize plants are hermaphrodites, having both male (pollen-producing) and female (seed-producing) flowers. Other plant and animal species have two sexes, such as males and females. From the title, “Density-dependent regulation of sex ratio in an annual plant�, I assumed that this week’s paper (by Marcel Dorken and John Pannell, published in American Naturalist) would be about how parent plants adjust the male:female ratio in their offspring, a topic I have discussed previously.

But no. Mercurialis annua is stranger than that. Its two “sexes� are male and hermaphrodite.

When a hermaphrodite plant pollinates itself or is pollinated by another hermaphrodite plant, all of its seeds grow up to be hermaphrodites. When a hermaphrodite is pollinated by a male plant, half its seeds are hermaphrodites and half are male.

Male plants don’t produce seeds, so an isolated population of only male plants will die out. An isolated population of only hermaphrodites will stay hermaphrodite-only. Real-world populations tend to be between 0 and 40% male. Maleness is genetically determined in this species, so it can evolve, changing the frequency of males over generations. But in which direction?

The authors hypothesized that males would become more common at high density, when lots of M. annua plants are crowded together, but less common at low density. One reason is that isolated hermaphrodites (the only sex that produces seeds, in this species) would mainly pollinate themselves, producing more hermaphrodites. At higher population density, however, hermaphrodites would receive more wind-blown pollen from nearby male plants. If the males produce enough pollen to swamp that from the hermaphrodites, up to half of the resulting seeds would be male.

But what if the hermaphrodites change their investment in pollen vs. seeds with changing conditions, as maize plants apparently do under drought? The authors hypothesized that hermaphrodites would produce less pollen at high than at low density. If so, then other plants (some of them males) would be responsible for a larger fraction of pollination at high density. More pollination by males, rather than self-pollination by hermaphrodites, would lead to a higher percentage of male seeds. So high density could lead to more males, in two apparently independent ways.

To test this hypothesis, the authors collected seeds from several locations and grew each batch of seed at high density (closely-spaced plants) and low density (widely-spaced plants). They measured pollen production and the sex ratio in seeds produced.

Consistent with their hypothesis, “hermaphrodites produced 3.6 times as much pollen per unit biomass under low densities.� Most populations also evolved in the predicted directions, increasing in percent male at high density and decreasing at low density.

The paper doesn’t seem to explain why wider spacing would increase pollen production by hermaphrodites. I can see that this would be beneficial if a hermaphrodite needed to supplement its neighbors’ pollen with more of its own, but wouldn't closer neighbors also increase the success rate for any pollen it sent drifting on the wind? Also, how can plants tell how many other members of their species are nearby? Are they just responding to the amount of nonself pollen they receive, or are other signals (perhaps gases) being exchanged?

Your homework assignment is to explain the implications of these findings for human sexual morality. Blank sheets of paper will receive full credit.

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