Tree pollen exploits surrogate mothers

A rare kind of desert tree can manage a bit of sexual wizardry that scientists have never seen before in a plant, reports an international research team.

An Algerian cypress releases pollen that can develop without fertilization, using another tree species’ female organs instead of a mate’s, says Christian Pichot of the French National Institute for Agronomy Research in Avignon, France. He and his colleagues discovered this talent of Cupressus dupreziana by examining what plant breeders had intended to be mixed-species hybrids. Instead, the offspring all turn out to be clones of dad, the researchers say in the July 5 Nature.

“It’s surprising from two points of view,” Pichot says. Researchers had found female gametes from flowering plants, or angiosperms, that don’t need fertilization to sprout into a perfect adult plant. However, Pichot says, “as far as we know, this is the first for a paternal gamete.” Also, the cypress represents the only known cone-making species in which either gender has managed the trick.

“It’s astounding,” exclaims Lynda Delph, a specialist in plant reproduction who is at Indiana University in Bloomington. “I’ve never heard of anything like it.”

This cypress is one of the most severely threatened trees in the world. Although some botanical collections include the cypress, only 231 trees survive in the wild. They live scattered across the parched Tassili N’Ajjer region in northwestern Africa, where rainfall averages about 30 millimeters a year. In the sparse Algerian population of C. dupreziana, one tree may stand several kilometers from its closest neighbor.

Pichot recalls realizing that the tree was up to something odd when he found that seeds collected from the desert have food-storage tissue that carries a double set of chromosomes, as adult tissue does. The reserves, or endosperm, of other cone-making species bear only one set, matching that in pollen and female gametes.

Pichot’s team found that the C. dupreziana endosperm similarly matches the pollen, which also has a double set of chromosomes. The pollen therefore could, in theory, develop into an adult tree independently.

Next, the researchers took a close look at a set of 15-year-old saplings that horticulturalists had created by dusting the desert cypress pollen onto the female parts of a sister species, Cupressus sempervirens. This species abounds near the Mediterranean but can’t survive the parched Tassili N’Ajjer.

From their examination, the researchers conclude that the crossbreeding attempt yielded just the father’s species. The youngsters have their dad’s cone size, pollen size, and flattened arrays of twigs, as well as pollen and endosperm with a double set of chromosomes. When the researchers studied two types of genetic markers, the offspring proved themselves clones of their dad.

Just what happens in the wild isn’t clear yet, Pichot explains. Studying the desert trees hasn’t been easy. However, he notes that the two cypress species’ current ranges don’t overlap, so he doubts that he’ll find an exact repeat of the induced surrogacy.

Instead, he speculates that C. dupreziana pollen might use trees of its own species as surrogate mothers. Plant geneticist Loren Rieseberg of Indiana University also finds that scenario plausible.

If they’re right, the trait’s discovery could humble botany, Rieseberg says. Not until a few decades ago did scientists culture a full plant from a pollen grain. “Oh, no,” he teases, “we’ve been beaten by evolution again.”