The Case of the Misunderstood Male

Agronomy student Melissa Gates examines turf varieties at the Valentine Turfgrass Research Center. Gates is studying apomixis, a process by which many species can produce offspring without the fertilization of the egg.

There's something mysterious going on in the world of genetics. Many plant species–and even some animals, such as fish and lizards–have developed a way to produce offspring without fertilization of the egg. This process, called apomixis, occurs most commonly throughout the plant kingdom. Scientists have identified more than 300 plant species from 35 families that can reproduce in this way as well as normally. The process seems to occur most frequently with Gramineae (grasses), Compositae (asters, daisies, and the like), and Rosaceae (roses and their kin). This phenomenon could have very practical implications for plant breeders. If geneticists can learn how apomixis works, they may be able to harness the process and produce by seed an unlimited number of whatever plant species or cultivar they choose–all exactly alike down to the last cell.

"The normal way that plants reproduce is called double-fertilization because it involves two sperm cells within one pollen grain containing genes from the male," explains Melissa Gates, a graduate student in agronomy who is studying the process in the laboratory of turfgrass scientist David Huff. "When pollen comes in contact with the female part of a flower, one sperm cell fertilizes the egg, which then develops into the embryo. The other sperm cell fertilizes a part of the ovary called the polar nuclei. The polar nuclei develop into the endosperm, the part of a seed that stores nutrients for the developing embryo. You can see what a plant embryo and endosperm look like by breaking open a large seed, such as a shelled peanut. Inside is a tiny plant, the embryo. The surrounding seed halves are the endosperm."

Apomictic reproduction is different from cloning, another method of reproduction that does not require fertilization of the egg. In horticulture, cloning is done by taking a segment of a plant–a leaf, a branch cutting, or just a few cells–and using it to grow a new plant identical to its parent. Anyone who has rooted a geranium cutting in a jar of water has produced a clone. "A plant produced by apomixis is identical to its maternal parent, just like a clone–but the difference is that it's produced from seed," says Gates. "It's not clear how this occurs, and that's what makes the process fascinating–how can a plant produce seed without being fertilized by pollen from the male?"

Gates, who earned a bachelor's degree in biology with an emphasis in genetics and developmental biology from Penn State's Eberly College of Science, knew of apomixis only as a footnote until she started her graduate work in agronomy. "Apomixis is so complicated that we never covered it in depth in my undergraduate classes," she says. "Working in Dr. Huff's lab is a great opportunity to learn about this unusual mechanism of plant reproduction as well as study an entirely different genus of plants. Some people tend to think grass is grass. Nothing could be further from the truth."

Gates is studying apomixis in Kentucky bluegrass, one of the most commonly used species in Pennsylvania's $1.5 billion-a-year turf industry. "It's an extremely tough, versatile grass that's ideal for high-use turf areas like golf courses, athletic fields, and home lawns," says Huff. "It also has a strong tendency to reproduce apomictically as well as sexually, making it a useful plant for comparing the two forms of reproduction."

"Pollen plays a role during apomixis in Kentucky bluegrass–but somehow the pollen is prevented from fertilizing the egg," Gates says. "Pollen does get to fertilize the polar nuclei, which in turn provide food for the embryo. An apomictically fertilized egg contains all the chromosomes of the mother plant, but half the chromosomes of a sexually fertilized egg are from the mother plant and half from the father."

Gates is cross breeding six different cultivars of the grass in every possible combination, totaling 36 different crosses. "I'm looking at the percentage of offspring that are produced apomictically and the percentage produced sexually," she says. "We can identify them visually because the apomictic offspring look exactly like the mother plant. But to be certain, I'm also identifying genetic markers in the mother plants so that I can examine the offspring at the molecular level. Those carrying the genetic marker are apomictic, and those not carrying it are sexually produced.

"Most flowering plants–and this includes grasses–have both male and female organs in the same flower," she adds. "So I can experiment using each grass cultivar as both a male parent and a female parent. One of the patterns I'm looking for is which cultivars father more apomictic offspring than sexually reproduced offspring. I'm also looking closely at pollen's role in apomictic reproduction. Even though pollen doesn't fertilize the egg, its role is still very significant. The polar nuclei provide nourishment to the embryo, and that suggests that when pollen fertilizes the polar nuclei, it contributes greatly to the plant's early development."

The main practical problem that plant geneticists have with apomictic plants is that the male doesn't contribute any of its genetic information to the offspring, and because of this, it's difficult to incorporate new traits into existing varieties. Gates envisions a time when scientists will be able to switch off a plant's apomixis, cross it with one that contains genes for a desired trait, such as drought resistance, then switch the plant's apomixis back on. All of the offspring would then predictably contain the desired trait.

Gates, who plans to earn her Ph.D. and continue her career in genetics research, feels that aside from having a lot of practical applications for breeders and growers, apomixis is worth studying just for the pure science of it. "The whole process is extremely intriguing," she says. "You could spend a lifetime studying it, and there still would be many unanswered questions."

–Rose Pruyne