Apomixis: Why Is It Taking So Long?

"The whole situation with apomixis research reminds me of the 1902 Georges Méliès movie, A Trip to the Moon. In the movie, they simply shot a large bullet from a giant gun at the moon, and after a short time it struck the giant cheese orb. Sixty-seven years later, we actually landed on the moon, but not until we had developed and fully understood a huge range of new technologies, as well as the basic scientific concepts involved. Now, in our apomixis work, we have reached the stage where we understand that our initial approach was too simple, and we need to know more."

Over 13 years ago, the Institut de Recherche pour le Développement (IRD) joined with CIMMYT to initiate work on creating "apomictic" maize. Hopes were high that by crossing maize with its wild apomictic relative Tripsacum, researchers could breed a maize plant that would produce clones of itself over generations (see "What Makes Apomixis a Valuable Trait?".

One need not be familiar with the terminology or the process to recognize the revolutionary potential of apomixis. Hybrid production could be greatly accelerated, breeding for niche environments (small environments with unique conditions) could be economically feasible, and poor farmers could recycle seeds that maintain hybrid characteristics.

Knowledge about apomixis has grown considerably, and so has impatience to develop apomictic maize. So exactly why has it taken so long?

"At the beginning," says research team leader Olivier Leblanc, "we were working on the premise that apomixis is a simple trait and that it should not be overly difficult to transfer a single-gene trait to maize with the existing technology. We pursued an applied breeding rather than a basic science approach. We weren't interested in the mechanisms and the molecular basis for the phenomenon. We just needed to find that one and only apomictic specimen that was hiding out there among half a million experimental plants. We never found it."

Therein lies much of the impatience. In plant breeding, if you identify a source of variability for a given trait, eventually, through step-by-step plant crosses, the desired trait can usually be incorporated into maize varieties or lines. As it turns out, apomixis is complex. It certainly did not yield to a step-by-step procedure.

Does this mean that those years of work were unfruitful? No. In science, as false leads are discarded, efforts are redirected based on the insights obtained, according to David Hoisington, director of CIMMYT's Applied Biotechnology Center, where the research on apomixis takes place.

"Because we have such a strong team, partnerships, and advances in science," Hoisington explains, "we can continue our progress toward the ultimate goal, even if we change roads every once in a while."

Although the creation of apomictic maize remains the team's clear goal, the route there has changed from a relatively mechanical approach—transferring the apomixis gene(s) directly from Tripsacum to maize—to exploring other options that require better understanding of the apomictic process in general.

Most teams working on apomixis, according to Leblanc, are investigating and manipulating the sexual pathways of plants to produce an apomictic outcome. The CIMMYT-IRD team remains committed to using an apomictic plant and a related crop plant. In doing so, they can draw on their long experience with Tripsacum, "a truly beautiful model plant for apomixis," says team member Daniel Grimanelli, "which is an original approach compared to those being pursued by other groups." They are investigating the cell biology and molecular genetics of the processes behind apomixis, as well as barriers within the maize genome to the transfer of the characteristic.

They also draw support from a consortium formed in 1999 to accelerate progress. The IRD and CIMMYT joined in a five-year agreement with Pioneer Hi-Bred, Groupe Limagrain, and Novartis Seeds (now Syngenta) to bring their diverse strengths to bear on the apomixis challenge. For the CIMMYT-IRD team this means access to useful biological material, databases, information, and experts, as well as additional financial resources.

The team is excited about its new direction. "We're working on novel approaches and have some nice stuff cooking," says Leblanc. "But it's too soon to talk about major achievements. We're out of the prediction game for good."

What Makes
Apomixis a Valuable
Trait?

Apomixis—asexual reproduction through seeds—results in plants that are exact clones of the mother plant. The trait occurs naturally and has been identified in more than 400 species of plants, including some varieties of Tripsacum, a wild relative of maize. With an apomictic mode of reproduction, exact copies of the chromosomes are transferred from the mother plant to the progeny, making each offspring a clone of its ancestor. This direct transfer of chromosomes (and therefore traits) continues generation after generation.

The implications of transferring the apomixis trait to a major cereal crop such as maize are tremendous. Breeders would be able to greatly reduce the time and expense required to produce new varieties, for example, by instantly "fixing" a desired genetic composition, which normally takes several seasons. Apomixis is of particular interest to CIMMYT because it would make niche breeding, the development of cultivars tailored to unique agroecological areas and very specific uses, more economically feasible.

Seed producers would be able to reduce the cost of producing hybrids, which could translate into lower seed prices for farmers. Farmers in developing countries who obtained improved seed carrying the apomixis trait would be able to recycle their seed indefinitely, while maintaining various yield-enhancing properties usually associated with hybrids, which cannot be productively recycled.

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