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How Do Plants Choose Mates?

Tasha Ladoux
Department of Biological Sciences
Claremont Graduate University
March 2002

Breeding systems of plants are an important aspect of nature for humans to understand. A breeding system simply refers to all the processes and mechanisms required for sexual reproduction to occur in any given plant species. The first process that must occur in most flowering plants is called pollination. This simply refers to the transfer of the male component, the pollen, to the female component, the stigma. This is usually done by insects, wind, or birds. Following pollination there are several steps the pollen must go through in order to reach the female egg, the ovule. The final step is called fertilization. This is where the union of gametes occurs, in other words, sexual reproduction. My research focuses on understanding the steps between pollination and fertilization.

There are many types of breeding systems in plants, one of which is called "self-incompatibility". This type of breeding system controls who the plant can mate with by not allowing the plant to self fertilize (mate with itself) or mate with close relatives. It may seem odd that any plant would be able to or want to self fertilize, however most plants are hermaphroditic which means they have both male and female parts within the same flower. Therefore, the close proximity of the sex organs makes it easy for the pollen to transfer onto the stigma thus promoting self fertilization. It is in the best interest of most plants to avoid self fertilization because it can promote disease or simply decrease the plants ability to survive. Over 60% of flowering plants have this "self-incompatibility" breeding system, however, there are several different forms throughout the many plant families. I am studying the different forms of "self-incompatibility" found in a certain family of plants, the Phlox family. The center of diversity for this family is in the Southwestern United States, which has several species that are considered rare or endangered.

Research which focuses on "self-incompatibility" has improved crop production of such things as rye, apples, and mustard. In addition, there are many plants which are extremely rare and are the focus of revegetation efforts. Knowing the type of breeding system controlling the species can greatly influence the rehabilitation of that species. For example, the Hawaiian Silversword was the focus of an enormous revegetation effort because there were only three left on the island. After revegetating the island with over 1500 plants they realized that none of the individuals could mate due to their breeding system. Thus, understanding the many forms of self-incompatibility can be useful in many regards.

In order to learn more about how the breeding system works in the Phlox family I work with living plants representing many different species within the family. There are two major steps that I follow for my research. The first step involves performing pollinations among all the individual plants and keeping track of which combinations actually produce fruits. If a fruit is produced this means the two plants are "compatible", if they do not produce fruits the two plants are "incompatible". If a plant is unable to produce fruits after self-fertilization occurs it is considered "self-incompatible", hence the name of the breeding system. By recording which plants are incompatible I can learn more about how the breeding system works.

The second step of my research involves observing the many steps a pollen grain must go through to reach the ovule. The ovule of a plant is located inside a protective casing. At the top of the casing is a receptive surface for the pollen to land, this is called the stigma. Once pollination has occurred, the pollen grain must find a pathway through the protective casing to reach the ovule, this pathway is called the style. As the pollen grain grows down through the style it forms a tube, like digging a tunnel, until it reaches the ovule where fertilization occurs. The breeding system of the plant acts like a gatekeeper throughout this entire process. The moment the pollen grain lands on the stigma the first gate must be opened. If the pollen grain is of the right "type" it is allowed to pass through the first gate, and so on until it finally reaches the ovule. By observing the pollen tube with a microscope I can assess how far the pollen tube was allowed to go, in other words, how many gates it passed. By comparing the number of gates and how these gates affect pollen tube growth among the different species I can learn more about the different forms of "self-incompatibility" systems. Understanding these different forms of breeding systems may produce better crops in the future and help us conserve the diversity of plant species on our planet.