BREEDING SYSTEMS

I. OUTBREEDING

Outbreeding is defined as crossing between different individuals. This is guaranteed in many animals by the presence of two sexes. Unisexual (dioecious) plants, however, are rather uncommon, with most plants having bisexual flowers or both sexes of flowers present on the same plant (monoecious). Obviously, dioecious plants are outbreeders. Plants with bisexual flowers and monecious plants have evolved a number of structures and biochemical features that help guarantee outbreeding. Some of these are:

A. Flower morphology
Example - Primula, Gelsemium

1. Heterostyly - pins & thrums - pollinators entering a pin get pollen on the middle of their body while those entering thrums get pollen on the end of their body. Notice how a pollinator entering a pin after visiting a thrum will have the pollen at the right position for its deposition on the stigma and vice versa. However, a pollinator visiting only thrums will not transfer pollen to the stigma and the same is true for an insect visiting only pins.

2. Pollen-grain size - pollen grains can be large or small

3. Stigma papilla size -stigma papilla can be large or small; the pollen grains and stigma papillae have to be matched in size for fertilization to occur.

B. Self-incompatibility reaction (biochemical in nature). Even if all the mechanical blocks to self-fertilization are overcome, self-incompability reactions will prevent seed development. These mechanisms can be pre-zygotic (before zygote formation) or post-zygotic (after zygote formation). The pre-zygotic mechanisms are better characterized than the post-zygotic. An example of a pre-zygotic self-incompatbility reaction is the prevention of pollen grain germination.

II. SELF-FERTILIZATION

Self-fertilization occurs when pollen fertilizes an ovule on the same plant. Each of the seeds in an ovary results from individual fertilization events. In some plants both outbreeding and self-fertilization can occur on the same flower so some seeds are the result of outcrossing and others the result of self-fertilization. Other plants, such as some members of the genus Viola produce two types of flowers. One occurs above ground and is available for pollen from other plants while the other flower is cleistogamous; it occurs below ground and never opens, guaranteeing self-fertilization.

Plants can vary from being nearly complete outbreeding to nearly complete self-fertilizating, with many intermediate stages. Plants can also switch between the two modes in response to environmental factors. The degree of outbreeding or self-fertilization was difficult to document until the development of techniques that allowed the comparison of proteins (isozymes) and DNAs of the parents and progeny.

III. APOMIXIS

Apomixis is reproduction without fertilization. It can occur in two ways:

A. Vegetative - this can occur simply by fragmentation, especially in aquatic plants but may involve specialized vegetative structures such as aerial bulbils that are easily detached from the parent. Many types of plants are capable of reproducing in this manner, including out-breeders.

B. Agamospermy - In this case normal seed is set but sexual fusion has not occurred. Some botanists would define agamospermous plants only as being apomictic. There are a number of mechanisms that can result in agamospermy but the final outcome is that the seed is not the result of the fusion of gametes. In some cases, the flower actually has to be pollinated before seeds will develop but these seeds are still clones of the maternal plant.

Plants can be obligate or facultative apomicts. Obligate apomicts can only reproduce via apomixis so they are essentially incapable of generating variability. Facultative apomicts can reproduce via apomixis or sexually so they have the capacity to generate either clones or genetically variable progeny. The ability to breed apomictic crop plants may eventually be very important because farmers could replant their seeds and be guaranteed that subsequent crops would be genetically identical to their first planting. This is not true of hybrid seeds where planting the seeds would result in highly variable and less valuable crops.

SUMMARY

There are evolutionary advantages to outcrossing, the main one is that it maintains high levels of diversity relative to self-fertilized and apomictic plants. If the maintenance of genetic diversity was of paramount importance in all cases, you would expect that virtually all plants would be outbreeders. However, outbreeding plants generally put more energy into reproductive efforts and there are cases in which the production of diversity would not be of any advantage to the plant, at least in the short term, such as in plants that are highly adapted to their habitats. On the other hand, obligate apomicts are essentially at an evolutionary deadend with somatic mutations the only source of variability. A change in the environment, new herbivores, diseases, etc. would have a devastating outcome on such genetically uniform plants.