Cross-pollination is the transfer of pollen between two separate plants, a process fundamental to plant reproduction and the creation of new varieties. This natural event, often mediated by wind or insects, can also be intentionally performed by breeders and gardeners. Whether two different plants can cross-pollinate depends entirely on their biological relationship. Success is governed by strict genetic rules that determine compatibility, making some crosses highly probable and others biologically impossible.
Understanding Genetic Compatibility
The ability for two different plants to successfully cross-pollinate and produce viable seeds is determined by how closely they are related on a genetic level. Compatibility is highest between different varieties or cultivars within the same species, such as crossing a beefsteak tomato with a cherry tomato, which is almost always successful.
Moving one step further out in the classification system, cross-pollination can sometimes occur between different species that belong to the same genus. For example, a plum and an apricot can sometimes be crossed because both belong to the Prunus genus, resulting in a hybrid like the pluot. These interspecific crosses often face significant biological hurdles, but the shared genetic heritage allows for the possibility of hybridization.
The genetic difference between species can lead to a phenomenon known as reproductive isolation. This isolation is enforced by several pre- and post-fertilization barriers that prevent the successful creation of a hybrid. A major barrier is the active rejection of foreign pollen by the recipient flower’s pistil, an event known as pollen-pistil incompatibility.
The female reproductive structure recognizes the pollen as “non-self” and prevents the growth of the pollen tube, which delivers the male genetic material for fertilization. If the pollen is too distantly related, this molecular incongruence stops the process before fertilization can occur.
Crosses between plants from different plant families, such as attempting to cross a rose with a tomato, are almost universally impossible due to the immense genetic distance. The difference in chromosome structure, number, and genetic instructions prevents the formation of a viable zygote, similar to how a cat and a dog cannot produce offspring. In essence, the success of a cross is a measure of genetic closeness, with the biological barriers becoming stronger as the relationship between the two parent plants becomes more distant.
The Manual Process of Cross-Pollination
When gardeners or breeders want to intentionally create a hybrid, they must bypass natural pollination mechanisms and perform the cross manually in a controlled environment. The process begins with the careful selection of two parent plants that possess desirable traits and are known to be genetically compatible. The pollen donor (male parent) is selected for its pollen, while the recipient plant is prepared to receive the foreign genetic material.
Preparation of the recipient flower is a meticulous step, particularly in plants that can self-pollinate, like tomatoes. To ensure that the chosen pollen is the only one to fertilize the flower, the male parts (stamens) of the recipient flower must be removed before they mature and shed their own pollen, a technique called emasculation. This preparation is performed while the flower bud is still closed or just beginning to open, preventing any accidental self-pollination.
Once the recipient flower is prepared, the breeder collects fresh pollen from the donor flower, often using a small, soft brush or a cotton swab. This pollen is then gently transferred and applied directly onto the stigma, the receptive tip of the female part of the recipient flower.
To protect the newly pollinated flower from natural agents like wind or insects that could introduce unwanted pollen, the flower is usually covered with a small paper bag and tagged with information about the cross. The paper bag remains on the flower until the fruit begins to develop and the stigma is no longer receptive, ensuring the integrity of the cross.
Outcomes of Successful Plant Crosses
The immediate result of a successful, intentional cross-pollination is the production of seeds within the fruit of the female parent. These first-generation offspring, known as F1 hybrids, carry a mix of genetic material from both parents. F1 hybrids are often highly valued in agriculture and gardening because they frequently exhibit hybrid vigor, a phenomenon where the offspring are more robust, productive, or disease-resistant than either parent.
The seeds saved from these F1 hybrid plants, however, behave differently in the next growing season. If a gardener plants the seeds collected from an F1 hybrid plant, the resulting F2 generation will not “come true” to the parent. This is due to genetic segregation, which is the reshuffling of the mixed genetic traits from the F1 parent according to the principles of Mendelian genetics.
The F2 plants will display a wide range of characteristics, often resulting in unpredictable variations in yield, size, or quality. Some F2 plants may exhibit the desirable traits of the F1 hybrid, but many others may regress toward the less vigorous characteristics of the original grandparents. For this reason, gardeners who desire consistency must purchase new F1 hybrid seeds each year, as the specialized vigor and uniformity are lost in the F2 generation.
In cases of wide crosses, where the parent plants are from different species or genera, the resulting hybrid may face postzygotic barriers that affect fertility. The hybrid offspring may be partially or completely sterile, often because the differing number of chromosomes from the two parents prevents proper formation of reproductive cells. This sterility is a common outcome in interspecific hybrids, such as the production of seedless fruits like some varieties of watermelon, which possess an odd number of chromosomes.