Plant Inbreeding and Outbreeding Spectrum

 

Different degrees of inbreeding (self-pollination) and outbreeding (cross-pollination) can be observed in the fertilization process of plants and the nature of their flowers. Plants can be categorized into four main groups on this spectrum, with some crops being somewhere between those groups: primarily inbreeding, strongly inbreeding, primarily outbreeding, and strongly outbreeding plants. This categorization will help future seed savers to be more familiar with the fertilization habits of plants grown for seed production, as well as with the implications of their respective management (including isolation).

Inbreeders

Inbreeding involves a process where the sperm of a plant fertilizes the ovule of the same plant, resulting in offspring genetically similar to its parent and well adapted to their environment. As it doesn’t rely on external vectors to transfer pollen from one plant to another, it is a safe and reliable way to ensure reproduction as well as the success of the next generation. In a seed production setting, inbreeders usually require shorter isolation distances and a smaller population size to maintain each variety’s genetic makeup.

Primarily inbreeding plants include individuals that possess perfect flowers that remain almost totally closed, and are therefore still capable of being pollinated through external vectors such as insects. On the other hand, strong inbreeders can be categorized as plants with perfect flowers remaining completely closed and relying almost exclusively on self-pollination to reproduce. In some cases, the flower’s anthers form a cone around the pistil, completely sealing off the stigma and preventing it from receiving any pollen other than its own (like most tomato varieties). Peas are another example, whereby the flower remain closed when the stigma is receptive to pollen, thus favoring the pollen from its own anthers for fertilization. When the pea flower opens, the fertilization process has already occurred.

There are instances where inbreeders can still cross with another plant of a different variety. Insects such as bees can for example tear open the flower’s petals and deposit pollen coming from a different plant. Under specific climatic conditions, the flowers will open while it is receptive, allowing the stigma to receive pollen from another plant before it has been self-pollinated by its own anthers. Human intervention can also induce cross-pollination through the transfer of pollen from one strong inbreeder to another.

Outbreeders

Outbreeding involves a process where the sperm of a plant fertilized the ovule of another plant of the same specie, resulting in offspring carrying a different gene pool compared to their parents, therefore giving birth to a more diverse plant population able to adapt to local, changing environmental conditions more rapidly.  It mainly relies on external vectors such as insects or wind to transfer pollen from one plant to another plant (or the same plant in case of monoecious crops). In a seed production setting, outbreeders usually require greater isolation distances and a larger population size to maintain each variety’s genetic makeup.

Primarily outbreeding plants include individuals that possess perfect flowers that open at sexual maturity, thus allowing them to be pollinated by insects or other vectors. However, inbreeding is still possible when self-pollination occurs. Some of these plants will inbreed at a lower rate as the male and female sexual structures will mature at different times. Individuals such as monoecious crops are often more likely to cross as the male and female flowers are physically separated, even though inbreeding is still possible when self-pollinated.

There are cases where some crop species–such as brassicas–have evolved to prevent self-fertilization through specific genetic mechanisms in order to encourage outbreeding, allogamy, and genetic diversity. Being self-incompatible, these crops will indeed halt any fertilization process initiated by one of their pollen grains, making the production of fruits or seeds impossible. Dioecious plants are probably the strongest outreebers of all, only having either male or female flowers on individual plants (i.e. payapa, spinach) and thus relying completely on cross-pollination to reproduce.