When used by botanists and plant breeders, xenogamy (also called outbreeding) generally refers to a form of cross-pollination. Xenogamy is also a term more broadly used in genetics to describe the union of genetically unrelated organisms within the same species. In all cases, xenogamy promotes genetic diversity and thus, also enhances the overall fitness of a species.
In some circumstances, xenogamy and outbreeding are also referred to as crossbreeding. Regardless of the exact terminology, the core concept involves an increase in genetic variability. With crossbreeding, genetically dissimilar or unrelated animals from the same breed can be crossed in a process known as outcrossing. True crossbreeding exists when, for example, differing breeds of cattle are allowed to mate and produce offspring. Extreme xenogamy exists with species crossing (a mating between organisms from two different species).
Induced xenogamy and crossbreeding are often attempts by scientists to genetically combine desirable traits from two differing species or breeds in order to produce offspring with more desirable characteristics. Successful crossbreeding, whether in plants or animals, usually results in increased hybrid vigor (a set of characteristics that can include increased fertility, faster growth rates, increased immunological tolerance, greater strength, and/or other desired characteristics of a hybrid species).
In addition to induced xenogamy, outbreeding is a fundamental part of natural selection and, by producing new and varied genetic combinations, an essential element of evolution. As such, outbreeding is an important tool in the continued survival and evolution of a species.
In terms of genes and alleles, xenogamy promotes genetic variability and vitality within a breeding population by reducing homozygosity (the state of being homozygous). Organisms that are homozygous carry identical alleles on both chromosomes of a pair of homologous chromosomes.
For example, using traditional notation to designate dominant and recessive alleles (e.g., "T" for tall stems and "t" for short stems in a particular plant species), a homozygous plant with a pair of corresponding chromosomes would be designated as either "TT" (two "T" alleles) or "tt" (two "t" alleles). With regard to phenotype (the outward expression of genotype), "TT" homozygous plants should normally produce tall-stemmed plants. The "tt" genotype should—under normal environmental conditions—result in a short stemmed plant. If the "T" allele is dominant, tall stemmed plants would be the normal expected phenotypic expression of "Tt" or "tT" genotypes.
Because homozygotes contain identical alleles on their chromosome, in the absence of mutation they can only produce gametes (sex cells) that contain those same alleles. For example, an organism with a homozygous "TT" genotype can contribute only a "T" allele to its offspring. Such organisms, when mated with other homozygotes, breed true with regard to a particular trait (e.g., stem length). In many cases, xenogamy allows the reintroduction of alleles—or the introduction of new alleles—into a population.
By uniting differing genotypes, xenogamy allows increases in genetic variability that, in turn, exert measurable influences on the frequency of genes, types of alleles, and traits within a population.
Although such simple examples as above serve to illustrate broad genetic principles, the degenerate nature of the genetic code (i.e., there are multiple codes that convey the same genetic instructions) means that homozygosity more specifically means that the products of the instructions contained in the homozygous genes are similar enough to produce identical visible expression (identical phenotypic expression). Because there are multiple codes that represent multiple sequences of bases in the nucleic acids (deoxyribonucleic acid [DNA] and ribonucleic acid [RNA]) to convey essentially identical instructions for the construction of proteins, multiple instructions can result in the formation of the same protein. As a result, although an individual may be homozygous for a particular gene, this does not necessarily mean that the base sequence found in those genes is identical.
In plant species, there are several natural mechanisms that can result in xenogamy, including self-incompatibility. With self-incompatibility, there is an inability on the part of sex cells (gametes) from the same species of plants to produce a viable embryo. With such species, it is usually the case that pollen, unable to induce fertilization on its own stigma, is able to successfully grow on the stigma of other plants of the same species. The process involving the transfer of pollen to a foreign stigma is termed allogamy. Regardless of the exact mechanism, self-incompatibility mechanisms promote xenogamy (outcrossing) and heterozygosity while acting to prevent inbreeding. Self-incompatibility is often the result of incomplete nuclear or genetic fusion. The failure of the fusion processes are usually traced to a single genetic locus (S-locus) that exists as multiple alleles.