Xenogamy

Xenogamy

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 ho-mozygous). 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 ho-mozygous plant with a pair of corresponding chromosomes would be designated as either "TT" (two "T" alle-les) 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 off-

spring. Such organisms, when mated with other homozy-gotes, breed true with regard to a particular trait (e.g., stem length). In many cases, xenogamy allows the rein-troduction of alleles-or the introduction of new alle-les-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.