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Introgression, a word often used in discussions of hybrid variation, has had various definitions. As originally defined by Anderson and Hubricht (1938: 396), it was the "infiltration of germ plasm from one species into another through repeated backcrossing of the hybrids to the parental species." This definition, however, is problematic because it involves the word species, which is not clearly defined. More recently, there has been an attempt to limit use of the term to situations involving permanent infiltration of genes from one population into another (Heiser 1973; Stebbins 1959). Thus, Rieseberg and Wendel (1993: 7) define it as the “permanent incorporation of alien alleles into a new, reproductively integrated population system.” But here, too, a problem arises with the definition of the term "integrated population system." There is also the problem of saying what constitutes "infiltration." In the case of many crosses, the genetic influence of natural hybridization is very local and affects only individuals in the immediate vicinity of the contact zone. In the case of others, the effect may be detectable hundreds of kilometers away. How should one draw a line between these two cases? What is "infiltration" and what is not?
In fact, the main difficulty with "introgression" is that it is thought of as an introduction of genes from one entity (usually thought of as a "species") into another. Actually, natural hybridization between two populations treated as species generally has a genetic effect only on individuals in the vicinity of the contact zone. No genes are introduced into other individuals elsewhere. Over the remaining geographic ranges of the two interbreeding populations, all individuals remain pure and genetically unaffected. The same distinction exists with the breeder's term introduction, the transfer of a trait via hybridization from a donor stock to a target stock. Here, the trait is introduced into only a subset of the target stock.
Indeed, the whole idea of introducing genes into a "genetic background" is probably misleading. In many cases, what is called "introduction into a target stock" is really crossing individuals of the target stock with individuals of some other type to produce hybrid individuals with genetic complements of mixed parental origin. Esuperanzi (2005: 302) suggests that birds produced by repeated backcrossing to a single parent should be regarded as belonging to that parental type only when they contain less than 1/16 blood from the other parent. But this is clearly arbitrary. Why not 1/8 or 1/32?
Even when the intention is to transfer a single gene to the target stock other genes often are transferred at the same time. For example, Carver and Taliaferro (1992: 131) note that even when the primary purpose is to transfer disease resistance to a crop, other traits, such as yield, may also be increased. If backcrossing occurs to one parental type or the other, the relative contributions of the two parents to the genetic complements of the resulting hybrids will vary in accordance with the amount of backcrossing that occurs. But no matter how many backcrosses to the target stock occur, so long as the traits of the offspring are discernibly mixed, they are hybrids, distinct from the target stock type.
For the foregoing reasons, the term introgression, though widely used in the literature on hybridization, is avoided on this website. NEXT PAGE >>
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