The introduction of alien genes, their adaptation and integration within a new genome

The expression and apparent heritability of a gene that is suddenly thrust into an unprepared genome of a complex organism such as corn is often not clear and not stable. Although the details of a trait's heritability, the type of gene action and interaction and the genetic location on the chromosomes may take years to discover, this does not preclude attempts to use said trait for crop improvement. First, however, some degree of heritability needs to be established, such as is the case with sporadic resistance or tolerance of plants to pests.

The sugary enhancer (se) gene is an example of a gene or complex of genes whose inheritance and location is not yet understood, and yet we go ahead and use it in sweet corn improvement. In the case of sh2, bt2, wx, o2 and several other endosperm mutants, their genetics was worked out long before their practical application. During the genetic experiments, the background of these mutants was usually adapted for increased viability by unconscious selection of modifying genes that affect the growth pattern of the pericarp.

Generally, the more divergent the background source of a gene, such as from an unrelated or exotic race of corn, the more difficult its integration into a new background. This is especially true with germplasm extracted from corn's wild relatives, teosinte and Tripsacum. In the case of teosinte, when its key traits are transferred to a background of modern maize they have unstable expression and variable linkage relationships (review, Galinat, 1974). Because the genes involved do not always behave in a clear-cut Mendelian manner, one taxonomist (Iltis, 1983) has concluded that they do not exist. Instead, Iltis would have "a catastrophic sexual transmutation, a unique macroevolutionary event," in which the secondary male traits of the teosinte tassel became the secondary female traits of the corn ear (see previous item).

In the case of experimental introgression from Tripsacum, penetrance of the key trait genes is even more obscure. Usually it is primarily the dominant alleles from Tripsacum masking the recessive marker genes in maize that find expression. With considerable difficulty, resistance (Ht3) to Helminthosporium turcicum was extracted from T. floridanum (Hooker). Tripsacum Toes not appear to have been involved in the origin of either corn or teosinte. Most of its chromosomes have a different genetic architecture from that of corn and will not pair with corn.

Walton C. Galinat
 
 


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