A Review of the Evidence - Search CIMMYT repository

only one is eventually used to produce commercial hybrids. To facilitate the task, groups ofinbreds are often crossed with one or more common testers selected from among thematerials that are known to combine well with the inbreds being evaluated (known as“heterotic partners”). The top-cross test may be done as early as the S1 generation or as lateas the S7 or S8 generation. Most commonly it is first done at some intermediate stage, suchas S3. Lines showing good combining ability are further inbred and then evaluated again inadditional hybrid combinations.After a superior hybrid has been identified, the final step involves the production ofcommercial seed. This is accomplished by planting both parents in the same field. Aproportionally greater area is planted to the female parent (or seed parent) which willproduce hybrid seed. Enough plants of the male parent are sown to ensure that sufficientpollen is produced to cross-pollinate the female parents. The female parents are detasseledto eliminate any possibility of self-pollination.The profitability of hybrid maize seed production depends critically on the seed yield ofthe female parent, which tends to vary greatly between hybrid types. Seed of single-crosshybrids is usually expensive to produce, because it is harvested from female parent linesthat are often low in productivity. Seed of three-way-cross and double-cross hybrids tendsto be less expensive to produce, because it is harvested from more productive femaleparent lines. Seed of non-conventional hybrids tends to be cheapest of all to produce,because it is harvested from female parents that are always full-vigor (non-inbred) plants.Theories of hybrid vigor and inbreeding depressionThe term hybrid vigor refers to the increase in performance of a hybrid compared to themean performance of its parents (with performance expressed in terms of one or moremeasurable characteristics, such as size, vigor, and/or yield). George Shull (1908, 1909,1952) coined the term heterosis – actually a contraction of “stimulus of heterozygosis” – todenote this increase in performance. Today, the terms hybrid vigor and heterosis areconsidered synonyms and are used interchangeably.Most geneticists view heterosis and inbreeding depression as opposite manifestations ofthe same phenomenon. In his classic text Introduction to Quantitative Genetics, Falconer(1989) states that “complementary to the phenomenon of inbreeding depression is itsopposite, ‘hybrid vigor’ or ‘heterosis’…. That the phenomenon of heterosis is simplyinbreeding depression in reverse can be seen by considering how the population meandepends on the coefficient of inbreeding.”Two major theories and several minor theories have been advanced to explain heterosis(and, by implication, inbreeding depression). Although several of these theories havegained large numbers of adherents, none offers a complete explanation of this complexphenomenon (Crow, 1997).The most widely accepted theory of heterosis (known as the dominant theory) is based onthe assumption that hybrid vigor results from bringing together favorable dominant genes.31