Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 285

The immediate effect of hybridization is the assembly of two different genomes into a newly created individual. Several genetic consequences may result from such union of diverse genomes, some of which may be desirable, whereas others may not be desirable. The key ones are as follows:

 Expression of recessive lethal geneCrossing may bring together recessive lethal genes (that were in the heterozygous state) into the expressible homozygous state. The resulting hybrid may die or lose vigor. By the same token, hybridization can also mask the expression of a recessive allele by creating a heterozygous locus. Individuals carry a certain genetic load (or genetic burden), representing the average number of recessive lethal genes carried in the heterozygous condition by an individual in a population. Selfing or inbreeding predisposes an individual to having deleterious recessive alleles that were protected in the heterozygous state to becoming expressed in the homozygous recessive form.

 Hybrid necrosisEspecially the crossing of parents that are somewhat distantly related (but still the same crop species), may result in the phenomenon of hybrid necrosis. Interactions between pairs of genes in both parents may work out unfavorably to the physiology of the plant. This phenomenon has been reported in wheat and rye, but also in Arabidopsis.

 HeterosisGenes in the newly constituted hybrid may complement each other to enhance the vigor of the hybrid. The phenomenon of hybrid vigor (heterosis) is exploited in hybrid seed development (see Chapter 19).

 Transgressive segregationHybrids have features that may represent an average of the parental features, or a bias toward the features of one parent, or even new features that are unlike either parent (transgressive segregates). When the parents “nick” in a cross, transgressive segregates with performance superseding either parent is likely to occur in the segregating population.

 Genome‐plastome incompatibilityPlastomes (the genetic material found in plastids such as in chloroplasts) and genomes in most genera function to form normal plants, regardless of the taxonomical distances between the plastid and nuclear genomes. However, in some genera, plastomes and genomes, having co‐evolved to a significant degree, are only compatible within a specific combination.