Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 322

 Genetic make‐upAll the progeny from an individual propagated asexually are genetically identical (clones) and uniform. Clones are products of mitosis. Any variation occurring among them is environmental in origin.

 Heterozygosity and heterosisMany species that are asexually propagated are highly heterozygous. They are highly heterotic. Consequently, they are susceptible to inbreeding depression. For those species that can be hybridized without problems, any advantage of asexual propagation is that heterosis, where it occurs, is fixed for as long as the cultivar is propagated asexually.

 PloidyMany known species that are asexually propagated are interspecific hybrids or have high ploidy.

 ChimerismClones are stable over many generations of multiplication. The only source of natural variation, albeit rare, is somatic mutation in the bud. Plant breeders may generate variability by the method of mutagenesis. Whether natural or artificial, somatic mutations are characterized by tissue mosaicism, a phenomenon called chimerism.A chimera or chimeric change occurs when an individual consists of two or more genetically different types of cells. Though heritable changes, these mosaics can only be maintained by vegetative propagation (not transferable to progenies by sexual means).

There are four basic types of chimeras.

1 SectorialThis chimera is observed in a growing shoot as two different tissues located side‐by‐side. The effect of this modification is that the stem develops with two distinct tissues on each half.

2 PericlinalThis type of chimera consists of two thin layers of different genetic makeup, one over the other.

3 MericlinalWhen an outer layer of different genetic tissue does not completely extend over the layer below, the chimera is mericlinal.

4 Graft chimerasUnlike the first three chimeras that have genetic origin, a graft chimera is a non‐heritable mixture of tissues that may occur after grafting is made.

Additional information on chimeras is found in Chapter 12 (12.8). Whereas they are undesirable in crop plants, chimeras may be successfully exploited in horticulture.