Polyploid induction II --- potential benefits to the breeder

Biochemical composition changes: As a plant takes light energy, water and carbon dioxide to create carbon-based molecules of varying sizes and structures, each step in a biosynthetic pathway is controlled by an enzyme.  Each enzyme is coded for by one or more genes --- specific DNA sequences.  In a diploid plant, there may be two forms or doses of a particular enzyme.  In a tetraploid, there can be up to four forms or doses of that particular enzyme.  This can allow a plant to create new and novel biochemicals, in addition to altering the amounts of a particular biochemical produced.  Sometimes, these new biochemicals can create novel plant pigments, and potentially a novel flower color.  Sometimes, there is simply more of a particular biochemical produced, and the concentration of that biochemical may increase in a tetraploid.  This has the potential of creating darker and more intense flower or leaf colors, for example.

If you are breeding to enhance phytochemical composition or to increase specific phytochemical yield, similar phenomena may occur: tetraploid conversions may produce more of a particular phytochemical, or produce novel phytochemicals, or may simply produce more of all phytochemicals within a particular class of interest.  

Dosage effects re doubleness: Dosage effects can occur in physical characters as well as in biochemical ones.  For example, in diploid Pelargoniums, flowers may have extra petals so that the flower appear to be double.  Single flower forms have no alleles for doubleness; semi-double forms have one dose (one allele, or are heterozygous); fully double forms (which resemble a rosebud) have two doses (or are homozygous double).  When a diploid is doubled to create a tetraploid, instead of three possibilities (as in a diploid) there are now five possible combinations from single-flowered to full double-flowered.  Each additional dose of doubleness increases the number of petals in the flower.  The most attractive zonal pelargonium flowers tend to occur in genotypes which have 2 doses (duplex) or 3 doses (triplex) of the double-flowered allele.  Under identical growing conditions and rowing from identically aged cuttings, you can frequently convince yourself that you can distinguish between simplex, duplex, triplex and quadraplex flowering forms.  However, doubleness in pelargoniums is extremely sensitive to the environment: light levels, temperature, daylength, plant nutrition --- all can affect the expression of doubleness.

Restoring fertility: Hybrids between two different species (known as interspecific hybrids, or ISHs) are frequently sterile.  Fertility can be restored in some ISHs by converting them to polyploids.  Sometimes this occurs naturally, as in the fully fertile Digitalis ISH, D. xmertonensis.  Mertonensis resulted from the hybridization of D. purpurea and D. grandiflora, which created a sterile ISH.  However, the chromosome number naturally doubled in the ISH, creating the fully-fertile D. xmertonensis. 

How did this happen?  The phenomenon is called the production of unreduced gametes.  What occurs is that meiosis --- the production of gametes or reproductive cells --- breaks down, and rather than producing haploid (1n) gametes, the plant produces a small number of 2n gametes (eggs and pollen).  When two 2n gametes combine, a 4n embryo --- a tetraploid --- is produced.  In certain ISHs, each cell contains one set of chromosomes from each species parent.  Because these chromosome sets are different, the chromosomes cannot pair and reduce during meiosis, and the ISH is sterile.  When the ploidy is doubled, there are now two sets of chromosomes for each parent species, normal pairing can occur, meiosis proceeds normally, and ... fertility is restored.  In practice, it rarely works that easily.  But if you can restore any degree of fertility, you can select for fertility, and breed for it over successive generations. 

Breaking self-incompatibility: In some forms of self-incompatibility (SI), doubling the ploidy can create self-compatibility (SC).  This can be convenient in a breeding program because once you break SI, self-pollination becomes possible, making inbreeding and traditional line development more straightforward.  However, in many species, once you create SC individuals, you will then have to work through the inbreeding depression issues that frequently occur.  

Facilitating interspecific hybrids (ISHs) between species of different ploidy levels (bridging): In Nature, spontaneous polyploidy in plants occasionally occurs during evolution, creating an immediate barrier to cross-fertilization, and the instant creation of a new species.  For a breeder, this can be a problem if you want to cross those related species.  However, if you can double the chromosome number in the lower-numbered species, you may then be able to make successful crosses at the higher ploidy level.  This can enable you to bridge between species when direct hybridization may not be possible.  

Creating sterile triploids:  Last but certainly not least, it may be extremely convenient for a breeder to produce sterile triploid cultivars by making tetraploid lines by conversion, and then crossing tetraploid x diploid to create triploids.  In concept, many of these triploids will be sterile.  If you do this in potentially invasive species, you can minimize the potential for a new cultivar to become invasive once released.  However, be aware that moving from concept to implementation is not always easy