Glossary Plant Breeding

polyploidization. In case, both the fusing gametes are unreduced ones, the process is
termed bilateral sexual polyploidization. Both types have been reported in the
progeny derived from crosses between Solanum spp. Sexual polyploidization via 2n
gametes is a distinctly separable process from somatic doubling. It has the advantage
of providing for heterosis, variability, minimal inbreeding, high fertility and
maximum heterozygosity (new intra- and inter-locus interactions). For example,
tetraploids derived from diploids via somatic doubling with colchicine have the same
or low yield compared to their undoubled counterparts and were inferior to tetraploids
derived from bilateral sexual polyploidization in potato.
Shattering. Falling out and the loss of seeds from a plant before harvest or during
harvesting operations. Resistance to shattering is important to prevent the loss in
crops like soybeans, mungbeans, and many other crops.
Shift. A chromosomal structural change resulting in the transposition of a chromosome
segment to another place in the same chromosome. It may be homo- or
heterobranchial depending on whether transposition occurs within the same or
different arms of the chromosome.
Shuttle Breeding. A breeding approach that aims at speeding up advancement of
segregating materials by moving the germplasm rapidly from place to place. Although
the primary objective is to get two or more generations per year, a valuable side effect
results in terms of incorporation of insensitivity to photoperiod in the selected
materials. This system was initially followed in breeding for stem rust resistance. It
led to selection of photo-insensitive wheat genotypes besides permitting wide-spread
adoption of CIMMYT cultivars. Thus wide-testing strategy of CIMMYT led not only
to the development of many widely grown wheat varieties, but also led to the concept
of megaenvironment and the use of very wide testing in many other crop species.
Shuttle Vector. A vector (for instance, a plasmid) constructed in such a way that it can
replicate in at least two different host species, allowing a DNA segment to be tested
or manipulated in several cellular settings.
Shotgun Technique. Cloning a large number of different DNA fragments as a prelude to
selecting one particular clone type for intensive study.
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