Glossary Plant Breeding

recurrent selection procedures are employed throughout on both the plant and seed
portions of each plant generation. The diallel and selective mating aspects of this
system infuse simultaneously multiple genotypes into a few central populations.
According to Jensen (1970), it involves a planning phase and an implementation
phase. The latter has four stages: a basic parent series of crosses (1) which set up the
F 1
diallel series of crosses (2) which when composited form P 2 ; spaced P 2 and
subsequent P 3 , etc. provide the populations within which selective mating (3) is
practiced on mass and recurrent selection principles. The final stage (4) is standard
line selection from the various F 5 composite populations. Specifically, the proposed
arrangements provide for broad use of germplasm, simultaneous input of parents,
creation of persistent gene pools, breaking of linkage blocks, freeing of genetic
variability, and general fostering of genetic recombination.
Dicentric Chromosome. A chromosome with two centromeres.
Dichogamous. Of flowers whose anthers and pistil mature at different times.
Dichotomous Classification. The classification applying to an event that can occur in
one of the two possible ways.
Diclinous. Having male and female germ cells in separate flowers either on the same
plant (monoecious) or on different plants (dioecious).
Differentiation. The changes in cell shape and physiology associated with the production
of the final cell types of a particular organ or tissue. It is the origin of differences
during embryonic development between spatial parts of an originally homogeneous
whole. In higher plants and animals, it is one of the spectacular aspects of
development.
Digenomic Species. Species containing two distinct genomes. Such species are
invariably amphidiploids. These are also called secondary species because they are
convergent products of primary species. In oilseed brassicas, B. juncea (AABB), B.
napus (AACC), and B. carinata (BBCC) are digenomic species. It is interesting to
note that these secondary species are basically self-pollinated compared to their
outbreeding diploid constituents. This is probably because genomic divergence makes
it unnecessary for continuous reshuffling of genes through cross-pollination.
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