Glossary Plant Breeding

reproduction or by mutation. If gene-for-gene relationships are common in host
specialized pathogen systems, VR and HR are extremes of continuum. With few
genes (each with large effects) operating, differential interactions are easily
discernible and the result is VR. With more genes (each with relatively small effects)
operating, differential interactions are less easy to discern, and the result is a mixture
of HR and VR. With many genes, each with still smaller effects, differential
interactions cannot be recognized, and the resistance is predominantly HR.
Vertifolia Effect (Van der Plank 1963, ’68). The loss of polygenic, partial resistance in
the process of breeding for vertical resistance. Van der Plank demonstrated it by
comparing potato cultivars with and without race-specific (R) genes for resistance to
Phytopthora infestans with races virulent on the R genes. The epidemic built up faster
on cultivars with race-specific than on those with race-nonspecific resistance.
Vertifolia effect, although occurring in some cases, is not a universal phenomenon.
For example, Parlevliet and Kupier (1977) reported that ‘Cebaka Capa’ barley carried
both race-specific resistance gene Pa 7 and a high level of polygenic, partial
resistance to leaf rust. Probably, the vertifolia effect does not occur universally for
two reasons: (1) very susceptible genotypes are constantly removed from breeding
nurseries, and in fact, parents used in crosses likely have some genes for partial
resistance, and (2) major race-specific genes often have an incomplete expression in
the field with their expression becoming more complete when backstopped by genes
for partial resistance.
Viability. The number of an individual’s offspring that survive to maturity and produce at
least one offspring each (in the broad sense); the probability that a fertilized egg will
survive and develop into an adult organism.
Vilmorin Principle. The selection of a line/individual from a population on the basis of
progeny test. It has come to be called so after the name of a French scientist who
successfully practised it.
Virion. The ultimate stage of virus development representing the static, inert form of a
virus. It is the extracellular state of a virus in which no reproduction and no
metabolism take place.
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