Agriculture Reference
In-Depth Information
time taken from planting an apple seed to the time that
fruit can be evaluated. With several other clonal crops,
even the time to develop a new apple cultivar can be
considered a relatively short time period in terms of
the life of a cultivar. For example, the most common
date palm cultivar in commercial production is a clonal
line derived from the Middle-east called 'Siguel'.To
our knowledge, no one knows the derivation of this
line, but the cultivar still predominates as the leading
cultivar around the world.
Clonal crop species have shown a high frequency of
exploited natural mutations compared to other crop
types, probably simply as a result of their clonal propaga-
tion making any 'variant' obvious and easily multiplied.
As a result many clonal cultivars are the result of natural
mutations rather than arising from selection following a
specific hybridization between parental lines. For exam-
ple, the potato cultivar 'Russet Burbank' is a mutation
from 'Burbank', and similarly 'Red Pontiac' is a natu-
ral mutation of 'Pontiac' and many apple cultivars are
simply fruit colour mutants.
has been the result of conscious human selection, based
on the argument noted above and where sexual seed
development can also cause a problem of 'volunteers' in
subsequent crops.
In clonal crops where the reproductive product is
used, there is of course no question about reducing
flowering, but nonetheless, reproductive peculiarities
and sterility problems are still very common. In gen-
eral, selection has favoured the vegetative part of fruit
development at the expense of seed production. In
the extreme, bananas are vegetatively parthenocarpic
(i.e. formation of fruit without seeds). Wild bananas
are diploid (
n
11) and reproduce normally produc-
ing fruit with large seeds. Commercially grown bananas
are triploid and hence sterile, so many banana cultivars
cannot be used as parents in a breeding scheme. Pineap-
ples are also parthenocarpic but self-incompatible, so
that clonal plantings give seedless fruits, even though
fruit would be seedy if pollinated by another genotype.
In addition, mangoes and some citrus sometimes pro-
duce polyembryonic plants (where multiple embryos
are formed from a zygote by its fission at an early devel-
opment stage, and hence in effect, results in clonal
pseudo-seedlings identical to the mother genotype)
which can be a great nuisance to breeders when they are
selecting amongst segregating populations of sexually
generated progeny.
In conclusion reproductive derangement in clonal
species can result in potential sexual crosses between
particular parent combinations not being possible, or
that individual parental lines cannot be used for sexual
reproduction. This limits the options open to breeders
of clonally reproduced crops.
=
Sexual reproduction in clonal crops
In crops that are reproduced from true botanical seed
there is definite selection for reproductive normality
and high productivity of sexual reproduction. In clonal
crops this has not always been the case and the result
can hinder the ability of breeders to generate variation
by sexual reproduction. There are two main types of
clonally reproduced crops (excluding apomicts):
•
Those that produce a vegetative product
•
Those producing a reproductive product (i.e. fruit)
Maintaining disease-free parental lines and
breeding selections
Those which produce a vegetative product have
almost all been selected to have reduced sexual repro-
ductive capacity and can exhibit problems in making
sexual crosses. This is probably the result of conscious
or subconscious selection for plants which do not '
waste
energy
' on aspects of sexual reproduction and will there-
fore put more energy in to the vegetative parts. Extremes
are found in yams and sweet potatoes in which many
cultivars never flower and in many cases they cannot be
stimulated to reproduce sexually. Modern potato culti-
vars have far less flowering than their wild relatives and
of those that do flower, many have very poor pollen
viability or are pollen sterile. In the case of potato this
Several decades ago it was thought that clonal cultivars
were subject to
clonal degeneration
, or a reduction in
productivity with time. It is now known that this degen-
eration is primarily the result of clonal stocks becoming
infected with bacterial or viral diseases. For example,
strawberries can be affected in this detrimental way by
infection by bacterial or viral diseases. As an additional
example, a bacterial disease causes stunting disease in
sugarcane ratoons and not, as was postulated, by genetic
'
drift
' causing degeneration. It should, however, be
noted that the cause of reduction in performance is often
