Towards a Domestication Strategy for Indigenous Fruit Trees in the Tropics - Indigenous Fruit Trees in the Tropics: Domestication, Utilization and Commercialization - page 43

The question of the number of clones to be deployed has two aspects: the
number of clones within the production population, and the number of clones
planted per site. Based on probability theory, 7-30 clones provide a reasonable
probability of achieving an acceptable final harvest. There is continued debate
on the wisdom of releasing a few rather than many clones to optimize gains
and minimize risk. The arguments about susceptibility to environmental
disasters when only a few clones are deployed may apply more rigorously to
agroforestry trees, given the risk-averseness of small-scale farmers and their
possible desire to maximize stability of production rather than production per
se. Consensus among tree breeders and forest geneticists indicates that
production population sizes of 100 genotypes or fewer are acceptable. Actually,
the absolute number of clones is less important than the range of genetic
diversity among the clones. Ten clones which share the same alleles for a
particular trait would express no genetic diversity, in contrast with five clones
each of which has different alleles for the same trait. Hence, by sourcing
material from diverse origins, the tree breeder must emphasize genetic diversity
for traits associated with survival and adaptation while exercising strong
selection pressure on production traits. Following rigorous testing, new clones
should be added to the production population each year. In the case of
dioecious species such as Sclerocarya birrea , it is important to include male
trees in the production population in order to ensure adequate pollination. This
is especially important for kernel production as there is some evidence that the
number of kernels per nut may be constrained by inadequate pollination
(Leakey et al ., 2005b).
There is perhaps one situation in which an exception to the above strategy
to broaden the genetic base of the production population may be acceptable.
This situation arises when there are good market reasons to preserve regional
variations in the quality of the product. For example, in the wine industry,
regional attributes of the wine ('appellation') are a result of gene combinations
specific to different regions. Nevertheless, within each region processes to
maintain a broad genetic base are still important.
2.5.3 Protecting some wild populations ( in situ conservation)
A conservation programme for any species would be incomplete without a
strategy for the protection of wild populations, which represent 'hotspots' of
genetic diversity. One advantage of this approach is that a species is conserved
together with its symbionts (e.g. mycorrhizal fungi), pollinators and other
associated species; something that is not so easy in ex situ conservation and
which is important for circa situ conservation. Molecular techniques now
provide a powerful tool to identify these hotspots rapidly (e.g. Lowe et al .,
2000), although of course the data can only be as good as the sampling
strategy will allow. In southern Africa, molecular studies have been completed
for U. kirkiana and S. birrea (Kadu et al ., 2006; Mwase et al ., 2006).