Towards a Domestication Strategy for Indigenous Fruit Trees in the Tropics - Indigenous Fruit Trees in the Tropics: Domestication, Utilization and Commercialization - page 41

2.5.1 Establishing a gene bank ( ex situ conservation)
The strategy for building up the base population will, to some extent, be
influenced by whether or not there has been a tree improvement programme
for the species in question, and how far it has gone towards identifying
genetically superior stock. If, from previous tree improvement work, the
population has been subdivided into provenances and progenies, it is easy to
ensure that a wide range of unrelated seed sources and clones are set aside for
the establishment of a number of living gene banks established at different sites
to minimize the risk of loss if a site fails or is destroyed by fire or some other
event. If possible, the material conserved in the gene bank should encompass
the full geographical range of the species and be obtained from sites with
differing soils, rainfall, altitude, etc. The latter is desirable for the subsequent
selection of clones that are appropriate for different sites.
In the same way, for species without an existing tree improvement
programme, clones for field testing should originate from seed collections
spanning the natural range of the species, particularly including populations on
the edge of the range and any isolated subpopulations that may be important
in a later breeding programme between selected clones. Ideally, each collection
area should be represented by identifiable half-sib progenies of individual
mother trees. The first round of germplasm collections of this sort were made
for Sclerocarya birrea and U. kirkiana in eight SADC (Southern Africa
Development Community) countries in 1996 (Kwesiga et al ., 2000) and
established in multilocational provenance trials (Akinnifesi et al ., 2004, 2006).
In both of the above instances, the individual trees to be included in the gene
bank should be selected at random from unrelated origins and vegetatively
propagated to form as large a clonal population as possible. The unused
seedlings should be established at different sites as living gene banks for future
use, whether directly by subsequent coppicing and cloning or indirectly through
their progeny. The identity of plants within gene banks of this sort should be
maintained and different seed-lots should be planted such that cross-pollination
between plants of different origins is likely. In this way the gene pool will be
maintained with maximum diversity.
Ex situ conservation can also be achieved in species producing seeds,
which retain their viability in storage, by creating seed banks. Species differ in
their amenability to seed storage at different temperatures and moisture
contents, including cryopreservation. The Genetic Resources Unit of the World
Agroforestry Centre (ICRAF) contains material of several fruit-tree species. In
southern Africa, clonal orchards have been established for the production of
superior U. kirkiana clones. Numerous studies have also been made on the
germination of indigenous fruit tree species (Maghembe et al ., 1994;
Phofeutsile et al ., 2002; Mkonda et al ., 2003; Akinnifesi et al ., 2006).
It needs to be appreciated that new material should be brought into the
gene resource (gene bank), selection and production populations whenever
possible (see Fig. 2.2), so that genetic diversity is being continually enhanced.