To allow hybridisation programs to be effective it is important to conserve as many genetically different forms as is possible. Generally, the primitive strains of a cultivated species is more variable in nature than the cultivars used in crop production. If the primitive strains cease to exist then the genetic material they contain is also lost. This is true of many species of domesticated animals. It is difficult to maintain stocks of primitive animal strains, since these stocks would be small in number and would tend to be subject to random genetic drift and inbreeding depression. A DNA bank has been set up to retain the genetic material of known species, but it is unlikely to be of direct value to the animal breeder.
Plants are much easier to maintain in long-term culture and gene banks have been organised, eg Institute for Plant Industry in Leningrad and National Seed Centre at Fort Collins. The aim of these institutions is to establish collections of crop plant lines which include at least one alternative allele for each locus. The international plant agencies collect samples of all crop plants, they assess the range of morphological and biochemical variation of these samples and publish this information for use by plant breeders. The conservation of plant material falls into three main methods:
1) maintenance of seed lines by random mating (outbreeding) or self-fertilisation, followed by seed storage. This allows the germplasm (the sum of the hereditary material of a species) to be retained for later study, or use in breeding programs. The seeds can be stored in three major ways:
a) base collection for long term storage at sub-zero temperatures and low seed moisture (3-6%). These seeds are not distributed.
b) active collection for medium term storage (20 years maximum) at freezing point and above. These seeds are distributed to breeders.
c) working collection held by breeder for use in their own selection programmes.
2) maintenance of perennial plants of long lived varieties with seed propagation. This is necessary for plants whose seeds cannot be stored for very long the so called recalcitrant seeds, eg oil palm, sugar cane, coconut.
3) Maintenance of perennials as clones (vegetatively propagated or in vitro meristem culture). This is used for types which do not produce seeds freely, eg banana, sugar cane, grape. The use of grape meristem culture has been employed successfully for many years, the advantages being that 2 m2 of lab space can incorporate as much material as could be grown whole on one hectare of land and this reduces maintenance time and prevents genetic contamination.