Wednesday, April 12, 2006

Conservation of plant genetic resources


Maintenance of biodiversity is an essential prerequisite for the continued production of new cultivars of current crops, for the development and exploitation of newer crops.

In situ conservation is the conservation in any habitat where the germplasm normally occurs, not only in natural habitats, but also in farms, gardens and other man-made habitats of relevant germplasm. Ex situ conservation on the other hand refers to any collection maintained outside the normal habitat of the germplasm including seed collections and in vitro tissue cul­ture, and also living collections in botanic gardens and collections of spe­cies with recalcitrant seed.

The objective of in situ conservation, at least for agricultural purposes, is to conserve the maximum possible number of alleles and/ or maximum possible diversity of genotypes whilst permitting continued evolution. This is of importance in generating new genes or genotypes, particularly: (i) in response to changing environments, e.g. genes for resistance to newly evolved strains of pathogens; and (ii) by continued selection of landraces by farmers or gardeners (at least where law still permits). Additional ben­efits include conservation of much more biodiversity - entire ecosystems - than just the targeted crop germplasm. Against this is the disadvantage that the germplasm cannot be efficiently utilized because characterized genotypes cannot be readily tracked.

The objective of ex situ conservation is to maintain a collection contain­ing as many alleles as possible, and/ or as diverse a range of gene combina­tions (i.e. genotypes) as possible, in a form that can be readily utilized for breeding and research. For efficient utilization, genetic variation within the collection must be appropriately characterized. For efficient conservation, ideally the collection should be as small as possible commensurate with conserving maximum diversity. In practice, in the face of rapid genetic ero­sion it is often necessary to collect germplasm before it can be ascertained that it contains genes or genotypes not already present in the collection: most collections are therefore considerably larger than strictly necessary for the diversity they contain.

There are three groups of implica­tions for efficient conservation.

First, efficient collection of diverse germplasm for ex situ conserva­tion depends on having good knowledge of the spatial distribution of genetic diversity. Inevitably it is not possible to know the exact location of every genotype. Instead, a good understanding of the factors that control the distribution of genetic diversity is necessary to devise a collecting strat­egy that maximizes the diversity sampled.

Second, in addition to efficient construction of an ex situ collection, effi­cient maintenance of the same also depends on good understanding of the factors that control the distribution of genetic diversity - in this case to control the genetic shifts that occur whenever a population is sampled, subsampled or regenerated.

Third, efficient in situ conservation depends on good knowledge of the distribution of genetic diversity in space and time, and of the factors that control its distribution. In particular, an in situ conservation area must have a size, heterogeneity and structure that maximize genetic variance maintained by evolution.

The efficient achievement of both the primary objectives requires a knowledge of the genetic nature of variability, population structure, the distribution of diver­sity and the factors that control them.