Conservation and Sustainable Use of the Biosphere - WBGU

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Conservation and Sustainable Use of the Biosphere - WBGU

82 D The use of genetic and species diversity

Box D 3.4-3

The gene pool concept

Harlan and de Wet (1971) first put forward the idea of the

gene pool. The concept divides cultivated plants and their

relatives and wild species into a primary, secondary and tertiary

gene pool. The measure of relationship is the possibility

of crossing the species:

• The primary gene pool contains all representatives of the

cultivated species.

• Plants in the secondary gene pool can be crossed with the

cultivated plant species under consideration and produce

fertile hybrids; the plants do not however belong to

the cultivated plant species under consideration.

• The tertiary gene pool comprises species that can no

longer be crossed in the classic sense with the cultivated

species, but only with the help of in-vitro techniques such

as embryo rescue or protoplast fusion.

Since genes can be transferred across species barriers using

genetic engineering methods, the gene pool concept has

been ‘diluted’. The ability of a species to hybridize with

another is no longer the exclusive criterion for whether

gene transfer is possible. Callow et al (1997) therefore discuss

whether hybridization with ‘cultivated species’ is still a

suitable criterion for regarding a species as a genetic

resource since genetic engineering methods in principle

allow us to see any organism as a source of valuable genes.

The gene pool concept however, does continue to provide a

basis for designing collection and conservation strategies

for genetic resources.

Theoretically, the following strategies may be

derived form those two options (Table D 3.4-3):

1. In-situ conservation in the case of agricultural

crops means more or less intensive management.

There are gradations of difference from ‘no management’

(eg core zones in a national park), over

‘minimal management’ to ‘intensive management’

(when conservation is only possible if the present

human-influenced state is also preserved at the

same time). In-situ or on-farm conservation relate

to conserving genetic resources in agricultural or

horticultural situations, such as the traditional

household garden (‘conuco’).

2. Ex-situ conservation takes place in gene banks,

botanical and zoological gardens and aquaria. For

the conservation of plants, there are many different

available measures. These include seed collections,

field collections, in-vitro tissue cultures,

pollen conservation, protoplast cultures and cryoconservation.

Animals are kept in captive populations;

sperm, eggs and embryos can be conserved

cryogenically.

With regard to conservation strategy, one can differentiate

between crops and livestock on the one hand

and the wild portion of agrobiodiversity on the other

hand. Whilst the latter have to be kept in situ, this

approach is only possible for the former in exceptional

cases since these generally depend on human

assistance for survival. Their long-term preservation

is possible in situ, on farm or ex situ (Kosak, 1996).

On-farm conservation generally only takes place in

‘traditional’ agriculture that is characterized by crop

diversity. In modern or industrial agriculture that is

characterized by genetically homogeneous highyielding

plant varieties and animal breeds of just a

few species, diversity must be conserved ex situ.

Brush (1995) proposes that in isolated areas in the

centres of domestication, in which small-scale farming

is still the only possibility, systematic in-situ and

on-farm conservation could be introduced to supplement

ex-situ conservation. This is not just the conservation

of the status quo, but rather allows evolutionary

processes to continue.

Examples of conservation strategies for

particular cultivated plant species

• Wheat: Worldwide almost 800,000 wheat (Triticum)

accessions are being kept ex situ (FAO,

1996b). Many of the accessions are historic farmers’

varieties. Although there are certainly some

duplicates among the 800,000 accessions, sheer

numbers clearly show that systematic in-situ or onfarm

preservation of this diversity would be

impossible from an organizational standpoint.

Two biological characteristics predestine wheat

for ex-situ conservation strategy:

1. Wheat is self-fertilizing; that means elaborate

measures to manage pollination or isolate plots

can be dispensed with.

2. The samples have only to be planted every 25

years for regeneration. That is sufficient to preserve

germinating capacity.

• Cassava: Cassava (manioc) is a species that reproduces

by vegetative propagation.Worldwide there

are around 28,000 accessions in ex-situ collections,

23 per cent of which are historic farmers’ varieties,

and 9 per cent are modern varieties or nucleus

stocks (FAO, 1996b). It may be assumed that the

collections constitute more of a random selection

than a complete collection. Furthermore, the cassava

clones have to be replanted every year. The

conservation of habitats in which a large diversity

of cassava is cultivated is therefore the best strategy

for conservation.

The two examples indicate how one might proceed in

order to define the optimum conservation strategy

for a given species.

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