The Harlan and deWet gene pool concept is one of the basic concept to develop an understanding of alien gene transfer and its use in evolution of domesticated crops.

1. DR. VIBHA KHANNA
ASSO. PROF. (BOTANY)
S.P.C. GOVERNMENT COLLEGE
AJMER (RAJASTHAN)

2. The Concept of Gene pool
[For Breeding Purpose]
The Harlan and deWet Gene pool Concept

3. Definition
• The concept of the “gene pool” was given by the Russian
geneticist Aleksandr Sergeevich Serebrovskii,
• In 1926 he used the term genofond (“gene fund” in English)
for ‘the complete set of different genes found within a
group of organisms’.
• The genofond, later came to be known as the “gene pool”
by Theodosius Dobzhansky
• It refers to “a reservoir of diversity that can be tapped into
by organisms to adapt to a changing environment, and by
scientists for plant breeding and crop improvement.”
• It is the collection of genes in an interbreeding population
that includes each gene at a certain frequency in relation to
its alleles: the genetic information of a population of
interbreeding organisms

4. Classification of the Gene-pool
• The wild relatives of a given crop are said to be in its gene
pool because, although they may be different species, they
‘can’ exchange genes with their cultivated relation
• But practically, not all wild relatives are equally competent
for such exchange of genes.
• Jack Harlan and Jan de Wet, divided the gene pool of Crop
Wild Relative (CWR) species.
• The Harlan and deWet (1971) gene pool concept, divides
CWR species into primary, secondary and tertiary gene
pools based on how easy it is for them to exchange genes
with the cultivated species to which they are related, i.e.,
the ease with which they may be used for breeding
purpose, (the primary and secondary gene pools being the
easiest to use).

5. Gene-pools Within The ‘Sexual
Compatibility Barrier’
Primary Gene-pool: Spp. that can be easily crossed;
complete chromosome pairing
Subspecies A: cultivated races
Subspecies B: Spontaneous races
Secondary Gene-pool: various spp. that can be
crossed with GP-1, with some difficulty (more
distant species);
Partial F1 sterlity
Gene transfer is possible with some difficulty;
incomplete chromosome pairing
Tertiary Gene-pool: Related spp.
Wherein gene transfer is not easy and
requires radical techniques like embryo-
rescue, protoplast fusion /culture,
Hybrids with GP-1 are anamoulous,
completely sterile or lethal.
GP-1
GP-2
GP-3

6. The Primary Gene pool (GP1)
• It’s like concentric circles around the crop (or a triangle
with several levels).
• The primary gene pool (GP1), the first ring, includes
species that can be directly mated with the crop to
produce lots of strong, fertile progeny.
• The primary gene pool consists of both cultivated and
wild varieties, including the perennial species. They can
even be referred to as the different sub-species of the
same species.
• A. ipaensis, A. duranensis, A. batizocoi, and A.
monticola belong to the primary gene pool of Arachis
hypogaea.

7. The Primary Gene pool (GP1)
• Genes flow easily within the primary gene pool but
since selection of cultivated plant species has led to
unique variation they also suggested that GP-1 be
subdivided into two subspecies.
– Cultivated races belong to subspecies A while
– wild types growing spontaneously fall into subspecies B.
• Both subspecies contain race and subrace distinctions.
• Harlan and de Wet (1971) define races to have a
“distinct cohesion of morphology, geographical
distribution, ecological adaptation and frequently of
breeding behavior” while subrace is simply a
“convenient division of race” to be used when drastic
variation is documented.

8. The Secondary Gene pool (GP2)
• The secondary gene pool (GP2) is composed of crop wild relatives
that are distinct from the cultivated species, but which are still so
closely related that they can cross with the crop to at least some
extent to produce some fertile offspring.
• It’s the next ring around the ‘bullseye’.
• It is more difficult to use species from the secondary gene pool,
because reproductive barriers of different kinds are present
between it and the crop.
• For example, Aegilops tauschii and Aegilops speltoides, two wild
relatives in the secondary genepool of bread wheat (Triticum
aestivum), are diploid. That means they have paired chromosomes,
whereas bread wheat is hexaploid (six copies). Such mismatches
create difficulties for breeders.
• In addition, some hybrids resulting from crosses with secondary
gene pool species are partly sterile or just weaklings.

9. The Secondary Gene pool (GP2)
• The secondary gene pool (GP-2) includes those that
have potential to cross but the progeny will likely result
in hybrids that are weak since chromosomes may pair
poorly if at all.
• Furthermore, it may be difficult to recover a specific
genotype in later generations.
• Success of gene transfer depends on barriers
respective to each crop species.
• Even though crossing between species in GP-2 is
somewhat of a challenge, it is an option for plant
breeders to consider when increasing the variation in a
population.

10. The Tertiary Gene pool (GP3)
• The tertiary gene pool (GP3) is made up of even
more distantly related crop wild relative species.
This is the ring outer to the GP2.
• Plants assigned to the tertiary gene pool (GP-3)
may be crossed using novel gene transfer
techniques.
• The hybrids are usually sterile, lethal, and/or
anomalous.
• Anything farther away, from GP3, will be beyond
the ‘sexual compatibility barrier’ and
biotechnology will be needed to transfer genes.

11. Modified Gene pool Models
• There have been a few arguments for
modifications to the gene pool model since it
was first described.
• Smartt (1984) argued that the tertiary gene
pool should be subdivided in two sections: A
and B.
– Section A would include taxa which can cross
fertilize the cultigen and produce a viable,
although sterile, hybrid.
– Conversely, section B would include those which
do not produce a viable hybrid.

12. Modified Gene pool Models
• Another modification that has been suggested is
the addition of a fourth gene pool (GP-4) to
accommodate genetically engineered plants.
• It was first recommended by Hammer (1998), but
Gladis and Hammer (2002) later concluded that
GE crops should fall into GP-3 and that GP-4
should be reserved for synthetic crops with
nucleic acids that are not normally found in
nature.
• The most recent gene pool model seen in
literature today was adapted from Gepts and
Papa (2003) where GE crops belong to GP-4

13. Different genepools beyond GP3
{taken from Michelmore (2003)}