genetic evolution of crops like wheat, peanut and brassica provide good examples of whole genome transfer in nature

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

2. Genetic Evolution of Triticum,
Arachis and Brassica
{Examples of The Transfer Of Whole
Genome }

3. The Evolution Of Bread Wheat
(Triticum aestivum)
• Einkorn, emmer, and spelt wheat types are the earliest cultivated
ones; hence, designated as ancient wheats.
• The hulled (glumed) wheats comprise all 3 polyploidy levels of
diploid (2x ), tetraploid (4x ), and hexaploid (6x ) present
in Triticum spp.
– Einkorn (T. monococcum L.) is a diploid wheat (AA; 2n = 2x = 14).
– Emmer (T. turgidum L. spp. dicoccum Schrank ex Schübler) is a
tetraploid wheat (AABB; 2n = 4x = 28) which is a domesticated form
of T. turgidum spp. dicoccoides (wild emmer wheat).
– Triticum turgidum ssp. durum (Desf) Husn. (durum wheat) originated
from the domesticated emmer.
• Spelt (T . aestivum subsp. spelta ) is a hexaploid wheat (AABBDD;
2n = 6x = 42) and is most likely the ancestor of the free‐threshing
common wheat.
• Thus, einkorn, emmer, and spelt represent the 3 cultivated species
of hulled wheat which include a bridging species between the
cultivated (bread wheat and durum wheat) and wild wheats.

5. Hybridizations events involved in the evolution
of bread wheat, Triticum aestivum
• Bread wheat (Triticum aestivum L.) is an allohexaploid species produced
from two separate hybridisation events.
• Each hybridisation was followed by chromosome doubling in the new
hybrid; this enables normal bivalent formation at meiosis and thus the
production of fertile plants.
• The initial hybridisation, that occurred approximately 10,000 years ago, is
believed to have been between the two grass species T. urartu (the A
genome donor), and Ae. speltoides (the B genome donor).
• This new species would have been tetraploid (four complete genome
complements)
• Hexaploid wheat arose as a result of a second hybridisation between the
new tetraploid and a third diploid species, Ae. tauschii (the D genome
donor).
• Again, chromosome doubling must have occurred in order to produce a
fertile individual.
• This new species would then have have 42 chromosomes; that is, six
complete genomes each of 7 chromosomes.

6. Triangle of U
{Genetic Evolution of Brassica}
• The triangle of U is a theory about the evolution
and relationships among members of the plant
genus Brassica.
• In 1935, Korean-Japanese botanist Woo Jang-
choon (the Japanese translation of this name is
“Nagahara U”) crossed the three “traditional”
Brassicas— B. nigra (black mustard), B. rapa
(turnip, Chinese cabbage), and B. oleracea (kale,
cabbage, broccoli, cauliflower, Brussels sprouts)–
to create three new hybrid species– B. juncea
(Indian mustard), B. napus (rapeseed, rutabaga),
and B. carinata (Ethiopian mustard).

7. Triangle of U
• The artificial interbreeding of three separate, yet closely
related, diploid species (each with two set of
chromosomes), to create three new tetraploid species (each
with four sets of chromosomes) is referred to as the
“Triangle of U,” in reference to Nagahara U.
• U's triangle illustrates the evolutionary relationship
between three cultivated elementary species (B. rapa, B.
oleracea, and B. nigra) and three amphiploid species (B.
napus, B. juncea, and B. carinata).
• Though the Triangle of U was only a theory in 1935, it has
now been confirmed by DNA mapping.
• The Triangle is significant in that it explains the creation (by
both natural and artificial means) of many of our most
important food crop species, and provides the genetic
understanding necessary to prevent undesired
hybridization of these species.

8. Triangle of U
• The concept of U’s triangle,
– revealed the importance of
polyploidization in plant
genome evolution,
– described natural
allopolyploidization events
in Brassica using three
diploids [B. rapa (A
genome), B. nigra (B),
and B. oleracea (C)] and
derived allotetraploids
[B. juncea (AB
genome), B. napus (AC),
and B. carinata (BC)].

9. Genetic Evolution of Arachis
• Most Arachis species have 2n = 2x = 20 chromosomes, but A.
hypogaea is an exception in having 40 chromosomes.
• The cultivated peanut (Arachis hypogaea L.) is of hybrid
origin and has a polyploid genome that contains essentially
complete sets of chromosomes from two ancestral species.
• It is a recent allotetraploid, most probably resulting from the
hybridization of two wild species followed by natural
chromosome duplication.
• It has two sets of chromosome pairs, one from each of the
ancestral species: a type of polyploid termed allotetraploid
(AABB-type genome; 2n = 4× = 40 chromosomes)
• A. hypogaea and A. monticola, are the two tetraploid species

10. Two Distinct Chromosome Pairs in
Arachis
• Husted observed the presence of two quite
distinct chromosome pairs in the cultivated
peanut:
– a pair he called A, with different staining behavior and
much smaller than the remaining chromosomes, and
– another pair, with a secondary constriction, he called
B.
• The occasional meiotic tetravalents, the
predominant bivalent pairing pattern of the
chromosomes indicates the allotetraploid (or
segmental allotetraploid) nature of A. hypogaea

11. Genetic Evolution of Arachis
• The cultivated peanut A. hypogaea is considered a segmental
allotetraploid composed of the A and B genomes, with probable
origin by amphidiploidization of an AB hybrid
• Cultivated peanuts (A. hypogaea) arose from a hybrid between two
wild species of peanut, probably, A. duranensis and A. ipaensis.
• The initial hybrid would have been sterile, but spontaneous
chromosome doubling restored its fertility, forming what is termed
an amphidiploid or allotetraploid.
• Genetic analysis suggests the hybridization event probably occurred
only once and gave rise to A. monticola, a wild form of peanut that
occurs in a few restricted locations in northwestern Argentina and
by artificial selection to A. hypogaea.
• The process of domestication through artificial selection made A.
hypogaea dramatically different from its wild relatives.

12. Genetic Evolution of Arachis

13. Origin of Arachis
• The initial domestication may have taken place in
northwestern Argentina, or in southeastern
Bolivia, where the peanut landraces with the most
wild-like features are grown today.
• From this primary center of origin, cultivation spread
and formed secondary and tertiary centers of diversity
in Peru, Ecuador, Brazil, Paraguay and Uruguay.
• Over time, thousands of peanut landraces evolved;
these are classified into six botanical varieties and two
subspecies {Two subspecies (hypogaea and fastigiata)
and six botanical varieties (hypogaea, hirsuta,
fastigiata, vulgaris, aequatoriana & peruviana)}.

14. A Polyphyletic Origin
• A possible area for the origin of the peanut would
be in the southeast of Bolivia and northwest of
Argentina, where natural populations of A.
ipaensis, A. duranensis, A. batizocoi, and A.
monticola could come together, corroborating the
hypothesis that ‘two wild sympatric species,
carrying the A and B genomes, were crossed by
bee pollination, generating a sterile hybrid that
would be naturally chromosome doubled’.
• Those fertile hybrids would have been
domesticated by the native people.

15. Biphyletic Origin
• A. hypogaea subsp. fastigiata Waldron
evolved from a diploid species such as A.
batizocoi and A. duranensis while
• A. hypogaea subsp. hypogaea evolved from
diploid species such as A. batizocoi and A.
villosa Benth.
{A. duranensis is the most probable donor of the A
genome, although A. villosa is only discarded on
geographic and morphological grounds.}

16. Genetic Evidence
• Arachis hypogaea is an allotetraploid species (2n = 4x = 40, AABB) with a
very large and complex genome.
• Cytologically, it behaves mostly as a diploid, but multivalents can result in
skewed genetic ratios.
• Peanut display both disomic and tetrasomic genetic recombination.
• A. hypogaea was derived from just two wild diploid species, and indeed
probably between very few or one individual of each diploid species. This is
supported by the very limited genetic variability among landraces and
commercial cultivars of A. hypogaea and from its molecular cytogenetics
• It is also apparent that the wild tetraploid A. monticola is very closely related
to A. hypogaea, indeed they most probably share the same origin. They have
very high crossability, cytogenetically the species are indistinguishable, and
molecular studies show they are very closely related and the same biological
species. They cannot be differentiated based on isozymes , Random
Amplified Polymorphic DNA (RAPD), or microsatellite markers.
• However, various studies based on AFLP, microsatellite, or Sequence-Related
Amplified Polymorphism (SRAP) markers have shown that A. monticola does
have enough genetic divergence to form a separate group and can be
considered the same biological species.