Wheat Evolution, Domestication, and Improvement - Wheat Science and Trade - page 7

mately 35 million years ago (MYA) and that the
Triticum group separated out about 11 MYA. The
formation of the various polyploid wheat species
within the Triticum genus began approximately
10,000 years ago. Since the early 1900s it has been
known that the wheat species and the entire Tri-
ticeae tribe have a basic chromosome number of
n = 1 x = 7. Cultivated wheat consists of diploid
(einkorn; 2 n = 2x = 14, AA), tetraploid (emmer,
durum, rivet, Polish, and Persian; 2 n = 4x = 28,
BBAA), and hexaploid (spelt, bread, club, and
Indian shot; 2 n = 6x = 42, BBAADD) species.
The various diploid genomes of the Triticeae
tribe appear to be highly conserved in gene order
along the seven pairs of chromosomes (Gale and
Devos 1998). The chromosomes (1 through 7) in
the various diploid genomes (B, A, and D) are
considered to be evolutionarily related, that is,
homoeologous in nature. When combined in the
same nucleus, homoeologues can be induced to
pair with each other. The importance of this fact
in controlling our ability to make interspecifi c
crosses and manipulate genes from one species to
another will be considered elsewhere in this and
other chapters.

Clearly, the diploid component of the Triticum
genus is composed of two defi ned species, T.
urartu and T. monococcum . Triticum monococcum is
the only cultivated diploid wheat species and was
fi rst found in Greece by Boissier (1884). Both T.
urartu and T. monococcum have been identifi ed in
natural habitats ranging from southwestern Iran,
northern Iraq, Transcaucasia, eastern Lebanon,
southeastern Turkey, western Syria, and beyond
into neighboring Mediterranean areas (Kimber
and Feldman 1987). The sterility of their hybrids
(Johnson and Dhaliwal 1976, 1978) confi rms that
they are valid biological species. It has been estab-
lished that T. urartu contains approximately
4.93 pg DNA (http://data.kew.org/cvalues/
introduction.html) and is the donor of the A
genome to all polyploid wheat species. Dvoˇák
et al. (1988) showed that variation in A-genome
repeated nucleotide sequences, present in both
tetraploid wheat species, was more related to the
A genome of T. urartu than to the A genome of
T. monococcum .

Origin of the B genome
Both the B and G genomes of tetraploid wheat
have undergone massive changes following ances-
tor divergence and polyploidization, and they are
widely considered to be modifi ed S genomes
having evolved from a common ancestor. Gu et al.
(2004) indicated that the B genome diverged
before the separation of the A and D genomes.
There has been considerable controversy over the
donor of the S-genome progenitor, but it was cor-
rectly identifi ed as an ancestor of Ae. speltoides
Tausch (2 n = 2 x = 14) in 1956 (Sarkar and
Stebbins 1956; Riley et al., 1958; Shands and
Kimber 1973; Dvoˇák and Zhang 1990; Daud and
Gustafson 1996) and contains 5.15 pg DNA
(http://data.kew.org/cvalues/introduction.html).
Cytoplasmic analyses have shown that Ae. speltoi-
des was the maternal donor of not only tetraploid
but also hexaploid wheat (Wang et al., 1997). It is
clear that the B genome has undergone signifi cant
intergenomic noncoded and coded DNA changes
(in both the diploid and the tetraploid wheats)
since the formation of tetraploid wheat. The B-
genome component of polyploid wheat is the

Origin of the A genome
It is apparent that the key to understanding the
evolution of wheat involves an elucidation of the
evolution of the tetraploid wheat species. Early
cytogenetic studies led to the conclusion that the
A genome of the tetraploid species, T. timopheevi
and T. turgidum , was contributed by T. monococ-
cum (Sax 1922; Kihara 1924; Lilienfeld and Kihara
1934). However, it became apparent that diploid
einkorn wheat actually comprised two biological
species, T. monococcum and T. urartu Tum. Ex
Gand. Chapman et al. (1976) determined that the
A genome originated from T. urartu . Konarev et
al. (1979) concluded, from studies of the immu-
nological properties of seed-storage proteins, that
the A genome in T. turgidum was contributed by
T. urartu , and the A genome of T. timopheevi
(Zhuk.) Zhuk. (GGAA) was contributed by T.
monococcum . However, Nishikawa et al. (1994)
suggested that the A genomes in both diploid
species were contributed by T. urartu .