Environmental Engineering Reference
In-Depth Information
and
teosinte glume architecture-1
(
tga1
), have indeed been found to account for two of the remark-
able changes from teosinte to maize: suppression of lateral branches that ended in tassels and their
transformation into ears instead (
tb1
), and exposure of the kernels on the surface of the ear which
were otherwise enclosed in a hardened casing or glumes (
tga1
). Except for
tga1
, which manifests its
phenotype more or less independent of the genetic background, the effects of other genes with major
influence on morphological features vary quantitatively depending upon the genetic background,
indicating the involvement of the epistatic effects.
On average, nucleotide diversity in maize at a given locus is approximately 70% of that of teo-
sinte, which is expected as a consequence of selective pressure. In the
tb1
locus, however, nucleotide
diversity was only 30% in the coding region and 2% in the 5'-upstream region, clearly indicating
strong selection, particularly for the regulatory region of the gene.
Based on polymorphisms in approximately 800 genes, an estimate of 2-4% was obtained for
the genes that were subjected to artificial selection, which translates into approximately 1000-2000
genes across the maize genome assuming ~55,000 total genes.
16.2.3 m
aizE
B
rEEding
ovEr
thE
p
aSt
c
Entury
Maize was introduced to North and South America by human migration and, naturally, the
farmers selected their own favorite lines that showed improved performance at the place of
migration, resulting in the origin of myriad landraces. Prior to initiation of organized breed-
ing by the Old World immigrants in the twentieth century in the USA, the local farmers had
been selecting for desirable traits, albeit in a primitive manner, for thousands of years. In the
mid-nineteenth century, farmer breeders selected high yielding, open-pollinated varieties (OPV)
from the unadapted lines. However, the yield gain stagnated in the beginning of the twentieth
century, at a time when the population of the USA was beginning to pick up and demand for food
was on the rise (Figure 16.3).
George Shull, a scientist at the Cold Spring Harbor Laboratory, realized in 1908 that the reduced
performance of two inbred lines that had undergone repeated selfing was recovered upon crossing
them into a hybrid, so much so that in some cases hybrid outyielded the OPV from which the inbred
lines were derived. At about the same time, Edward East at Harvard University who conducted
12
Biotech
traits
10
8
6
4
Double cross
hybrids
Open pollinated
varieties
2
0
1890
1910
1930
1950
Ye ar
1970
1990
2010
FIGure 16.3
Grain yield of maize over time in the United States. (From U.S. Department of Agriculture/
National Agricultural Statistics Service,
Agricultural Statistics Database
, 2009. Available at http://www.fas.
usda.gov/wap/current/toc.asp)
