Biology Reference
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to the advent of molecular markers that were reported to be controlled by
one or two genes. The cultivated sunflower is a diploid species with 17
pairs of chromosomes, and early efforts to establish linkage relationships
among the genes controlling the aforementioned traits met with little success.
As a result, no classical linkage map of morphological markers was available
for this crop. Instead, only a few pairs of linked genes governing
morphological traits were reported. Leclercq (1966) discovered a tight linkage
between a nuclear male sterility gene (
Ms
10
) and an anthocyanin pigment
gene (
T
)
and proposed a scheme to use male sterility to produce sunflower
hybrids without the tedious emasculation process of the seed-bearing female
plants. This enabled commercial hybrid seed production
in several European
countries including France and Romania prior to the discovery of the
cytoplasmic male sterility system. Hockett and Knowles (1970) reported
that the single gene
Y
for green versus yellow growing point color was
found to be linked with the gene
Br
3
for branching with a recombination
frequency of 11.6 ± 1.0%.
3.2.2 Biochemical Markers
3.2.2.1 Isozyme Markers
Isozymes are enzymes that have different amino acid sequences but catalyze
the same chemical reaction in living organisms. The technique of
electrophoresis of tissue extracts on starch gel followed by specific staining
for enzyme activity in the gel was first described by Hunter and Merkert
(1957). Markers generated by this technique were widely used in many
biological research applications. In plants, isozyme markers had been
extensively used by population geneticists to investigate the causes and
effects of genetic variation within and between populations before the advent
of DNA-based markers.
During the 1970s, plant geneticists started to map isozyme loci in the
genetic maps of different crop plant species. Corn was the leading crop
plant for which isozyme markers were mapped. Harris and Harris (1968)
located an esterase locus,
E4
, on chromosome 3. Schwartz (1971) placed an
alcohol dehydrogenase locus,
Adh
1
, on the long arm of chromosome 1,
approximately 1.6 map units from lemon white gene,
lw,
conditioning the
white seedling, pale yellow endosperm phenotype (Robertson 1961). By
1980, thirty isozyme loci had been mapped to nine of the ten chromosomes
(Goodman et al. 1980). The chromosome locations of these mapped isozyme
markers shed light on gene duplication in the maize genome evolution.
Additionally, isozyme studies in tomato substantially influenced the
application of molecular markers to the genetic improvement of crop plants.
After discovering the association of an acid phosphatase 1 locus,
Aps-1
,
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