Biology Reference
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with the root-knot nematode
resistant gene,
Mi
, on chromosome 6 of tomato,
Rick and Fobes (1974) reasoned that evaluation for nematode resistance
can be done more efficiently by electrophoresis than by parasite inoculation.
Furthermore, the co-dominant fashion of the zymogram of
Aps-1
will allow
tomato breeders to distinguish between heterozygous and homozygous
resistant seedlings. Based on the availability of a linkage map of 22 isozyme
loci mapped to nine of the 12 tomato chromosomes, Tanksley and Rick
(1980) discussed many potential uses of isozyme markers in basic plant
genetics research and applied plant breeding including gene mapping and
marker-assisted selection (MAS).
The first study on a sunflower isozyme was carried out by Torres (1974a,
b, c) who investigated the genetics of alcohol dehydrogenase (ADH) in great
detail. A total of 12 distinct bands of sunflower alcohol dehydrogenase
were resolved electrophoretically with starch gels. The slowest- and the
fastest-migrating sets of three bands were allozymic dimers, which are the
products of two genes designated
Adh
1
and
Adh
2,
respectively. There were
two co-dominant alleles, F (for fast) and S (for slow) at each locus, and each
heterozygote produced three bands as expected with a dimer molecule:
Adh
1
FF
,
Adh
1
FS
,
Adh
1
SS
and
Adh
2
FF
,
Adh
2
FS
,
Adh
2
SS
. The homozygotes produced
just one band each, consisting of FF or SS homodimers (Torres 1974a, b). The
remaining six bands were intergenic dimers or developmental artifacts
(Torres 1974c). It seemed possible that a third allele, E (for early), exists at the
Adh
1
locus and the expression of
Adh
1
allele is under developmental control
since it only appears in the developing seeds but not in the mature seeds. A
survey on allele frequency indicated that
Adh
1
S
is a rare allele in the
cultivated sunflower. Out of over 6,000 seeds belonging to 70 collections
surveyed, no
Adh
1
SS
homozygote was found and only three heterozygotes
(
Adh
1
FS
) were identified among 422 seeds of one collection. Heterozygote
Adh
1
FS
was seen infrequently and homozygote
Adh
1
SS
was even rarer in the
seven wild populations sampled. The dissociation-recombination
experiments using bands excised from starch gels revealed that an
intermediately migrating isozyme dimeric intergenic product consisting of
Adh
1
F
and
Adh
2
S
subunits could be formed. However, the hybrid isozyme
was unstable in vitro because its monomers spontaneously dissociated and
recombined to produce
Adh
1
FF
and
Adh
2
SS
isozymes (Torres 1974b). Genetic
studies indicated that
Adh
1
and
Adh
2
were not linked.
In sunflower, isozymes have been used for investigating genetic
variability in both domesticated and wild populations (Dry and Burdon
1986; Rieseberg and Seiler 1990; Cronn et al. 1997) for assessing phylogenetic
relationships and speciation mechanisms within the genus
Helianthus
(Rieseberg 1991, 1998) and for identifying interspecific hybrids (Carrera et
al. 1996). There have been several reports on genetic and linkage studies of
various isozymes in sunflower. Kahler and Lay (1985) studied the inheritance
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