The Ac element can transpose inside another gene and
alter gene expression.
The ovary wall is transformed into the pericarp. The
pericarp is part of the mother plant of the previous
sporophytic generation. The embryo is the new sporophyte.
Brink, R.A., and R.A. Nilan 1952. The relation between
light variegated and medium variegated pericarp in maize.
Genetics 37:519-544.
Brink, R.A. 1958. A stable somatic mutation to colorless
from variegated pericarp in maize. Genetics 43:435-447.
Brink and Nilan (1952) determined the modulator gene
(MP) was closely associated with the RR pericarp allele
at the P locus. The P locus in maize influences pericarp
and cob color. The first V in VV, indicates the affect
on the pericarp; the second V of VV indicates the affect
of the VV allele on cob color.
VV - variegated pericarp and cob color
WR - white pericarp and reb cob color
WW - white pericarp and white cob color
RR - red pericarp and red cob color
Brink and Nolan (1952) showed that the RR allele was
not directly associated with the Mp transposable element.
They identified two discreet classes of variegated pericarp;
light and medium variegated. RRMP resulted in medium
variegated pericarp color due to the MP (or Ac) transposable
element which was inserted within the RR allele. The
light variegated pericarp class was due to the RRMP
association + MP, the second MP element need not be
inserted within the RR allele.
Twin sectors on a medium variegated pericarp ear consisted
of light variegated and red sectors which are adjacent
groups of kernels. They explained these twin sectors
by the action of the Mp element transposing from one
sister chromatid to another during mitosis. (See pg.
540).
Mitosis
The above model is based on the hypothesis that the
MP element is inserted within the RR allele. During
mitosis the MP element can be transposed to another
chromosome. If both MP elements, the RR MP complex and
the transposed MP, end up in the same somatic cell the
result is twin sectors of red and light variegated pericarp
on a medium variegated ear.
Brink and Nolan (1952) did the following genetic analysis:
Testcross F1:
VV/VV(male) x WR/WR(female)
or
VV/VV(male) x WW/WW(female)
Progeny*
10.7% light variegated pericarp kernels
72.4% medium variegated pericarp kernels
16.8% red pericarp kernels
* Same ratios for inbred testers of
different backgrounds is evidence that result is not
due to a modifier gene.
Backcross 1
VV/WR(male) x WW/WW
VV/WW and WR/WW (BC1 progeny)
Backcross 2
VV/WW (BC1 progeny) x WW/WW
BC2 progeny
or
WR/WW (BC1 progeny) x WW/WW
BC2 progeny
Medium variegated kernals produced about 90% medium
variegated plants when VV/WR plants were backcrossed
to WR/WR plants. This is because the VV allele is actually
of the RRMP complex. The remaining 10% of kernels that
are not medium variegated are due to Mp transposing
away from RRMP. -Discarded WR/WR plants.
The following ratios were repeated in successive backcrosses
of medium variegated kernals to WR/WR testers:
RRMP 90% = VV/WR
RR 5% = VV/WR
RRMP + MP 5% = VV/WR
VV/WR plants only grew F1
plants of VV/WR x WR/WR cross
When VV/WR genotypes with light variegated pericarp
kernels were repeatedly backcrossed to WR/WR testers,
the relative proportion of segregants remained constant
within a family, but varied between families. The explanation
is that the second Mp element is linked to the RRMP
complex. Different families have different linkage relationships
between RRMP and the second Mp element.
VV/WR(light variegated) = RRMP + MP
X WR/WR
progeny 36.4% light var., red cob
X WR/WR (BC1)ny
56.6% medium var., red cob
X WR/WR progeny
7.0% red pericarp, red cob
The BC1 light variegated kernels were again backcrossed
to WR/WR.
VV/WR(light variegated) = RRMP + MP
X WR/WR
progeny 36.8% light var., red cob
X WR/WR (BC2)ny
58.8% medium var., red cob
X WR/WR progeny
4.4% red pericarp, red cob
The proportion of the three classes was relatively
constant when light variegated kernels were repleatedly
backcrossed to a tester because the WR-MP linkage remained
the same across generations.
