Variability
of
crossing
over
frequency
in
high
-
&
low
heterosis
F
1
hybrids
of
tomato
under
continued
exposure
to
low
temperatures
Ursul
S
.
V
.
,
Ursul
N
.
A
.
Institute
of
Vegetable
Breeding
and
Seed
Production
(VNIISSOK),
p/o
Lesnoy
Gorodok,
Odintsov
Region,
Moscow
District,
143080,
RUSSIA
.
(E
-
mail:
vniissok@cea
.
ru)
.
Introduction
Induced
recombinogenesis
is
of
great
practical
importance
for
plant
breeding
as
an
effective
method
of
increasing
genotype
diversity
in
the
progeny
(Elliot,
1961;
Zhuchenko,
Korol,
1985;
Zhuchenko,
1988)
.
The
change
in
temperature
was
one
of
the
first
methods
for
experimental
recombinogenesis
(Dishler,
1983)
.
The
choice
of
this
agent
is
not
casual,
as
temperature
is
the
basic
limiting
factor
of
environment,
and
also
its
use
is
simple
and
accessible
.
In
previous
studies
of
effects
of
a
thermal
factor
action
on
recombination,
as
a
rule,
short
stresses
were
applied
or
the
conditions
of
an
external
environment
were
used
at
the
expense
of
a
variation
by
terms
of
sowing,
but
in
this
case
the
temperature
conditions
were
changed
both
in
a
hothouse,
and
in
the
field
(Mock,
1973;
Zhuchenko,
Korol,
1985;
Zhuchenko,
Uschapovski,
1989)
.
The
temperature
influences
are
usually
accompanied
by
a
recombinogenic
effect
(Dishler,
1983),
however,
the
degree
of
response
largely
depends
on
the
genotype
(Straub,
1938;
Wilson,
1959)
.
Such
differences,
apparently,
are
caused
by
peculiarities
both
in
the
genetic
control
system
of
vegetative
development
(
F
),
and
in
the
system
determining
recombination
(
R
),
and
also
their
interaction
at
each
particular
genotype
.
Also
it
is
important
to
note,
that
the
increase
of
morphometric
parameters
at
heterosis,
as
a
rule,
is
accompanied
by
increase
of
organism's
fitness
(Mather,
Jinks,
1985)
and
probably
it
is
related
to
the
higher
“buffer”
ability
of
its
F
system
in
relation
to
R
.
Therefore
the
problem
was
to
determine
how
the
cultivation
of
F
1
tomato
hybrids
under
constant
subextreme
temperature
conditions
for
a
long
time
would
influence
the
recombination
system
depending
upon
the
level
of
hypothetical
heterosis
on
morphometric
traits
of
the
hybrids
.
Materials
and
methods
.
Four
F
1
hybrids
heterozygous
for
the
markers
ful
-
e
(chromosome
4)
and
hl
-
a
(chromosome
11)
and
differing
(by
a
factor
of
2
.
5
to
5
under
optimal
conditions)
in
the
degree
of
hypothetical
heterosis
(HH)
for
plant
height
and
the
sum
of
lengths
of
the
first
two
true
leaves
were
examined
(Ursul,
1992)
.
Hypothetical
heterosis
(HH)
was
estimated
as:
HH
=
[(F
1
-
P)
/
P]
x
100%,
where
P
=
(P
1
+
P
2
)
/
2
.
These
had
been
derived
from
crosses
of
the
multimarker
line
Mo628
(
L
.
esculentum
)
with
two
varieties
and
two
wild
species:
Nevsky
—
high
heterosis
(116%,
);
Breakaday
—
low
heterosis
(24%,
¯
);
L
.
es
.
var
.
racemigerum
—
high
heterosis
(82%,
);
L
.
cheesmanii
—
low
heterosis
(31%,
¯
)
.
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