more variable in an inconstant environment than the heterozygotes are’, a situation incompatible
with a complete genetic control of canalization. The second is that characters are generally nor-
mally-distributed in wild populations in spite of a strong genetic and environmental variability Bar-
ton and Turelli, 1989. The third is that genetic information is not completely controlling develop-
ment. There is rather a temporal alternance be- tween self-organized and direct genetic control,
while self-organization is largely involved in the achievement of major developmental and physio-
logical events Sachs, 1994; Amzallag, 2000a,b.
Beyond these
considerations, canalization
strongly differs from developmental homeostasis in the capacity of development to tend towards
the optimal end-result according to the internal or environmental constraint Waddington, 1957, pp.
43 – 44. This adaptive aspect of canalization was clearly observed in plants Bradshaw, 1965;
Moran et al., 1981; Sultan, 1992 and was termed adapti6e determinism by Seligmann and Amzallag
1995.
Existence of a ‘developmental buffer’ seems very important for integration of complex physio-
logical regulations, and evolution would require extremely rare combinations of simultaneous mu-
tations in its absence Warburton, 1955. In spite of the evidence, canalization was ‘unfairly under-
played in evolutionary discussions’ Alberch, 1980. It was also excluded from developmental
and physiological considerations. This obscure sit- uation does not only result from a deterministic
approach of development centered on expression of a pre-existing information, but we must also
recognize that the phenomenon of canalization confounds investigation. The high velocity and
efficiency of this process prevents any opportunity to identify transitory phases towards the ‘develop-
mental resolution’. Fortunately, this process be- comes visible when it takes place over an extended
period. For example, Oono 1985 described a dwarf mutation in some individuals regenerated
from cultured cells of rice. Although this trait is due to an homozygous mutation, a ‘chimeric re-
version’ towards normal stem height was observed in progeny of regenerated plants, and it remained
stable for at least three successive generations. This change was obviously directed, because a
normal-to-dwarf transition has never been ob- served after plant regeneration Oono, 1985. The
requirement for two sexual generations suggests the occurrence of intermediate states in the devel-
opmental reversion, which may be analyzed.
A similar opportunity exists in Sorghum bicolor. In this species, a 3-week exposure to a moderate
concentration of NaCl induces an ability to grow and set seeds at an otherwise lethal salinity. This
response was defined as salt-adaptation Amzallag et al., 1993. All the individuals of the salt-treated
population were able to grow at the NaCl concen- tration lethal for non-treated plants, but induction
of salt-adaptation was accompanied by a consid- erable increase in phenotypic diversity Amzallag
et al., 1995; Amzallag, 1999a. Salt-adapted plants displayed many perturbations in reproductive de-
velopment Amzallag, 1996, 1998, suggesting that canalization was disrupted by expression of salt-
adaptation. The reproductive development was also disturbed in progeny of salt-adapted plants
Amzallag, 1996, even for individuals grown in absence of NaCl Amzallag et al., 1998. More-
over, the evidence for a ‘developmental reversion’ in reproductive development of progeny of salt-
adapted plants was already suggested by previous observations Amzallag and Seligmann, 1998.
However, the latter analysis was complicated by re-exposure of the successive generations to a
salt-adaptation treatment. The purpose of this study is to analyze the phenomenon of ‘develop-
mental reversion’ in offspring of one or two suc- cessive generations of salt-adapted Sorghum that
are grown in absence of NaCl.
2. Materials and methods
2
.
1
. Plant material and growth conditions
2
.
1
.
1
. Parental generations Seeds from the MP610 genotype of S. bicolor
inbred line, male parent of the commercial hy- brid 610 were soaked overnight and germinated
in vermiculite moistened with tap-water. Five days after imbibition, the seedlings were trans-
ferred to 15 liter containers 12 seedlings per container, density of 80 plants m
− 2
filled with aerated half-strength Hoagland’s solution, accord-
ing to Amzallag et al. 1993. Some plants, defined as control, were maintained in these conditions,
were selfed and set seeds. Seeds from five control plants were harvested, and defined five lines of
control progeny. On day 8 following imbibition, another subpopulation was exposed to 150 mM
NaCl by six daily increases of 25 mM according to Amzallag 1998. These plants, defined as A0
plants, were maintained in this NaCl concentra- tion throughout their life cycle. They were selfed
and set seeds. Seeds from six of these A0 plants defined six A0 lineages. A year later, seeds from
these six lineages were germinated separately and treated for salt-adaptation as described for their
A0 parents Fig. 1. These A1 plants, also exposed to 150 mM NaCl until the end of their life cycle,
were selfed and set seeds. For each A0 lineage, seeds of two A1 plants were harvested separately.
All these plants were grown hydroponically throughout their life cycle. Media were changed
weekly, and deionized water was added daily to the root medium in order to compensate for
evapotranspiration. All the experiments were per- formed in a climatized greenhouse, under natural
light intensity, and during the photoperiod of June – September in Jerusalem.
2
.
1
.
2
. Field-grown progenies Fifteen seeds from each one of the five control
plants, the six A0 plants and the 12 corresponding A1 plants were soaked in a field at a density of
four plants m
− 2
. The 15 seeds from each line were separated in three groups of five seeds each,
which were randomized in the different rows of the experiment. The plants were drip-irrigated
with non-saline water approx. 5 mM NaCl throughout their life cycle. This experiment was
performed for the three populations at the Gilat Experimental Station, from May to August of the
same year. Field-grown progenies of the control, A0 and A1 plants are referred as the FC, FA0
and FA1 populations, respectively Fig. 1. All the field-grown progenies issued from the same A0
ancestor one FA0 line and two FA1 lines defined a lineage. The six lineages have been
separated into two classes of three lineages ac- cording to a criteria detailed in results below.
2
.
2
. Measurements The main stem of each plant was harvested
after complete senescence of the shoot. After re- moving the seeds, the shoot was dried for two
weeks at 60°C, weighed after removing all the blades, and defined as the shoot DW SDW. The
blade of the four last leaves was also weighed and
Fig. 1. Origin of the FC, FA0 and FA1 plants. ----, hydroponic conditions greenhouse without NaCl; + , hydroponic conditions greenhouse with 150 mM NaCl; – – , field condition, without NaCl.
Table 1 Comparison of mean 9 SD values of FC, FA0 and FA
populations
a
Population FA0
FC FA1
6.37 a Leaf DW g LDW
7.31 b 7.19b
9 2.49 9 2.02
9 2.41 32.8 a
Stem DW g SDW 42.5 b
39.9 b 9 10.9
9 15.6 9 13.9
97.1 a 88.2 a
96.5 b Stem height cm SH
9 5.4 9 11.0
9 12.2 1.54 a
1.00 a 1.48 a
Visible stem cm 9 4.71
9 6.73 VissS
9 6.37 43.4 a
49.8 b Tot seed weight g
48.9 b TSW
9 19.3 9 24.1
9 26.6 Number of seeds NS
2212 a 2216 a
2218 a 9 879
9 812 9 1068
19.25 a 22.05 b
Avg seed weight mg 21.73 b
9 3.42 9 4.72
9 5.54 ASW
SH:SDW ratio 2.99 a
2.71 a 2.75 a
9 1.52 9 1.03
9 1.16 55.4 b
NS:SDW ratio 56.2 b
68.7 a 9 25.3
9 19.3 9 26.4
6.86 a TSW:LDW ratio
6.78 a 6.74 a
9 2.42 9 2.87
9 3.24 112
Population size 123
296
a
A same letter following two values in the same row indicates that they are not distinguishable by a t-test at
PB0.05.
SV
L
C = 2[CL-C
FC
] CL + C
FC
where CL is the mean value of the character C for the line L, and C
FC
is the corresponding value calculated for the whole population of control
plants see Table 1. The absolute value of SV
L
C [abbreviated as ASV
L
C] quantifies the deviation of the line L from the FC population for expres-
sion of the character C, and the sign negative or positive indicates direction of this discrepancy.
For each character C, the difference in ASV between FA1 and their corresponding FA0 line
abbreviated as DASV
A1,A0
C was calculated as follows:
DASV
A1,A0
C =
1mn
n i = 1
m k = 1
ASVFA1
k i
C − ASVFA0
i
C
n
where n is the number of lineages considered, and m is the number of FA1 lines related to each FA0
line n = 3 and m = 2 for classes I and II in the current experiment.
3. Results
