J. Embryo/, exp. Morph. Vol. 26, 3, pp. 5S7-598, 1971
Printed in Great Britain
587
Reproduction of Dugesia tigrina under short-day
and long-day conditions at different temperatures
I. Sexually derived individuals
By C. YOWINCKEL 1 AND J. R. MARSDEN 1
From the Department of Biology, McGill University
SUMMARY
Dugesia tigrina was reared under various combinations of long- or short-day length with
three temperature levels, 20, 23 and 26 °C. The resulting asexual and sexual reproduction was
recorded.
Animals reared under short days exhibited shorter generation length and a higher number
of worms hatching per cocoon than animals reared under long days.
Changes from short to long day or vice versa, experienced by animals during development,
resulted in retardation or complete absence of sexual reproduction. Cocoons laid by these
individuals showed a high degree of infertility.
D. tigrina was also collected in cocoons from the natural habitat shortly before hatching.
When exposed to completely different light regimes they responded with a uniform generation
length.
It is concluded that generation length, apart from temperature influences, is governed by
photoperiod during early development of individuals, probably already in the maternal body.
The number of worms hatching from one cocoon, likewise, is under photoperiod influence
which acts at some time after hatching of the parent.
INTRODUCTION
Our knowledge of photoperiod control over development and reproduction of
the lower metazoa is both scanty and recent. Planarians, being among the most
primitive Bilateria, have been widely investigated with respect to their regenerative abilities. They deserve more detailed investigation into their reproductive
physiology, its regulation and control, especially since aspects of asexual reproduction and regeneration are so closely related.
A general review of the literature relating to the induction of sexual development in planarians was made by Vowinckel (1968, 19706), and therefore is not
repeated here.
It has recently been demonstrated (Vowinckel, 1970«, b) that changes in both
photoperiod and temperature can independently induce sexual development in
Dugesia tigrina. The effect of illumination appears to be profound and needs
1
Authors' address: Department of Biology, McGill University, P.O. Box 6070, Montreal
101, Canada.
588
C. YOWINCKEL AND J. R. MARSDEN
more detailed study. Our earlier work suggested that the inductive effect of
photoperiod may take place early during development of a planarian.
We started with the assumption that exposure to short-day or long-day
illumination combined with different levels of constant temperature should have
different effects on reproduction. We report below on the results of experiments
designed to test these assumptions.
MATERIAL AND METHODS
Material. Two groups of approximately 75 sexually developed Dugesia tigrina
were collected from a population in the St Lawrence River in the autumn of
1969 and maintained in the laboratory at a photoperiod of LD (light: dark)
16:8 at 20 °C. When cocoon laying started one population was transferred to
LD 12:12 at 20 °C. The cocoons laid by these two populations provided the
material for the populations of series 1 and 2 described below. They were
collected 5 times per week and each series was fully established before the next
was started. The age of individuals within each series varies by less than 2 weeks
with the exception of the first long-day series at 20 °C (L]20), which was the
first series to be established and took 6 weeks to complete.
The cocoons that formed the five populations of series 3 were all collected in
the natural habitat on the same day in July 1970.
Experimental design. Populations were established from cocoons. The latter
were raised in six incubators which represented all possible combinations between long- and short-day illumination and three temperature levels. Long day
was represented by LD 16:8 (On from 6 a.m. to 10 p.m. EST - eastern standard
time). This approximates the daylength at the time of cocoon deposition and
hatching of the wild population. LD 12:12 (On from 8 a.m. to 8 p.m. EST) was
chosen to represent short days. It occurs in the natural habitat at a time when
most worms have completed their sexual development. The lowest temperature
level was 20 °C at which cocoon deposition takes place in the river, 23 °C
typically represents the temperatures of the natural habitat during July and
August and 26 °C approximates the highest temperatures which the natural
population is likely to encounter.
There were three main series of populations: one series in which each population experienced only one type of daylength throughout the extent of the
experiment, another series where populations were exposed to changes of daylength during their development and a third series of populations deriving from
cocoons laid in the natural habitat.
Series 1 consisted of 11 populations (the twelfth could not be completed). Two
parallel populations were raised under each of six temperature-daylength combinations in the following manner:
Sx20 and
Lx20 and
S220 at short day and 20 °C
L220 at long day and 20 °C
Reproduction o/Dugesia tigrina. /
SX23 and
LX23 and
SX26 and
L26
S223 at
L 2 23 at
S226 at
at
589
short day and 23 °C
long day and 23 °C
short day and 26 °C
long day and 26 °C
Since the cocoon donors for these experimental series were maintained at 20 °C
the four populations at 20 °C experienced constant temperatures, while the seven
populations at 23 and 26 °C experienced one initial change of temperature from
20 °C to 23 and 26 °C respectively on the day the cocoons were laid.
Series 2 consisted of 17 populations. These were populations which experienced
a change in photoperiod regime during development in the cocoon or shortly
after. Changes were from short to long day or vice versa and took place either (a)
within 24 h after the cocoon was laid, or (b) within 24 h after the worms hatched
from the cocoon, or (c) 2 weeks after hatching. Ideally, this series would have
included populations for each of the six types of shifts (plus duplicates) for all
three temperatures. However, the cocoon donors stopped laying and only the
following populations were established:
Temperature
20 °C
23 °C
26 °C
Shifted S-L (short to long day)
a,b
Shifted L-S (long to short day) a,b,c,c
a,b
a,b,c,c
a,b
a,b,c
All cocoons laid under experimental conditions by any population of series 1
and 2 were maintained separately under room temperature (air-conditioned,
approximately 21 °C). Photoperiod was not controlled. The number of worms
hatching from these cocoons were counted regularly and then discarded.
Series 3 consisted of five populations. These were all established from cocoons
collected on the same day in the natural habitat. One population (L-S/) was
consequently maintained on a shaded float in the natural habitat until the age of
85 days (21 September 1970). It was then brought to the laboratory and installed
in an incubator at 20 °C and LD 12:12. The other four populations were
established directly in the laboratory. They were installed in incubators at 23 °C
in the following manner:
Temperature
Population
s«
L-Sm
L-S»2
(°Q
23-20
23-20
23-20
Light regime
LD 16:8
LD 12:12
LD 16:8 reduced within
8 weeks after hatching
to LD 12:12
The groups that were installed in incubators all experienced a retardation of
On and Off times by approximately 1 h on the first day while the short-day
population was exposed, at this time, to a shift in photoperiod from long day to
short day. Worms began to hatch the day after arrival in the laboratory.
590
C. VOWINCKEL AND J. R. MARSDEN
The photoperiod of the last two populations was reduced, after all worms had
hatched, by twice weekly steps of 15 min, 1\ min in the morning and at night.
When LD 12:12 was reached by the two L-S u groups all five populations of
series 3 were switched on the same day and in one step to 20 °C (21 September).
Maintenance. All populations were maintained in incubators at controlled
temperatures and photoperiods.
Both temperature and light were continuously monitored by a multi-channel
a.c.-d.c. recorder. Photoperiod was automatically Regulated. The light source
was a shaded 9 in (22-8 cm) fluorescent light of less than 750 lux intensity at
10 cm distance. Diurnal temperature variations stayed below ±0-5 °C. Due to
the long span of the experiment (over 1 year) and consequent slow changes of
room temperature, the overall variation was only just below ± 1 0 °C. Since
these changes affected all incubators in a parallel fashion we do not think that
our results were much influenced by them.
Animals were maintained in glass containers at population densities not above
0-1 worm/ml in non-chlorinated river water (Vowinckel, 1910a). All containers
were covered with non-translucent material. Beef liver was fed to all populations
4 to 5 times weekly on an all or none basis. Cultures were cleaned on the same
day and water replaced at the same temperature.
Collection of records. Population counts were taken approximately every 8
weeks. All additions to populations (fission products and cocoons) were
recorded daily except at weekends and removed. All losses from the population
were likewise recorded. They resulted mostly from individuals that crawled
under the lid and dried up. Diseased animals were very rare. Animals that
fissioned anterior to the pharynx were removed entirely and counted as losses.
Notes on the degree of sexual development of populations, as visible externally, were kept with the daily record and all individuals were submitted twice to
an examination under the dissecting microscope during the first 80 days of their
life. Cocoons from experimental series, after hatching, were investigated under
the dissecting microscope for remaining individuals. Infertile cocoons were
counted at the end of the hatching period.
Statistical evaluation. All records of fission products and cocoon deposition
were recalculated and expressed as percentage of the presently existing population. They were then averaged (fission products) or summed (cocoons) over 5
days.
Since a population's generation length, measured as time passed from laying
of first parental to laying of first filial cocoon, is based on the performance of one
or two individuals only the data for the two L20 populations were combined as
were those of the two S20 populations, the difference not being significant. All
cocoons laid up to 50% cocoon production (and therefore representing one
cocoon laid for every second animal) were then used to evaluate the difference
between L20 and S20 by a Mann-Whitney U-test after Siegel (1956).
A ?-test was used to evaluate the significance of the difference between the
Reproduction o/Dugesia tigrina. /
591
number of worms hatched from all fertile cocoons laid under long-day conditions at 20 °C (including shifted series no. 2) as against those laid under shortday conditions at 20 °C.
RESULTS
Reproduction at 20 °C. The reproductive activity, both sexual and asexual, of
all populations at 20 °C is summarized in Fig. 1 and Tables 1-3. All populations
reproduced asexually, and all but one population reproduced sexually. The
generation length of populations differs very obviously. When measured from
Li
L2
Si
L-S c2
S-L
26
I
I
I
I
I I
50
I
100
150
200
250
300
Age of populations (days)
Fig. 1. Asexual and sexual reproduction of groups at 20 °C in percent of population producing one fission product or cocoon. Asexual: representing 5-day means,
in white; sexual: representing 5-day sums, in black. Asterisk: end of record. The
figures on the right indicate the size of each population. All L under long-day, all S
under short-day illumination. Values below 1 % are entered as 1 %. Black bars are
added above white bars.
the first parental to the first filial cocoon of each population, the two short-day
(S20) groups have a generation length of 87 and 97 days. The value for the two
long-day (L20) populations are 110 and 129 days. Since such a comparison is
a
L-S 6
L-S c
L-S c
S-L
S-L „
sL2- S
L2
Sx
Li
94
22
34
30
50
46
0
1
24
44
N
60
19
40
31
73
40
30
12
30
26
43
59
62
0
32
41
0
0
4
11
2-2
2-6
4-9
0
3-7
4-9
0
0
10
2-6
0-49
0
116
117
0
0
—
0-94
108
0-84
mean
S.D.
of
110
121
87
97
149
124
0
310
253
191
Generation
length
a
L-S b
L-S c
L-S c
S-L a
S-L b
s2
L-S
L!
Lx
36
9
52
30
16
18
21
25
19
16
N
9
2
48
15
8
0
3
42
0
13
.
12
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
2-2
0
0
0
Total
cocoons
Mean,
%
fertile worms/
investigated cocoons cocoon
23 °C
0
0
0
0
0
0
117
0
0
0
mean
of
S.D.
S-L a
S-Lj
L-S a
L-S b
L-S c
s2
Sx
Li
r
30
—
27
11
25
41
24
—
8
15
14
—
7
20
57
15
27
—
53
36
7
—
0
35
26
0
19
—
51
3
10
—
0
1-7
40
0
2-6
—
4-7
20
Total
cocoons
Mean,
%
fertile worms/
investiN
gated cocoons cocoon
26-20 °C
20
56
68
168
128
78
138
—
118
203
98
—
—
—
—
—
—
123
65
0
16
20
10
0
0
3
11
40
96
was
On day 63
the equivalent
of % population which
had fissioned
L-S a
L-S b
L-S c
L-S c
S-L a
S-L b
s2
Sx
U
Lx
68
70
60
37
63
63
60
55
42
53
78
83
69
43
72
82
68
63
52
57
An equivalent An equivalent
of 50% of
of 100% of
population
population
had fissioned had fissioned
on day
on day
23 °C
36
17
67
311
51
53
78
104
177
120
On day 63
the equivalent
of % population which
had fissioned
was
—
2-86
0
—
—
1-22
1-45
0
—
mean
S.D.
of
53
47
32
47
45
45
75
40
S-L a
S-Lb
—
St
S2
L-S o
L-S b
L-S c
Lx
60
—
53
38
54
59
50
—
80
48
166
—
215
381
215
176
207
—
0
232
On day 63
An equivalent An equivalent the equivalent
of % popuof 50% of
of 100% of
lation which
population
population
had fissioned had fissioned had fissioned
was
on day
on day
26 °C
Table 2. Asexual reproduction of all groups at 50 and 100% of population and on day 63, for all temperatures
AT
investigated
Mean,
worms/
cocoons cocoon
%
fertile
An equivalent An equivalent
of 50% of
of 100% of
population
population
had fissioned had fissioned
on day
on day
L-S 6
L-S c
L-S c
S-L a
S-L b
sL-S
2
Sx"
Lo
Lx
Population
Total
cocoons
20 °C
Table 1. Cocoon production and generation length of groups at all temperatures
o
m
C/5
r
O
2,
o
<
•
o
592
Reproduction o/Dugesia tigrina. /
593
based on the performance of one or two individuals only, 50% cocoon production of the combined L populations was tested against the combined S
populations by a Mann-Whitney U-test. Based on these statistics the difference
in generation length between L20 and S20 populations is highly significant with
a probability of less than 0000001 that this difference arose accidentally. Since
the differences in generation length between the S20 or L20 groups and the
shifted populations are even greater, further U-tests were not calculated.
Table 3. Generation length of five populations all deriving from the same
batch of cocoons collected in the natural habitat
L-S/ remained there till autumn. All others in lab. at 23 °C. Ln long day, Sn short
day, L-S,, day length successively shortened from long to short day. All groups maintained at 20 °C after 21 September.
Population
L«
Sn
L-Sni
L—O n n
L-S/
78
67
51
70
137
Arrival in
laboratory
First
hatching
14 July
14 July
14 July
14 July
21 Sept.
15 July
16 July
15 July
16 July
16 July
First filial
cocoon laid
Generation
length (days)
17 Feb.
12 Feb.
22 Feb.
22 Feb.
27 Jan.
239
233
244
243
217
Approximately 50 % of the cocoons laid by the series with constant photoperiod were fertile with the exception of the second short-day population (S2)
which for some unknown reason only laid sterile cocoons.
The generation length of populations subjected to a photoperiod change early
in life differs considerably from that of populations kept at constant illumination. Groups shifted from long day to short day spent most or all of their posthatching life under short-day conditions. However, their generation length in
every case is much longer than that of the constant short-day populations. In
each case cocoon deposition is postponed, by 8 and 4 weeks respectively, leading
to generation lengths of 149 days (L-Sft, shifted at laying) and 124 days (L-S 6 ,
shifted at hatching). When the shifting occurs two weeks after hatching (L-Sc)
cocoon laying is postponed to 310 days, while the parallel population still had
not laid after 325 days. Periodic evaluations of the two latter groups showed that
nearly all individuals were fully developed sexually after 150 days.
A photoperiod shift in the other direction, from short day to long day,
affected the generation time even more drastically. As compared with the constant long-day population cocoon deposition was retarded by approximately 18
weeks (generation length 253 days) when shifted at laying (S-La) and by approximately 10 weeks (generation length 191 days) when shifted at hatching (S-L6). In
addition, these cocoons were all sterile in contrast with those laid after an L-S
shift (Table 1). In general, cocoon deposition was least retarded if the shifting
took place at hatching time.
594
C. VOWINCKEL AND J. R. MARSDEN
The populations of series 3 were all raised from cocoons laid in the natural
habitat and under natural long-day conditions. They experienced completely
different light regimes after hatching. In spite of this their generation lengths are
remarkably uniform (Table 3).
One interesting side effect of the postponement of cocoon deposition after
the reproductive organs have developed was the occurrence, in most worms, of
supernumerary copulatory complexes. Up to four such complexes in the same
animal have been observed several times, the hindmost being the smallest and
the anteriormost the largest and functional. The production of a very small and
empty cocoon from a supernumerary gonopore was observed. An impression of
the generality of this effect can be gained by the following figures: Of 234
Dugesia tigrina (cocoons collected from the St Lawrence in July 1969) raised
under LD 16:8 at 20 °C, 43-6 % had one gonopore, 44-9 % had two, 10-7 % had
three and 0-9 % had four gonopores.
. Asexual reproduction of cocoon-hatched populations is condensed in Table 2.
A comparison shows that all populations at 20 °C that grew up under long-day
conditions (including shifted groups) reached 50 % cocoon production much
earlier than short-day populations with the exception of the first group at L20
which is the only population with a wide age range (see Material and Methods).
Reproduction at 23 °C. After an initial few fission cycles worms in general at
this temperature oscillated between sexual and asexual reproduction. A full set of
reproductive organs including copulatory complex and filled seminal vesicles
would develop and then be lost again by fission, this cycle to be repeated several
to many times. Eventually, however, sexual reproduction asserted itself in most
populations. This is summarized in Table 1. Reproduction was largely irregular,
both in generation length and the number of cocoons laid, most of which were
infertile. Only one population showed over 100 % cocoon production with 12 %
of these fertile.
As was to be expected asexual reproduction of cocoon-hatched groups was
considerably higher at 23 °C than at 20 °C. It also started earlier (Table 1). At
this temperature there was no difference in fission rate between long- and shortday series.
Reproduction at 26 °C. This temperature corresponds to the highest summer
values encountered in the natural habitat from which our stock is derived. No
external sexual development was observed in any population under these conditions. After very high initial fission rates (Table 2) for approximately 2 months
the constant long-day group (L, no shifts) showed signs of exhaustion. The
fission rate declined, some worms developed very long tails (tail twice as long as
body anterior to fission zone) and many began to fission anterior to the pharynx,
often at ovary level. After a month of this abnormal behavior worms began to
die without evident signs of disease. At this point all 26 °C populations, including those exposed to photoperiod changes were shifted to 20 °C to see if
they were still capable of sexual reproduction. Their record (Table 1) shows that
Reproduction o/Dugesia tigrina. /
595
all populations laid some cocoons but that with the exception of two groups,
they were largely infertile.
Number of worms per cocoon. At 20 °C the number of worms contained in
each cocoon varied drastically with the day length at which the cocoon was laid
(Table 1). The mean number of worms for all 58 fertile cocoons laid under long
days was 2-25 (<x, ± 0-91) and for all 56 fertile cocoons laid under short days was
4-54 (cr, ± 1-21). The probability that this difference arose by chance is much less
than 0001. Individuals which hatched from cocoons with few worms were rather
larger than those hatched from cocoons with many worms. These larger individuals fissioned much earlier than small worms, sometimes as soon as a few
days after hatching.
At 23 °C not enough fertile cocoons were laid to evaluate the effect of daylength. All 26 °C populations experienced one 6 °C drop in temperature. The
cocoons laid by these groups did not show clear-cut differences.
DISCUSSION
Our experiments further confirm our previous results (Vowinckel, 19706) that
the daily illumination period decisively influences planarian reproductive
physiology and that environmental temperature and photoperiod combine in
the determination of the pathway reproduction is to take at any time. How then
do these two factors exert their influence?
At the three temperature levels that were used we find completely different
pathways of reproduction. Throughout the populations of one temperature
level, however, reproductive behavior is fairly uniform.
At 20 °C cocoons were laid by nearly all populations, and asexual reproduction was limited to a minimum. While growth at this temperature continued
due to high food intake, much of the gain seemed channelled into supernumerary reproductive structures. Jenkins (1970) described the occurrence of
supernumerary gonopores in D. dorotocephala, and found that the presence of
four to six gonopores interfered with reproductive success while two or three
gonopores did not. In our stock there were very few individuals with four
gonopores. It is conceivable, though, that supernumerary gonopores somewhat
contributed to the number of infertile cocoons laid by our populations at 20 °C.
At 26 °C asexual reproduction continued toward exhaustion and sexual reproduction did not take place.
At 23 °C reproduction oscillated between development of sexual structures
and fission. One gets the impression that both reproductive pathways fight for
expression. In most populations sexual reproduction eventually resulted in some
cocoon laying but these cocoons, with two small exceptions, were all infertile.
This general infertility of cocoons at 23 °C may be due to a failure of sperm to
survive at this temperature. This was suggested by Cowles (1965).
It is well known that temperature changes can have an inductive effect. For
38
E M B 26
596
C. YOWINCKEL AND J. R. MARSDEN
D. tigrina Vowinckel (1970 a) showed that temperature oscillations can be
correlated with increases in the number of germ cells forming testicular primordia. This is in agreement with our present set of experiments in which the
effect of relatively constant temperature levels, as we interpret it, is largely noninductive, limiting and permissive.
The differences between populations at the same temperature level, on the
other hand, can all be correlated with differences in illumination regimen.
The daily illumination period appears to play a regulating role in planarians
much as it does in higher metazoa. That is to say, we witness the effects of a
biological clock. In D. tigrina the photoperiod seems to determine the time of
cocoon deposition. Maintenance under long photoperiods leads to later cocoon
deposition than under short photoperiods. The natural generation length of
D. tigrina, at our latitude, is approximately 365 days. Physiological activity in
the cold winter months presumably comes to a near standstill leading among
other things to an arrest of the biological clock. In the laboratory, in the absence
of low temperatures, we therefore arrive at much shorter generation lengths than
under natural conditions.
The photoperiod also influences the number of worms hatching from a
cocoon. We do not know at what time this influence is exerted. Worms which
experienced changes in photoperiod directly after hatching laid cocoons with a
sibling number corresponding to the daylength which they had been shifted to.
It would follow, therefore, that the number of offsprings contained in a cocoon
was determined after the mother had hatched. One could speculate that the
photoperiod influences the number of ova released from the ovary. Long days
would reduce this number while short days would increase it. Long days, in
general, seem to retard sexual development and encourage asexual reproduction.
Following a 6 °C drop in temperature this correlation between daylength and
sibling number/cocoon is lost. We know already that germ cells respond to
temperature oscillations with an increase in number. It seems plausible to suggest
that temperature might also influence the number of worms hatching per cocoon
and thus cause the loss of a simple correlation.
Under natural conditions cocoons are laid during the longest days of summer.
After hatching individuals soon fission several times before sexual development
commences. Cocoons laid under long-day conditions contain fewer worms of
greater size, which fission earlier than short-day cocoons. Therefore, the fission
period of sexually derived individuals would, under natural conditions, tend to
occur earlier if cocoons are laid at the height of summer than if cocoons were
laid in spring, that is under short-day conditions. Apart from this size factor
long-day illumination by itself seems to promote faster and higher fission rates at
20 °C. The S-L shifted groups were laid under short days but they nevertheless
fission earlier than the L-S populations. All long-day groups, finally, reach
100 % fission production while none of the short-day groups do so (Table 2).
We suggested that there exists during ontogeny a sensitive period during
Reproduction 0/Dugesia tigrina. /
597
which the worm responds to daylength resulting in the determination of a
corresponding generation length. This, presumably, would take place via the
medium of some neurosecretory factor. Evidence of the existence of such factors
was brought by Lender (1964) and Ude (1964). On the basis of our results we
can, at present, only make some educated guesses as to what phase in the life cycle
of D. tigrina is receptive to photoperiod induction. One of us (C.V.) suggested
earlier (1970a) that it might be found during early development of the worm.
Worms were exposed to changes of photoperiod during their early development. This resulted without exception in a retardation of the onset of sexual
reproduction in individuals which hatched from such cocoons. If we assume that
this retardation represents interference with a mechanism which has been
established or is in the process of being established we can consider several
possibilities.
If induction took place directly after hatching, then all populations that were
shifted before hatching should show the same generation length and fertility as
the undisturbed groups of the daylength they were shifted to and the populations
that were shifted 2 weeks after hatching should show signs of interference by
retarded generation length and decreased fertility. According to our results such
induction directly after hatching cannot have taken place. The populations that
were shifted from long day to short day before hatching both have much longer
generation lengths than the undisturbed short-period groups. The discrepancy is
even larger with a shift in the opposite direction.
If induction had taken place later than 2 weeks after hatching, we would
expect that all shifted groups should show the generation length of the daylength
to which they had been shifted. Our results obviously do not agree with these
premises either.
If induction took place during development in the cocoon we would expect the
generation length of those groups that were shifted at laying to coincide with that
of undisturbed groups of the photoperiod towards which they were shifted, and
all later shifts to show interference. Again there is no evidence that this took place.
If, however, photoperiod acted on the maternal organism and the generation
length was already determined before cocoons were laid we would expect to find
evidence of interference in the generation length and fertility of all populations
that were shifted. Our results obviously agree best with this assumption. In
general, the later during ontogeny our shifts took place and the more they contradicted natural conditions the more severe was the interference. Possibly this
means that at the onset of development this mechanism is still somewhat flexible
to changes (especially around hatching time) while later on it appears to consolidate and loses its adaptability.
The populations derived from cocoons laid under natural conditions provide
a convincing example of the importance of early development for photoperiod
induction. After hatching these groups were exposed to widely different illumination regimes but responded with a very uniform generation length. The group
38-2
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C. VOWINCKEL AND J. R. MARSDEN
that had no photoperiod interference during development reproduced earlier by
about 2 weeks while the generation length of all populations shifted to incubators before hatching falls within 238 ± 6 days. Considered in combination with
our other results this to us represents convincing evidence that the period which
is sensitive to photo-induction of generation length is indeed found during the
early development of D. tigrina.
It must be assumed that photoperiod control over reproduction has evolved
most often in species or races whose environment permits reproduction only on a
seasonal basis. Jenkins & Brown (1963) have described a race of D. dorotocephala which lives in a constant temperature spring at 18-5 °C and produces
cocoons permanently, apparently without seasonal variations. Obviously such
an environment would favour the suppression of any seasonal timing of reproduction. The same may be true for a race of Curaforemanii which in our laboratory produces cocoons throughout the year.
This work was supported by a grant from the National Research Council of Canada.
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(Manuscript received 13 May 1971)