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Production and characterisation of an early dwarf mutant in soybean (glycine max (L.) merrill)
223
Mutation Research, 43 (1977) 223--230 © Elsevier/North-Holland Biomedical Press
P R O D U C T I O N AND CHARACTERISATION OF AN E A R L Y DWARF MUTANT IN SOYBEAN (GLYCINE MAX (L.) MERRILL)
LEE CHOO KIANG and G.M. HALLORAN
School of Agriculture and Forestry, University of Melbourne, Parkville 3052 (Australia) (Received October 10th, 1976) (Accepted October 24th, 1976)
Summary An early dwarf mutant condition was induced in the M1 generation after treatment of the soybean cultivar Wayne with ethyl methanesulphonate (0.05 M for 6 h at pH 7). Segregation in the M2 indicated the earliness ::-haracter to be a dominant mutation for lowered sensitivity to photoperiod (approx. 14.5 h) compared with Wayne. The relationship of the dwarf mutant condition with earliness in M2 segregation was not clear, indicating the possibility of either separate mutations for earliness and dwarfness, two types of earliness (one associated with dwarfness, and the other not) or pleiotropism of a single mutant gene which confers a physiological relationship of height and maturity different from that normally found in soybean. The implications have been outlined of being able to induce maturity variation, b y chemical mutagenesis, in soybean introduction programmes.
Introduction Increased worldwide interest in the attainment of higher yield and improved quality in soybean has led to greater efforts by breeders to evaluate wide ranges of introductions as potential parents for breeding for improvement in these characters. A great limitation in the evaluation of the potential usefulness of many introductions, particularly those from widely different latitudinal sources, is their inappropriate maturity in their new environment. Too late or early a maturity can " m a s k " the expression of yield and quality characters and hence the genetic worth of the introduction cannot be accurately evaluated in introduction trials. The use of induced mutation in soybean introduction programmes to produce mutants for maturity more appropriate to that desired in the new environment poses as a potentially useful means of gaining more ready access
224 to the genetic variability for those characters influenced by the environment. In this respect chemical mutagens, which appear generally to have a less serious disruptive effect on chromosomal organization than ionizing radiation, might enable a more rapid and appropriate evaluation of m u t a n t progeny to be carried out. Mutations for maturity alteration in soybeans have been previously reported as induced with both ionising radiation and chemical mutagens. Zacharias [5] used X-rays on the soybean cultivar Heinkraft 1 and found a diversity of mutants, including those for early maturity, in studies of M1, M2, and M3 populations. Rawlings et al. [4] found that both thermal neutrons and X-rays gave increased genetic variance in Ms populations for yield, time to maturity and seed weight. Gamma irradiation in soybean has likewise been found to produce m u t a t i o n for earliness, and a m u t a n t line 15 days earlier than the mother line was released as the cultivar Raiko in Japan in 1969 [2]. Chemical mutagens have been less widely used on soybeans to obtain maturity mutants. However, Borejko [1], using nitroso ethyl urea and nitroso methyl urea on the two cultivars Kirovograd-2 and -4, obtained mutants which matured 15--30 days earlier than the parent cultivars. In most reports of induced mutation for maturity alteration in soybeans, little attempt has been made to describe the mutants in terms of either their genetic or physiological basis. The present study was aimed at investigating the potential usefulness of the chemical mutagen ethyl methanesulphonate (EMS) in inducing mutations for maturity alteration in soybeans. If such mutants were produced, it was hoped that both the mutation frequency and their genetical and physiological characteristics might be determined. Materials and methods The present study was conducted on the soybean cultivar Wayne. From previous studies, appropriate concentrations and times of treatment with EMS had been obtained. A concentration of 0.05 M EMS (in McIvaine standard buffer pH 7.0) for a period of 6 h was used for the m u t a t i o n treatment. Six hundred seeds were treated with the mutagen while an additional 200 seeds were allowed to imbibe distilled water for 6 h and a further lot, of 100 seed, was allowed to imbibe McIvaine standard buffer pH 7.0 for 6 h. These treatments were to ascertain possible physiological or genetical differences between the mutation treatment and either water alone or buffer solution alone. All soaking of seed in these solutions was carried out in petri dishes at a constant temperature of 21 ° C. During the treatment period the petri dishes were shaken hourly to ensure adequate aeration of the treatment solution. After treatment, all seeds were washed in running tap water and then rinsed in three changes of de-ionised water. They were then sown in trays (28 cm × 34 cm × 6 cm deep), containing an artificial potting medium, for germination. Three weeks after emergence, seedlings were transplanted into 25-cm diameter pots (8 per pot) containing a soil-sand (50/50 by volume) mixture. The M~ generation was grown in a heated glasshouse at the School of Agriculture and Forestry, University of Melbourne, over the winter period. M1 plants were harvested individually and the M2 seeds were sown as M2
225 lines in pots just like the M1 seed. The M: generation was grown in a cooled glasshouse over the summer at temperatures ranging from 19--30°C. Detailed observations were made of maturity as node of first flower and time to first flower on both the M1 plants and individual plants in the M2 lines. A range of variation in time to maturity was observed in both populations b u t for the present study the behaviour of only one mutant, designated 6CD3, is reported.
Time to flower under different daylengths Studies of flowering time and height segregation of progeny of the mutant line 6CD3 were made in conjunction with a number of soybean cultivars from different maturity groupings. This was done to enable closer characterisation of mutant progeny, particularly for time of flowering. The cultivars Dorman, Flambeau, Harosoy, Lindarin 63 and Wayne plus 9 M2 plants of the line 6CD3 were grown in the University of Melbourne School of Agriculture and Forestry p h y t o t r o n under a constant temperature of 20°C and natural daylength which ranged from 14.73--14.87--13.73 h (30th November to mid-February). The cultivar Wayne, together with 3 M: plants of the mutant line 6CD3, were grown in the p h y t o t r o n at constant temperature of 20°C and a daylength of 10 h. The 10 h photoperiod was natural daylight over the hours 8.00 a.m.-6.00 p.m. Observations were made of node to first flower, days to first flower and plant height on the cultivars and the M2 plants of line 6CD3 under the two photoperiodic regimes. Results
The mutant plant described in this study was recognised in the MI plant population as being strongly deviant in both final height and maturity in that it was 15 days earlier in flowering that the cultivar Wayne. The possibility of this early variant having arisen by an intermixture or cross pollination within the parent cultivar is not at all likely. Careful precautions were taken in the production of seed for the mutagen treatment that either cross-mixing or out-pollination did not occur. A comparison of this plant with the mean values for the control population of the cultivar Wayne for certain distinguishing characters are shown in Table I. Eighteen seeds were harvested from the M1 plant, 6CD3, of which 15 were viable and were used for further study of possible segregation for a suspected
TABLEI
Height Node number of first flower Number of days to first flower Yield -- number of pods
Line 6CD3 (M 1 p l a n t )
Mean values for control population (cultivar Wayne)
14.5 cm 5.0 75 9
26.2 cm 7.4 90 11.7
226 mutation for earliness and height. The relationship of days to first flower and node number at first flower for the four cultivars compared with that obtained for nine M: plants of the line 6CD3, under natural daylength, is shown in Fig. 1. The 6CD3 M2 mutant plants under normal photoperiod could be considered to fall into three maturity types. Two lines flowered on the 41st day after planting at node number 3. Five plants flowered either on the 43rd or 44th days after planting at node number 5. The final two plants flowered later at 50 and 54 days after planting. The mutant plants, unlike the plants from all other control cultivars, exhibited large variation in both days to first flower and node of first flower. As the number of days to first flower is not highly variable in any of the cultivars studied, the mutant plants can be said to exhibit flowering time behaviours similar to three cultivars from three maturity groups, the earliest plants resembling group 00, the intermediate-group II and the latestgroup III. Mutant plants which came to flower on the 41st day and 43rd or 44th days could be regarded as similar in this attribute b u t the node number to first flower of these plants was divergent. The earliest mutant (i.e. 41 days to flower) flowered at a node number of 3 while the other two plants (i.e. 43 and 44 days to flower) flowered on node number 5. This behaviour is somewhat at variance with that exhibited by the four cultivars (Fig. 2) in that on the basis of their flowering time the mutant plants would have been expected to have flowered on node number 4. The late flowering 6CD3 segregates which came to flower after 50 days, at node 5, resemble the parental type and are in con-
o Dorman
7 .J
• • o~nVanm Yne l, 0
o
0 0 Z
Harosoy
=-,Flambeau
•
3L0
4'0
Progeny of 50
6'0
Line 6CD3
7'0
80
T O F I R S T FLOWER DAYS Fig. 1. R e l a t i o n s h i p b e t w e e n p l a n t h e i g h t and d a y s to first f l o w e r f o r s o y b e a n cultivars a n d M 2 p l a n t s of t h e m u t a n t line 6 C D 3 , of t h e cultivax W a y n e , u n d e r n a t u r a l d a y l e n g t h .
227 50
ODorman
O >I'n, • Wayne
30
~E oHarosoy AFlambeau
II--
m ILl
20
•m
"1"
Iz< ._1 "10
•
310
410
DAYS
Progeny of 5'0
TO
510
FIRST
Line 6CD3
7[0
8J0
FLOWER
Fig. 2. R e l a t i o n s h i p b e t w e e n d a y s to, a n d n o d e n u m b e r of, first f l o w e r in s o y b e a n c u l t i v a r s a n d M 2 p l a n t s of t h e m u t a n t line 6 C D 3 , of t h e c u l t i v a r W a y n e , u n d e r n a t u r a l d a y l e n g t h .
formity with the general relationship of node number and flowering time exhibited the cultivars of the study.
Dwarf phenotype The relationship of final plant height with days to first flower for the four cultivars compared with that for 9 M: plants of the line 6CD3 is shown in Fig. 2.
T A B L E II MATURITY NODE NUMBER TO FIRST FLOWER, AND HEIGHT OF M 2 PLANTS OF THE MUTANT L I N E 6 C D 3 A N D T H E P A R E N T C U L T I V A R W A Y N E G R O W N U N D E R 10-h P H O T O P E R I O D P r o g e n y o f line 6 C D 3
D a y s to 1st f l o w e r
N o d e t o 1st f l o w e r
Height (cm)
6CD*10 6CD-11 6CD-12 Mean of Progeny of 6CD3
32.0 32.0 33.0 32.33 + 0.19
3 3 3 3 ±0
9.0 8.8 8.9 8.9 ± 0 . 0 3
Mean of p a r e n t cultivar
35.33
3.33 -+ 0 . 1 9
17.0 ± 0 . 7 6
228 TABLE
III
MATURITY, NODE TO FIRST LINE 6CD3 AND THE PARENT
FLOWER, HEIGHT, AND YIELD OF M 2 PLANTS OF THE MUTANT CULTIVAR WAYNE GROWN UNDER NORMAL DAYLENGTH
Progeny of 6CD3
D a y s to 1st f l o w e r
N o d e t o 1st f l o w e r
H e i g h t at h a r v e s t ( c m )
Yield (pod no.)
6CD3-1 6CD3-2 6CD3-3 6CD3-4 6CD3-5 6CD3-6 6CD3-7 6CD3-8 6CD3-9 Mean of progeny of 6CD3
41 43 43 44 43 44 41 50 54 44.7 ± 1.45
3 5 5 5 5 5 3 5 5 4.55 i 0.31
16.5 19.5 21.7 19.4 21.5 19.7 7.5 18.0 20.5 18.3 ± 1.36
17 18 18 11 19 12 7 8 17 14.1 ± 1.53
Mean of parent cultivar Wayne
4 8 . 8 0 ~- 0 . 1 4
5.0 ± 0
32.0 ± 0.92
15.67 ± 1.68
The M: plants in line 6CD3 were all shorter than the control cultivars with a range in height of 7.5 to 21.7 cm compared to a mean value for Wayne of 32.0 cm. Moreover, two of the plants were shorter than the rest. These two plants, although differing greatly in height between themselves (7.5 and 16.5 cm) are classed together because they also share the same very early flowering phenotype. It is possible that the shorter plant may have been checked in early growth, resulting in the very short stature. The rest of the seven plants were not different in their heights. These seven plants included the 5 early flowering types and two late flowering types. The three plants grown at 10-h photoperiod appeared to breed true for the dwarfness character (Table II). Under normal photoperiod they were considerably shorter than the control cultivars and very much shorter than the cultivar Wayne (Table III). Discussion
Present evidence points to an association of the very early m u t a n t character with t h a t of dwarfness. However, the occurrence of the dwarf character in the late phenotype suggests that if both the characters height and maturity are being conditioned by the one gene and the physiological association of lateness and tallness normally found in soybean is somewhat broken in this situation. The presence of a separate mutation for a gene controlling height cannot be rejected, but the association of earliness with extreme dwarfness points to the possibility of a pleiotropic effect of one gene rather than two. Accepting the postulation that there existed three maturity classes in the progeny of the M2 dwarf early m u t a n t line, the observed maturity class (very early, late) proportions fit a 1 : 2 : I ratio as shown in Table IV. Regarding the possibility of the very early and early maturity classes being of 'similar maturity, the ratio of early to late maturity classes approximates a 3 : 1 ratio as also indicated in Table IV. Another possible explanation is t h a t two types of earliness may have
229 T A B L E IV SEGREGATION
RATIOS FOR MATURITY
T Y P E IN T H E M 2 G E N E R A T I O N
OF THE MUTANT
LINE
6CD3 Maturity type segregation in M 3 Postulated segregation ratio 1:2:1 3:1
Very Early 2 --
Early 5 7
X2 p r o b a b i l i t y Late 2 2
0 . 9 7 5 ) P :> 0 . 9 0.9 ) P ~ 0.5
been present, one associated with dwarfness and one not associated with the dwarf condition. The absence of M2 plants of the line 6CD3 approaching the height of the parental genotype, Wayne, restricted the genetic interpretation of this segregation. It is believed that the absence of plants of the height of Wayne could possibly be merely statistical in that t o o small a number of segregates was available for analysis. However, there is the possibility of a more complex genetic situation of height and maturity segregation. The presumed double recessive class of plants for late maturity (i.e. Wayne genotype) has a mean height significantly shorter than Wayne, which suggests independent assortment of height and maturity genes and the possibility of a double mutation, one for each of these characters in this line. Another aspect of interest is the apparent departure of the relationship of plant height with days to first flower amongst the M2 mutant plants from the type of relationship exhibited by the four cultivars as illustrated in Fig. 2. The relationship of node number of first flower and days to first flower amongst the M2 plants appears to be in general conformity with the relationship exhibited b y the four soybean cultivars examined (Fig. 1). One possible explanation of these results could be the segregation for a gene influencing earliness which exerted a pleiotropic effect on plant height to a different degree than the normal range of maturity alleles in soybean cultivars. Further studies of segregation for maturity and height within the line 6CD3 are at present in progress. The earliness expressed in the mutation studied, appeared to be one conditioning a changed photoperiodic sensitivity over that of the parental response. This change was in the direction of increased insensitivity to increased daylength. All maturity comparisons between cultivars and mutant progeny in the present study were conducted at 20°C and hence differences in maturity within and between the two daylength regimes can be considered to be due essentially to differences in photoperiodic sensitivity. The appearance of the early mutant condition in the M1 plant and the subsequent demonstration in the M2 plants of a 3 : 1 (early : late) segregation ratio indicates that this photoperiodic insensitivity is recessive and monogenically controlled. Previous reference has been made to a dominant gene (designated E) giving earliness in soybean [3]. Evidence on the frequency of occurrence of this early mutant, its genetic nature, and physiological influence could have important implications in soybean introduction programmes. Introduced genotypes which were of extremely divergent maturity, particularly those which were very late maturing, could be treated with mutagen and the M1 generation screened for the possible presence
230
of early variants. The frequency of approx. 3% for the occurrence of the very early mutation in the M~ in the present study suggests that excessively large populations need not have to be handled in order to have a good change of obtaining desirable early flowering M1 plant(s). References 1 B o r e j k o , A.M., P r o d u c t i o n of i n d u c e d m u t a t i o n s in s o y b e a n , G e n e t i c a M o s k a , 6 ( 1 9 7 0 ) 1 6 7 - - 1 6 9 . 2 I s h i k a w a , M., in I n d u c e d M u t a t i o n s a n d P l a n t I m p r o v e m e n t , I . A . E . A . / F . A . O . P r o c . , B u e n o s Aires, I . A . E . A . Press, V i e n n a 1 9 7 0 , p. 2 8 6 . 3 J o h n s o n , H.W. a n d L, B e r n a r d , S o y b e a n g e n e t i c s a n d b r e e d i n g , in A . G . N o m a n (ed.) " T h e S o y b e a n " A c a d e m i c Press, N e w Y o r k - L o n d o n 1 9 6 3 , p. 13. 4 R a w l i n g s , J . O . , D . G . H a n w a y a n d C.D. G a r d n e r , V a r i a t i o n in q u a n t i t a t i v e c h a r a c t e r s o f s o y b e a n a f t e r seed i r r a d i a t i o n , A g r o n . J. 5 0 ( 1 9 5 8 ) 5 2 4 - - 5 2 8 . 5 Z a c h a r i a s , M., in A . G . N o r m a n (ed.) " T h e S o y b e a n " A c a d e m i c Press, N e w Y o r k - L o n d o n , 1 9 6 3 , p. 6 1 .
Mutation Research, 43 (1977) 223--230 © Elsevier/North-Holland Biomedical Press
P R O D U C T I O N AND CHARACTERISATION OF AN E A R L Y DWARF MUTANT IN SOYBEAN (GLYCINE MAX (L.) MERRILL)
LEE CHOO KIANG and G.M. HALLORAN
School of Agriculture and Forestry, University of Melbourne, Parkville 3052 (Australia) (Received October 10th, 1976) (Accepted October 24th, 1976)
Summary An early dwarf mutant condition was induced in the M1 generation after treatment of the soybean cultivar Wayne with ethyl methanesulphonate (0.05 M for 6 h at pH 7). Segregation in the M2 indicated the earliness ::-haracter to be a dominant mutation for lowered sensitivity to photoperiod (approx. 14.5 h) compared with Wayne. The relationship of the dwarf mutant condition with earliness in M2 segregation was not clear, indicating the possibility of either separate mutations for earliness and dwarfness, two types of earliness (one associated with dwarfness, and the other not) or pleiotropism of a single mutant gene which confers a physiological relationship of height and maturity different from that normally found in soybean. The implications have been outlined of being able to induce maturity variation, b y chemical mutagenesis, in soybean introduction programmes.
Introduction Increased worldwide interest in the attainment of higher yield and improved quality in soybean has led to greater efforts by breeders to evaluate wide ranges of introductions as potential parents for breeding for improvement in these characters. A great limitation in the evaluation of the potential usefulness of many introductions, particularly those from widely different latitudinal sources, is their inappropriate maturity in their new environment. Too late or early a maturity can " m a s k " the expression of yield and quality characters and hence the genetic worth of the introduction cannot be accurately evaluated in introduction trials. The use of induced mutation in soybean introduction programmes to produce mutants for maturity more appropriate to that desired in the new environment poses as a potentially useful means of gaining more ready access
224 to the genetic variability for those characters influenced by the environment. In this respect chemical mutagens, which appear generally to have a less serious disruptive effect on chromosomal organization than ionizing radiation, might enable a more rapid and appropriate evaluation of m u t a n t progeny to be carried out. Mutations for maturity alteration in soybeans have been previously reported as induced with both ionising radiation and chemical mutagens. Zacharias [5] used X-rays on the soybean cultivar Heinkraft 1 and found a diversity of mutants, including those for early maturity, in studies of M1, M2, and M3 populations. Rawlings et al. [4] found that both thermal neutrons and X-rays gave increased genetic variance in Ms populations for yield, time to maturity and seed weight. Gamma irradiation in soybean has likewise been found to produce m u t a t i o n for earliness, and a m u t a n t line 15 days earlier than the mother line was released as the cultivar Raiko in Japan in 1969 [2]. Chemical mutagens have been less widely used on soybeans to obtain maturity mutants. However, Borejko [1], using nitroso ethyl urea and nitroso methyl urea on the two cultivars Kirovograd-2 and -4, obtained mutants which matured 15--30 days earlier than the parent cultivars. In most reports of induced mutation for maturity alteration in soybeans, little attempt has been made to describe the mutants in terms of either their genetic or physiological basis. The present study was aimed at investigating the potential usefulness of the chemical mutagen ethyl methanesulphonate (EMS) in inducing mutations for maturity alteration in soybeans. If such mutants were produced, it was hoped that both the mutation frequency and their genetical and physiological characteristics might be determined. Materials and methods The present study was conducted on the soybean cultivar Wayne. From previous studies, appropriate concentrations and times of treatment with EMS had been obtained. A concentration of 0.05 M EMS (in McIvaine standard buffer pH 7.0) for a period of 6 h was used for the m u t a t i o n treatment. Six hundred seeds were treated with the mutagen while an additional 200 seeds were allowed to imbibe distilled water for 6 h and a further lot, of 100 seed, was allowed to imbibe McIvaine standard buffer pH 7.0 for 6 h. These treatments were to ascertain possible physiological or genetical differences between the mutation treatment and either water alone or buffer solution alone. All soaking of seed in these solutions was carried out in petri dishes at a constant temperature of 21 ° C. During the treatment period the petri dishes were shaken hourly to ensure adequate aeration of the treatment solution. After treatment, all seeds were washed in running tap water and then rinsed in three changes of de-ionised water. They were then sown in trays (28 cm × 34 cm × 6 cm deep), containing an artificial potting medium, for germination. Three weeks after emergence, seedlings were transplanted into 25-cm diameter pots (8 per pot) containing a soil-sand (50/50 by volume) mixture. The M~ generation was grown in a heated glasshouse at the School of Agriculture and Forestry, University of Melbourne, over the winter period. M1 plants were harvested individually and the M2 seeds were sown as M2
225 lines in pots just like the M1 seed. The M: generation was grown in a cooled glasshouse over the summer at temperatures ranging from 19--30°C. Detailed observations were made of maturity as node of first flower and time to first flower on both the M1 plants and individual plants in the M2 lines. A range of variation in time to maturity was observed in both populations b u t for the present study the behaviour of only one mutant, designated 6CD3, is reported.
Time to flower under different daylengths Studies of flowering time and height segregation of progeny of the mutant line 6CD3 were made in conjunction with a number of soybean cultivars from different maturity groupings. This was done to enable closer characterisation of mutant progeny, particularly for time of flowering. The cultivars Dorman, Flambeau, Harosoy, Lindarin 63 and Wayne plus 9 M2 plants of the line 6CD3 were grown in the University of Melbourne School of Agriculture and Forestry p h y t o t r o n under a constant temperature of 20°C and natural daylength which ranged from 14.73--14.87--13.73 h (30th November to mid-February). The cultivar Wayne, together with 3 M: plants of the mutant line 6CD3, were grown in the p h y t o t r o n at constant temperature of 20°C and a daylength of 10 h. The 10 h photoperiod was natural daylight over the hours 8.00 a.m.-6.00 p.m. Observations were made of node to first flower, days to first flower and plant height on the cultivars and the M2 plants of line 6CD3 under the two photoperiodic regimes. Results
The mutant plant described in this study was recognised in the MI plant population as being strongly deviant in both final height and maturity in that it was 15 days earlier in flowering that the cultivar Wayne. The possibility of this early variant having arisen by an intermixture or cross pollination within the parent cultivar is not at all likely. Careful precautions were taken in the production of seed for the mutagen treatment that either cross-mixing or out-pollination did not occur. A comparison of this plant with the mean values for the control population of the cultivar Wayne for certain distinguishing characters are shown in Table I. Eighteen seeds were harvested from the M1 plant, 6CD3, of which 15 were viable and were used for further study of possible segregation for a suspected
TABLEI
Height Node number of first flower Number of days to first flower Yield -- number of pods
Line 6CD3 (M 1 p l a n t )
Mean values for control population (cultivar Wayne)
14.5 cm 5.0 75 9
26.2 cm 7.4 90 11.7
226 mutation for earliness and height. The relationship of days to first flower and node number at first flower for the four cultivars compared with that obtained for nine M: plants of the line 6CD3, under natural daylength, is shown in Fig. 1. The 6CD3 M2 mutant plants under normal photoperiod could be considered to fall into three maturity types. Two lines flowered on the 41st day after planting at node number 3. Five plants flowered either on the 43rd or 44th days after planting at node number 5. The final two plants flowered later at 50 and 54 days after planting. The mutant plants, unlike the plants from all other control cultivars, exhibited large variation in both days to first flower and node of first flower. As the number of days to first flower is not highly variable in any of the cultivars studied, the mutant plants can be said to exhibit flowering time behaviours similar to three cultivars from three maturity groups, the earliest plants resembling group 00, the intermediate-group II and the latestgroup III. Mutant plants which came to flower on the 41st day and 43rd or 44th days could be regarded as similar in this attribute b u t the node number to first flower of these plants was divergent. The earliest mutant (i.e. 41 days to flower) flowered at a node number of 3 while the other two plants (i.e. 43 and 44 days to flower) flowered on node number 5. This behaviour is somewhat at variance with that exhibited by the four cultivars (Fig. 2) in that on the basis of their flowering time the mutant plants would have been expected to have flowered on node number 4. The late flowering 6CD3 segregates which came to flower after 50 days, at node 5, resemble the parental type and are in con-
o Dorman
7 .J
• • o~nVanm Yne l, 0
o
0 0 Z
Harosoy
=-,Flambeau
•
3L0
4'0
Progeny of 50
6'0
Line 6CD3
7'0
80
T O F I R S T FLOWER DAYS Fig. 1. R e l a t i o n s h i p b e t w e e n p l a n t h e i g h t and d a y s to first f l o w e r f o r s o y b e a n cultivars a n d M 2 p l a n t s of t h e m u t a n t line 6 C D 3 , of t h e cultivax W a y n e , u n d e r n a t u r a l d a y l e n g t h .
227 50
ODorman
O >I'n, • Wayne
30
~E oHarosoy AFlambeau
II--
m ILl
20
•m
"1"
Iz< ._1 "10
•
310
410
DAYS
Progeny of 5'0
TO
510
FIRST
Line 6CD3
7[0
8J0
FLOWER
Fig. 2. R e l a t i o n s h i p b e t w e e n d a y s to, a n d n o d e n u m b e r of, first f l o w e r in s o y b e a n c u l t i v a r s a n d M 2 p l a n t s of t h e m u t a n t line 6 C D 3 , of t h e c u l t i v a r W a y n e , u n d e r n a t u r a l d a y l e n g t h .
formity with the general relationship of node number and flowering time exhibited the cultivars of the study.
Dwarf phenotype The relationship of final plant height with days to first flower for the four cultivars compared with that for 9 M: plants of the line 6CD3 is shown in Fig. 2.
T A B L E II MATURITY NODE NUMBER TO FIRST FLOWER, AND HEIGHT OF M 2 PLANTS OF THE MUTANT L I N E 6 C D 3 A N D T H E P A R E N T C U L T I V A R W A Y N E G R O W N U N D E R 10-h P H O T O P E R I O D P r o g e n y o f line 6 C D 3
D a y s to 1st f l o w e r
N o d e t o 1st f l o w e r
Height (cm)
6CD*10 6CD-11 6CD-12 Mean of Progeny of 6CD3
32.0 32.0 33.0 32.33 + 0.19
3 3 3 3 ±0
9.0 8.8 8.9 8.9 ± 0 . 0 3
Mean of p a r e n t cultivar
35.33
3.33 -+ 0 . 1 9
17.0 ± 0 . 7 6
228 TABLE
III
MATURITY, NODE TO FIRST LINE 6CD3 AND THE PARENT
FLOWER, HEIGHT, AND YIELD OF M 2 PLANTS OF THE MUTANT CULTIVAR WAYNE GROWN UNDER NORMAL DAYLENGTH
Progeny of 6CD3
D a y s to 1st f l o w e r
N o d e t o 1st f l o w e r
H e i g h t at h a r v e s t ( c m )
Yield (pod no.)
6CD3-1 6CD3-2 6CD3-3 6CD3-4 6CD3-5 6CD3-6 6CD3-7 6CD3-8 6CD3-9 Mean of progeny of 6CD3
41 43 43 44 43 44 41 50 54 44.7 ± 1.45
3 5 5 5 5 5 3 5 5 4.55 i 0.31
16.5 19.5 21.7 19.4 21.5 19.7 7.5 18.0 20.5 18.3 ± 1.36
17 18 18 11 19 12 7 8 17 14.1 ± 1.53
Mean of parent cultivar Wayne
4 8 . 8 0 ~- 0 . 1 4
5.0 ± 0
32.0 ± 0.92
15.67 ± 1.68
The M: plants in line 6CD3 were all shorter than the control cultivars with a range in height of 7.5 to 21.7 cm compared to a mean value for Wayne of 32.0 cm. Moreover, two of the plants were shorter than the rest. These two plants, although differing greatly in height between themselves (7.5 and 16.5 cm) are classed together because they also share the same very early flowering phenotype. It is possible that the shorter plant may have been checked in early growth, resulting in the very short stature. The rest of the seven plants were not different in their heights. These seven plants included the 5 early flowering types and two late flowering types. The three plants grown at 10-h photoperiod appeared to breed true for the dwarfness character (Table II). Under normal photoperiod they were considerably shorter than the control cultivars and very much shorter than the cultivar Wayne (Table III). Discussion
Present evidence points to an association of the very early m u t a n t character with t h a t of dwarfness. However, the occurrence of the dwarf character in the late phenotype suggests that if both the characters height and maturity are being conditioned by the one gene and the physiological association of lateness and tallness normally found in soybean is somewhat broken in this situation. The presence of a separate mutation for a gene controlling height cannot be rejected, but the association of earliness with extreme dwarfness points to the possibility of a pleiotropic effect of one gene rather than two. Accepting the postulation that there existed three maturity classes in the progeny of the M2 dwarf early m u t a n t line, the observed maturity class (very early, late) proportions fit a 1 : 2 : I ratio as shown in Table IV. Regarding the possibility of the very early and early maturity classes being of 'similar maturity, the ratio of early to late maturity classes approximates a 3 : 1 ratio as also indicated in Table IV. Another possible explanation is t h a t two types of earliness may have
229 T A B L E IV SEGREGATION
RATIOS FOR MATURITY
T Y P E IN T H E M 2 G E N E R A T I O N
OF THE MUTANT
LINE
6CD3 Maturity type segregation in M 3 Postulated segregation ratio 1:2:1 3:1
Very Early 2 --
Early 5 7
X2 p r o b a b i l i t y Late 2 2
0 . 9 7 5 ) P :> 0 . 9 0.9 ) P ~ 0.5
been present, one associated with dwarfness and one not associated with the dwarf condition. The absence of M2 plants of the line 6CD3 approaching the height of the parental genotype, Wayne, restricted the genetic interpretation of this segregation. It is believed that the absence of plants of the height of Wayne could possibly be merely statistical in that t o o small a number of segregates was available for analysis. However, there is the possibility of a more complex genetic situation of height and maturity segregation. The presumed double recessive class of plants for late maturity (i.e. Wayne genotype) has a mean height significantly shorter than Wayne, which suggests independent assortment of height and maturity genes and the possibility of a double mutation, one for each of these characters in this line. Another aspect of interest is the apparent departure of the relationship of plant height with days to first flower amongst the M2 mutant plants from the type of relationship exhibited by the four cultivars as illustrated in Fig. 2. The relationship of node number of first flower and days to first flower amongst the M2 plants appears to be in general conformity with the relationship exhibited b y the four soybean cultivars examined (Fig. 1). One possible explanation of these results could be the segregation for a gene influencing earliness which exerted a pleiotropic effect on plant height to a different degree than the normal range of maturity alleles in soybean cultivars. Further studies of segregation for maturity and height within the line 6CD3 are at present in progress. The earliness expressed in the mutation studied, appeared to be one conditioning a changed photoperiodic sensitivity over that of the parental response. This change was in the direction of increased insensitivity to increased daylength. All maturity comparisons between cultivars and mutant progeny in the present study were conducted at 20°C and hence differences in maturity within and between the two daylength regimes can be considered to be due essentially to differences in photoperiodic sensitivity. The appearance of the early mutant condition in the M1 plant and the subsequent demonstration in the M2 plants of a 3 : 1 (early : late) segregation ratio indicates that this photoperiodic insensitivity is recessive and monogenically controlled. Previous reference has been made to a dominant gene (designated E) giving earliness in soybean [3]. Evidence on the frequency of occurrence of this early mutant, its genetic nature, and physiological influence could have important implications in soybean introduction programmes. Introduced genotypes which were of extremely divergent maturity, particularly those which were very late maturing, could be treated with mutagen and the M1 generation screened for the possible presence
230
of early variants. The frequency of approx. 3% for the occurrence of the very early mutation in the M~ in the present study suggests that excessively large populations need not have to be handled in order to have a good change of obtaining desirable early flowering M1 plant(s). References 1 B o r e j k o , A.M., P r o d u c t i o n of i n d u c e d m u t a t i o n s in s o y b e a n , G e n e t i c a M o s k a , 6 ( 1 9 7 0 ) 1 6 7 - - 1 6 9 . 2 I s h i k a w a , M., in I n d u c e d M u t a t i o n s a n d P l a n t I m p r o v e m e n t , I . A . E . A . / F . A . O . P r o c . , B u e n o s Aires, I . A . E . A . Press, V i e n n a 1 9 7 0 , p. 2 8 6 . 3 J o h n s o n , H.W. a n d L, B e r n a r d , S o y b e a n g e n e t i c s a n d b r e e d i n g , in A . G . N o m a n (ed.) " T h e S o y b e a n " A c a d e m i c Press, N e w Y o r k - L o n d o n 1 9 6 3 , p. 13. 4 R a w l i n g s , J . O . , D . G . H a n w a y a n d C.D. G a r d n e r , V a r i a t i o n in q u a n t i t a t i v e c h a r a c t e r s o f s o y b e a n a f t e r seed i r r a d i a t i o n , A g r o n . J. 5 0 ( 1 9 5 8 ) 5 2 4 - - 5 2 8 . 5 Z a c h a r i a s , M., in A . G . N o r m a n (ed.) " T h e S o y b e a n " A c a d e m i c Press, N e w Y o r k - L o n d o n , 1 9 6 3 , p. 6 1 .