Artificial synthesis of Brassica juncea Coss

Genetica (1973) 44 :24%263

ARTIFICIAL SYNTHESIS OF BRASSICA JUNCEA COSS

SHYAM PRAKASH Cummings Laboratory, Indian Agricultural Research Insti tute,

New Delhi, India

Received September 9, 1972 / Accepted January 30, I973

Commonly cult ivated mustard, Brassica ]uncea Coss, is an amphidiploid having in its genetic system the full 20-chromosome A genome (Brassica campestris) and the 16-chromosome B genome (Brassica nigra). Considerable natural variabil i ty exists under the A genome. These variat ions have been exploited for the artificial synthesis of B. juncea in order to breed improved mustard. The different combinations were studied both in their Ft ' s and advanced amphidiploid generations in respect of their morphology, meiotic behaviour and fertility. Amphidiploids from leafy and rapiferous groups were generally bushy having arboreal habit . Some combinations from the leafy group result in types with luxur iant vegetative growth and can be used for fodder purposes. The amphiploids of ssp. rapi/era did not give a swollen and enlarged root like the mother parent. None of the combinations from these two groups was promising in respect of oil and seed yield. Amphi- diploids from the oleiferous group were both high seed and oil yielders and thus provide evidence t ha t it formed one of the const i tuent parental species in the formation of oil yielding B. juncea.

Introduction

Numerous species have been recognised in the genus Brassica. Most of these are diploids and only three are polyploids-tetraploids. Mustard or Brassica iuncea Coss, a 36-chromosome species is a natural amphidiploid containing the complete 20-chromosome complement of the A-genome (Brassica campestris) and the 16-chromosome complement of the B-genome (B. nigra). Naturally occurring types of B. iuncea show regular formation of 18 bivalents at meiosis and have disomic inheritance. Since the findings of MORINAGA (1934) and U (1935) about its amphidiploid origin, it has artificially been synthesized in different countries from its putative parents by RAMANUJAM & SRINIVASACHAR, 1943; FRANDSEN, 1943; OLSSON, 1960; SRINI- VASACHAR, 1965 and more recently by PRAKASH, 1969. There is a t remendous variability within the A genome of Brassica which could

250 SHYAM PRAKASH

be broadly put under three groups viz. leafy, rapiferous and oleiferous purely on morphological grounds but within the limits of homology. An investigation on the extent to which this variability may be utilized for the improvement of B. ]uncea is reported.

Material and Methods

The different species of Brassica used in the present investigations are listed in Table 1. Young flower buds of B. campestris were emascu- lated in the evening and pollinated with B. nigra pollen the next morning. Hybrids were grown in early winter and polyploidized with 0.2 per cent aqueous colchicine solution. The amphidiploid progenies were raised and analysed in subsequent years.

TABLE 1

DIFFERENT SPECIES AND VARIETIES OF BRASSICA USED IN THE INVESTIGATION

B. nigra Local

B. nigra E.C. 24678

B. nigra E.C.

B. campestris B. campestris B. campestris B. campestris B. campestris B. campestris B. campestris B. campestris B. campestris B. campestris B. campestris B. campestris B. campestris B. campestris B. campestris B. campestris ssp.

B, campestris ssp.

B, carapestris ssp.

B. campestris ssp.

B. campestris ssp.

B. campestris ssp.

24692

ssp. narinosa E.C. 24685

ssp. narinosa E.C. 34506

ssp. chinensis E.C. 34504

ssp. pekinensis vat. cylindrica ssp. pekinensis E.C. 25357

ssp. pekinensis var. Ta Pai Tsai

ssp. japonica E.C. 25352

ssp. rapi/era Local

ssp. rapilera vat. sylvestris ssp. rapi/era var. annua ssp. olei/era var. yellow sarson Type Local

ssp. oleilera var. yellow sarson Type 470

ssp. olei~era var. yellow sarson Type 487

ssp. olei/era var. yellow sarson Type 975

ssp. olei/era var. b rown sarson Type Lotni

olei[era vat. b rown sarson Type I.B. 307

olei/era vat. b rown sarson Type I.B. 315

olei~era vat. toria Type K a r m a h a

olei/era vat . toria Type Creamy

olei~era vat. toria Type 22

olei/era vat. toria Type 47

SYNTHESIS OF BRASSICA JUNCEA 251

For meiotic studies, anthers were fixed in 3:1 alcohol-acetic acid for 24 hours, stored in 70 per cent alcohol and stained in 2 per cent acetocarmine,

Results

All the hybrids were made in the winter of 1963, '64 and '65. The results of the crossings are given ill Table 2.

TABLE 2

CROSSABILITY RELATIONSHIPS BETWEEN BRASSICA CAMPESTRIS AND B. NIGRA

Cross Flowers Seeds Seeds Mat ro - H y b r i d %

polli- ob ta ined sown morph i c p l a n t s Hybr id

h a t e d p l an t s

B. campestris ssp, narinosa x B. nigra I827 94 50 37 13 26,0

B. campestris ssp. chinensis X B. nigra 1674 232 200 147 53 26.5

B. campestris ssp. pekinensis x B. nigra 1684 264 97 84 13 13.2

B. campestris ssp. ]aponica x B. nigra 2472 121 100 84 3 3.4

B. campestris ssp. rapitera × B. nigra 907 87 87 67 5 6.9

B. campestris ssp. olei/era var. yel low sa r son x B. nigra 2017 103 71 59 11 15.4

B. campestris ssp. olei/era var . b rown sa r son X B, nigra 1723 87 72 58 7 10.7

B, campestris ssp. olei/era var . toria x B. nigra 2173 174 100 91 9 9.0

M O R P H O L O G Y OF T H E P A R E N T S A N D D E R I V E D A M P H I D I P L O I D S

B. campestris ssp. narinosa Plant erect. Radical leaves glabrous, unwinged, oblong-ovate,

petioles thick, broad and succulent. Stem-leaves clasping. Leaves light green. Pods medium. No. of seeds: 9-20.

B. nigra Plant hispid-hairy. Leaves petiolate, lyrate pinnatifid. Flowers

clustered corymbosely. Petals very small. Pods very small and closely appressed to the stem. No. of seeds: 7-11.

252 SHYAM PRAKASH

B. campestris ssp. narinosa x B. nigra Plant erect with a number of branches at base. Joints of branches

characterized by conspicuous purplish tinge. Radical leaves petiolate, lyrate with large upper terminal lobe, occupying roughly half the leaf. Lower lobes few in number and very small in size. Lower stem leaves petiolate and lyrate. All leaves dark green and strongly hairy on undersurface, midrib and petiole. Upper stem leaves petiolate. Flowers bright yellow. Pods medium. No. of seeds: 9-16.

B. campestris ssp. chinensis Radical leaves shiny, glabrous, dark green. Petioles thick, broad,

succulent, white or greenish-white, unwinged, blades orbicular, usually curved inward. Stem leaves clasping. Pods long. No. of seeds: 13-27.

B. campestris ssp. chinensis x B. nigra Plants slender, tall with profuse vegetative growth giving bushy

appearance. Radical leaves petiolate, lyrate with only a few lower lobes arranged irregularly. Lower portion of leaf almost wingless. Lower stem leaves one-lobed, ovate and entire with white, somewhat flattened petiole at base and very much resembling those of B. campestris ssp. chinensis in shape. Leaves dark green and completely smooth. Flowers lemon yellow. Pods medium long. No. of seeds: 9-17.

B. campestris ssp. pekinensis Radical leaves forming a rosette, glaucous, blades suborbicular

with a number of prominent veins, dark green. Petioles greenish white, scarcely bearing leaf fragments. Stem leaves clasping. Pods medium long. No. of seeds: 8-20.

B. campestris ssp. pekinensis X B. nigra Plants erect with profuse vegetative growth. Radical leaves

petiolate, lyrate pinnatisect and very large. Upper terminal lobe large. Subsequent lobes gradually narrowing towards base. Lower stem leaves petiolate and large. Leaves dark green and smooth. Pods medium. No. of seeds: 8-16.


B. campestris ssp. japonica Radical leaves strongly developed, petiolate, crinkled, obovate.

Petiole very broad, flat. Stem leaves clasping. Pods medium sized.

No. of seeds: 9-20.

B. campestris ssp. iaponica × B. nigra Plants tall with a rosette of leaves at base. Radical leaves very

large with very broad, flattened and greenish white mid rib, lyrate with a number of lobes arranged irregularly on either side of mid rib.

Lower stem leaves similar to radical. Upper stem leaves almost sessile, lanceolate and entire. All leaves dark green, thick and crisp. Flowers light yellow. Pods medium long. No. of seeds: 8-17.

B. campestris ssp. rapi/era Root characteristically large, softish. Radical leaves petiolate,

deeply cut into small lobes at base, with a large terminal lobe. Stem

leaves clasping but not auriculate. Pods medium sized to small. No.

of seeds : 9-18.

B. campestris ssp. rapi/era x B. nigra Plants slender having very thick and stout stem with profuse

branching. Height of plants varied, some with arboreal habit reaching upto 263 era. Radical leaves petiolate, lyrate pinnatisect with upper terminal lobe broadly oval. Lower lobes small, few in number. Lower

stem leaves also petiolate and lyrate pinnatisect. All leaves dark green with hairs on undersurface. Upper stem leaves lanceolate and

smooth. Flowers pale yellow. Pods small. No. of seeds: 9-17.

B. carnpestris ssp. old/era var. yellow sarson Radical leaves forming a rosette, petiolate, lyrate. Stem leaves

auriculate clasping. Leaves dark green and smooth. Flowers deep yellow. Pods medium sized to long. No. of seeds: 9-20. Seeds yellow.

B. campestris ssp. old[era var. yellow sarson X B. nigra Plants medium high with a rosette of leaves at base. Radical

leaves petiolate, lyrate with crenate-dentate margin. Upper lobe broadly oval in shape. Lower lobes few in number. Upper stem

254 SHYAM PRAKASH

leaves sessile and lanceolate. Leaves dark green and strongly hairy. Flowers bright yellow. Pods medium sized. No. of seeds: 9-18.

B. campestris ssp. old/era var. brown sarson Radical leaves petiolate. Stem leaves auriculate clasping with a

large terminal lobe and several pairs of small lobes. Pods medium

sized. No. of seeds: 8-19.

B. campestris ssp. old/era var. brown sarson x B. nigra Plants tall, ranging from 165 to 205 cm in height. Stem strongly

hairy. Radical leaves petiolate and lyrate. Upper lobe very prominent

and oval in shape. Lower lobes rudimentary and few in number arranged irregularly. Lower stem leaves petiolate and lyrate, dark green and strongly hairy. Upper leaves sessile and lanceolate. Flowers

deep yellow. Pods medium sized. No. of seeds: 9-16.

B. campestris ssp. old~era var. toria Plant medium high, stem weak. Radical leaves petiolate, lyrate.

Stem leaves auriculate clasping. Leaves dark green and hairy. Pods

small. No. of seeds: 9-20.

B. campestris ssp. old~era var. toria x B. nigra Plants similar to var. brown sarson x B. nigra plants in gross

morphology except former small in size. Pods medium sized. No. of

seeds: 8-18.

CHROMOSOMAL BEHAVIOUR OF INTERSPECIFIC HYBRIDS

Meiosis was characteristic of sterile interspecific hybrids. All the hybrids had a predominance of 18 univalents at meta-anaphase (Fig. 1). A variable number of bivalents (Fig. 2) were encountered as is evident from Table 3 where range and mean bivalent frequency for different hybrids are given. The most unexpected and rare case was the occurrence of 8 bivalents and 2 univalents in one of the sporocytes of B. campestris ssp. old[era var. brown sarson X B. nigra hybrid (Fig. 3). Bivalents were found to be lying in the metaphase plate and parallel to each other. The bivalents were all rod shaped, monochiasmate and sometimes heteromorphic too. The univalents


Figs, 1-4: M e t a p h a s e I s tages of F1 h y b r i d s and syn thes i zed amphid ip lo ids

(1) B. campestris × B. nigra F1 hybr id , 181; (2) B. campestris x B. nigra F1 hybr id , 6ii + 61; (3) B. campestris ssp. olei/era brown sa r son × B.

nigra FI hybr id , 811 + 2i; (4) B. campestris X B. nigra amphid ip lo id , 1811.

T A B L E 3

CHROMOSOME ASSOCIATION AT MI OF MEIOSIS IN THE HYBRIDS BETWEEN BRASSICA CAMPESTRIS AND B. N1GRA

Cross Cells P l a n t s B iva l en t s Un iva l en t s

s tud ied used Mean R a n g e Mean R a n g e

B. campestris ssp. narinosa x B. nigra I00 7 1.51 1-6 3.8 6-18

B. campestris ssp. chinensis x B. nigra 100 13 1.35 1-6 4.4 6-18

B. campestris ssp. pekinensis x B. nigra 100 9 1.33 1-6 4.9 6-18

B. campestris ssp. ]aponica× B. nigra 100 3 1.23 1-6 5.2 6-18 B. campestris ssp. rapi[era X B. nigra 100 3 1.05 1-6 6.1 6-18 B. campestris ssp. olei/era var. yellow s a r s o n × B. nigra 100 8 0.99 1-5 5.6 8-18 B. campestris ssp. olei/era var. b r o w n s a r s o n × B. nigra 100 5 1.13 1-8 6.3 1-18 B. campestris ssp. olei[era var . loria X B. nigra 100 5 0.67 1-8 6.7 6-18

256 SHYAM PRAKASH

were generally found in groups at the polar regions, however, at

times lying near the equator or distributed haphazardly throughout the cytoplasm. Occasionally these univalents were observed in secondary associations i.e. side-to-side or end-to-end. The disjunction

of all the bivalents was not synchronous. A few chromosomes which did not show polar movement, were not included in any of the ana-

phase I groups. At the end of the first meiotic division a small percen- tage of the cells contained restitution nuclei. The second division was highly irregular resulting in the formation of dyads, triads,

tetrads and pentads along with micronuclei. The whole process was

so irregular and disturbed that the meiotic system might have broken down completely resulting in the formation of non-viable and shrivel- led pollen.

MEIOTIC B E H A V I O U R OF AMPHIDIPLOIDS

Meiosis in the A1 generation of all artificially synthesized amphidiploids was disturbed to a considerable degree. Although cells with 18 bivalents were often observed, a varying number of univalents,

sometimes as many as eight were recorded (Table 4). No multivalent

associations were observed in any of the plants. An improvement in chromosome pairing was noticed in the A2 generation but only to a limited extent as the number of nnivalents was reduced to four.

Again in the As and A4 generations, a tendency to increased chromosome pairing was noticed in all the amphidiploids and in most cases the number of univalents decreased significantly. A further improvement in meiotic st£oilization was observed in the A5 generation where the chromosomes appeared as bivalents only (Fig. 4) and occasionally dissociating in univalents.

F E R T I L I T Y IN THE INTERSPECIFIC H Y BRID S

All the hybrids were pollen and seed sterile. Anthers were replaced by staminodes and they had 0-1.7 per cent good pollen. Small num- bers of pods also developed which were mostly empty but sometimes contained one or two seeds. They all gave rise to 36-chromosome plants and probably arose as a result of parthenogenesis. These have not been included in this study, however.

S Y N T H E S I S O F BRASSICA JUNCEA

T A B L E 4

CYTOLOGICAL BEHAVIOUR AT MI OF MEIOSIS OF ARTIFICIAL BRASSICA AMPHIDIPLOIDS IN DIFFERENT GENERATIONS

257

A m p h i d i p l o i d s A1 A2 A3 A4 A5

I I I No. II I No. II I No. II I No. II I No,

ce l l s ce i l s ce l l s ce l l s ce l l s

B. campestrisssp, narinosa 18 - - 61 18 - - 78 18 - - 9 2

x 17 2 2 4 17 2 2 2 17 2 8

B. nigra 16 4 15

B. campestrisssp, chinensis 18 - - 3 9 18 - - 72 18 - - 89

× 17 2 23 17 2 19 17 2 10

B. nigra 16 4 13 16 4 9 16 4 1

15 6 14

14 8 11

B. campestrisssp, pekinensis 18 - - 55 18 - - 81 18 - - 93 18 - - 98

X B. nigra

B. campestris s sp . japonica ×

B. nigra

B. campestris s sp . rapi/era ×

B. nigra

B. campestris s sp . olei/era v a r . y e l l o w s a r s o n ×

B. nigra

B. campestris s sp . olei/era v a r . b r o w n s a r s o n X

B. nigra B. campestris s sp , oteifera v a r . toria ×

B. nigra

17 2 19 17 2 12 17 2 7 17 2 2

16 4 13 16 4 7

15 6 13

18 - - 45 18 - - 5 7 18 - - 72

17 2 29 17 2 27 17 2 15

16 4 18 16 4 16 16 4 13

15 6 8

18 - - 5 2 18 - - 82 18 - - 94

17 2 2 4 17 2 13 17 2 6

16 4 2 0 16 4 5

15 6 4

18 -- 65 18 -- 74 18 -- 82 18 -- 94 18 -- 98

17 2 19 17 2 18 17 2 13 17 2 6 17 2 2

16 4 9 16 4 8 16 4 5

15 6 7

18 -- 64 18 -- 72 18 -- 86

17 2 23 17 2 17 17 2 12

16 4 13 16 4 11 16 4 2

18 -- 67 18 -- 78 18 -- 84 18 -- 93 18 -- 98

17 2 14 17 2 11 17 2 10 17 2 7 17 2 2

16 4 11 16 4 11 16 4 6

15 6 8

258 SHYAM PRAKASH

F E R T I L I T Y IN T H E AMPHIDIPLOIDS

B. campestris ssp. narinosa × B. nigra Pollen and seed fertility in A1, 54% and 37% resp. Pollen fertility

in A2 and A3, 61% and 87% resp. Average seed fertility in As variable: 42%-55% in different families. In Aa up to 73%.

B. campestris ssp. chinensis × B. nigra Pollen fertility in A1 and A~, 29% and 63% resp. In A3 variable:

70%-88%. Seed fertility in A1 7%-13%; A2 31%-37%. In A3 49%- 78% in different families.

B. campestris ssp. pekinensis × B. nigra Pollen fertility in A1 17%-31%. In A2, A3 and A4, 47%, 54% and

76% resp. Seed fertility in A1 variable: 7%-240/o. In A2 and A3 35% and 47% resp., in A4 55%-63%.

B. campestris ssp. iaponica × B. nigra Pollen fertility in A1 27%, in A2 28%-46% and A3 58%. Seed

fertility in A1, A2 and A3 18%, 34% and 46% resp.

B. campestris ssp. rapi/era × B. nigra Pollen fertility in A1 42%, in A2 67%--76%, in As 98%. Seed

fertility in A1 37%-41~/o, A2 59%, wide variation in As: 73O/o-900/o .

B. campestris ssp. old/era var. yellow sarson x B. nigra Pollen fertility in A1, 260/0-430, in As, As, A4 and A5 54%, 72%,

81% and 96% resp. Seed fertility in A1, A2, A, and A4 24%, 31%, 53% and 67% resp. In A5 variable: 76%-94%.

B. campestris ssp. old/era var. brown sarson × B. nigra Pollen fertility in A1 and A2 34% and 58% resp., in As up to 67%.

Seed fertility in A1 23%-27%, in A2 variable: 43%-51%, in As 62%.

B. campestris ssp. olei/era var. toria × B. nigra Pollen fertility in A1 42%-54%, in As 72%, in A3, A4 and A5

76%, 83% and 97% resp. Seed fertility in A1 26%, in As variable: 43%-62%, in A3 69% and in A4 and A5 up to 96%.


Discussion

B. iuncea is an important oil seed crop of India. This species evolved in nature as an amphidiploid between two elementary species viz. B. campestris and B. nigra whose chromosome sets since then remained structurally unaltered. The utilization of the prevailing variation among 20-chromosome Brassica species for the improvement of B. ~uncea has remained so far mostly unexploited. The morphological variability occurring at the 20-chromosome level (A genome) has formerly been a reason for distinguishing different species. These were taken together into one species 13. campestris by OLSSON (1954) on the basis of cytological studies. These variations can be classified into three groups purely on a morphological basis into oleiferous, leafy and rapiferous. B. campestris ssp. old/era var. yellow sarson, var. brown sarson and var. toria are important oleiferous ecotypes under cultivation in northern India. Leafy types are recent intro- ductions in this country and generally used for vegetable purposes in east Asia while tile rapiferous type has a wide distribution. The object in the present investigation has been, by combining representatives of the above groups with the other diploid parent (B. nigra) to obtain variability at the amphidiploid level for further breeding and selection. These crosses are also likely to reveal the particular group of 20-chromosome species which were involved in the making of oleiferous B. ~uncea.

MORPHOLOGICAL CONSIDERATIONS

All the synthesized amphidiploids were intermediate between the constituent parents for most of the characters and resembled natural B. iuncea in all general respects. In combinations obtained from leafy types, a remarkable uniformity regarding the height of the plants was noted. They all had arboreal habit, sometimes reaching 276 cm as was the case with ssp. narinosa × B. nigra amphidiploid. All these plants had a very pronounced vegetative growth, giving a bushy appearance. The amphidiploids originating from oleiferous types were slender with less vegetative growth as compared to leafy combinations and were of medium height ranging from 130-170 cm. Besides height variations, differences in the general appearance of

260 SHYAM P R A K A S H

the plants caused by other factors were also recorded e.g. number of primary and secondary branches and their length. Synthetic amphidiploids of ssp. rapi/era X B. nigra were vegetatively vigorous as compared to oleiferous combinations but not as much as those described from the leafy types although they bore bushy appearance.

In respect of leaf characters, these amphidiploids looked like the natural species B. ]uncea. I t is characterized by the presence of petiolate, lyrate and strongly lobed radical leaves with crenate- dentate margin. The lower stem leaves are petiolate with few lobes while the upper stem leaves are lanceolate and entire with a narrow petiole. The radical leaves of the mother plant (B. campestris) are petiolate while the lower and upper stem leaves are strongly clasping. On the other hand, radical as well as stem leaves of B. nigra are strongly petiolate. The derived amphidiploids all had petiolate radical as well as stem leaves. As for the size of the leaves in the synthetic amphidiploids, it was, in leafy combinations, very large and quick growing showing a positive heterosis over their parents. Sometimes the leaves resembled the mother parent more e.g. the amphidiploids from ssp. ]aponica had radical and stem leaves with broad, flattened and greenish white petiole and midrib, almost like that of ssp. ]aponica. Similarly the lower stem leaves of the ssp. chinensis x B. nigra amphidiploids were very much like those of ssp. chinensis in shape, being suborbicular. Also the leaves of the ssp. pekinensis x B. nigra plants were exceptionally big in size especially the radical ones while those of ssp. narinosa x B. nigra were intermediate. The radical and lower stem leaves of oleiferous and rapiferous combinations agreed well with natural B. ~uncea with no marked variations among them. The upper stem leaves in all these amphidiploids were lanceolate, entire with a short petiole, an intermediate condition between the two parents and resembling natural B. ~uncea.

One of the desirable characters of ssp. rapi/era having swollen and enlarged root was not observed in the derived amphidiploids, one object for which the cross was attempted in the present investigations. Such types of nmstards, however, with enlarged roots viz. B. ~uncea var. megarrhiza occur in nature (SUN, 1946) which probably arose by occasional hybridization of B. campestris ssp. rapi/era with B. nigra around the Mediterranean. The inflorescence in all the parental


species and their derived amphidiploids was corymbose-raceme, but the arrangenlent of the buds and flowers differs both in 16 and20- chromosome species and their resulting amphidiploids. As pointed out by OLSSON (1960), the buds of B. ~,igra have very short pedicels, are densely crowded and stand above the flowers while in the 20- chromosome species flowers are pedicellate with long pedicels and stand higher than the buds. All the artificially synthesized amphidiploids were intermediate in respect of this character having flowers and buds standing more or less at the same level with the medium pedicel. The sepals in all the 20-chromosome species form an angle, at the time of anthesis, of approximately 30-45 ° with the central axis while in B. uigra they are perpendicular. All the amphidiploids resemble the latter in this respect, howevel. Much importance has been given to the arrangement and general appearance of the pods and number ot seeds per pod in classifying the genus Brassica (SUN, 1946). The pods of the 20-chromosome species are long with long pedicels standing more or less at right angles to the stem, containing 20-30 seeds. In B. ,,igra the pods have very short pedicels and are closely appressed to the stem with a short beak having 7-I2 seeds. All the amphidiploids raised artificially were intermediate in this character, having pods at an angle of approximately 45 ° from the main axis, containing 15-20 seeds in fully fertile pods, all of which quite comparable with natural B. iuncea.

CHROMOSOME PAIRING BEHAVIOUR

The analysis of the pairing behaviour of the hybrids involving the three groups and B. nigra showed that leafy × B. nigra hybrids displayed more genetical affinity than oleiferous × B. nigra and rapiferous × B. nigra. The failure of the chromosomes of the two genomes to undergo pairing in these hybrids appears to result simply from the lack of homology i.e. the occurrence of differences in gene content of the pairing partners. Genetic modifyers and asynaptic genes are known to prevent or greatly reduce the level of chromosome pairing but they do not play a role here, because of the fact that normal pairing is resumed in most of the cells of the artificially synthesized amphidiploids derived from these hybrids. Had these systems been operating, it would have been difficult to get normal

262 SHYAM PRAKASH

bivalent formation. Consequently, it can safely be assumed that the capacity of the chromosomes of the two genomes to pair in the hybrids is due to the extent of their homology (RILEY & LAW, 1965; RILEY, 1966).

Moreover, from the comparative studies of cross-homologies in the haploid of B. juncea and the F1 hybrids of B. campestris and B. nigra, it can be inferred that the chromosomes of the natural B. funcea are still very similar structurally to their homologues in the natural diploid species (MoRINAGA, 1934 ; U, 1935) and whatever little differentiation has taken place in them is mainly a result of gene mutation, selection and incorporation of adaptive gene complexes in the course of evolution while major chromosome reorganization has played a minimal part in the transformation of raw amphidiploids into finished products.

The improvement in fertility of the amphidiploids in subsequent generations was prabably due to a more regular meiosis and balanced interactions between the two parental genotypes.

EVALUATION OF AMPHID1PLOIDS

The different artificially synthesized amphidiploids of B. juncea were evaluated for their seed and oil yield and forage value, the details of which will be reported elsewhere. But as a preliminary information, it appears that the amphidiploids from only oleiferous × B. nigra combinations offer better prospects for application as

far as oily types are concerned and it is only this combination which seems to have gone in the formation of natural oil yielding B. juncea grown in India. The other two combinations (with the leafy and rapiferous types) do not have any direct utility as they lack oil to a considerable extent but, being promising in certain components of economic characters, can be exploited in introducing high yield in- to existing types through further breeding and selection.

The author is grateful to Dr. G6STA OLSSON of Sval6f, Sweden for critically going through the mainuscript and helpful suggestions and to Dr. H. K. JAIN, Head, Division of Genetics, I.A.R.I. for providing facilities.


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Artificial synthesis of Brassica juncea Coss