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Somatic Reduction and Chromosome Elimination
Cases are known where either spontaneously or due to specific treatments, the chromosome number was reduced to half in the somatic tissues, a phenomenon described as somatic reduction or reductional mitosis. Swaminathan and Singh (1958) induced a haploid branch on a watermelon by irradiation of the seed used. This must have resulted by the reduction of chromosome number in the somatic tissue through an unknown mechanism (perhaps due to spindle organizer abnormalities). Similarly, colchicines treatment has also been found to lead to the reduction in chromosome number. Franzke and his associates at South Dakota State University (USA) conducted a series of interesting studies in Sorghum vulgare.
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They found that in Sorghum vulgare, somatic tetraploid (2n = 4x) cells responded to colchicines treatment and gave rise to diploid cells, which took over the growing point completely thus giving rise to diploid (dihaploid) individuals.
In general, attempts to induce somatic reduction in other species by treating seeds for seedlings with colchicines have failed or met with only partial success as in flax (Dirks et al., 1956). There are also a number of other chemicals like chloramphenicol and para-fluorophenylalanine (PFPA, an amino acid analogue) which have sometimes been successfully used for production of haploids in a number of materials.
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Barley haploids through crosses with Hordeum bulbosum
A situation analogous to somatic reduction, occurs when in an interspecific or intergeneric hybrid, chromosomes of one parental gnome are preferentially eliminated. This method has been extensively utilized for production of haploids in barley and also in wheat. The production of haploids in barley in this manner involves interspecific hybridization of Hordeum vulgare (2n=14) with H. bulbosum (2n=14), a cross pollinated, self incompatible perennial species. Most progeny (95%) are barley haploids, remainder being diploid hybrids. A cross between 2x barley and 4x H.bulbosum mainly produces triploid hybrids.Cytological examination of early embryo and endosperm in VV × Bb or BB × VV hybrids revealed gradual elimination of chromosomes. While most genotypes of barley readily produce haploids, chromosome elimination seems to be controlled by genotypes of both parents.
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Scheme of monoploid and double monoploid barley plants using wide hybridization (bulbosum Method involving elimination of hordeum bulbosum chromosomes
| 1. Hordeum Vulgare (VV) (2n = x = 14) |
2. Gametes |
3. Crossing |
| 4. Hordeum bulbosum (BB) (2n = 2x = 14) |
5. Cross (VxB) or (BxV) |
6. Male gametes |
| 7. Bulbosum chromosome eliminatin |
8. Embryoculture (haploid = V) |
9. Monoploid barley (2n = x = 7) |
| 10. Chromosome doubling (colchicine treatment) |
11. Doubled monoploid (2n = 2x = 14) homozygote |
12. Female gametes |
| 13. VxB zygote |
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Results of crosses between Hordeum vulgare (2x = VV, 4x = VVVV) and H. bulbosum (2x = BB, 4x = BBBB) (After Lange, 1971).
| Cross |
Progeny |
Hybrid: Haploid ratio |
VV × BB |
2 (VB) : 1 (V) |
BB × VV |
4 (BV) : 2 (V) |
VV × BBBB |
145 (VBB) : 1(V) |
BBBB × VV |
12 (BBV) : 0 (V) |
VVVV × BBBB |
11 (VVBB) : 15 (VV) |
BBBB × VVVV |
5 (BBVV) : 3 (VV) |
Kasha and Kao (1970) summarized the findings for haploid production using the above technique as follows: (i) a very high frequency of haploids can be obtained and these haploids are not associated with any diploid hybrids; (ii) these haploids can be obtained with any cultivated barley and would resemble cultivated barley genome in reciprocal crosses. Contrary to these observations, Lange (1971) showed that haploids obtained due to chromosome elimination are associated with hybrids.
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With embryo culture technique, he was able to obtain viable hybrids form several cross combinations, although frequency of hybrids was low except in case of VBB hybrids, which were quite frequent.
Following is the subsequent summary of characteristics associated with chromosome elimination, keeping in view the above results (Kasha 1974). (i) Elimination occurs in hybrid embryos following interspecific or intergeneric hybridization. (ii) Both haploids an species hybrids may be produced from a cross, and the haploids can have female or male origin, so that androgenetic haploids can also be produced. (iii) Genotypic variation may be found within parental species and different species may differ in their response to chromosome elimination. (iv) Initial seed set is good followed by shriveling, which requires embryo rescue. (v) Hybrids obtained in such crosses exhibit chromosome instabilitya nd aneuploidy, in somatic and meiotic cells.
Haploids of other Hordeum species from interspecific hybrids
Chromosome elimination is also quite common among other Hordeum interspecific hybrids not involving H. vulgare, and has been reported in about 20 such hybrids. For instance, H. bulbosum (4x) × H. secalinum (4x) gave haploids of H. bulbosum, (2x and 4x) and H. secalinum (4x) × H. vulgare (2x) gave haploids of H. secalinum along with hybrids.
Wheat Haploids through crosses with H. bulbosum.
Haploids in hexaploid wheat (var. Chinese Spring) can also be produced through chromosome elimination in hybrids of wheat with H. bulbosum (both 2x and 4x). A frequency of 13.7% grain set in hybrids with 2x bulbosum and 43.7% grain set with 4x bulbosum, were obtained. Inorder to explore the possibility of further use of this technique in wheat breeding programmes, studies were conducted at Cambridge (U.K.) on the crossability of wheat with H. bulbosum. It was found that most of the European varieties did not cross, but four of the six Australian varieties were successfully crossed. A similar crossability patter of wheat varieties was observed with rye also so that i was concluded that crossability with rye and H. bulbosum was controlled by same genetic factors. Through the use of chromosome substitution lines, it was also shown that in the variety Hope 1B, 1D, 2D, 3A, 3D, 4A, 4B and 6D have genes promoting crossability, while 5A and 5B have genes reducing crossability. In view of the limitation of the crossability of H. bulbosum with only few wheat varieties, H. bulbosum can not be extensively used for production of haploids in common wheat.
Wheat haploids from wheat × maize crosses
M.D. Bennett and his coworkers in Cambridge, U.K. (now at Kew, U.K.) demonstrated that wheat × maize crosses can be made successfully, and hybrid embryos can be rescued. Due to elimination of maize chromosomes, wheat haploids can be obtained. Wheat × maize crosses are now extensively used at CIMMYT, Mexico and elsewhere for the production of haploids in bread wheat, particularly with an objective of producing doubled haploids (DH) for developing mapping populations.
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