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Polyploidy from Twin Seedlings by Arne Muntzing Svalof, Sweden

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Polyploidy from Twin Seedlings by Arne Muntzing Svalof, Sweden 1937 211 Polyploidy From Twin Seedlings By Arne Muntzing Svalof, Sweden 1. Introduction Recently two Japanese scientists, NAMIKAWA and KAWAKAMI (1934), have briefly reported some rather interesting results con cerning the chromosome numbers of twin seedlings in wheat. In crosses between varieties of Triticum, vulgare the occurrence of twin seedlings was observed. The chromosome numbers of these twin plants were determined and in some cases were found to differ from the normal number. Of a total of 29 twin pairs studied, 19 were found to be normal in chromosome number, both twins in each pair having 2n=42. In 10 cases, however, the twins had different chro mosome numbers, one member of the pair generally having 2n=63, the other one 2n=42. This was found in eight cases. In another two pairs the chromosome numbers of the twin plants were 42-21 and 42-84 respectively. Consequently, by selection of twin plants NAMIKAWA and KAWAKAMI, in a rather limited material, succeeded in obtaining wheat plants with 21, 63 and 84 chromosomes, besides normal plants with 2n=42. Being occupied with problems of polyploidy, especially in culti vated plants, the present writer decided to repeat, if possible, the work of NAMIKAWA and KAWAKAMI. Further, as selection of poly ploids from twin plants might be possible also in other cultivated plants than wheat, the work was also extended to include other cereals and different species of forage crop plants, etc. The twin material studied was in part produced from germina tion experiments of our own, performed in the laboratory of the cyto-genetic department at Svalof. Seeds of winter wheat and rye were germinated (between moist blotting paper) in the autumn of 1935, summer wheat, barley and oats in the spring of 1936. Most of the material, however, was obtained from the seed control stations at Svalof and Akarp, Sweden. For this material I am very much indebted to Dr. H. CHRISTOFFERSSON, Dr. O. TEDIN and Mr. A. GARSNER. From these gentlemen I learned that twin seedlings are not infrequently met with during seed control work. Collections of twin seedlings of wheat, oats and barley had even been brought together and preserved by Dr. CHRISTOFFERSSON. In view of this 14* 212 A. MUNTZING Cytolcgia, Fujii jub. vol. information the possibility of obtaining twin plants in various species of cultivated plants seemed to be rather promising, and during the winter of 1935-1936 rather large numbers of twin seedlings were in fact obtained from the seed control stations mentioned. The twin plants received were separated from each other at an early stage, simply by cutting the two plants apart with the aid of a sharp knife. After separation, each plant was placed in a separate pot, and root tips were fixed in chrom-acetic formalin. As most of the plants were received during the winter time they were kept growing under electric light, the installation of electric light for winter cultures, available at Svalof, being utilized for that purpose. 2. The frequency of twin plants Up to now twin seedlings have been obtained from the following species: Triticum vulgare, (winter and summer wheat), Triticum turgidum, Secale cereale, (winter and summer rye), Avena sativa, Hordeum vulgare, Phleum pratense, Dactylis glomerata, Lolium perenne, Poa pratensis, Agrostis stolonifera, Festuca pratensis, Fes tuca ovina, Festuca rubra, Festuca duriuscula, Cynosurus cristatus, Medicago sativa, Trifolium pratense, Beta vulgaris, Daucus carota and Solanunz tuberosum. The ratio of normal: twin seedlings could not be calculated this year for the material coming from the seed control stations, but data are available from our own germinations. These, however, only include the cereals, wheat, rye, barley and oats. The frequency of twin seedlings in this material is given in Table 1. In Linum, KAPPERT (1933) found that strains producing a high proportion of twin seedlings could be selected in the progeny of crosses between parent strains, giving a low frequency. Further, as the twin seedlings of wheat, studied by NAMIKAWA and KAWAKAMI (l. c.), were found in F3- and F4-progenies of certain intervarietal crosses, a similar heterozygous material of winter wheat was kindly placed at my disposal by Prof. A. AKERMAN, Svalof. This material consisted of seeds from F2-, F3, F4-, F5- and F6- progenies of different crosses between varieties of Triticum vulgare. For comparison seeds of the commercial variety "Skandia"-wheat, which may be considered to be completely or almost completely homozygous, were also germinated. As is evident from Table 1 there is no significant difference between the percentage of twin seedlings in the homozygous and heterozygous material. In both categories the frequency is low, approximately one twin pair being obtained from one thousand seeds. The exact values are 0,89•}0,27 per mille for Skandia-wheat and 1937 Polyploidy from twin seedlings 213 0,99•}0,21 per mille for the F2-F6 material. A comparison be tween the frequency of twin seedlings in the different generations Table 1. The frequency of twin seedlings in some cereals 214 A. MUNTZING Cytologia, Fujii jub. vol. (F2-F6) of the latter material did not reveal any significant dif ferences either. In summer wheat a similar frequency of twin seedlings as in winter wheat was obtained, 8960 seeds from four different commer cial varieties giving 6 twin pairs. If all data for Triticum vulgare are added, the resulting figures will be 44410 seeds and 40 twin pairs. This gives a frequency value of 0,90•}0,14 per mille. In Triticum turgidum only 3 seeds out of 18856 gave twin plants, which corresponds to the value 0,16•}0,09 per mille. This value is much lower than that found for Triticum vulgare (0,90•}0,14), and, evidently, the difference is statistically significant. In rye three different commercial varieties were tested. A similar or slightly higher percentage was found, 25 twin pairs being obtained from 17364 seeds. This corresponds to 1,44•}0,29 per mille. The variety "Stalrag" gives a higher proportion of twins than "Kungsrag" , the difference being significant (D/m=3,78). Also in barley and oats a low proportion of twin seedlings was obtained from germination experiments with a number of commercial varieties (cf. Table 1). The average values were found to be 0,17•} 0,06 per mille in barley and 0,07•}0,07 in oats. Oats and barley thus seem to give a lower proportion of twin seedlings than Triticum vulgare and rye, the data hitherto accumu lated demonstrating significant differences. The main point, how ever, is the fact that all the cereals tested have given quite a low proportion of twin seedlings, generally less than one twin pair per mille being obtained. 3. Chromosome counts The entire twin material obtained from the seed control stations and from our own germinations has not yet been completely investi gated as to the chromosome numbers. However, the data hitherto accumulated (cf. Table 2) are sufficient to show that selection of twin seedlings is an important method of obtaining heteroploid plants not only in wheat but also in several other species. The results may be given under the following two headings: A) species in which all the twins had the same chromosome number and B) species in which some twins had deviating chromosome numbers. A. Species in which all twin plants had the same chromosome number a) Dactylis In Dactylis glomerata the chromosome number was found to be 28 or •}28 in each member of 40 twin pairs. In seven cases one of the twins in each pair had died before fixation but the remaining 1937 Polyploidy from twin seedlings 215 single plants all had •}28 chromosomes, which is the normal number of the species. b) Hordeum In barley all the members of seven complete pairs had 2n=14, just as four single plants, of which the corresponding twins had died at an early stage. One single plant of the same category had tetraploid sectors in some roots but died before a re-investigation could be made. Tetraploid sectors in root tips are frequent after heat treatment (unpublished results) and may even occur spontane ously in exceptional cases. Probably the sectors found in the plant mentioned had nothing to do with its origin as a twin seedling. c) Festuca Four complete pairs and one single plant of Festuca pratensis had all the normal chromosome number 2n=14. One pair and two single plants of Festuca ovina had 2n=42 and one pair of Festuca rubra 2n=•}56. A single twin plant from another biotype of the same species had 2n=42. d) Scattered observations from other genera In the following species all twin plants studied had also un changed chromosome numbers: Agrostis stolonifera, 2n=•}42, (4 pairs, 1 single twin plant); Cynosurus cristatus, 2n=14, (1 pair, 1 single plant); Beta vulgaris, 2n=18, (1 triplet, 1 pair and 2 single twin plants). B. Species in which heteroploid twin plants were found a) Triticum As the results of NAMIKAWA and KAWAKAMI (1934) in wheat was the direct cause of the present investigation, it seemed desirable in the first place to procure twin material of Triticum vulgare. Such twins were in fact obtained in the autumn of 1935, in part from germination experiments of our own, in part from the seed control stations. Unfortunately, however, these winter wheat twins were obtained rather late in the season, and therefore mortality among this material was very high. About half of the twin plants of T. vulgare did not survive the winter, though they were growing in boxes, placed in a hot-bed. Only part of the material could be fixed in the autumn.
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