Cytogenetics of ditelotetrasomics for short arms of four chromosomes of barley (Hordeum vulgare L.)

Summary Plants with a pair of extra homologous telocentric chromosomes in addition to the normal chromosome complement are called ditelotetrasomics. Six types of ditelotetrasomics of barley have been obtained. Four types obtained in the selfed progenies of telotrisomics (Triplo 2S, Triplo 5S, Triplo 6S, and Triplo 7S) are reported in this paper. The ditelotetra 2S showed a stronger expression of the diagnostic characteristics of Triplo 2S. It was weak and small, with narrow, short, dark-green leaves, and was almost completely pollen and seed sterile. However, three other ditelotetrasomics (ditelotetra 5S, 6S, and 7S) did not show specific diagnostic characteristics and were similar to normal diploid plants, with the exception of ditelotetrasomic 5S, which showed some effects. At meiotic diakinesis and metaphase I, these ditelotetrasomic plants showed chromosome configurations of 6_II+1_IV, 7_II+1 telo_II, 6_II+1_III+1telo_I, or 7_II+2 telo_I. Most of the sporocytes at anaphase and telophase in the first and second meiotic divisions showed almost normal chromosome behavior. Quartets were mostly normal with no micronuclei. Approximately 30% of the selfed progenies of these three ditelotetrasomics were ditelotetrasomics and almost 50% were telotrisomics, indicating a high percentage of male and female transmission of the extra telocentric chromosomes.     

The allotetraploidization of maize

Summary Four barley telotrisomics (Triplo 3S, 5S, 6S, and 7S) were studied. No major qualitative differences in morphology between the telotrisomics and their diploid sibs were found. The pollen and seed fertility of these telotrisomics was comparable to their diploid sibs. The meiotic study showed that the average frequency of 6_II + 1_III at diakinesis and metaphase I was 84.2% and 71.7%, respectively. The normal chromosome separation ranged from 77.2% to 89.4% at anaphase I through telophase II. The transmission rate of the extra telocentric chromosomes averaged 28.4% upon selfing and 28.7% through the female. All four telotrisomics showed various degrees of pollen transmission, the average being 3.6%. Ditelotetrasomic plants (2n = 14 + 2 homologous telocentrics) were obtained in the progenies of selfed monotelotrisomic plants of all four types. These ditelotetrasomic plants were viable and showed various degrees of seed fertility.Contribution from the Department of Agronomy. Supported by USDA-CSU Cooperative Research Project No. 58-82HW-6-8 and CSU Hatch Project.     

Cytogenetic studies on natural intergeneric hybridization inAster alliances

Natural intergeneric hybrids betweenAster ageratoides subsp.ovatus (2n=36) andKalimeris incisa (2n=72) were found. All of the hybrids studied were found to have 2n=72, 18 more chromosomes than a regular F₁ hybrid. The hybrids were found to be of two types: one having 18 large chromosomes ofovatus, and the other having 9 large chromosomes of the same subspecies. In meiosis of the PMCs of the hybrid with 18 large chromosomes, a regular chromosome configuration, 36_II, was observed. In PMCs of the hybrid with 9 large chromosomes an irregularity of chromosome pairings was observed, showing varied chromosome configurations: 35_II+2_I, 34_II+4_I, 33_II+6_I, I_III+33_II+3_I, 1_IV+32_II+4_I, 32_II+8_I, 31_II+10_I, 29_II+14_I, 3_III+29_II+5_I. Most univalents were large, but a few were small. The hybrids with 18 large chromosomes were found to be partial amphidiploid and possessing double chromosome complements ofovatus. The hybrids with 9 large chromosomes were found to be the first backcrossed generation between the hybrid with 18 large chromosomes andK. incisa.     

Primary trisomics obtained from autotriploid by diploid reciprocal crosses in cucumber

To promote cytogenetical studies on cucumber (Cucumis sativus L., 2n = 2x = 14), the reciprocal crosses were made between autotriploid and diploid for selecting the primary trisomics. Meanwhile, chromosome behavior during meiosis in autotriploid cucumber was investigated to look for cytological evidences for origin of primary trisomics. Many viable F₁ seeds were obtained from reciprocal crosses between autotriploid and diploid. The number of chromosomes of 56 surviving progenies varied from 14 to 28, with plants having 2n = 15 occurring at the highest frequency (51.8%). Primary trisomics were firstly obtained in this study. Four types of primary trisomics were isolated and they could be distinguished from each other, as well as diploid. Variable chromosome configurations, e.g. univalent, bivalents and trivalents were observed in many pollen mother cells of the autotriploid at metaphase I. Binomial chromosome distribution was observed at anaphase I and frequency of 8/13 was 6.25%. The meiosis of autotriploid, especially the class of gametes with eight chromosomes, gave the cytological evidence of producing 2x + 1 type gamete and could be induced into primary trisomic plants from progeny of autotriploid–diploid crosses. These studies have established a ground work for selecting a series of primary trisomics, and further using them for associating linkage groups with specific chromosomes in cucumber.     

Cytogenetic studies on microsporogenesis and male gametophyte development in autotriploid cucumber (Cucumis sativus L.): implication for fertility and production of trisomics

To understand the correlation between chromosomes behavior and fertility in autotriploid cucumber (Cucumis sativus L.), microsporogenesis in pollen mother cells (PMCs) and male gametophyte development were studied using improved staining and chromosome preparation techniques. Meanwhile, for more efficient selection of trisomics from the progeny of autotriploid-diploid crosses, fertilization rates of ovules from reciprocal crosses were counted to observe the transfer rate of gametes in the autotriploid cucumber. Variable chromosome configurations, e.g. multivalents, quadrivalents, trivalents, bivalents and univalents were observed in the most PMCs of the autotriploids at metaphase I. Chromosome lagging and bridges at anaphase in both meiotic divisions resulted from irregular chromosome separation and asynchronization was frequently observed as well, which led to formation of micronuclei and inviable gametes. The frequency of normal PMCs in autotriploids at the stage of tetrad was only 40.6%. Among those normal microspores, most of them (91.2%) could develop into normal gametophytes with 2 cells and 3 germ pores. Stainability and germination rate of pollen grains were only 18.8 and 13.5%, respectively. However, chromosomes separated to form gametes with 8 chromosomes at anaphase I, suggesting a possible method for the production of primary trisomics from the progeny of autotriploid-diploid crosses. Fruit set of 3n × 2n and 2n × 3n were 80 and 70%, respectively. It obtained an average of 6.2 plump seeds per fruit in 3n × 2n, while 4.9 in 2n × 3n crosses. Transfer rates of gametes through the gastrula or the pollen in autotriploids were 13.4 and 10.4%, respectively. Some aneuploid gametes (n + 1 = 8, n + 2 = 9) also have capability of setting seed and sexual reproduction besides normal gametes containing whole chromosome sets (n = 7, 2n = 14). Further, some primary trisomic plants were selected from the progeny of autotriploid-diploid crosses. Based on the results obtained we suggest that abnormal meiosis in PMCs was the cytogenetic reason for low fertility of autotriploid cucumber pollen. 3n × 2n cross was more efficient for selecting primary trisomic plants in cucumber.     

CYTOTAXONOMIC STUDIES IN ELEOCHARIS SUBSER. PALUSTRES: CENTRAL UNITED STATES TAXA

Comparative chromosomal and morphological studies indicate that four species are present in the area surveyed. Eleocharis smallii Britt. is primarily diploid with 2n = 16, although sporadic polyploids with 2n = 36 also occur. E. macrostachya Britt. is morphologically similar with unstable polyploid numbers ranging around 2n = 38 and multivalents and univalents present in meiosis. E. xyridiformis Fern. & Brack., a species generally synonymized with E. macrostachya, is shown to be a morphologically distinct species with 2n = 18, 19, and 20. The 19-chromosome types are trisomic for one of the long chromosomes, the three homologs pairing in meiosis as a large chain trivalent. The trivalent separates equationally in the first division and preferentially in the second so that only 9- or 10-chromosome pollen grains containing an extra chromosome are formed. Trisomic cytotypes may potentially produce normal (18), reconstituted trisomies (19), or tetrasomic (20) plants, although tetrasomics have not been found. The 20-chromosome cytotype is not the expected tetrasomic, as it is karyotypically distinct from either the 18 or 19 cytotypes. In all species somatic mutations including translocations, translocation-retranslocations, and chromosome fragmentation (agmatoploidy) have been observed of which the significance, if any, has not been determined.     

Cytological relationships in theConvolvulus pluricaulis complex

InConvolvulus pluricaulis Chois. two forms (2n=18, 36) along with one colchicine-autotetraploid have been investigated morphologically and cytologically. The diploid regularly formed 9_II although in the “off-season” plants certain irregularities were observed including formation of unreduced pollen grains. The natural tetraploid and the colchicine-autotetraploid had mean frequencies of configurations of 1.56_IV+14.04_II+1.48_I and 2.95_IV+0.56_III+11,43_II+0.8_I, respectively. The mode of chromosome association in the two types of tetraploids was comparable. A comparison of the morphological characters of the two tetraploid types, further, suggested close similarity. In addition they were indistinguishable from the diploid from except being gigas. From these data it is inferred that the natural tetraploid presumably arose as a result of direct duplication of the diploid form, through the chance fusion of unreduced spores. The lower quadrivalent frequency of the natural polyploid is ascribed to a gradual shift towards bivalent association accompanying natural selection for fertility. The taxonomic status of the two forms (2x, 4x) is discussed and the varietal status accorded to the tetraploid form is supported.     

Embryo-callus-regenerated hybrids and their colchicine-induced amphiploids between Elymus canadensis and Secale cereale

Summary Intergeneric hybrids recovered through plant regeneration from embryo callus culture and their colchicine-induced amphiploids were obtained from a cross of Elymus canadensis with Secale cereale (cv Gazeller). The embryo-callus-regenerated F₁ plants grew vigorously to maturity and regrew well after clipping, while the embryo-rescued F₁ died of hybrid necrosis before maturity. Most of the morphological characters of the sterile F₁ hybrids were intermediate between the parents, but tiller number and dry matter yield were higher than the parents. Amphiploids from these F₁ plants had improved fertility but were less vigorous than the F₁ plants. The predominance of univalents in the F₁ and bivalents in the amphiploids indicated that the genomes S, H, and R were distinct. However, the occasional occurrence of multivalents reflected a random, intergenomic or intragenomic pairing. The mean chromosome associations of the F₁ (2n=21, SHR), the C₀ amphiploids (2n=42, SSHHRR), and the C₁ amphiploid (2n=40) at metaphase I (MI) were 16.51_I+2.05_II+0.06_III+0.02_IV+0.02_V, 2.20_I+19.87_II +0.02_IV, and 7.10_I+16.37_II+0.04_III+0.02_IV, respectively. These amphiploids could be exploited as new germplasm for forage crop improvement by controlled introgression and backcrosses to the parents.     

An unusual chromosome system in a leafhopper (Homoptera auchenorrhyncha)

An unidentified species of the genus Austroagalloides is shown to have 7_II+1_IV instead of the normal 11_II+XO configuration at metaphase I of meiosis in males. The quadrivalent manifests two types of pairing which ensures regular disjunction; normal chiasma-type pairing and distance pairing. It is suggested that the 7_II+1_IV form is derived from the 11_II+XO form by a series of fusions.     

Aneuhaploids and tetrasomics in rice (Oryza sativa L.) derived from anther culture of trisomics.

Eight types of aneuhaploids (Aneuhaplo 4, 5, 6, 8, 9, 10, 11, and 12) and eight types of tetrasomics (Tetraplo 4, 5, 6, 7, 8, 9, 10, and 12) of rice have been obtained from anther culture of trisomics. This paper reports the plant morphology of these aneuploids and their chromosome behavior at metaphase I. Aneuhaploids for different chromosomes are distinguishable from each other and are morphologically similar to the parental trisomics, suggesting that the extra chromosome has similar genetic effects on plant morphology at the haploid level as at the diploid level. Similarly, tetrasomics with different extra chromosomes are distinguishable from each other and are similar morphologically to the parental trisomic. However, stronger changes in morphological characters were observed in tetrasomics compared with trisomics having the same extra chromosome, as a result of a dosage effect of the extra chromosomes. Comparing plant size between aneuhaploid, tetrasomic, and trisomic with the same extra chromosome, it was shown that the trisomic was the largest, the tetrasomic was of medium size, and the aneuhaploid was the smallest, except for those plants with an extra chromosome 8 in which plant size is dramatically decreased in both the aneuhaploid and the tetrasomic. At metaphase I, aneuhaploids showed chromosome configurations of 1 II + 11 I and 13 I. The frequency of the 1 II + 11 I configuration is higher than 70%, indicating that homologous chromosomes in aneuhaploids tend to stay associated in meiosis. Intragenome chromosome pairing (2 II + 9 I), so called secondary association, was observed in the aneuhaploid for chromosome 5. Tetrasomic plants showed 5 kinds of chromosome configurations: 1 IV + 11 II, 1 III + 11 II + 1 I, 13 II, 12 II + 2 I, and 11 II + 4 I. A chromosome configuration of 13 II was often observed in tetrasomics with shorter extra chromosomes and a chromosome configuration of 1 IV + 11 II was often observed in tetrasomics with longer extra chromosomes. Aneuhaploids had complete seed sterility. Tetrasomics showed very poor pollen fertility and complete seed sterility, except for a few shriveled seeds that were observed in Tetraplo 6 and 9. This is the first report in rice where many aneuhaploids and tetrasomics have been characterized. This information will help to further unravel rice aneuploidy and cytogenetics. The aneuploids obtained here will be very useful tools for the study of genetics and breeding in rice.     

鱼腥草花粉母细胞减数分裂特征及其育性研究

为深入了解鱼腥草花粉母细胞的减数分裂特征与花粉育性的关系,该研究采用卡宝品红染色法对2个鱼腥草居群花粉母细胞的减数分裂过程进行观察,并采用氯化三苯基四氮唑(TTC)染色法、I2-KI染色法、B-K培养基培养法及荧光显微镜观察法来检测鱼腥草花粉的活力及萌发率。结果发现:(1)鱼腥草减数分裂的进程与花序大小、花药颜色、花药长度均有密切的关系。(2)2个居群的鱼腥草中花粉母细胞减数分裂过程正常占88.2%,有11.8%的花粉母细胞减数分裂异常。(3)减数分裂异常表现在减数分裂过程中出现微核、落后染色体、染色体桥、不均等分离、多分体等现象,并发现在二分体阶段及单核花粉发育过程中存在细胞融合。(4)2个居群的鱼腥草花粉活力均不超过1.5%,花粉几乎不萌发。研究认为,鱼腥草花粉育性低的主要原因是单核花粉的发育过程异常,而非鱼腥草花粉母细胞减数分裂异常所致。     

Heterochromosomes in Mytilicola intestinalis Steuer (Copepoda)

The male and female diploid complements in Mytilicola are formed both by 22 chromosomes. Two different somatic metaphase figures are found: with two minute-size chromosomes and with three minute-size chromosomes. The study of meiosis has shown that the female sex is heterogametic. The chromosome complement is 10_II+XX in the male and 10_II+XY in the female. A bivalent formed by two different-length homologues, is to be seen in the female meiosis at pachytene; it shows, sometimes, a light positive hetero-pycnosis.     

PARTHENOGENESIS IN TETRAPLOID LILIUM LONGIFLORUM

Emsweller , S. L., and Joseph Uhring . (U.S.D.A., Beltsville, Md.) Parthenogenesis in tetraploid Lilium longiflorum. Amer. Jour. Bot. 49(9): 978–984. Illus. 1962.—Nine maternal polyhaploids from 1 capsule and 1 tetraploid from another were produced following pollination of 2 tetraploid Lilium longiflorum plants with pollen from diploids of the same species. One of the 9 plants had 25 chromosomes; the extra chromosome was identified as a modified D. Two other plants had 2 new chromosomes each and the remaining 6 had 24 unmodified chromosomes. Translocations in meiosis of the tetraploid produced the new chromosomes. One plant obtained from a second capsule had 48 chromosomes. The 9 plants were smaller than diploids and the 48-chromosome plant was considered a diploid until mitosis was observed. The 9 plants originated from unfertilized eggs of the tetraploid, and the 48-chromosome plant presumably from chromosome doubling of an egg cell.     

Cytogenetics of an unstable trisomie in barley (Hordeum vulgare).

The origin, identification, meiotic chromosome behavior, and breeding behavior of an unstable trisomic barley were studied. The extra chromosome originated by breakage and fusion of an acrocentric chromosome 3 in a plant from an F2 population of a cross between acrotrisomic 3L3S (2n = 14 + 1 acro3L3S) and a balanced lethal stock, xc. (xantha) ac (albino). The F2 population segregated only for the albino trait. The genotypic constitution of the trisomic plant was ac ac (for both normal chromosome 3) and Ac (for the unstable metacentric chromosome). The unstable extra metacentric chromosome was designated as metacentric 3B (abbreviated as meta3B). Meiotic chromosome behavior in plants with 2n = 14 + 1 meta3B differed from plant to plant and within spikes. Some plants showed only trisomic cells with a chromosome configuration of 1 III + 6 II and 7 II + 1 I at metaphase I, whereas other plants showed both trisomie and disomic cells (7 II) that resulted from the elimination of the extra meta3B. The frequency of ring trivalents was low (6.8%). An average transmission rate of unstable meta3B ranged from 4.3 to 12.9%. The elimination of meta3B, and hence loss of the dominant Ac allele, resulted in albino seedlings as well as white stripes on plants, leaves, and spikes. Chromosome numbers of albino seedlings in the progeny of 2n = 14 + 1 meta3B were all diploid (2n = 14), while green seedlings contained 2n = 14 + 1 meta3B. However, progenies of some spikes of one trisomic plant showed a low frequency of green diploids and metatrisomics (2n = 14 + 1 meta3B), which was attributed to crossing-over.     

Cytological investigation of Triticale

Summary A cytological investigation of 15 different 56-chromosome Triticale and 16 Triticale with 42 chromosomes was carried out. 4 were primary Triticale and 12 were secondary Triticale. Chromosome pairing was not disturbed; 21 and 28 bivalents were found in the hexaploids and octoploids, respectively. Meiotic irregularities were established, however, in all the Triticale studied; in octoploids the frequency of the irregularities was 22–88% and in hexaploids it was 12–87%.In metaphase and anaphase asynchronous separation of chromosomes was noted. Incompatibility between wheat and rye genomes and the inactivation of single loci of rye chromosomes are suggested as the main causes of the irregularities in meiosis.Mitotic disturbances were found in all the amphidiploids. The frequency of anomalies in mitosis was considerably lower than in meiosis: in octoploids they made up 5%–11% and in hexaploids 6.2%–15.2%. In all the amphidiploids studied chimera plants were found containing pollen mother cells with different chromosome numbers. The chromosome number in the aneuploid cells varied from 8–48 in hexaploids and from 8–62 in octoploids. Octoploid Triticale had 29.4%–72.9% aneuploid pollen mother cells, while hexaploid Triticale had 5-2%–55-7%.     

HYBRIDS AND INDUCED AMPHIPLOIDS OF ELYMUS CANADENSIS X AGROPYRON LIBANOTICUM

North American Elymus canadensis L., 2n = 28, and Asian Agropyron libanoticum Hack., 2n = 14, crossed with ease and yielded vigorous but sterile F₁ hybrids, 2n = 21. Chromosome pairing in the hybrids averaged 9.47^I, 5.38^II, and 0.26^III in 150 metaphase-I cells. One genome of E. canadensis is more or less homologous with the A. libanoticum genome. Treatment of the F₁ hybrids with colchicine produced 42-chromosome amphiploids, C₀, which were advanced through two seed generations, C₁ and C₂. More than half of the metaphase-I cells in the C₀ amphiploids contained 21^II; and average associations were 1.09^I, 20.16^II, 0.07^III, and 0.09^IV in 116 cells. Meiosis became increasingly irregular beyond metaphase-I; nevertheless, the C₀ amphiploids produced 68% stainable pollen and averaged 0.75 seed per spikelet. Multivalent frequencies increased in advanced generations, and the C₂ amphiploids averaged 1.11^I, 19.00^II, 0.23^III, and 0.55^IV in 100 metaphase-I cells. Meiosis was essentially regular in the C₁ and C₂ amphiploids beyond metaphase I, and the C₂ amphiploids averaged 73% stainable pollen and 2.28 seeds per spikelet. The amphiploids have an excellent chance of developing into a meiotically stable, fertile, new species. Forage characteristics of the amphiploids indicate that they have considerable economic potential as a forage grass.     

Interspecific hybridization between Vigna pubescens and V. unquiculata (L.) Walp through embryo rescue

Interspecific hybridization was achieved between cowpea (Vigna unquiculata [L.] Walp) and a hairy wild relative (V. pubescens). Hybrid embryos which otherwise would have shrivelled and failed to germinate were excised prematurely and cultured on a solidified MS medium. The F₁ plants were vigorous in growth, partially sterile, slightly hairy and showed some intermediate features between the two parental types. Cytological investigations on F₁ pollen mother cells showed average chromosome associations per cell of 0.66_I+10.00_II+0.34_IV.     

Allopolyploid of tetraploidphaseolus species ×p. calcaratus cross

Allopolyploids were raised by oolchicine treatment of vegetative buds in the triploid hybrid of tetraploidPhaseolus species xP. calcaratus cross. They had 70 per cent pollen fertility and produced many viable seeds. The average chromosome configuration per cell at MI in them was 1.56_IV + 0.01m + 29.30_II + l.llr. It has been concluded that the increased fertility in allopolyploids was due to more regular meiosis and the low frequency of quadrivalents indicated the occurrence of preferential pairing.     

Tissue culture in Haworthia

Summary Plants regenerated on two different media (NK and I) from the calluses of simple or cloned subcultures, which were originated from a single stock callus of Haworthia setata derived from its flower bud, were observed for eight characters, i.e., somatic chromosome number in root tips, growth vigor, leaf shape, leaf color, number of stomata per unit leaf area, esterase zymogram, chromosome association at meiotic metaphase I in pollen mother cells, and pollen fertility. From these regenerates plants with different characters from those of the parental plant were obtained. With regards to chromosomal aberrations, tetraploids, aneuploids, plants with a part of the chromosome segment deleted, with reciprocal and non-reciprocal translocations, or with paracentric inversions and those showing sub-chromatid aberrations at meiosis were obtained. The NK medium tended to regenerate more tetraploids and less plants carrying translocation than the I medium.Chromosome variabilities in somatic cells of the regenerates correlated with those of the calluses, from which they regenerated, while they did not correlate with either the meiotic irregularities (chromosome association at MI) or pollen fertility of the regenerates. From these facts, it was concluded that a rather large number of callus cells participate in the regeneration of an individual plant, although, however, only a few limited types of the cells form its germ line.Polyploidy affected growth vigor, leaf shape, stomata number and chromosome association at MI, but its effects were not detected on other characters. Chromosomal aberrations at the diploid level produced no clear changes in the regenerate's phenotype except in meiotic chromosome configuration and pollen fertility.Most chromosomal variants obtained in the present study are already reported in plants collected from wild populations, but plants with the deletion of a whole chromosome (karyotype 7L+6S) or chromosome segment (7L+1M+6S and 14L+2M+12S) have never been reported: this fact suggests that tissue culture is a powerful tool for producing plants with novel karyotypes.Contribution from the Laboratory of Genetics, Faculty of Agriculture, Kyoto University, Japan, No. 436     

Chromosome elimination in trisomics of Coix aquatica Roxb.

Summary Somatic chromosome elimination was identified and its patterns studied in a trisomic (2n=11) with marker genes in Coix aquatica Roxb. In a cross between a recessive trisomic with green base and white style (ccc ii ss) and a dominant disomic having purple base and purple style (CC II SS), all the F₁ seedling progeny were purple based because of the presence of C, I and S. For C. to be expressed in seedling base, either I should be absent or S should be present with I. In style colour, however, irrespective of the presence of I and S, C produces purple phenotype.In one trisomic (Ccc ii ss) plant (designated as 4–15) of the F₁ progeny, a part of the seedling base was green. All the tillers coming up from the green side of the main tiller also had green base, and those arising on the purple side were purple based. Similarly, the pistillate spikelets on the green side of the main culm and on the tillers with green base were white styled, and the male spikes showed 10 chromosomes. Female spikelets on the purple side of the main tiller and on the tillers with purple base were mostly purple styled and the male spikes had 11 chromosomes. In some of the purple based tillers, however, there were both 11 and 10 chromosomes in different regions or different inflorescence clusters on the tiller. In these tillers, where the chromosome number was 11, style colour was purple, and white style occurred when there were 10 chromosomes. In one tiller, the style colour was purple but the chromosome number was 10.The recessive phenotype of the style in the trisomic conceivably resulted from an elimination of the extra chromosome carrying the dominant allele C. On the basis of the morphological features of the extra chromosome, such as length, centromere position and distribution pattern of the hetero and eupycnotic regions, it was identified as chromosome No. 2 in the complement. It was therefore possible to place with certainty the gene c on this chromosome. Sometimes, however, the extra chromosome carrying c also was eliminated giving 10 chromosomes and purple style.In the other trisomic plants of the F₁ progeny, one plant showed 11 chromosomes but in a tiller there were only 10 chromosomes and white styles. In two other plants, although the chromosome number was 11 throughout, white style was present in a single cluster of inflorescences in one plant, and in one pistillate spikelet in the other. In the latter two cases, white style was believed to have arisen as a result of a mutation from C to c or somatic crossing over, giving the genotype ccc in the affected regions. In a single plant, chromosome elimination was observed in only one cell.Apparently the 10-chromosome sectors arose from the 11-chromosome condition by selective elimination of the extra chromosome during mitosis in the primordium giving rise to these sectors. In the affected plants, elimination did not obviously occur at the same stage but at different times in their ontogeny. Instability is probably governed by one or a few major genes, associated with a number of modifiers, exhibiting incomplete penetrance and variable expression. Chromosome elimination did not apparently follow any particular pattern but was erratic. Probably some intracellular environment is necessary to trigger the mechanism governing the elimination into action. The unstable system, occurring in combination with other favourable features like the functional nature of the aneuploid gametes, sexual reproduction, monoecious condition favouring cross pollination and tolerance of extra chromosomes by the sporophyte, could be an important factor in the cytogenetic evolution of the species.