Hexaploid Hybrids Research Articles

Interspecific hybridization in the cultivated, non-tuberous Solanum species

Relationship between five cultivated Solanum species (3 diploids and 2 hexaploids) was studied. Fertile diploid and hexaploid hybrids were obtained from the crosses S. macrocarpon x S. melongena and S. nodiflorum x S. nigrum, respectively. The diploid hybrid of S. macrocarpon x S. incanum was partially fertile. All the crosses involving S. incanum as female parent did not produce ovary stimulation. The 2x×6x or reciprocal crosses yielded non-viable seeds. F2 plants derived from S. macrocarpon x S. incanum and S. macrocarpon x S. melongena were quite variable, but those from S. nodiflorum x S. nigrum were vigorous and remarkably uniform. The BC1 plants more closely resembled the recurrent parent. The morphological comparison of these species, the intererossability and fertility of their hybrids indicate that S. macrocarpon is more closely related to S. melongena than to S. incanum, while the differences between the two hexaploid species are more varietal than specific.

Gametophyte Sex Expression in Some Species of Dryopteris

The study of sexuality in fern gametophytes has stimulated research in the genetic consequences of gametophyte sex expression (Klekowski, 1971) and in the physiology of sex determination in ferns (Dopp, 1950; Voeller, 1971). A class of antheridia-inducing hormones, antheridogens, has been discovered in several genera of ferns including Dryopteris (Dopp, 1950; Voeller, 1964). Homosporous ferns differ fundamentally from heterosporous plants in their potential for intragametophytic selfing, which eliminates heterozygosity by the union of genetically identical egg and sperm derived mitotically from a single gametophyte (Klekowski & Lloyd, 1968). Homeologous pairing in polyploids is a possible mechanism for expression of heterozygosity following intragametophytic selfing (Klekowski & Hickok, 1974). Intragametophytic selfing and intergametophytic crossing are genetically equivalent to self pollination and cross pollination in heterosporous plants. The occurence of bisexual, male, and female gametophytes in multispore cultures of Dryopteris ludoviciana suggested that gametophytes of this genus are not regularly bisexual (Cousens & Horner, 1970). This paper extends the observation of gametophyte sex expression to seven additional species of Dryopteris. Various patterns of sex expression may be adaptive in allowing different balances of intraand intergametophytic mating in different environments (Klekowski, 1972). Dryopteris is a large genus of ferns notable for frequent hybridization between species (Wagner, 1970). Three tetraploid species studied, D. campyloptera, D. spinulosa, and D. celsa, probably arose by chromosome doubling of sterile hybrids; their ancestral taxa are either known or hypothesized (Wagner, 1970; Walker, 1961, 1962). Dryopteris clintoniana is a hexaploid hybrid of D. goldiana and D. cristata (Wagner, 1970). Dryopteris intermedia, D. goldiana, D. marginalis, and D. ludoviciana are all sexual diploids. Cultures of Dryopteris gametophytes were initiated by inserting a sterile wooden applicator into 25 ml spore vials and transferring 400-600 adherent spores to a 5 ml beaker which was then filled with sterilized water to which a trace of detergent had been added. Each species studied was represented by spore samples from two or more sporophytes. After the spores were dispersed in the liquid, the suspension was divided among three sterile 60 mm petri dishes filled with sterile clay-loam soil. Only cultures which resulted in a gametophyte population density of 1-3 gametophytes per cm2 were used because previous work has shown that culture density modifies sex expression (Cousens & Horner, 1970; Rashid,

Chromosome relationships between Lolium and Festuca (Gramineae)

With a view to eclucidating chromosome relationships between Lolium perenne (Lp), L. multiflorum (Lm) and Festuca pratensis (Fp), chromosome pairing in different diploid (2n=14), auto-allotriploid (2n=3x=21), trispecific (2n=3x=21), amphidiploid (2n=4x=28) and auto-allohexaploid (2n=6x=42) hybrids between them was analysed. At all these levels of ploidy there was very good chiasmate pairing between the chromosomes of the three species and, on the whole, there was little evidence of preferential pairing of the chromosomes of a particular species in the triploid, tetraploid and hexaploid hybrids. A critical test for this also came from the synaptic ability of the chromosomes of the single genome with those of the duplicated genome in the auto-allotriploids which formed predominantly trivalents with 2, 3 or even 4 chiasmata. Moreover, the homology between the Lp and Lm chromosomes seems strong enough to pass the discrimination limits of the B-chromosomes which do not suppress homoeologous pairing in the Lp LmLm triploid and LpLm diploid hybrids. — The triploids having two genomes of a Lolium species and one of F. pratensis had some male and female fertility which suggested genetic compatibility of the parental chromosomes resulting, presumably, in compensation at the gametic level. Also, the occurrence of comparable chiasma frequencies in the auto-allotriploids and trispecific hybrids showed that they were not markedly affected whether two doses of one genome and one of the other or all the three different genomes from the three species were present. From the trend of chromosome pairing in all these hybrids it is concluded that there is little structural differentiation between the chromosomes of the three species, no effective isolation barrier to gene-flow between them, and that they are closely related phylogenetically, having possibly evolved from a common progenitor. Taxonomic revision of the two Lolium species is suggested.

Reproduction in Crested Wheatgrass Pentaploids1

Pentaploid, 2n=35, crested wheatgrasses [Agropyron cristatum (L.) Gaertn.] and related taxa., were derived by crossing natural and induced tetraploids, 2n=28, with hexaploids, 2n=42. The pentaploids backcrossed readily to tetraploids and hexaploids, but they crossed only rarely with diploids, 2n=14. Chromosome numbers in 5x‐2x, 5x‐4x, 5x‐5x, and 5x‐6x hybrids ranged from 2n=21 to 25, 2n=28 to 35, 2n=28 to 42, and 2n=35 to 60, respectively. Three hyperoctoploid hybrids—2n=58, 59, and 60—from 5x‐6x crosses probably resulted from union of unreduced male gametes, n=42, and reduced eggs, n=16 to 18. The pentaploids produced functional reduced gametes with n=14 to 21 chromosomes, and the gametic chromosome numbers were distributed somewhat normally, with a mean between n=17 and 18. High multivalent associations at metaphase‐I in some hybrids were caused by a combination of autoploidy and structural heterozygosity. Fully fertile tetraploid and hexaploid hybrids were recovered immediately from 5x‐4x and 5x‐6x crosses, respectively. Grass breeders should not find it difficult to transfer genes, chromosomes, or even whole genomes between tetraploid and hexaploid crested wheatgrasses. Genetic transfer between diploid and polyploid races is somewhat more difficult.

Brassica napocampestris L. (2n=58). 1. Synthesis, cytology, fertility and general considerations

The synthesis of leafy, forage forms of Brassica napocampestris (2n=58, aaaacc) by colchicine treatment of hybrids between B. napus (2n=38, aacc) and B. campestris (2n=20, aa) is described. The cytology of both triploid and hexaploid hybrids was studied. B. napocampestris proved fairly stable meiotically although multivalents were formed and some aneuploids detected in later generations. Seed fertility proved adequate for commercial production. The variation within the parental species and their easy crossability suggests that several distinct forms of B. napocampestris could be created, some of which may have agronomic potential. B. napocampestris may be used as a stepping stone in elaborating the range of variability within B. napus.

THE ORIGIN OF AGROPYRON LEPTOURUM

Colchicine‐induced amphiploids, 2n = 42, of diploid Agropyron libanoticum Hack., 2n = 14, X tetraploid A. caninum (L.) Beauv., 2n = 28, were morphologically and cytologically similar to A. leptourum (Nevski) Grossh., 2n = 42. Both A. leptourum and the induced amphiploids were self‐fertilizing. The induced amphiploids crossed readily with A. leptourum and gave rise to partially fertile hexaploid hybrids. Chromosome pairing in the hybrids averaged 0.60I, 18.29II, 0.36III, 0.58,v, 0.02v, and 0.22VI in 90 diakinesis or metaphase‐I cells. The genomes of the induced amphiploids are essentially homologous with those of A. leptourum except for two or more reciprocal translocations. The morphological, cytological, fertility, and crossing data provide conclusive evidence that A. libanoticum and A. caninum, or their close relatives, are the parents of A. leptourum. The genome formulas of A. libanoticum, A. caninum, and A. leptourum may be written as SS, SxSxHxHx, and SSSxSxHxHx, respectively, where S is the basic A. libanoticum genome and H is a genome derived from Hordeum.

INTERSPECIFIC CROSSES INVOLVING ALFALFA. VII. MEDICAGO SATIVA × M. RHODOPEA

On crossing Medicago sativa L. with M. rhodopea Velen., two hybrids were obtained. One was a triploid 2n = 24. It was produced by crossing a self-incompatible, diploid (2n = 16) M. sativa plant with an artificially chromosome-doubled (2n = 32) M. rhodopea plant. In this hybrid almost no fully analyzable MI plates were found. The range of observed univalents in 57 MI plates examined was one to seven per plate; in 38 of these plates one to four trivalents were seen. The chromosomes were doubled in the triploid to produce a hexaploid hybrid (2n = 48) which was self-sterile. This was probably the influence of the self-incompatible parent. The amount of plasma-filled pollen was approximately 64% in the hexaploid hybrid, as compared to less than 20% in its triploid progenitor. On backcrossing the hexaploid hybrid to hexaploid and tetraploid alfalfa, seeds were readily obtained. The other hybrid, which was obtained by crossing a tetraploid (2n = 32), male-sterile M. sativa with the chromosome-doubled M. rhodopea; had 2n = 31. It is assumed that in this and similar rarely successful crosses, some incompatibility factor/s is eliminated with the eliminated chromosome or the genic ratio is changed favoring compatibility. Transfer of M. rhodopea genic material to cultivated alfalfa may be through crossing over at meiosis involving chromosomes of the two species. Such occurrences were indicated on examination of meiosis in the triploid hybrid. Another way to include M. rhodopea chromosome complements in alfalfa would be to produce hexaploid auto-alloploids consisting of two sets of M. rhodopea (Rp) and four sets of M. sativa (S) chromosomes.

Origin of Fragaria Polyploids. I. Cytological Analysis

The genomic relationships between modern Fragaria diploids (European and American F. vesca and F. viridis—2n = 14) and octoploids (F. virginiana and F. chiloensis—2n = 56) were investigated. This contrasts with previous studies where cultivars have represented the octoploids. Critical information on genomic pairing was obtained by comparing chromosome behavior of pentaploid hybrids with that of hexaploid hybrids (obtained by using a recent colchiploid of European F. vesca). Supporting data were also used from an F. chiloensis × Potentilla glandulosa hybrid (2n = 35). A revision of the AABBBBCC (Federova) genomic formula for the octoploids to AAA'A'BBBB is proposed based upon cytological evidence of homology between the AA and “CC” genomes. The phylogeny of the octoploids is postulated.

ORIGIN OF FRAGARIA POLYPLOIDS. I. CYTOLOGICAL ANALYSIS

The genomic relationships between modern Fragaria diploids (European and American F. vesca and F. viridis—2n = 14) and octoploids (F. virginiana and F. chiloensis—2n = 56) were investigated. This contrasts with previous studies where cultivars have represented the octoploids. Critical information on genomic pairing was obtained by comparing chromosome behavior of pentaploid hybrids with that of hexaploid hybrids (obtained by using a recent colchiploid of European F. vesca). Supporting data were also used from an F. chiloensis × Potentilla glandulosa hybrid (2n = 35). A revision of the AABBBBCC (Federova) genomic formula for the octoploids to AAA'A'BBBB is proposed based upon cytological evidence of homology between the AA and “CC” genomes. The phylogeny of the octoploids is postulated.

Reproductive capacity and mode of death of yeast cells.

The technique of micromanipulation was used to observe the number of daughter cells produced by individual cells of two yeasts, one a brewing strain and the other a hexaploid hybrid. The mode in which these cells died was also recorded. An average reproductive capacity of 34 daughter cells was found for the brewing yeast and of 17 daughter cells for the hexaploid strain. Two distinct modes of death were observed, one in which the final daughter cell appeared normal and the other where the last daughter cell could not be detached from its mother and both cells died. A correlation was obtained between the mode of death of a cell and its reproductive capacity. A number of final daughter cells (the 28th - 46th buds of their mother cell) was also observed through a considerable number of divisions and these cells were found apparently normal in their reproductive ability. It is suggested that cessation of budding is a consequence of reduction of the active surface to volume ratio because of the lower metabolic activity of scar tissue.

THE EVOLUTIONARY SIGNIFICANCE OF NATURAL FRAGARIA CHILOENSIS × F. VESCA HYBRIDS RESULTING FROM UNREDUCED GAMETES

A natural nonaploid hybrid (2n = 63) from fusion of a reduced F. vesca male gamete with an unreduced F. chiloensis gamete, and a partially fertile natural hexaploid hybrid (2n = 42) from fusion of an unreduced F. vesca (2n = 14) male gamete with a reduced F. chiloensis (2n = 56) gamete were discovered in separate mixed colonies along with over 20 additional pentaploid hybrids (2n = 35). Plants of all hybrids aggressively compete with F. chiloensis. This relatively high percentage of hybrids from unreduced gametes may mean that they have higher survival value than pentaploids, since some evidence indicates that unreduced gametes may not function that often in successful F. chiloensis × F. vesca hybridization. The partial fertility of the hexaploid shows that intermediate fertile levels of ploidy need not be derived exclusively from lower ploidy species, and unreduced gametes from such hybrids should produce a high percentage of decaploid offspring in natural backcrossing to F. chiloensis. The successful functioning of unreduced vesca and chiloensis gametes in natural hybridization justifies the postulation that other euploid levels also may occur naturally, including triploids, tetraploids, decaploids, 12 ploids, and 16 ploids. Of these, the even multiples—tetraploids, decaploids, 12 ploids, and 16 ploids—should be at least partially fertile.

Crossability, fertility and cytogenetic studies in Solanum acaule × Solanum bulbocastanum

From 538 cross combinations made between 20 accessions of S. acaule ♂ and 28 clones of S. bulbocastanum 150 different F1's were obtained. Average berry set was 11%, average number of seeds per berry 0.7. The accessions of both parent species could be divided into distinct groups on the basis of the crossability. Of the 72 F1-clones examined 59 were triploid and 13 were tetraploid. Evidence is presented that the 4x-F1's originated from unreduced gametes produced by two bulbocastanum accessions. Pollen stainability of the 6x-, 8x-, 4x- and 3x-F1's was 85, 29, 12 and <5% respectively. The genomic constitutions of the F1's adequately accounted for these fertility relations. Selfing of F1's was successful with 6x-F1's only. Crosses between the F1's and S. tuberosum, tuberosum-haploids and S. demissum did not succeed in spite of the large number of pollinations made. A breeding programme is recommended to be carried out within the hexaploid hybrid populations which aims at late blight resistance and crossability with S. tuberosum. The quadruple cross (acl × blb) × (acl × tbr) and the direct cross between S. tuberosum and S. bulbocastanum are discussed.

THE CYTOLOGY OF A HYBRID BETWEENGOSSYPIUM HIRSUTUMANDG.LONGICALYX

T h e specics of Gossypiz~nz havc been categorized by Beasley (1940) into five cytologically and geographically defined diploid genome groups (A, Africa and Asia; B, northern and southern Africa; C, Australia; Dl North and South America; and E, northeastern Africa to Pakistan) and one amphidiploid group (AD, tropical America, Hawaii, and cultivated). Interspecific, intragenomic hybrids gcncrally show cn. 13 bivaleilts at meiotic metaphase I, whereas the uumbcr of bivalints in intergenomic hybrids ranges from less than 1 (for some hybrids involving E genome species) to 11 (A X B hybrids). In 1958 Hutchinson and Lee described a new species of Gossypiz~m from central Tanganyika as G. longicnlyx; collections of this species have also been made in Uganda and southcrn Sudan. They placed the new species in section Stocltsiana (which is coincident with Beasley's E genome group), citing simililrities in thc geographical range and morphology of G. lo~zgicnlyx with species of this scction of the genus. Saunders (1961) later assigned this species the E: subeenome dcsienation. U U Attempts to hybridize G, lo7zgicrrlyx with other Gossypizf~~z taxa have been generally unrewa;ding, but recently a hybrid involving the natural amphidiploid, G. hi.rs.lrtznn, and G. lo7lgicalyx has bcen obtained. T h e present report describcs the cytology of this triploid, and its synthetic hexaploid, which we interpret as evidence that G. longicnlyx represents a new Gossypiz~nz cytotype. Methods and Materials The G. hi7'~~lt~17?~ (and pistillate) parent of the G. hirs~rtz~7?z X lo7zgicnlyx hybrid is accession #820 of the Gossypiz~nz collection of Dr. S. G. Stephens of this institution. Seeds of G. lo~zgicnlyx were reccived through the Cotton and Cordage Fibers Branch, U.S. Department of Agriculture. Three hybrid seeds were obtained: two of the resulting plants vrovided the materials used for the I I study of triploid cytology, the third plant was converted into a hexaploid via the colchicine-tragacanth method of Schwanitz ( 1919). T h e cytological analyses of the two triploid plants were similar, and have been combined in TabIe I. The cytological analyses of PhlC's were made at meiotic metaphase I, following fixation in Carnoy's fluid (4: 3: 1) and storage in 70% ethanol. Results and Discussion The results of the G. hirszrtzr77z X longicrrlyx triploid and hexapIoid cytological analyses are contained in Tablcs I and IT, respectively. A summary of the pertinent literature on the cytology of triploid and hexaploid hybrids combining the AD tetraploids (G. hirszrtunz and G. bnrbacle7zse L.) with species of the A, B, C, D, and E genome groups is presented in Table 111. Comparable data on triploid bivalent frequency and hexaploid multivalent frequency for G. hirszrtzr7n X longicnlyx (6.75 and 2.00, per cell, respectively) and AD X E (0.55 and 0.19, per cell, respectively), indicate that G. lo7zgicalyx

Cytogenetic studies on the artificially raised trigenomic hexaploid hybrid forms in the genus Brassica

Cytogenetic studies on the artificially raised trigenomic hexaploid hybrid forms in the genus Brassica

CYTOGENETIC STUDIES IN THE TRIBE TRITICEAE

Two cases of natural interspecific hybridization in Japanese Agropyron were observed in Misima. One is a pentaploid hybrid between Ag. ciliare and Ag. tsukushiense, and the other is a hexaploid hybrid between Ag. humidum and Ag. tsukushiense.The pentaploid hybrid was highly sterile and no seed setting was observed. It is assumed that in this hybrid combination introgressive hybridization would not occur in natural conditions.As to the hexaploid natural hybrid, two cases were observed; i.e., one allopatric and the other sympatric. In the allopatric case Ag. humidum and the common type of Ag. tsukushiense were involved, while in the sympatric case Ag. humidum and the early ecotype of Ag. tsukushiense were the parental species. In both cases low seed setting was found from the examination of a considerable number of hybrid clones in natural conditions. From progeny tests of backcrossed plants obtained from the sympatric natural hybrids between Ag. humidum and the early ecotype of Ag. tsukushiense, the following conclusions are drawn: It is quite possible that introgression of characters, such as waxiness, from one species to the other was taking place in the natural populations through backcrossing to the parental species of the F1 hybrids followed by segregation of the characters concerned. Rather quick restoration of fertility in the hybrid progenies made possible the establishment of hybrid swarms in a state of nature having intermingled characteristics of the parental species. However, so far no introgressants have been found yet in the natural sympatric populations of these two species. High sterility of F1 hybrids might be a major cause of restricted introgression.

MORPHOLOGY AND CYTOLOGY OF SYNTHETIC HYBRIDS OF AGROPYRON TRICHOPHORUM X AGROPYRON CRISTATUM

Dewey, Douglas R. (Utah State U., Logan.) Morphology and (cytology of synthetic hybrids of Agropyron trichophorum X Agropyron cristatum. Amer. Jour. Bot. 50(10): 1028–1034. Illus 1963.—Three hybrids were obtained from controlled crosses of pubescent wheatgrass, A. trichophorum (2n = 42), and hexaploid crested wheatgrass, A. cristatum (211 = 42). The hybrids were intermediate between the parent plants for all vegetative and spike characteristics observed. Under open pollination, 2 of the hybrids set 2 seeds each, and the other hybrid produced 60 seeds. Meiosis in the parent plants was basically regular. Average motaphase‐I chromosome associations were 0.09 I, 20.56 II, 0.05 III, and 0.16 IV per cell in the A. trichophorum parent, which was described as a segmental autoallohexaploid. The hexaploid A. cristatum parent averaged 0.18 I, 7.44 II, 0.81 III, 2.86 IV, 0.08 V, and 2.11 VI per cell at diakinesis and was described as an autohexaploid. Chromosome pairing in the hexaploid hybrid averaged 5.08 I, 8.94 II, 4.33 III, 1.11 IV, 0.27 V, and 0.05 VI per cell. On the basis of chromosome pairing in the parent species and their hybrids, it was concluded that 1 of the A. trichophorum genomes was partially homologous with the 3 genomes of hexaploid A. cristatum. Genome formulae for hexaploid A. cristatum, A. trichophorum, and their hybrids were represented as AAAAAA, A1A1B1B1B2B2, and AAAA1B1B2 respectively.

The occurrence of hybrid semi-lethals and the cytology ofTriticum machaandTriticum vavilovi

1. A semi-lethal gene combination has been found in hexaploid species hybrids involving some strains ofTriticum macha, but not in comparable hybrids involving other strains ofT. macha. The production of these semi-lethal hybrids can be explained by the interaction of two genes designated as maand mb.2. The gene mais carried in some strains ofT. macha. The gene mbis carried in the third chromosome set of all other hexaploid species ofTriticum, inAegilops squarrosaand inAeg. cylindrica.3. The semi-lethal gene combination has been found between species which would otherwise produce fertile hybrids and between species which would otherwise produce sterile hybrids. The semilethal gene combination occurs between species which do not overlap geographically.4. The distribution of the genes maand mbshows that the gene maoriginated inT. machaand that the gene mboriginated inAeg. squarrosaand was introduced from there, during the course of evolution, into the hexaploid species ofTriticum(probably excludingT. macha) and intoAeg. cylindrica.5. Meiosis in semi-lethal plants was comparable to meiosis in normal plants. But the semi-lethal plants showed a reduction in fertility, probably as a result of their poor vegetative growth.6. The chromosomes ofT. machaandT. vavilovihave been no more structurally differentiated than the chromosomes of the other hexaploid species ofTriticum.7. The present study has confirmed that the third chromosome set of the genusTriticumhas been phylogenetically derived fromAeg. squarrosa.

POLYGENOMIC HYBRIDS IN GOSSYPIUM. II. MOSAIC FORMATION AND SOMATIC REDUCTION

IN THE COURSE of a study of species relations in Gossypium, two series of synthetic amphiploids have been produced, tetraploids from doubling F1 hybrids between diploid species, and hexaploids from doubling of hybrids between natural allotetraploid and diploid species. These synthetic amphiploids may be crossed among themselves, and with natural *species, both diploid and tetraploid. Hence it has so far been possible in Gossypium to obtain series of hybrids combining complete chromosome sets from two, three or four different species, and having the diploid, triploid, tetraploid, pentaploid or hexaploid multiple of the basic chromosome number 13. During a cytological and morphological investigation of these complex hybrids, a number of instances of unexpected and striking color mosaics have been noted. In some types of polygenomic hybrids involving three or four species, mosaic formation is a characteristic morphological feature, while in a number of others it occurs sporadically. In addition, one clear case of somatic reduction has been found. These two phenomena will be presented together, since recent investigations of Huskins and his coworkers (Huskins, 1948a, b; Wilson and Cheng, 1949; Huskins and Cheng, 1950) suggest that somatic segregation and somatic reduction may, in some cases, have a common cytological basis. For a detailed explanation of genome symbols -and the genome formulae of hybrids used in this paper, based on Beasley's nomenclature (1940) . the reader is referred to the first paper of this series (Brown and Menzel, 1952). Letters A, B, C, D, and E represent cytologically differentiated groups of genomes (n = 13), while subscripts identify particular species of a genome group. (AD) refers to the genomes of the natural allotetraploid species (n = 13 + 13 _26). Thus both the chromosome number and the number of species combined may be inferred from the genome formula of a given plant. MOSAIcs.-One series of complex trispecies hybrids has been obtained by intercrossing allohexaploids derived from hybrids of G. hirsutum X a diploid species. The hexaploid hybrids so far studied, comprising 18 different species combinations, all share one marked morphological peculiarity: the universal occurrence among them of color mosaics in petals and leaves. This is most striking and was first observed in six hybrids having the hexaploid 2(AD) 1C1 as one parent. In all of these, the mauve petal color carried by the C1 genome is

CHIASMA FREQUENCY IN SPECIES AND SPECIES HYBRIDS OF AVENA

(i) Chiasma frequency at metaphase has been studied in the following species and species hybrids: A. strigosa Schreb.; A. brevis Roth; A. Wiestii Steud.; A. barbata Pott; A. abyssinica Hochst. (2 strains); A. sterilis L. (2 strains); A. sativa L. var. Radnorshire Sprig; a triploid hybrid, A. barbata × A. strigosa; a pentaploid hybrid, A. abyssinica "naine" × A. sterilis maxima; and a hexaploid hybrid, A. sterilis (white) × A. sativa var. Radnorshire Sprig.(ii) The various factors that may affect chiasma frequency are discussed and it is concluded that there may be some parallelism between closeness of relationship and similarity in chiasma formation, as genetical control is one factor affecting the process. The results are examined in the light of this conclusion.(iii) The homogeneity of species in respect to proportion of bivalents with zero, one, two, three, or four chiasmata is tested by means of χ2. The test indicates no significant difference between the closely related diploid species, A. brevis and A. strigosa, but both differ significantly from the less closely related A. Wiestii. The differences between the tetraploid species are of questionable significance, those between the hexaploids, insignificant. It is suggested that the lack of significant differences among these last-mentioned forms may be due to a multiplication, in the polyploid, of genes favoring high chiasma frequency. In this way a maximum effect might be brought about, and inter-specific differences with respect to this character would consequently be lacking.(iv) High chiasma frequency in the hybrid might be regarded as strong evidence of homology, and consequently of relationship between parental forms. The triploid hybrid, A. barbata × A. strigosa shows a high chiasma frequency at metaphase, a condition consistent with the close relationship of the parental forms. The pentaploid hybrid, as might be expected, has a low chiasma frequency. The chiasma frequency of the hexaploid hybrid, A. sterilis × A. sativa var. Radnorshire Sprig does not differ significantly from that of its low-frequency parent, A. sterilis. Cytological observations in this hybrid, therefore, reveal only a slight degree of non-homology between parental chromosomes. It is noted that there are certain characters of A. sativa var. Radnorshire Sprig that suggest affinity with A. byzantina, a species commonly associated with A. sterilis in relationship schemes.

The Genetics and Cytology of Certain Cereals

1. One triploid hybrid, Avena barbata×strigosa, two pentaploid hybrids, A. barbata×fatua and A. barbata×sterilis, four hexaploid hybrids, A. fatua×sativa, A. fatua×sterilis, A. sativa×byzantina and A. sterilis× byzantina, and their parents were studied morphologically and cytologically.