Diploid Forms Research Articles

THE CYTOGENETICS OF HYBRIDS IN BROMUS. I. HYBRIDS WITHIN THE SECTION CERATOCHLOA

ONE OF the commonest, most widespread, and variable groups of grass species in western North America is the subgenus Ceratochloa of the genus Bromus, of which the best known species native to the United States are B. carinatus H. & A. and B. marginatus Nees. The problem of the number of species actually present in this group has been a constant puzzle to systematists, as is evident from the discussion of Hitchcock (1935, p. 38). In this manual he recognizes four native species of Ceratochloa, but the list of synonyms under the various species names (op. cit., pp. 811-817) indicates that fourteen different species have been described by various authors. Cytologically, the great bulk of the forms are octoploids, with 2n 56 (Stebbins and Love, 1941), but hexaploid (2n = 42) and decaploid (2n 70) counts have been recorded (Stiihlin, 1929; Nielsen and Humphrey, 1937; Nielsen, 1939), while there is one doubtful record of a diploid form (Stiihlin, 1929). In an attempt to secure this purported diploid, the senior author obtained seed of B. carinatus from Stiihlin's source, the Munich Botanical Garden, but the strain received was a typical octoploid. In view of the evidence presented below as to the probable origin of the B. carinatus complex, the presence of a diploid in this complex seems highly unlikely. In addition to its interest from the systematic standpoint, this complex contains forms of considerable forage value (Hitchcock, 1935, p. 33; Sampson and Chase, 1927, pp. 21-22). B. marginatus has been recommended by Stewart, Walker, and Price (1939) for reseeding range lands of the intermountain region. Among a large series of grass species sown in experimental range plots throughout California, B. carinatus has proved, one of the most readily established of any species native to the state (Burle Jones, unpublished data). Therefore, hybrids between different forms of this complex might be of value both as an aid to understanding the species problem as it relates to this group, and as a source of new varieties for establishment on range lands. For the latter purpose another series of hybrids was considered promising-between the B. carinatus complex and an eVen more valuable forage grass belonging to the section Ceratochloa, the South American B. catharticus Vahl. (= B. unioloides HBK, cf. Hitchcock, 1935). The work reported in the present paper, however, has shown that these latter hybrids are of more interest from the phylogenetic and systematic than from the agronomic point of view. MATERIAL AND METHODS.-The strains used in the present hybridization experiments are part of a large collection assembled by the senior author and

Polyploids are More Variable than their Original Diploids

STUDYING the modification of a large number of experimentally produced polyploid plants of various species and families (Solanaceae, Gramineae, Compositae, etc.) within the single plants and in the polyploid lines in respect to the modification of their diploid forms, when the plants grew in equal environmental conditions, I found the following regularities. Cell dimensions showed in the majority of the cases studied a greater modification in the tetraploid than in the diploid plants. Cell dimensions of the experimentally produced octoploids were more variable than the cell dimensions of tetraploid and diploid forms. The numbers of chloroplasts per pair of stomatal cells were more variable in the tetraploids than in the diploids and much more variable in the octoploids than in the tetraploids and diploids. Counting, for example, the chloroplasts1 in 100 pairs of stomatal cells of each form of Nicotiana alata—diploid, tetraploid and octoploid, the following values for the standard deviations were respectively found: σ2n = 1·81, σ4n = 4·70, and σ8n = 7·03. (It was found that polyploidy does not affect the size of the chloroplasts1 2. It also does not significantly affect the variability of the chloroplast diameter.) The modification of the cell dimensions decreases somewhat gradually in the subsequent polyploid generations, that is, it decreases with the increase of the polyploid generations. But in the cases studied tetraploids still had more variable cell dimensions in the fifth generation than the corresponding diploids.

Cyto-genetical Studies in the Genus <i>Solanum</i>. I

1. Among the plants generally termed as S. nigrum by the systematists the author found two types, karyologically distinct. One of them is n=12, and the other n=36.2. Close examination proved that both types are not exactly alike, differing in many points of their morphological characters. The type with 36 chromosomes is the typical S. nigrum, but, the other with 12 chromosomes deserves to receive a new specific name, S. photeinocarpum.3. S. nigrum is generally distributed throughout the northern part of Japan, whereas S. photeinocarpum occurs in the south. S. nigrum rarely found in Taiwan is of recent introduction, and its distribution is limited only to a few locales.4. The chromosome behavior observed in meiosis proved that S. photeinocarpum is a diploid form of the genus Solanum, while, S. nigrum is an autohexaploid.5. Size of the vegetative and reproductive organs of S. nigrum is markedly larger than that of S. photeinocarpum.6. The comparison of morphological characters and the survey of geographical distribution suggest that, there is an intimate relationship between both types with respect to the evolution of species.

Different Results in Reciprocal Crosses between Diploid and Triploid Allium Sch” noprasum L

IN the summer of 1934, crosses were made between diploid forms of chive (2n=16) and artificial triploids (2n=24)1. The somatic chromosome numbers in the progeny of these crosses have now been determined for 100 plants and are shown in the accompanying table.

Cytological Study and Hybridization in Two Sections of <i>Verbena</i>

The species of Verbena studied here fall into two cytological classes, one wich a basic number n=5 chromosomes, the other wich n=7. The chromosomes of the n=7 dass were approximately half to one sixth as large as those of the n=5 dass. This classification was found to correspond to taxonomical classification. The genus is divided taxonomically into two sections: Glandularia and Verbenaca. The species of the latter seetion are further divided into two series: Pachystachyae and Leptostachyae. All the species with n=5 and n=15 chromosomes belong to the Glandularia section. The polyploid forms of the n=7 dass and V hispida, a diploid form, are classified under Pachystachyae, and the other diploid species under Leptostachyae of the Verbenaca section.Hybridization was attempted between all the species under investigation. Hybrids were obtained, first between the species with n=5 chromosomes and between the species wich n=5 and n=15 chromosomes; secondly, between any two species wich n=7 chromosomes and between the species with n=7 and n=14 chromosomes, but not between n=7 and n=21, nor between n=14 and n=21 chromosomal species. Hybrids were not obtained from crossing species of the two classes referred to above with a basic difference of n=5 and n=7 chromosomes. In all Gases the hybrid plants were intermediate in general characteristics between their parent plants.A diploid and a hexaploid species in Glandularia section were crossed resulting in a tetraploid hybrid which was highly sterile. In the section Verbenaca successful Grosses were made between a diploid and two tetraploid species. In one Gase (V. hispida×V. bonariensis) the hybrid plants were triploid while in another (V. hispida×V. litoralis) only pentaploid hybrids were obtained, a total of five plants, two of which resembled closely the female parent and three the male parent.Hybrids were obtained between V hybrida and V. erinoides each with n=5 chromosomes. The chromosomes in the former are somewhat Tonger than in the latter. These differences could be observed both in root-tip sections and meiotic preparations.Suggestions are presented as to the mode of origin of tetraploid and hexaploid species.Two types of Gell size differences are presented, in one of which increase in volume appears to be physical, in the Sense Navaschin has described, and the other genetical.

CYTOLOGICAL STUDIES ON THE INFLUENCE OF LOW TEMPERATURE UPON THE POLLEN FORMATION IN <i>DISPORUM SESSILE</i>

1. This paper deals with the effect of low temperature artificially acted upon the pollen formation in the diploid form of Disporum sessile (n=8).2. The reduction division occured very irregularly at low temperature and the restitution nuclei and dyad cells were often produced.3. In a large pollen grain produced at low temperature, the diploid number of chromosomes was counted. They showed, considering from their size and shape, the duplication of the haploid set of chromosomes. From such a pollen grain and a normal egg an autotriploid plant may be expected to occur.4. Thus an experimental basis was given to the assumption that the autotriploid form (2n=24) of this species existing in the nature may have been formed from the diploid form by the abnormal meiosis caused by exceptionally low temperature often coming in warm season when the normal meiosis goes on.5. In a pollen grain two extra chromosomes were found together with a normal haploid set. The chromosome categories of the formers were identified according to their size and shape. From such a pollen and a normal egg, a double trisomic plant may be produced.6. In a pollen grain a normal haploid set and an extra chromosome were observed. The latter was recognized to have arisen by the fragmentation of some longer chromosome at its constriction point.

The Chromosomes of a New Haploid Oenothera

A new haploid Oenothera, derived by parthenogenesis from the diploid form as a result of cross-pollination of a flower of the diploid, has been described with special reference to its meiosis. The diploid (Oe. blandina) is homozygous, having seven pairs of chromosomes. The haploid plant shows seven univalents in 80% of the pollen mothercells, and various bivalent, trivalent and quadrivalent configurations of chromosomes in the renaining 20% of cells. Corresponding multivalents have been seen in the diploid. The associations of chromosomes in the haploid prove the presence of reduplicated segments in the chromosome complement. In one case, the reduplicated segment is terminal in one chromesome and interstitial in the other. The unequal configuration produced proves that chiasma formation is associated with and is a direct result of cytological crossing-over; such a configuration would lead to reduplication of end segments.The significance of the reduplications in relation to segmental interchange, translocation and the origin of a self-perpetuating ringmechanism is discussed. Proofs of various other principles are given, including (i) the random segregation of chromosomes at anaphase I in the haploid, (ii) the disturbance in the random distribution caused by lagging of chromosomes and subsequent formation of restitution nuclei, (iii) terminalisation of chiasmata in the diploid, (iv) random occurrence of chiasma failure in the diploid. The arrest of terminalisation, due to a change in the linearr homologies of the chromosomes, is clearly established by the pairing phenomena in the haploid.

The Secondary Association of Chromosomes

1. A detailed account is given of the behaviour of the chromosomes from diplotene to second anaphase in Dahlia Merckii (2n=36), D. coccinea (2n=32), D. coronata (2n=32) and D. variabilis (2n=64).2. Two kinds of chromosome association are found in these species:a) Primary Association, arising from particulate pairing at zygotene and determining segregation at metaphase.b) Secondary Association, arising at pro-metaphase and due to the general affinity of homologous chromosomes. This does not affect segregation. The chromosomes first associate at pro-metaphase when they are in close proximity to each other.3. As first described by DARLINGTON, the primary association of chromosomes at metaphase is shown to be solely due to the mainten-ance of chiasmata formed at pachytene. Only those chromosome pairs or multivalents which are materially connected by chiasmata survive the strong repulsion phase of diakinesis. This repulsion results in the radial dispersion of the bivalents to the periphery of the nucleus, coupled with an equal aversion between the pairs and the members of each pair. The repulsion is fully maintained until mid-diaknesis, and then gradually diminishes to pro-metaphase, when the close proximity and general affinity of homologous chromosome results in groups of secondarily associated chromosomes at metaphase. The frequency and size of the group is characteristic for a given species. The importance of the diakinetic Phase in differentiating primary and secondary association is emphasised.4. The theory of secondary association is discussed and evidence from the literature presented. It is claimed that secondary association is evidence of more remote affinities than can ever be expressed in primary association in a poly-ploid. This is proved by the occurrence of true (primary) association in triploid Pyrus between chromosomes which only show secondary association in the diploid form.5. The effects of fixation, the correlation between allopolyploidy and secondary association and finally the value of secondary association as a criterion of homology are discussed.

Structural Variatin in the Chromosomes of Campanula persicifolia

CHROMOSOME behaviour at meiosis has been described in Campanula persicifolia to show the relationships of diploid, triploid, and tetraploid forms.1 The haploid number is eight. In the diploid, as a rule the chromosomes are associated at both ends forming rings, but occasionally association at one end gives a rod. In the tetraploid, the majority of the chromosomes still associate in pairs, but this is varied by the formation of rings and chains of four as well as by failure of pairing. In the triploid the third chromosome is occasionally associated with its homologues in a chain, but more usually free. The end-to-end association always found at metaphase follows terminalisation of from two to five interstitial chiasmata found at a diplotene stage characteristic of parasynapsis. This terminalisation accounts for the failure of quadrivalents by the cancellation of chiasmata.2 We may therefore speak of the chromosomes as being joined at metaphase by terminal chiasmata with an approximate average of two to each chromosome.

Chromosome Studies in Tulipa and some related Genera.

The basic number of chromosomes in Tulipa is twelve and diploid, tetraploid, and hexaploid varieties and species occur. Great differences in size of chromosomes are found among the diploid species. The size of the nucleus is correlated with the size and number of the chromosomes it contains. Correlation of nuclear and somatic size only holds between tetraploid and diploid forms of the same species. Tulipa galatica Fresn. has sixteen chromosomes of which four are very small and unrepresented in the most nearly allied species. The increased number is regarded as being due to transverse fragmentation. Differences between the species exist in the form of the chromosomes and in their size relative to one another, even where no difference of number is present. Two pairs of chromosomes with sub-median attachment are found in Lilium, Fritillaria, and in most species of Tulipa. The doubtful correctness of the position of Calochortus in the Tulipeæ is emphasized by the cytological differences that distinguis it from the other genera. The chromosome numbers found are seven, nine, and ten. Well-marked satellites occur in this genus and in conjunction with differences in size and attachment enable each chromosome in the haploid set to be distinguished individually. Calochortus Vesta, a tetraploid series, has four representatives of each of the seven kinds of chromosomes. In the species (T. primulina, T. australis, T. Orphanidea) especially studied, synapsis involves the lateral pairing the leptotene stage of threads representing whole chromosomes. Later the split separating the conjugants reappears, and each conjugant becomes longitudinally split giving a tetrad consisting of four chromatids, which become separated in the first and second meiotic divisions. The forms take on by the tetrads in diakinesis -rings, crosses, etc.–are regarded as being due to the opening out alternately of the reductional and equational splits. No cytological proof of the existence of crossing over has been found. This work has been carried out whilst holding a studentship of the John Innes Horticultural Institution, and I have to record my gratitude to the late Dr. Bateson for his unfailing interest and encouragement.

Sex in the Salicaceæ and its Modification by Eriophyid Mites and other Influences

ABSTRACT The species of the two genera Populus and Salix are ordinarily diœcious. In the whole of the British Salix species and hybrids examined a preponderance of female plants, slight in the diploid forms, but great in the polyploids, was discovered. In the diploids this excess, if significant, probably arises from the slower growth of pollen tubes carrying the male determining gametes. In the polyploids that influence is assisted (a) by the occurrence ofapomixis with thelytoky; (b) by pseudo-fertilisation with thelytoky; (c) by circumstances arising from peculiarities marking the chromosome complexes of polyploid diœcious plants. Hybrid Salices, when combinations of diploid species, invariably possess males. Other hybrids of which at least one parent is polyploid, are exclusively female as far as observation goes or, at best, produce but few males. These latter features may arise from circumstances detailed in (4), or there may be a deliberate and permanent switch-over of the heterogametic sex as in my Lycia-Nyssia crosses. Certain hybrids produce, in addition to plants of the normal male and female types, androgynous or monœcious forms. These are explained as arising through environmental influences temporarily switching the metabolism of a male plant over from its normal condition to that characteristic of the opposite sex. If that be the correct view the switch-over must occur, with a subsequent reversal, annually. In the Capreœ group intersexual florets occur. Such are induced by the attacks of parasites in the form of mites of the Eriophyid group, in particular of Epitriments salicobius. They act by effecting local changes in the metabolism of the plant. The phenomenon is thus one of parasitic castration. Many similar cases are known, for which, for the most part, fungi are responsible.