CHROMOSOMES AND CROSSING BEHAVIOR OF SOME SPECIES OF SANSEVIERIA

Menzel , Margaret Y. (Florida State U., Tallahassee), and James B. Pate . Chromosomes and crossing behavior of some species of Sansevieria. Amer. Jour. Bot. 47(3) : 230—238. Illus. 1960.–Approximately 20 species (28 clones) studied were diploids, tetraploids or hexaploids of the basic numbers x = 20; about 40% of the taxa were polyploid. All species had similar karyotypes, except for chromosome number. Five of 12 combinations of diploid species gave fertile F₁ hybrids; 4 studied cytologically showed 20 bivalents at metaphase I. Two triploid interspecific hybrids showed high trivalent frequencies. In contrast, multivalent formation in polyploid species was variable but rather low. Morphological relationships appeared reticulate among and between diploids and polyploids and did not coincide with barriers to crossing or to hybrid fertility. The following tentative hypothesis concerning relationships in the genus is proposed: Sansevieria is monophyletic and speciation has proceeded through genetic variation and hybridization at the diploid level and by allopolyploidy (of the segmental type) ; a low level of chromosome differentiation has accompanied speciation such that complete pairing occurs in diploid hybrids, but considerable preferential pairing occurs in allopolyploids. The occurrence of both polyploid and hybrid vigor, the fertility of hybrids between species differing greatly in morphology and physiology, and the high potential for vegetative propagation make the genus a favorable subject for breeding based on interspecific hybridization.     

HOMOEOLOGOUS CHROMOSOME PAIRING AND RESTRICTED SEGREGATION IN THE FERN CERATOPTERIS

The segregation of a marker characterized by pale green gametophytes was monitored within an inbreeding study of the polyploid fern Ceratopteris. Although all of the sporophytes showing segregation were derived from the self-fertilization of haploid gametophytes, a low overall frequency of 2.5% pale gametophytes was observed in the F₃–F₅ generations. A model based upon a duplicated locus and homoeologous chromosome pairing can explain the segregational behavior within the study. The overall level of homoeologous pairing was determined to be 10%. Occasionally, green gametophytes that were presumably heterozygous for the marker contained pale sectors. This behavior may involve mitotic crossing-over between homoeologous chromosomes.     

Genetic regulation of diploid-like chromosome pairing in the hexaploid species,Festuca arundinacea Schreb. and F. rubra L. (Gramineae)

The basis of diploid-like chromosome pairing in hexaploid (2n=6x=42) Festuca arundinacea Schreb. and hexaploid F. rubra L. has been investigated. On the combined evidence derived from chromosome pairing in some euploid (2n=42) and monosomic (2n=41) hybrids from a diallel set of crosses between ten geographically diverse ecotypes of tall fescue, intergeneric hybrids involving tall fescue as well as red fescue, and euploid (2n=56) and aneuploid (2n=52, 53, 54, 55) amphiploids between Lolium multiflorum and F. arundinacea, it is concluded that diploid-like meiosis in these hexaploid species as well as in other natural polyploid species of Festuca is under genetic control. It is further inferred that this diploidizing gene(s) system must at least be disomic in dosage to be effective in suppressing homoeologous pairing and, therefore, had no influence upon pairing in haploid complements of the hybrids, i.e., it is haplo-insufficient or hemizygous-ineffective. — It has also been shown that sterility in hybrids between some geographically isolated ecotypes of tall fescue results from irregular meiosis due to the breakdown of the regulatory mechanism, rather than from chromosomal differentiation of the parental ecotypes as widely believed so far. The evolutionary significance of such a gene-repressing effect of certain genotypes or genes is indicated. — It is further suggested that the hemizygous ineffectiveness of the genetic control of bivalent pairing is of evolutionary significance and could have major implications on the cytogenetic relationships and the breeding of the entire Lolium-Festuca complex.     

Transfer of Ph I genes promoting homoeologous pairing from Triticum speltoides to common wheat

Diploid-like chromosome pairing in polyploid wheat is controlled by several Ph (pairing homoeologous) genes with major and minor effects. Homoeologous pairing occurs in either the absence of these genes or their inhibition by genes from other species (Ph ^I genes). We transferred Ph ^I genes from Triticum speltoides (syn Aegilops speltoides) to T. aestivum, and on the basis of further analysis it appears that two duplicate and independent Ph ^I genes were transferred. Since Ph ^I genes are epistatic to the Ph genes of wheat, homoeologous pairing between the wheat and alien chromosomes occurs in the F₁ hybrids. Using the Ph ^I gene stock, we could demonstrate homoeologous pairing between the wheat and Haynaldia villosa chromosomes. Since homoeologous pairing occurs in F₁ hybrids and no cytogenetic manipulation is needed, the Ph ^I gene stock may be a versatile tool for effecting rapid and efficient alien genetic transfers to wheat.Contribution no. 93-435-J from the Kansas Agricultural Experiment Station, Kansas State University, Manhattan, KS 66506-5502, USA     

Chromosomes and polyploidy in Lenophyllum (Crassulaceae)

Gametic chromosome numbers of 22, 32, 33, and 44 in five species of Lenophyllum suggest that they may be polyploids on a basic 11, but this number has not been found. Three species have 8-12 distinctively large chromosomes that do not pair with each other in their hybrids and probably belong to the same genome. In hybrids of many polyploid Mexican Crassulaceae preferential pairing occurs between corresponding chromosomes of their multiple genomes, which indicates that they are autopolyploids. However, little or no preferential pairing occurs between chromosomes of Lenophyllum in its hybrids, and its species appear to be allopolyploids. The putative parents are unknown.     

NONHOMOLOGOUS CHROMOSOME PAIRING IN THE FERN CERATOPTERIS

The homosporous fern Ceratopteris thalictroides forms completely homozygous offspring sporophytes in one generation of selfing. Such homozygotes do not breed true and can exhibit structural heterozygosity for chromosomal mutations in some meiocytes. This is due to occasional nonhomologous (presumably homoeologous) chromosome pairing at meiosis in these polyploid ferns.     

HYBRIDIZATION STUDIES IN PERENNIAL ZINNIAS

Results of hybridization studies among the 6 species of subgenus Diplothrix (Zinnia-compositae) are reported. Also included are analyses of chiasmata frequency in the parental species and 2 diploid hybrids. Pairing relationships and chiasmata frequency of the chromosomes in the hybrids of the diploid species indicate their genomes are homologous. Analyses of F₂'s of Z. juni-perifolia X acerosa (2n) show that production of pigment in ray flowers, C, from Z. juniperifolia segregates as a simple dominant over colorless, cc, from Z. acerosa. Morphological studies of the hybrids produced to date indicate that the origin of the polyploid taxa has been more circuitous than simple hybridization followed by chromosome doubling. The genomic constitution of Z. juniperifolia is designated A₁A₁, that of Z. acerosa A₂A₂, and that of Z. oligantha A₃A₃. Morphological data and chromosomal pairing in the polyploid hybrids suggest that the tetraploid species should be tentatively assigned genomic formulae as follows: A₁A₁A₂A₂ or A₁A₁A₃A₃ for both Z. citrea, and Z. grandiflora, and A₂A₂A₃A₃ for 4n Z. acerosa.     

The genetics of meiotic chromosome pairing in Lolium temulentum x Lolium perenne tetraploids

Summary The degree of preferential pairing of homologous chromosomes was estimated in a series of tetraploid hybrids of Lolium temulentum x Lolium perenne by means of cytological and genetic analyses. The correlations between the frequency of bivalents at first metaphase of meiosis in the hybrid tetraploids and the degree of preferential pairing calculated from the segregation pattern of isozyme alleles in a test cross was extremely high. The results showed clearly that suppression of heterogenetic pairing in these Lolium tetraploids is achieved by a genetic system involving the A chromosomes as well as the B chromosome system which has been known for some time. Certain similarities with the genetic system controlling pairing in polyploid wheats are discussed.     

GENOME RELATIONS OF AGROPYRON SERICEUM,HORDEUM JUBATUM AND THEIR HYBRIDS

Cytogenetic investigation of microsporogenesis in Agropyron sericeum, Hordeum jubatum, their spontaneous hybrid, Agrohordeum pilosilemma, its amphiploid, and the backcross of the amphiploid to A. sericeum, B₁, elucidated the genome relationships of A. sericeum and H. jubatum. The tetraploid parental species share a partially homologous genome which affects the pairing relationships evidenced in their hybrids. The genome formulae assigned to these plants are: A. sericeum, A“A”BB; H. jubatum, AAA'A‘; Agrohordeum pilosilemma, AA'A“B; the amphiploid, AAA'A‘A”A“BB; and B₁, AA'A”A“BB. Observed pairing configurations were compatible with the expected maximum pairing configurations predicted under the assumption of genetic control of pairing with dosage effects. This is interpreted as further support for the hypothesis that pairing in the hybrids of H. jubatum is controlled by the A genome, one dose of A allowing homeologous pairing and two doses of A promoting homeologous association.     

VARIATION IN CHROMATOGRAPHIC PATTERNS IN THE TRAGOPOGON DUBIUS-PRATENSIS-PORRIFOLIUS COMPLEX (COMPOSITAE)

Natural populations of the diploid species, Tragopogon dubius, T. pratensis and T. porrifolius, the F₁ hybrids of these species, and the two amphidiploids, T. mirus and T. miscellus, were obtained from the same localities used for previous morphological and cytological studies of the evolutionary relationships of this complex. Inter- and intra-populational comparisons were made utilizing 2-dimensional chromatograms of these taxa. Members of the complex shared a group of 18 flavonoid-type pattern components, no one of which was completely species specific. Therefore, species patterns were expressed for a population rather than an individual with pattern components being expressed as a frequency. Patterns for F₁ hybrids and amphidiploids, expressed in the same manner, were relatable to the parental species. Diploid species and F₁ hybrid patterns were the most variable in areas of greatest sympatry, indicating considerable genetic interchange between species. The distribution of specific components in certain populations was interpreted as chemical introgression. Evidence was presented for separate origins of T. mirus in two localities. The results confirm previous interpretations of the evolutionary relationships in this complex, and when compared to those obtained for Baptisia, they implicate the different breeding systems as important factors in establishing the kind of variation observed.     

Genome differentiation in Magonoliaceae as revealed from meiotic pairing in interspecific and intergeneric hybrids

The cross compatibility within and between Yulania Spach and Michelia L.(Magnoliaceae) is relatively good and various such hybrids,obtained by conventional artificial hybridization,are available.The aim of the present study was to determine the extent of genome differentiation between the species involved in these crosses through the observation of chromosome pairing during meiosis in pollen mother cells (PMCs) of the hybrids.Chromosome pairing behavior was studied in five species (2n =38) and two interspecific hybrids of Michelia,eight species (2n =38,76 and 114) and 10 interspecific hybrids of Yulania,and three intergeneric hybrids between Michelia and Yulania.The results showed that chromosome pairing was normal with bivalent formation in diploid parental species and in interspecific hybrids.In addition to bivalents,multivalents were encountered in polyploid parental species and polyploid interspecific hybrids.In the intergeneric hybrids between a tetraploid Yulania and two diploid Michelia,19 chromosomes,most likely originating from Michelia,were unable to synapse from zygotene to metaphase I.Meiotic chromosome pairing indicated a high degree of homology between species within Michelia and Yulania and less homology between the genomes of these two genera.The differentiation of morphological characters and the distinctness of natural distribution also support the conclusion that these two genera are likely independent monophyletic groups.This suggests that the two genera were split at early evolution of Magnoliaceae and the overlapping characteristics in external morphology and internal structures of the two genera may be the result of parallel evolution or ancient common ancestry.     

Cytogenetics of the hybrid Gilia millefoliata × achilleaefolia

Summary Gilia millefoliata andG. achilleaefolia, two annual diploid (n=9) species ofPolemoniaceae, crossed readily in certain combinations but not in others. The F₁ hybrids were vigorous but sterile. They gave rise, apparently by the union of unreduced gametes, to an F₂ generation of tetraploids, which were mostly fertile.Chromosome pairing in the hybrids varied markedly according to the state of nutrition of the plants. The F₁ hybrids formed fewer clear diakinesis figures, fewer bivalents, fewer chiasmata per bivalent, and more attenuated or stretched bivalents when grown in 2 pots of sand than when grown in rich soil (Table 3). A pot-bound allotetraploid individual derived from this hybrid showed the same meiotic irregularities as the starved F₁s until irrigated with a solution of mineral nutrients, after which its chromosomes paired regularly in bivalents (Table 2, Fig. 38).The capacity of the F₁ hybrids to produce polyploids also differed strikingly in the two cultures. The rate of polyploidy of the stunted sand-grown hybrids was 2381 viable tetraploid zygotes per million flowers, while the corresponding figure for the luxuriant field hybrids was only 2.7 per million flowers.For the production of polyploid progeny by diploid parents — a process which should be clearly distinguished from normal fertility — the termpolyploidy rate is proposed. It is suggested that starvation of a structural hybrid may sometimes increase its polyploidy rate by reducing chromosome pairing to the point where restitution nuclei and hence unreduced gametes can be formed.     

Genomic analysis in the genus Aegilops.

Summary Hybrids between different Aegilops species and Secale cereale were studied at metaphase I by means of a C-banding technique. On the basis of differences in the C-banding patterns of some of the chromosomes of these hybrids it was possible to carry out an accurate analysis of several types of Aegilops-Aegilops and Aegilops-Secale chromosome associations and, consequently, to establish intraspecific and intergeneric genome relationships. Genomes present in the majority of polyploid Aegilops species are shown to maintain similar patterns of evolutionary affinity to those reported for their proposed diploid parents although in some species there are differences indicating either that differentiations occurred during the evolution of the polyploid species or, on the contrary, that the diploid donors proposed are not the correct ones. On the other hand, differences in the relationships not only between the R genome and different Aegilops genomes but also among different homoeologous groups have been found.     

Interspecific hybridization and the analysis of meiotic chromosome pairing inDahlia (Asteraceae — Heliantheae) species with x = 16

The successful production of a large number of artificial hybrids betweenDahlia species based on x = 16 has allowed a detailed study of their genomic relationships. Chromosome behaviour in these artificial hybrids was extremely similar to that observed in parental species suggesting that there is a considerable degree of homology between the genomes of theseDahlia species. Using GISH it can be demonstrated that in these hybrids bivalent formation involved pairing only between parental genomes. The ability of GISH to differentiate between parental genomes in artificial hybrids was variable, indicating that molecular divergence of highly repeated sequences has accompanied the evolution of these species. However, the extent of chromosome pairing and chiasma formation in the hybrids does not reflect the differences that can be detected by GISH. Seyeral of the new hybrid combinations have resulted in horticulturally interesting plants.     

Use of <Emphasis Type="Italic">2n</Emphasis> gametes for the production of sexual polyploids from sterile Oriental × Asiatic hybrids of lilies (<Emphasis Type="Italic">Lilium</Emphasis>)

Sixteen Oriental and 12 Asiatic cultivars were crossed in 158 different combinations. A total of 708 F₁ hybrids were obtained from 86 of the different combinations of 15 Oriental and 11 Asiatic cultivars. Because the Lilium cultivars (2n=2x=24) used for the production of these hybrids belong to two different taxonomic sections—Archelirion (O) and Sinomartagon (A), respectively—the F₁ hybrids (OA) could be obtained only through embryo, embryo sac rescue, ovary slice or ovule culture. Most of the F₁ hybrids were highly sterile (did not produce viable n gametes) due to the failure of chromosome pairing. However, in a few cases F₁ plants were found that produced viable 2n pollen at variable frequencies. These 2n pollen grains were successfully used for the production of backcross progenies. Using genomic in situ hybridization we found intergenomic recombinant chromosomes in the sexual polyploid progenies. These results indicate that there are effective prospects for combining important horticultural traits from the two main groups of cultivars of lilies through sexual polyploidization.     

The effect of Ph gene alleles on synaptonemal complex formation in Triticum aestivum × T. kotschyi hybrids

Summary Chromosome pairing at zygotene-pachytene was studied in Triticum aestivum × T. kotschyi hybrids (2n=5x=35, genomic constitution ABDC^US^v) by electron microscopy of synaptonemal complexes in spread microsporocyte nuclei. Hybrids carrying either the Ph allele or the ph allele, which differ markedly in metaphase I pairing, are both capable of greater than 90% pachytene pairing, although pairing in the Ph hybrids appeared slower or less synchronised. In both genotypes branched synaptonemal complexes were formed by intra-and interchromosomal pairing. The Ph gene control on homoeologous pairing does not act on the ability to pair into synaptonemal complexes. It may act on the rate of pairing or the time of crossing over.     

POLYPLOIDY AND DIPLOIDY: NEW PERSPECTIVES ON CHROMOSOME PAIRING AND ITS EVOLUTIONARY IMPLICATIONS

The last widely accepted classification of polyploidy recognized three major categories: autoploidy, segmental alloploidy, and true alloploidy. Criteria for recognizing these types were based largely on chromosome pairing in F₁ hybrids, but until very recently there were no quantitative criteria to distinguish autoploids from segmental alloploids. This is theoretically possible, and Jackson and Casey (1982) and Jackson and Hauber (1982) have presented models and methods to quantitatively predict the frequencies of the different meiotic configurations in autotriploids through autooctoploids. The expected numbers of configurations then can be statistically tested for goodness of fit with observed data. Synthetic and natural autotetraploids have been shown to fit the models well. However, the exceptions that do not fit are of particular interest because they indicate the presence of Ph-like (Pairing homoeologous) genes that cause fewer multivalents and more bivalents than expected in the autoploid models. The effect of Ph-like genes at the nuclear level apparently involves placement of chromosome attachment sites on the nuclear membrane such that different genomes occupy slightly different sites. This would be far enough apart in true alloploids so that no pairing occurred in the original F₁ hybrid. In true autoploids and normal diploids, homologues are equally close so that pairing obeys probability laws that make it possible to describe expected pairing events with appropriate models and equations. From the foregoing statements, one can deduce that the terms autoploid, segmental alloploid, and true alloploid need not and indeed should not be equated with results of crosses between any taxonomic levels. Rather, they represent conditions that can be attained by one or very few genes acting on nuclear membrane attachment sites. It is possible to deduce from hybrids at the diploid level the type of polyploid that can be obtained, and such data can have great agronomic value. A generally held dogma is that the inability of chromosomes to pair in F₁ hybrids is due to considerable genetic divergence and perhaps numerous structural changes at the light microscope and/or ultrastructural level. Data from various sources show that this need not be true, and there is no reason to believe this is a factor in what has been called genome divergence in the classical sense. Genome divergence in terms of pairing ability can occur at the population level due to one or a few genes.     

ABNORMAL REDUCTIONAL AND NONREDUCTIONAL MEIOSIS IN CERATOPTERIS: ALTERNATIVES TO HOMOZYGOSITY AND HYBRID STERILITY IN HOMOSPOROUS FERNS

Natural and synthesized hybrids of Ceratopteris were investigated to determine the effect of hybridization on the genetic system. Studies indicated that the hybrids exhibited massive spore abortion and pairing abnormalities at meiotic prophase, characteristic of “sterile diploids and triploids” reported in hybridization studies of other fern genera. However, a small percentage of viable spores also was produced by the hybrids. Cytological investigations indicated the presence of previously unreported meiotic adaptations that allowed the production of unreduced spores and reduced spores exhibiting chromatid heterozygosity. The reduced spores allow haploid gametophytes to form heterozygous zygotes in spite of intragametophytic selfing. The unreduced spores were shown to be responsible for the fertility of the “sterile” hybrid and allowed the subsequent production of up to three generations of sporophytes. The literature suggests that these meiotic adaptations are present in other fern genera and may play a significant role in evolution through hybridization.     

Chromosomal polymorphism in Festuca arundinacea

Within F. arundinacea ten exotic populations each crossed with an indigenous bred form S. 170, and their hybrids, were grouped into fertile, partly fertile and sterile categories. Four were fertile hybrids with bivalent pairing (20.75 to 20.99/cell) and high pollen fertility. Two were partly fertile with univalents, and bivalents (18.26 to 19.54/cell) and rarely multivalents. The pollen fertility was low. Four hybrids were completely sterile with low or nil pollen fertility. The bivalent formation was low (12.94 to 14.25/cell) while the frequency of univalents and multivalents was high. There was evidence of structural changes of both gross (multivalents formation due to translocation; univalents due to inversions and deletions) and cryptic types from these hybrids. This diversity in the chromosomal constitution of several populations could be the result of several factors, including wide geographical distribution, climatic and edaphic diversity of the population, polyploid nature of the species, restriction of the gene pool and successful mode of vegetative propagation. In some of the populations an isolating mechanism has been effectively established and may well represent an initial stage in speciation.     

THE ORIGIN AND RELATIONSHIPS OF LASTHENIA BURKEI (COMPOSITAE)

Lasthenia burkei (Compositae) is a narrowly restricted California endemic closely related to L. conjugens and L. fremontii. These three species differ from each other by pappus and phyllary characters and in geographical distribution. All are freely intercrossable, but L. fremontii forms rather sterile artificial hybrids with its two relatives which, in turn, form fairly fertile artificial hybrids with each other. Lasthenia burkei and L. conjugens have homologous chromosomes, four of which are homologous with four of those of L. fremontii. The remaining two chromosomes probably have reciprocal translocations which lead to multivalent formation during meiosis in interspecific hybrids. Pollen viability is restored in most F₂ generations, suggesting a close genetic relationship among the three species. The evolutionary relationship among these species may be a linear one with L. burkei occupying an intermediate position between L. fremontii and L. conjugens, although the direction of this linear phylogeny is not certain, or it may be one in which L. burkei has been derived from hybridization between its two relatives. Support for the latter hypothesis comes from the appearance of some individuals in F₁ progenies of L. conjugens × L. fremontii that are morphologically indistinguishable from L. burkei (although fairly sterile). The apparently rather simple genetic basis for the morphological characteristics of each of the species in this trio suggests that the morphologically heterogeneous genus Lasthenia may be considerably more homogeneous genetically than might be suspected. Because of the diverse kinds of relationships among these three Lasthenias, possible alternative taxonomies for the group are dependent upon those relationships that a taxonomist wishes to communicate. Nevertheless, the patterns of diversification in this group have led to reaffirmation of an earlier decision that three species should be recognized.