Genome relationships between Hordeum vulgare L. and H. bulbosum L.

Interrelationships between H. vulgare (2x=14) and H. bulbosum (2x=14; 4x=28) were estimated on the basis of the karyotypes and the pairing behaviour of the chromosomes in diploid, triploid and tetraploid hybrids obtained with the aid of embryo culture. — A comparison of the karyotypes of the two species revealed similarities as well as differences. It was concluded that at least 4 or more of the chromosomes were similar in morphology and probably closely related. — Diploid and tetraploid hybrids are rarely obtained and their chromosome numbers tend to be unstable whereas triploid hybrids (1 vulgare + 2 bulbosum genomes) were stable and relatively easy to produce. In the diploid hybrid only 40% of the meiotic cells contained 14 chromosomes while the numbers ranged from 7 to 16 in other cells. All hybrids exhibited pairing between the chromosomes of the two species. Diploid hybrids had a mean of 5.0 and a maximum of 7 bivalents per cell in those cells having 14 chromosomes. Triploid hybrids from crosses between 2x H. vulgare and 4x H. bulbosum exhibited a mean of 1.5 and a maximum of 5 trivalents per cell. In a hexaploid sector found following colchicine treatment of a triploid the mean frequencies of chromosome associations per cell were: 5.5_I+8.0_II+0.7_III+3.7_IV+0.3_V+0.4_VI. One unstable 27 chromosome hybrid obtained from crosses between the autotetraploid forms had a mean of 1.1 and a maximum of 4 quadrivalents per cell. The chromosome associations observed in these hybrids are consistent and are taken as evidence of homoeologous pairing between the chromosomes of the two species. Interspecific hybridization between these two species also reveals that chromosome stable hybrids are only obtained when the genomes are present in a ratio of 1 vulgare2 bulbosum. Based upon the results obtained, the possibility of transferring genetic characters from H. bulbosum into cultivated barley is discussed.     

The effect of B chromosomes on meiotic and pre-meiotic spindles and chromosome pairing in Triticum/Aegilops hybrids

Diploid populations of Aegilops mutica and Aegilops speltoides containing B chromosomes have been used as male parents in crosses with aneuploid genotypes of Triticum aestivum to investigate the effect of B chromosomes on meiotic homologous and homoeologous chromosome pairing. F₁ hybrids of T. aestivum/Ae. mutica and T. aestivum/Ae. speltoides segregated into four classes with regard to the degree of meiotic chromosome pairing, irrespective of the presence of B chromosomes. The B chromosomes do not introduce factors altering the level of pairing other than that due to the natural allelic and gene variation occurring in the diploids. Similarly no reduction in pairing of homologous chromosomes was observed in genotypes in which pairs of homologues co-existed with B chromosomes. However, a significant drop in chiasma frequency was observed in F₁ hybrids of T. aestivum × Ae. mutica with B chromosomes and T. aestivum × Ae. mutica nullisomic for wheat chromosome 5D with B chromosomes, in temperature regimes of 12° C. No asynapsis occurred in similar hybrids in the absence of Mutica B chromosomes at low temperatures. The low-temperature sensitive phase lies early in the pre-meiotic interphase. In this instance the Mutica B chromosomes are interacting with specific gene loci of the A chromosomes. Synaptic pairing has been observed between A and B chromosomes in Ae. mutica. A high frequency of pollen mother cells with twice the number of chromosomes was observed in hybrids in the presence of Mutica B chromosomes due to failure of spindle formation at the last pre-meiotic mitosis. Meiotic spindle irregularities occurred in hybrids containing Speltoides B chromosomes. Hybrids of Ae. speltoides + B's X Ae. mutica + B's displayed the mitotic and meiotic spindle abnormalities introduced by the presence of the B chromosomes of each parent.     

The pattern of polytene chromosome conjugation and crossing-over in interspecific hybrids of Drosophila

Summary The pairing of polytene chromosomes was investigated in the hybrids between three closely related species of Drosophila belonging to the virilis species group. It was found that within the same hybrid different chromosome bands lost the ability to pair by differing degrees. Furthermore, the same chromosome sections paired with different frequencies depending on the hybrid involved. This study revealed that poor polytene chromosome pairing in the hybrids is not due to specific genetic interaction in the hybrids, but depends solely on the properties of the homologous loci themselves. It was also of interest to find whether the pattern of polytene chromosome somatic pairing resembled in some way the picture of chromosome synapsis during meiosis. To obtain evidence for this, crossing-over in the hybrid 5th chromosome was analyzed both genetically and cytologically (from salivary gland chromosome observations). It was found that the sections of the fifth chromosome which were characterized by a high frequency of conjugation in the salivary glands of hybrids also exhibited a high frequency of crossing-over in hybrid females. It may be concluded that sections of the polytene chromosome characterized by a low frequency of conjugation behave in the same manner in meiosis, and thus rarely take part in genetic recombination.     

The effect of B chromosomes on homoeologous pairing in species hybrids

The effect of B chromosomes on chromosome pairing at meiosis was investigated in the species hybrid Lolium temulentum x L. perenne at both the diploid and tetraploid level. The presence of B chromosomes drastically reduced association of homoeologous chromosomes in both the diploids and tetraploids. This was evident from the high frequency of univalents recorded in PMC's of diploid hybrids with B's and from the predominantly bivalent association of homologous chromosomes in tetraploids of this type. In the absence of B's homoeologous pairing was extensive giving a high frequency of bivalents in the diploids and multivalents as well as bivalents and univalents in the tetraploids.     

Conserved autonomy of replication of the X-chromosomes in hybrids of Drosophila miranda and Drosophila persimilis

The chromosome complement of hybrid males from the cross between Drosophila miranda female and D. persimilis male provides an interesting chromosomal situation where an autosome, the 3rd chromosome of D. persimilis, coexists with a homologue that developed into a sex chromosome, the X₂ in D. miranda. Except for certain inversions and a few minor translocations, these two chromosomes (X₂ and the 3rd) still look alike as polytene elements. However, in hybrid males pairing of the two chromosomes, the X₂ and 3rd, is rare, while in female hybrids it occurs frequently. — ³H-TdR labeling shows that while the X₂ and 3rd chromosomes replicate synchronously in hybrid female, in the hybrid male the former completes its replication earlier than the 3rd chromosome, as do the two arms of the X₁ (XL and XR). The frequency and relative intensity of ³H-TdR labeling of each site of the X₂ and that of the 3rd chromosome in hybrid males closely agree with those of the corresponding sites in the X₂ of the miranda male and the 3rd chromosome of the persimilis male (or female), respectively. The results suggest that timing and rate of replication of the X₂ are determined autonomously and follow the pattern in the respective parental species.     

The Alteration of the Chromosome Structure in Ovarian Nurse Cells of Drosophila melanogaster upon Hybrid Dysgenesis

The impact of hybrid dysgenesis on the chromosome structure of Drosophila melanogaster ovarian nurse cells was studied. In the examined lines and interlinear hybrids (including those yielded by dysgenic crosses in the P–M and I–R systems of hybrid dysgenesis), disturbed chromosome synapsis was revealed. The disturbance was somewhat similar to that observed in interspecific hybrids. Quantitative analysis showed that the mean frequency of nuclei with defective chromosome pairing ranged from 60.4 to 76%. FISH analysis of ovarian nurse chromosomes of Canton S × Berlin hybrids showed differences in the label localization in asynaptic homologs of arm 2L, which probably results in disrupted homolog pairing and reveal interlinear differences in localization of mobile genetic elements. Our results conform to Sved's model stating that hybrid dysgenesis is based on disorganization of the germline nuclear space.     

Homoeologous relationships of Aegilops speltoides chromosomes to bread wheat

 Homoeologous pairing at metaphase I was analysed in the standard-type, ph2b and ph1b hybrids of Triticum aestivum (AABBDD) and Aegilops speltoides (SS). Data from relative pairing affinities were used to predict homoeologous relationships of Ae. speltoides chromosomes to wheat. Chromosomes of both species, and their arms, were identified by C-banding. The Ae. speltoides genotype carried genes that induced a high level of homoeologous pairing in the three types of hybrids analyzed. All arms of the seven chromosomes of the S genome showed normal homoeologous pairing, which implies that no apparent chromosome rearrangements occurred in the evolution of Ae. speltoides relative to wheat. A pattern of preferential pairing of two types, A-D and B-S, confirmed that the S genome is very closely related to the B genome of wheat. Although this pairing pattern was also reported in hybrids of wheat with Ae. longissima and Ae. sharonensis, a different behaviour was found in group 5 chromosomes. In the hybrids of Ae. speltoides, chromosome 5B-5S pairing was much more frequent than 5D-5S, while these chromosome associations reached similar frequencies in the hybrids of Ae. longissima and Ae. sharonensis. These results are in agreement with the hypothesis that the B genome of wheat is derived from Ae. speltoides. Received: 8 January 1998 / Accepted: 4 February 1998     

Inheritance of Arabidopsis DNA in offspring from Brassica napus and A. thaliana somatic hybrids

 Chromosome counts and RFLP markers mapped to Arabidopsis thaliana were used to determine the proportion of eliminated chromosomes and retained A. thaliana DNA in the back-crossed (BC) progeny derived from symmetric and asymmetric somatic hybrids between Brassica napus and A. thaliana. All plants were analysed for the presence of two RFLP markers per chromosome, preferably with one located on each chromosome arm. A reduction in both A. thaliana RFLP markers and chromosome numbers was found in the BC₁ and BC₂ generations of the symmetric hybrids as well as in the BC₁ generation of the asymmetric hybrids. In the symmetric hybrids, two back-crosses to B. napus were required to reduce the frequency of retained A. thaliana loci to 42.4% and mean chromosome number to 39.4. In comparison, the BC₁ progeny of the asymmetric hybrids had 16% of the analysed A. thaliana loci present and an average of 38.4 chromosomes maintained. When the frequency of A. thaliana chromosomes with both analysed loci maintained was compared with the frequency of chromosomes with one locus lost and one kept, a reduction in the number of complete chromosomes between BC₁ and BC₂ derived from the symmetric hybrids was observed. Among the BC₁ plants in the asymmetric group the situation was different, with higher amounts of incomplete donor chromosomes compared to whole chromosomes. The results indicate that A. thaliana chromosome fragments are more often found in the progeny of irradiated hybrids, while back-crossed symmetric hybrids have more complete chromosomes. Received: 2 April 1998 / Accepted: 14 July 1998     

Production and cytogenetics of hybrids of Triticum aestivum x Leymus innovatus

Summary Hybrid plants were obtained between Triticum aestivum (2n=6x=42, AABBDD) and Leymus innovatus (2n=4x=28, JJNN) at a frequency varying from 0.4% to 1.2% of the pollinated florets. Improvement of the embryo culture medium resulted in a higher frequency of embryo rescue. Eight of ten hybrids had the expected chromosome number of 35 (ABDJN). Meiotic analysis indicated that there was no homology between the genomes of the two species. Two hybrids had only 28 chromosomes. Comparison of chromosome pairing between the two types of hybrids suggested that Leymus innovatus carries genes that affect chromosome pairing and behavior. The relatively high occurrence of spontaneous doubling in the meiocytes of these hybrids may indicate that backcrossing of the hybrids to wheat should be possible, although frequent chromosome irregularities observed in the meiocytes of the hybrids may decrease the probability of success of this step, which is essential to the process of gene transfer from L. innovatus to wheat.Contrib. no. 366     

THE PROBLEM OF PLOIDY IN ECHEVERIA (CRASSULACEAE) I. DIPLOIDY IN E. CILIATA

The largely Mexican genus Echeveria is characterized by an extensive series of dysploid chromosome numbers, with every gametic number from 12 to 34 known in at least one species. Within this nearly three-fold range of numbers, the boundary between diploidy and tetraploidy is not immediately apparent. However, species of Echeveria can be hybridized in an extraordinary number of combinations, both among themselves and with related genera, and study of the morphology of the hybrids and the pairing of their chromosomes provides information that helps to identify the ploidy of the parents. This paper reports observations from study of 80 hybrids between E. ciliata (n = 25) and 73 other species and/or cytotypes. Hybrids between E. ciliata and definite diploids are all nicely intermediate morphologically, whatever the chromosome numbers. In these same hybrids, most chromosomes become involved in pairing at meiosis, and the number of paired elements (bivalents and multivalents) approaches or equals, but never exceeds, the number of chromosomes received from the lower-numbered parent. In most cells, relatively few univalents are present, sometimes none. These observations are considered to indicate that all paired elements include at least one chromosome from each parent and therefore that pairing occurs between chromosomes of different parents only (allosyndesis). Since none of the 25 gametic chromosomes of E. ciliata is able to pair with any other, although they do pair very extensively with chromosomes from many other species having a wide range of numbers, E. ciliata is considered to be diploid in spite of its relatively high chromosome number. On the other hand, hybrids of E. ciliata with definite polyploids resemble the latter much more closely in their morphology, and at meiosis most or all pairing occurs by autosyndesis between chromosomes received from the polyploid parent, while the chromosomes from E. ciliata generally remain unpaired. In these respects most, but not all, species of Echeveria having as many as 34 gametic chromosomes have the same properties as E. ciliata and also are considered to be diploid. The ancestral chromosome number in the genus is not clear, but it is probably near the upper end of the series of dysploid numbers.     

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.     

Pairing and recombination at meiosis of Brassica rapa (AA) × Brassica napus (AACC) hybrids

Interspecific crosses contribute significantly to plant evolution enabling gene exchanges between species. The efficiency of interspecific crosses depends on the similarity between the implicated genomes as high levels of genome similarity are required to ensure appropriate chromosome pairing and genetic recombination. Brassica napus (AACC) is an allopolyploid, resulting from natural hybridization between Brassica rapa (AA) and Brassica oleracea (CC), both being diploid species derived from a common ancestor. To study the relationships between genomes of these Brassica species, we have determined simultaneously the pairing and recombination pattern of A and C chromosomes during meiosis of AAC triploid hybrids, which result from the interspecific cross between natural B. napus and B. rapa. Different AAC triploid hybrids and their progenies have been analysed using cytogenetic, BAC-FISH, and molecular techniques. In 71% of the pollen mother cells, homologous A chromosomes paired regularly, and usually one chromosome of each pair was transmitted to the progeny. C chromosomes remained mainly univalent, but were involved in homoeologous pairing in 21.5% of the cells, and 13% of the transmitted C chromosomes were either recombined or broken. The rate of transmission of C chromosomes depended on the identity of the particular chromosome and on the way the hybrid was crossed, as the male or as the female parent, to B. napus or to B. rapa. Gene transfers in triploid hybrids are favoured between A genomes of B. rapa and B. napus, but also occur between A and C genomes though at lower rates.     

Genomic divergence of allopatric sibling species studied by molecular cytogenetics of their F1 hybrids

Despite their similar karyotype morphology and close taxonomic affinity, the genomes of allopatric sibling species, Gibasis karwinskyana and Gibasis consobrina, are clearly distinguished in metaphases of their F₁ hybrids by genomic in-situ hybridization (GISH). The reduced ability of chromosomes from one species to bind labelled total DNA from the other involves almost the whole chromosome complement, and is equally pronounced in euchromatin and heterochromatin. The only region strongly conserved in the two species is an AT-rich band proximal to each nucleolus organizer. Molecular differentiation is accompanied by chromosome pairing failure in the F₁ interspecific hybrids, although the reason remains open to question. The two species also differ in their numbers of detectable sites for rRNA genes. The greater number of such sites in G. consobrina may be linked with a propensity for interchange heterozygosity. The ability to discriminate rapidly and reliably between the chromosomes of close relatives with almost identical karyotypes makes GISH invaluable in preliminary studies of phytogeny. Detection of even small conserved chromosome bands using GISH confirms the sensitivity of the technique and demonstrates its potential use in evolutionary cytogenetics. This will allow rapid re-evaluation of many important genetic systems exposed by classical cytogenetics in previous decades.     

The genotypic control of homoeologous chromosome association in Lolium temulentum × Lolium perenne interspecific hybrids

Two contrasting genotypes of Lolium perenne and two inbred lines of L. temulentum were examined with regard to their effect on homoeologous chromosome pairing in interspecific hybrids derived from them. Substantial differences in chiasma frequency were observed between the hybrid progeny of the different parental types. The background genes involved were found to operate in the presence and in the absence of B chromosomes. The combination of A chromosome genes present in some of the 0B hybrids was found to result in a considerable suppression of chiasma formation at the diploid level, and the restriction of pairing to strict homologues at the tetraploid level. It appears, therefore, that genes are present within the diploid species of the genus Lolium which are capable of performing a function similar to that of the Ph locus in wheat.     

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.     

Hybrids between tetraploid Italian and perennial ryegrass

Summary Hybridization frequency was investigated between tetraploid perennial and Italian ryegrass (Lolium perenne X multiflorum) without emasculation by using genetic markers. The Italian phenotypes, fluorescentroots and awned florets, were dominant. About 82% of the plants in perennial X Italian and nearly 93 % of the plants in the reciprocal crosses were hybrids. The hybrids had a high multivalent frequency and involved homoeologous chromosome pairing. Aneuploids with 2n = 26, 27 and 29 chromosomes were present. The hybrids were highly fertile. The cytogenetic behaviour of these allopolyploids suggested that the genomes of the parental species have undergone little repatterning and have free genetic exchanges. The species maintained their self-incompatibility and cross-compatibility at the tetraploid level.     

Molecular cytogenetic evidence for a high level of chromosome pairing among different genomes in Triticum aestivum–Thinopyrum intermedium hybrids

Intergeneric hybrids (ABDJJ^sS genomes) were made between Triticum aestivum cv. Chinese Spring (CS) and Thinopyrum intermedium. Genomic in situ hybridization (GISH) using genomic DNA probes from Pseudoroegneria libanotica (Hackel) D.R. Dewey (genome S, 2n = 14) was used to study chromosome pairing among J, J^s, S and wheat ABD genomes in the hybrids. It was shown that in the hexaploid (ABDJJ^sS) hybrids, high pairing occurred among wheat chromosomes and among Thinopyrum chromosomes. A closer relationship was observed among the three genomes of Th. intermedium than among the three genomes of T. aestivum. It was further discerned that S genome chromosomes paired with J- and J^s-genome chromosomes at a high frequency. The frequency of heterologous pairing between S and J or S and J^s chromosomes was higher than those between J and J^s chromosomes, indicating that the S-genome was more closely related with these two genomes. Our results provided direct molecular cytogenetic evidence for the hypothesis that S-genome chromosomes are genetically similar to the J-genome chromosomes and, therefore, genetic exchange between these genomes is possible. The discovery of a close relationship among S, J and J^s genomes provides valuable markers for molecular cytogenetic analyses using S-genomic DNA probes in monitoring the transfer of useful traits from Thinopyrum species into wheat. Received: 23 August 2000 / Accepted: 5 September 2000     

Intraspecific hybridization and its bearing on chromosome evolution inVicia narbonensis (Fabaceae)

Nine accessions ofVicia narbonensis, considered to be the wild progenitor of faba bean (Vicia faba), were investigated to ascertain the nature and extent of intraspecific karyotypic polymorphism. The chromosome complements resolved into four distinct types (A, B, C, D), and the meiotic data of F₁ hybrids (A × B, B × C, A × C) revealed that alteration in chromosome morphology is the result of segmental interchanges. The interchange complexes indicate that the parents differ from each other by 1 to 2 interchanges. It is also evident that karyotype B, and not A as previously reported, is the normal karyotype of the species, and A and C are single homozygotes for unequal interchange. The comparative karyomorphology of the parents and the hybrids, and of two interchange heterozygotes of four chromosomes each in F₁ hybrids of A × C shows that the chromosomes involved in the single interchange homozygotes (A, C) are not common and the breaks in both interchanges occurred in short and long arms of the involved chromosomes. Identification of the interchanged chromosomes in the complements and the frequency of ring and chain quadrivalents in the heterozygotes enabled location of the breakpoints. The present results provide probably the first example indicating that interchange homozygosity (A) is not only firmly established but also has enabled the species to spread further by adapting to a wide range of habitats. — The genetic relationships between A and D are very different. All seven chromosome pairs in D could be distinguished from A, and for that matter, B and C as well. From the meiotic pairing properties it is also amply clear that genome D is well differentiated from A and possibly B, and C, and deserves special status.     

Direct evidence for high level of autosyndetic pairing in hybrids of Thinopyrum intermedium and Th. ponticum with Triticum aestivum

 Genomic in situ hybridization (GISH) was used to distinguish autosyndetic from allosyndetic pairing in the hybrids of Thinopyrum intermedium and Th. ponticum with Triticum aestivum cv ‘Chinese Spring’ (CS). All hybrids showed high autosyndetic pairing frequencies among wheat chromosomes and among Thinopyrum chromosomes. The high autosyndetic pairing frequencies among wheat chromosomes in both hybrids suggested that Th. intermedium and Th. ponticum carry promoters for homoeologous chromosome pairing. The higher frequencies of autosyndetic pairing among Thinopyrum chromosomes than among wheat chromosomes in both hybrids indicated that the relationships among the three genomes of Th. intermedium and among the five genomes of Th. ponticum are closer than those among the three genomes of T. aestivum. Received: 19 September 1996 / Accepted: 18 April 1997     

Spontaneous hybridization between a male-sterile oilseed rape and two weeds

Spontaneous interspecific hybrids were produced under natural conditions (pollination by wind and bees) between a male-sterile cybrid Brassica napus (AACC, 2n = 38) and two weeds Brassica adpressa (AdAd, 2n = 14) and Raphanus raphanistrum (RrRr, 2n = 18). After characterization by chromosome counts and isozyme analyses, we observed 512 and 3 734 inter-specific seeds per m² for the B. napus-B. adpressa and B. napus-R. raphanistrum trials respectively. Most of the hybrids studied had the expected triploid structure (ACX). In order to quantify the frequency of allosyndesis between the genomes involved in the hybrids, their meiotic behavior was compared to a haploid of B. napus (AC). For the B. napus-B. adpressa hybrids, we concluded that probably no allosyndesis occurred between the two parental genomes, and that genetic factors regulating homoeologous chromosome pairing were carried by the B. adpressa genome. For the B. napus-R. raphanistrum hybrids, high chromosome pairing and the presence of multivalents (in 9.16% of the pollen mother cells) indicate that recombination is possible between chromosomes of different genomes. Pollen fertility of the hybrids ranged from 0 to 30%. Blackleg inoculation tests were performed on the three parental species and on the interspecific hybrids. BC₁ production with the weeds and with rapeseed was attempted. Results are discussed in regard to the risk assessment of transgenic rapeseed cultivation, F₁ hybrid rapeseed variety production, and rapeseed improvement.