RFLP marker analysis supports tetrasonic inheritance in Lotus corniculatus L.

Lotus corniculatus is a tetraploid (2n=4x=24) perennial forage legume and has been reported to have tetrasomic inheritance for several traits, although it has also been reported to show disomic inheritance. Molecular markers were used to clarify whether tetrasomic inheritance, disomic inheritance, or a combination of both, was found within an F₂ population arising from a cross between two diverse L. corniculatus accessions. The inheritance of ”tetra-allelic” RFLP markers (markers with four segregating bands) indicated that disomic inheritance could not account for the phenotypic F₂ classes observed, and that only tetrasomic inheritance would explain the observed results. Goodness of fit tests for ”tetra-allelic” and ”tri-allelic” (three segregating bands) RFLP marker data suggested support for chromosomal-type tetrasomic inheritance. RFLP genotypes interpreted from autoradiographic signal intensity provided additional support for tetrasomic inheritance and the occurrence of preferential pairing between parental chromosomes. Bivalent pairing was predominant in the two parental lines and their F₁ hybrid in cytological analyses. L. corniculatus has been classified as both an autotetraploid and an allotetraploid species. RFLP evidence of tetrasomic inheritance gives support for L. corniculatus being classified as an autotetraploid species. Even though bivalent pairing occurs, as seen in other autotetraploid species, pairing between any of the four homologous chromosomes is possible. Preferential pairing in the F₁ hybrid suggests that genome differentiation appears to be minimal between homologs within an accession, while genome differentiation is greater between homologs from different accessions of this genetically diverse species. Received: 16 November 1999 / Accepted: 14 July 2000     

Chromosome pairing in haploids of Brassica campestris

Summary The maximum chromosome pairing observed in haploids of Brassica campestris was two bivalents plus one trivalent but differences were observed in the chromosome pairing frequencies of the four haploids studied. This pairing supports the theorem that the species is hexasomic for one chromosome, tetrasomic for two and disomic for three others but it is emphasized that some of the observed pairing might be explained by a phenomenon other than homology.     

Inheritance in tetraploid yeast revisited: segregation patterns and statistical power under different inheritance models

In their recent article, Albertin et al. (2009) suggest an autotetraploid origin of 10 tetraploid strains of baker’s yeast (Saccharomyces cerevisiae), supported by the frequent observation of double reduction meiospores. However, the presented inheritance results were puzzling and seemed to contradict the authors’ interpretation that segregation ratios support a tetrasomic model of inheritance. Here, we provide an overview of the expected segregation ratios at the tetrad and meiospore level given scenarios of strict disomic and tetrasomic inheritance, for cases with and without recombination between locus and centromere. We also use a power analysis to derive adequate sample sizes to distinguish alternative models. Closer inspection of the Albertin et al. data reveals that strict disomy can be rejected in most cases. However, disomic inheritance with strong but imperfect preferential pairing could not be excluded with the sample sizes used. The possibility of tetrad analysis in tetraploid yeast offers a valuable opportunity to improve our understanding of meiosis and inheritance of tetraploids.     

Evidence for a Polymorphism in Gametic Segregation Using a Malate Dehydrogenase Locus in the Tetraploid Treefrog HYLA VERSICOLOR

Artificial cross combinations of tetraploid Hyla versicolor were analyzed electrophoretically using a polymorphic malate dehydrogenase locus (MDH-1) to determine the mechanism of chromosome segregation. Models for differentiating between disomic and tetrasomic inheritance are presented and tested. In some crosses progeny genotypes fit a disomic mode of segregation. In other crosses there is only evidence for a tetrasomic mode of segregation. Additional crosses produced genotypic ratios which conformed to either a disomic or tetrasomic mode of segregation. The same type of inheritance was demonstrated for any individual when used in multiple cross combinations. These results suggest that there exists in H. versicolor a polymorphism with respect to segregation of gametes, resulting from differences in chromosome pairings during meiosis I.     

Genetic segregation in relation to chromosome pairing in tetraploid hybrids between Lolium perenne and L. multiflorum

Summary Segregation at one of the loci controlling tiller-base pigmentation was studied to determine the mode of inheritance in tetraploid hybrids between Lolium perenne and L. multiflorum. The results could be explained by tetrasomic inheritance and thus did not support previous reports of a degree of preferential chromosome pairing in this material. However, double reduction and aneuploidy may to some extent have masked any tendency to disomic segregation brought about by preferential pairing. Moreover, there was significant heterogeneity between families in the segregation ratios which may indicate genetically controlled differences in pairing behaviour. The results are related to previous cytological and genetic studies.     

Gene segregation in induced tetraploid rainbow trout: genetic evidence of preferential pairing of homologous chromosomes

Gene segregation at six protein loci was analysed in progeny from tetraploid males and females obtained by suppression of first mitosis. The triploid full-sib families from five tetraploid males and the diploid gynogenetic lines from four tetraploid females were examined. The proportions of heterozygous gametes (0.83 on the average) were significantly higher than expected from tetrasomic inheritance (0.667) at all the loci studied. This was explained by preferential pairing of homologous chromosomes. The proportions of heterozygous gametes were significantly different between loci, but the variations were not correlated with the gene--centromere distances. Our results showed that, at least for one locus, the homozygous gametes mainly resulted from pairing of homologous chromosomes rather than from pairing of homologous chromosomes, quadrivalent formation, and chromatin exchanges between homologous chromosomes.     

Meiotic behaviour of single-arm tetrasomic combinations of isochromosomes,telocentrics and normal chromosomes in rye (Secale cereale L.)

The isochromosome studied was derived from the short arm of the satellite chromosome of rye (Secale cereale, 2n=14); the telocentrics represent both the short and long arms of the same chromosome. Three different combinations, tetrasomic for the short arm, have been composed and studied: I: 2 isochromosomes (short arm) + 2 telocentrics (long arm) + 6 normal pairs. II: 1 isochromosome + 2 telocentrics (short arm) + 2 telocentrics (long arm) + 6 normal pairs. III: 1 isochromosome + 1 telocentric (short arm) + 1 normal satellite chromosome + 1 telocentric (long arm) + 6 normal pairs. — Over 20,000 cells were analysed. Simple mathematical models describing the frequencies of the different types of MI configurations in terms of frequency of chiasmata in the different pairing combinations of the polysomic arms, and of the frequency of multivalent pairing of this arm, were developed. They were used to derive estimates for chiasma frequencies and multivalent pairing frequencies in the different chromosome constitutions from the observations on configuration frequencies. Variation between plants and within plants was studied, and it was concluded that much of the within plant heterogeneity was due to regulatory variation expressed independently in different chromosomal segments. There was also a significant genetic component. Analysis of the reasons for the models to fail under certain conditions led to suggestions for extension of the models.     

Evidence for non-disomic inheritance in a Citrus interspecific tetraploid somatic hybrid between C. reticulata and C. limon using SSR markers and cytogenetic analysis

Artificial tetraploid somatic hybrids have been developed for sterile triploid citrus breeding by sexual hybridization between diploid and tetraploid somatic hybrids. The genetic structure of diploid gametes produced by tetraploid genotypes depends on the mode of chromosome association at meiosis. In order to evaluate tetraploid inheritance in a tetraploid interspecific somatic hybrid between mandarin and lemon, we performed segregation studies using cytogenetic and single sequence repeat molecular markers. Cytogenetic analysis of meiosis in the somatic hybrid revealed 11% tetravalents and 76% bivalents. Inheritance of the tetraploid hybrid was analyzed by genotyping the triploid progeny derived from a cross between a diploid pummelo and the tetraploid somatic hybrid, in order to derive genotypes of the meiospores produced by the tetraploid. A likelihood-based approach was used to distinguish between disomic, tetrasomic, and intermediate inheritance models and to estimate the double reduction rate. In agreement with expectations based the cytogenetic data, marker segregation was largely compatible with tetrasomic and inheritance intermediate between disomic and tetrasomic, with some evidence for preferential pairing of homoeologous chromosomes. This has important implications for the design of breeding programs that involve tetraploid hybrids, and underscores the need to consider inheritance models that are intermediate between disomic and tetrasomic.     

Secondary Tetrasomic Segregation of Mdh-B and Preferential Pairing of Homeologues in Rainbow Trout

We examined the inheritance of allelic variation at an isozyme locus, MDH-B, duplicated by ancestral polyploidy in salmonid fishes. We detected only disomic segregation in females. Segregation ratios in males were best explained by a mixture of disomic and tetrasomic inheritance. We propose a two-stage model of pairing in male meiosis in which, first, homologous chromosomes pair and recombine in the proximal region of the chromosome. Next, homeologous chromosomes pair and recombine distally. We suggest that this type of tetrasomic inheritance in which centromeres segregate disomically should be referred to as ``secondary tetrasomy' to distinguish it from tetrasomy involving entire chromosomes (i.e., ``primary tetrasomy'). Differences in segregation ratios between males indicate differences between individuals in the amount of recombination between homeologous chromosomes. We also consider the implication of these results for estimation of allele frequencies at duplicated loci in salmonid populations.     

Tetrasomic segregation for multiple alleles in alfalfa

Evidence of tetrasomic inheritance in alfalfa, Medicago sativa L. and M. falcata L., for multiple codominant alleles at three isozymic loci is reported in this study. The locus Prx-1 governing anodal peroxidase and the loci Lap-1 and Lap-2 governing anodal leucine-aminopeptidase were studied by starch gel electrophoresis in seedling root tissue or seeds. The progenies from several di-, tri- or tetra-allelic plants belong to the species M. sativa and M. falcata and their hybrids were studied for the segregation of the three genes. In all cases, tetrasomic inheritance of chromosomal-type segregation was observed. In another progeny resulting from the crossing of two plants involving four different alleles at locus Lap-2, tetrasomic segregation with the possible occurrence of double reduction was observed. This study presents direct evidence of autotetraploidy and the existence of tetra-allelic loci in alfalfa. It also supports the concept that the species M. sativa and M. falcata are genetically close enough to be considered biotypes of a common species.     

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

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

Cytogenetic evidence of mixed disomic and polysomic inheritance in an allotetraploid (AABB) Musa genotype

Background and Aims

Edible bananas originated mainly from two wild species, Musa acuminata Colla (AA) and Musa balbisiana Colla (BB), and triploid cultivars with an AAA, AAB or ABB genome are the most widely used. In the present study, chromosome pairing affinities are investigated in a sterile AB Indian variety and in its fertile colchicine-induced allotetraploid (AABB) derivative to determine the inheritance pattern of the tetraploid genotype. The potential implications of interspecific recombination and chromosomal composition of diploid gametes for Musa improvement are presented.

Methods

The pairing of different chromosome sets at diploid and tetraploid levels was investigated through a combination of conventional cytogenetic and genomic in-situ hybridization (GISH) analyses of meiotic chromosomes, leading to a likelihood model of the pairing behaviour. GISH analysis of mitotic chromosomes was also conducted to reveal the chromosome constitution of hybrids derived from crosses involving the allotetraploid genotype.

Key Results

Analysis of chromosome associations at both ploidy levels suggested that the newly formed allotetraploid behaves as a ‘segmental allotetraploid’ with three chromosome sets in a tetrasomic pattern, three sets in a likely disomic pattern and the five remaining sets in an intermediate pattern. Balanced and unbalanced diploid gametes were detected in progenies, with the chromosome constitution appearing to be more homogenous in pollen than in ovules.

Conclusions

Colchicine-induced allotetraploids in Musa provide access to the genetic background of natural AB varieties. The segmental inheritance pattern exhibited by the AABB allotetraploid genotype implies chromosome exchanges between M. acuminata and M. balbisiana species and opens new horizons for reciprocal transfer of valuable alleles.     

Hierarchical and factorial mating designs for quantitative genetic analysis in tetrasomic potato

Plant breeders need to quantify additive and non-additive components of genetic variance in order to determine appropriate selection methods to improve quantitative characteristics. Hierarchical and factorial mating designs (also known as North Carolina mating designs I and II, respectively) allow one to determine these variance components. The relative advantages of these two designs in the quantitative genetics of tuber yield in tetrasomic potato were investigated. Likewise, the number of female parents to include in design I was also investigated. Data were collected from two independent experiments at two contrasting Peruvian locations: La Molina in the dry coast and San Ramon in the humid mid-altitude. In the first experiment, although design I gave a negative digenic variance (σ² _D), this design provided almost the same estimate of narrow-sense heritability (h²) for tuber yield as that obtained in design II (0.291 and 0.260, respectively). Therefore, design I appears to be appropriate for quantitative genetics research in tetrasomic potato, a crop in which some clones are male sterile. The easy handling of crosses (distinct random females included in the crossing scheme) is another advantage of design I relative to design II. In the second experiment, 12 males were crossed with either two or four females following a design-I mating scheme. The additive genetic variance (σ² _A) was zero (or negative) when two females per male were included but was positive with four females. These results suggest that two females per male may not be enough for design I in tetrasomic potato. Four females per male are preferable to determine σ² _A in design I for this tetrasomic crop. Received: 19 March 2001 / Accepted: 3 July 2001     

Most of the homoeologous pairing at metaphase I in wheat-rye hybrids is not chiasmatic

The use of telomeric C-bands in wheat-rye hybrids has made it possible to distinguish three types of wheat-wheat (1B^L) and wheat-rye associations (a, end-to-end extremely distal; b, end-to-ed distal; and c, interstitial) between homoeologous chromosomes at different metaphase I stages (early, middle and late) and also to estimate the actual recombination frequencies for such associations at anaphase I. There was a decrease of the a and b association frequencies during the different metaphase I stages, whereas the c type remained without variation in all stages. A good fit between the frequencies of c associations at metaphase I and the number of recombinant chromosomes at anaphase I, assuming a maximum of one chiasma per bond, was found; however, there was no correspondence between metaphase I and anaphase I data when all associations (a + b + c) were considered. In addition, rye-rye homologous pairing was observed at metaphase I, but no evidence for rye-rye recombination was found at anaphase I. The results indicate that most of end-to-end (a and b) homoeologous and nonhomologous associations are actually nonchiasmatic and are a remnant of prophase pairing.     

Genetic control of bivalent pairing in the Avena strigosa polyploid complex

Bivalent pairing in the tetraploid oat A. barbata, the main tetraploid form of A. strigosa polyploid complex, was found to be determined by a single recessive gene in quadriplex condition. This gene segregated in tetrasomic fashion in the A. barbata × A. strigosa autopolyploid, which indicates conspicuously autopolyploid origin of A. barbata and close relationships between the two chromosome sets of this oat, and the chromosomes of the diploid oat A. strigosa. It was speculated that the gene affecting bivalent pairing in A. barbata was evolved already at the diploid level in one of the A. strigosa races and had been recovered in quadriplex state following polyploidization of an intervarietal A. strigosa hybrid.     

Bayesian Procedures for Discriminating among Hypotheses with Discrete Distributions: Inheritance in the Tetraploid Astilbe Biternata

Discrimination between disomic and tetrasomic inheritance aids in determining whether tetraploids originated by allotetraploidy or autotetraploidy, respectively. Past assessments of inheritance in tetraploids have used analyses whereby each inheritance hypothesis is tested independently. I present a Bayesian analysis that is appropriate for discriminating among several inheritance hypotheses and can be used in any case where hypotheses are defined by discrete distributions. The Bayesian approach incorporates prior knowledge of the probability of occurrence of disomic and tetrasomic hypotheses so that the results of the analysis are not biased by the fact that there is a single tetrasomic hypothesis and multiple disomic hypotheses. This analysis is used to interpret data from crosses in the tetraploid Astilbe biternata, a herbaceous plant native to the southern Appalachians. The progeny ratios from all crosses favored the hypothesis of disomic inheritance at both the PGM and slow-PGI loci. These results support earlier cytogenetic evidence for the allotetraploid origin of Astilbe biternata.     

The quantitative analysis of chromosome pairing and chiasma formation based on the relative frequencies of M I configurations

In autotetraploids, chromosome pairing may be in the form of quadrivalents or bivalent pairs. Whether or not the quadrivalents are maintained until first meiotic metaphase depends on the formation of chiasmata. The relative frequencies of M I configurations thus contain information both on pairing and on chiasma formation. With distal chiasma localisation six configurations can be recognised and their relative frequencies determined: ring quadrivalents, chain quadrivalents, trivalents (with univalent), ring bivalents, open (rod) bivalents, univalent pairs. These represent five degrees of freedom permitting five parameters to be estimated: the frequency (f) of quadrivalent pairing; the frequencies of chiasmate association of the two ends (arms in metacentrics), a′, b′, after quadrivalent pairing, and a, b after bivalent pairing. — The appropriate formulae have been derived and applied to observations on Tradescantia virginiana (4n=24) which has pronounced distal chiasma localisation. Slight modifications make the model applicable to autotetraploids with interstitial in addition to distal chiasmata. In T. virginiana, chromosome pairing appeared to be random between homologues (65.8% quadrivalent pairing; 55.4% observed at M I). After quadrivalent pairing chiasmate association is frequent in the “average long” arm (95.0%) and much less so in the other arm (60.5%). This is attributed to partner exchange. After bivalent pairing chiasma frequencies are still different for the two arms (93.8% and 83.5% association respectively) but much less pronounced. Various complications are discussed.     

Deficiency of X and Y chromosomal pairing at meiotic prophase in spermatocytes of sterile interspecific hybrids between laboratory mice (Mus domesticus) and Mus spretus

The normal association between the X and Y chromosomes at metaphase I of meiosis, as seen in air-dried light microscope preparations of mouse spermatocytes, is frequently lacking in the spermatocytes of the sterile interspecific hybrid between the laboratory mouse strains C57BL/6 and Mus spretus. The purpose of this work is to determine whether the separate X and Y chromosomes in the hybrid are asynaptic, caused by failure to pair, or desynaptic, caused by precocious dissociation. Unpaired X-Y chromosomes were observed in air-dried preparations at diakinesis, just prior to metaphase I. Furthermore, immunocytology and electron microscopy studies of surface-spread pachytene spermatocytes indicate that the X and Y chromosomes frequently fail to initiate synapsis as judged by the failure to form a synaptonemal complex between the pairing regions of the X and Y Chromosomes. Several additional chromosomal abnormalities were observed in the hybrid. These include fold-backs of the unpaired X or Y cores, associations between the autosome and sex chromosome cores, and autosomal univalents. The occurrence of abnormal autosomal and XY-autosomal associations was also correlated with cell degeneration during meiotic prophase. The primary breakdown in hybrid spermatogenesis occurs at metaphase I (MI), with the appearance of degenerated cells at late MI. In those cells, the X and Y are decondensed rather than condensed as they are in normal mouse MI spermatocytes. These results, in combination with the previous genetic analysis of spermatogenesis in hybrids and backcrosses with fertile female hybrids, suggest that the spermatogenic breakdown in the interspecific hybrid is primarily correlated with the failure of XY pairing at meiotic prophase, asynapsis, followed by the degeneration of spermatocytes at metaphase I. Secondarily, the failure of XY pairing can be accompanied by failure of autosomal pairing, which appears to involve an abnormal sex vesicle and degeneration at pachytene or diplotene.by C. Heyting     

Meiotic gene conversion and crossing over: their relationship to each other and to chromosome synapsis and segregation

The yeast mer1 mutant produces inviable spores and is defective in both meiotic recombination and chromosome pairing. A gene called MER2 partially suppresses the mer1 phenotype when present in high copy number. Both gene conversion and chromosome pairing are completely restored in mer1 strains overexpressing MER2; however, reciprocal crossing over and spore viability are not restored. The data presented are consistent with a model in which chromosome pairing is a direct consequence of a homology search mediated through gene conversion. Analysis of random viable spores indicates that the crossovers that occur in mer1 strains overexpressing MER2 are more effective in ensuring meiosis I disjunction than those that occur in mer1 strains. One interpretation of this result is that only those crossovers that occur in the context of the synaptonemal complex lead to the establishment of functional chiasmata. The MER2 gene product is essential for meiosis.     

Chromosome pairing in meiosis of partially fertile wheat/rye hybrids

The normal course of meiosis depends on regular pairing of homologous chromosomes. In intergeneric hybrids, including those of wheat, there is no chromosome pairing because there are no homologs. In F₁ wheat/rye hybrids, pairing is largely prevented by the pairing homoeologous1 (Ph1) gene. In its presence, there are only rare instances of pairing; most chromosomes are univalent, and their orientation at metaphase I initiates different pathways of the meiotic cycle. The meiotic-like pathway includes a combination of the reductional and the equational + reductional steps at AI followed by the second division. The resulting gametes are mostly non-functional. The mitotic-like pathway involves equational division of univalents at AI and the absence of the second division. Any fertility of wheat/rye hybrids depends on the production of unreduced gametes arising from meiotic restitution (mitotic-like division). We examined the meiotic pairing in wheat/rye hybrids created from wheat lines with single rye chromosome substitutions and Ph1 present. This guaranteed F₁ meiosis with one pair of rye homologs. All hybrids formed bivalents, but proportions of meiocytes with bivalents varied. In the meiocytes where bivalents were present, there was a higher tendency for the meiotic-like pathway, while in meiocytes where bivalent pairing failed, the tendency was stronger for the mitotic-like pathway. Among the equationally dividing cells, we observed more than 90 % of meiocytes without bivalents, where rye homologs did not form bivalents, too. The data indicate a potential application of wheat/rye lines in producing genetic stocks of amphidiploids with designated genomic constitutions.