The allotetraploidization of maize

Summary Allotetraploidization is the creation of artifical allotetraploids from a normally diploid species. The possible value of allotetraploid maize has been discussed in Section I of this series. Allotetraploidization of maize can be achieved by restructuring a maize genome so that its chromosomes will not pair with those of the standard maize genome. This restructuring can be done by concentrating differential pairing affinity (DPA) factors into a single line by a recurrent selection type of breeding program. Because the divergence of the maize genome is a gradual process, it is necessary to devise a model for chromosome pairing and gene segregation in segmental allotetraploids. This has been done by considering pairing in each arm separately and then combining paired arms to form pairing configurations for whole chromosomes. The chromosome disjunction patterns are hypothesized and genetic ratios in relation to different levels of DPA are suggested.Contribution from the Science and Education Administration, U.S. Department of Agriculture, and the Agronomy Department, University of Missouri, Columbia, Missouri, Agricultural Experiment Station Journal Series No. 8090     

The allotetraploidization of maize

Summary Allotetraploidization is the creation of artificial allotetraploids. Allotetraploidization of maize can be accomplished by concentrating differential pairing affinity (DPA) factors into lines by a recurrent selection breeding system. Selection will be based on changes in genetic ratios which are the result of changes in the relative frequencies of various pairing configurations caused by DPA. Part 1 of this series gave extensive data on gene segregation in trisomic and tetraploid heterozygotes. Some of these tetraploids behaved like segmental allotetraploids. Part 2 presented a model for gene segregation in segmental allotetraploids. This paper presents an analogous model for gene segregation in trisomic heterozygotes. The pairing configurations of trisomes are analyzed by considering pairing in single arms which then are combined to obtain pairing configurations for whole chromosomes. The chromosome disjunction patterns of the various pairing configurations are hypothesized and expected genetic ratios are given that result from different levels of DPA expressed in several hypothetical trisomes. The model analyzes the effect of random pairing in one arm and non-random pairing in the other arms. Also, the effect of crossing over is taken into account. Because crossing over rates are affected by the environment, part of the variability in the data (Part 1) is explained. In addition, an hypothesis is advanced to explain the frequent enhancement of pairing affinity following x-irradiation.Contribution from Agricultural Research/Science and Education Administration, US Department of Agriculture, University of Missouri, Columbia, Missouri, Missouri Agric. Exp. Sta. Jounal Series No. 8 670     

The allotetraploidization of maize

Summary Allotetraploidization is the creation of synthetic allotetraploids. The allotetraploidization of maize can be accomplished by concentrating DPA (differential pairing affinity) factors into stocks by a recurrent selection breeding system. Selection is based on pairing configuration frequencies and altered genetic ratios that reflect DPA. Both an observed decline in the quadrivalent frequency per meiocyte from 8.10 to 7.31 and genetic data disclosing a reduction in the average frequency of recessive waxy (wx wx) pollen from Wx Wx wx wx plants from 17.48% to 13.35%, indicate considerable progress has been made toward allotetraploidization. A simple model for the effect of DPA on chromosome pairing and genetic ratios is presented.Contribution from the Agricultural Research Service, U.S. Department of Agriculture, University of Missouri, Missouri Agricultural Experiment Station, Journal Series No. 9806     

Tripsacum-maize interaction: a novel cytogenetic system

The genera Zea and Tripsacum cross readily when they are not isolated by gametophytic barriers, and it has been postulated that intergeneric introgression played a role in the evolution of maize. The basic x = 9 Tripsacum and x = 10 Zea genomes have little cytological affinity for each other in hybrids that combine 10 Zea with 18 Tripsacum chromosomes. However, one to four Tripsacum chromosomes sometimes associate with Zea chromosomes in hybrids between Z. mays (2n = 20) and T. dactyloides (2n = 72). These hybrids with 10 Zea and 36 Tripsacum chromosomes frequently produce functional female gametes with 36 Tripsacum chromosomes only. When they are pollinated with maize, their offspring again have 36 Tripsacum and 10 maize chromosomes, but the Tripsacum genome is contaminated with maize genetic material. In these individuals, intergenome pairing is the rule, and when they are pollinated with maize, their offspring have 36 Tripsacum and 10, 12, 14, 16, 18, or 20 Zea chromosomes. Plants with 36 Tripsacum and 20 Zea chromosomes behave cytologically as alloploids, although the Tripsacum genome is contimated with maize, and one basic maize genome is contaminated with with Tripsacum genetic material. When they are pollinated with maize, offspring with 18 Tripsacum and 20 Zea chromosome are obtained. Further successive backcrosses with maize selectively eliminate Tripsacum chromosomes, and eventually plants with 2n = 20 Zea chromosomes are recovered. Many of these maize plants are highly "tripsacoid." Strong gametophytic selection for essentially pure Zea gametes, however, eliminates all obvious traces of Tripsacum morphology within a relatively few generations.     

Meiotic pairing in the hybrid (Zea diploperennis x Zea perennis) x Zea mays and its reciprocal

Genomic formulae, fertility, chromosome pairing, and the cryptic intergenomic pairing (induced by using diluted colchicine solution) were analysed in the tri-hybrid (MDP), obtained by crossing DP40 (2n=40, which was inferred in previous studies to have originated from the fusion of an unreduced gamete of Zea diploperennis with a normal gamete of Z. perennis) with the maize inbred line Zm40 (2n=40). MDP (2n=40) showed a higher fertility (90% of the seeds are viable) than Zm40 (60%) and DP40 (80%). A regular migration of 20 chromosomes to each pole occurred in 92% of the cells in anaphase I, while bridges were observed in the other 8% of the cells. When Zm40 was used as female of the crossing (Zm40 x DP40), ears were similar to corn. Conversely, ears resembled those of the wild species when cytoplasm was donoured by Zd. Then, it can be stated the existence of cytoplasmic influence on MDP ear type. MDP had almost no I or III, with an average of 0.04I + 10.90II + 0.01III + 4.50IV. The most frequent meiotic configuration was 10II + 5IV (43.93% of the cells). On average, 33.81 chiasmata/cell were observed (17.34, 0.05 and 16.42 average numbers of chiasmata/cell in bivalents, trivalents and tetravalents, respectively). It can be inferred that the 5IV were the product of homoeologous chromosome pairing of A genomes from the three species. On the other hand, the 10II configuration suggests separate pairing of the 5 homologous B chromosomes from maize and the 5 homoeologous B chromosomes from Zp and Zd.     

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     

Flow cytometric sorting of maize chromosome 9 from an oat-maize chromosome addition line

Large numbers of maize chromosome 9 can be collected with high purity by flow cytometric sorting of chromosomes isolated from a disomic maize chromosome addition line of oat. Metaphase chromosome suspensions were prepared from highly synchronized seedling root-tips of an oat-maize chromosome-9 addition line (OM9) and its parental oat and maize lines. Chromosomes were stained with propidium iodide for flow cytometric analysis and sorting. Flow-karyotypes of the oat-maize addition line showed an extra peak not present in the parental oat line. This peak is due to the presence of a maize chromosome-9 pair within the genome of OM9. Separation of maize chromosome 9 by flow cytometric sorting of a chromosome preparation from a normal maize line was not possible because of its size similarity (DNA content) to maize chromosomes 6, 7 and 8. However, it is possible to separate maize chromosome 9 from oat chromosomes and chromatids. An average of about 6×10³ chromosomes of maize chromosome 9 can be collected by flow-sorting from chromosomes isolated from 30 root tips (ten seedlings) of the oat-maize addition line. Purity of the maize chromosome 9, sorted from the oat-maize chromosome addition line, was estimated to be more than 90% based on genomic in situ hybridization analysis. Sorting of individual chromosomes provides valuable genomic tools for physical mapping, library construction, and gene isolation. Received: 28 February 2000 / Accepted: 14 July 2000     

The pattern of pairing that is effective for crossing over in complex B-A chromosome rearrangements in maize

The study of the mechanism of meiotic homolog pairing, approached by comparing chiasma frequencies in rearranged segments that differ in relative length and intrachromosomal location, is substantially extended here. For the first time, two kinds of evidence were found that centers specialized for alignment pairing may exist in maize chromosomes: (1) for two segments, higher than average crossover frequency per unit length was maintained when these were located in several different chromosomal positions with respect to centromere and telomere, and in fact apart from their own normal centromeres and telomeres. High crossover frequencies in these segments regardless of position are considered to reflect innate capacity for alignment pairing due to relatively strong pairing center content. (2) For a short rearranged segment, chiasma frequency was drastically reduced, and evidence suggests that all of the chiasmata found there depended upon juxtaposition made possible by the completion of the zip-up pairing process in the other arms of the translocation configuration. This short segment is thought to be essentially devoid of pairing center content. It seems possible that crossover frequency depression in short rearranged segments may usually not be due, as commonly supposed, to mechanical difficulties inherent in formation of contorted configurations, but rather to absence of pairing centers within them and the relative rarity (compared to the normal sequence situation) of enabling zip-up pairing. Evidence also indicates that pairing which leads to crossing over must frequently occur between internal translocated segments and their normal sequence counterparts in a way which cannot be dependent upon zipping-up of two-by-two pairing initiated at or near telomeres. Pairing centers in maize are probably numerous and widely dispersed, since coarse direct proportionality is found when chiasma frequency is compared for an array of segment lengths.     

Genomic rearrangement in long-term shoot competent cell cultures of hexaploid wheat

Summary By use of a method for regenerating wheat plants (Triticum aestivum L.) from cells from long-term suspension culture, the chromosome complement and stability of cultured cells of cv. Mustang were examined. Massive chromosome restructuring and genomic rearrangements were detected by HCl−KOH-Giemsa banding techniques. Chromosome structural variations involved mainly heterochromatin and centromeric regions. These included B genome chromosome elimination; heterochromatin amplification; megachromosomes and extrachromosomal DNA particles; translocations and deletions; telocentric, dicentric, and multicentric chromosomes; and somatic pairing and crossing over. At least 65 break-fusion sites were identified. Most of the sites were located in the B genome chromosomes (42 sites, 64.6%); 36.9% (20 sites) were located in the A genome chromosomes; and the fewest (3 sites, 4.6%) were detected in the D genome. Most of the chromosome break-fusion is in the heterochromatin and centromeric regions. The B genome chromosomes appeared to be eliminated nonrandomly, and the stability of the genome may vary among the genotypes and depend on culture duration. We also checked chromosome number of 1-year-old shoot-competent cells. Only 20% of the cells still had 2n=42 chromosomes. Most of the cells (60%) were hyperploid. These observed variations describe the types of tissue-culture-induced variations and suggest the unsuitability of using wheat cells from long-term cultures for genetic transformation experiments.     

Origin of reciprocal translocations and their effect in Clarkia speciosa

Reciprocal translocations occur in high frequencies in Clarkia speciosa and closely related species. Observations from C. speciosa suggest this species is predisposed to translocations involving breaks in or adjacent to the centrochromatin (centromeric chromatin) due to the characteristic association of all nonhomologous centrochromatin in the genome during early meiotic prophase. Translocation heterozygote multiples involving six different breaks were examined for homologous pairing and in each case the euchromatic arms were completely paired, the change in homologous pairing occuring within the nonhomologous centrochromatic association. Such a proximal exchange point precludes the possibility of a structurally determined interstitial or differential region and, therefore, any genetically differential regions that might exist must be maintained solely by means of distal localization of crossing over. — The frequency of chromosomal nondisjunction (adjacent segregation) was found to be positively correlated with the number of chromosomes in the translocation multiple. Rings of four chromosomes had an average disjunction of over 99% and therefore had little affect on fertility whereas the largest multiples of 16 chromosomes had an average disjunction of about 10% and correspondingly low fertility.     

Maize meiotic mutants with improper or non-homologous synapsis due to problems in pairing or synaptonemal complex formation

During meiotic prophase homologous chromosomes find each other and pair. Then they synapse, as the linear protein core (axial element or lateral element) of each homologous chromosome is joined together by a transverse central element, forming the tripartite synaptonemal complex (SC). Ten uncloned Zea mays mutants in our collection were surveyed by transmission electron microscopy by making silver-stained spreads of SCs to identify mutants with non-homologous synapsis or improper synapsis. To analyse the mutants further, zyp1, the maize orthologue of the Arabidopsis central element component ZYP1 was cloned and an antibody was made against it. Using antibodies against ZYP1 and the lateral element components AFD1 and ASY1, it was found that most mutants form normal SCs but are defective in pairing. The large number of non-homologous synapsis mutants defective in pairing illustrates that synapsis and pairing can be uncoupled. Of the ten mutants studied, only dsy2 undergoes normal homologous chromosome recognition needed for homologous pairing. The dsy2 mutation fails to maintain the SC. ZYP1 elongation is blocked at zygotene, and only dots of ZYP1 are seen at prophase I. Another mutant, mei*N2415 showed incomplete but homologous synapsis and ASY1 and AFD1 have a normal distribution. Although installation of ZYP1 is initiated at zygotene, its progression is slowed down and not completed by pachytene in some cells and ZYP1 is not retained on pachytene chromosomes. The mutants described here are now available through the Maize Genetics Cooperation Stock Center (http://maizecoop.cropsci.uiuc.edu/).     

Chromosome pairing in tetraploid rye with monosomic-substitution wheat chromosomes

In tetraploid rye with single-substitution wheat chromosomes - 1A, 2A, 5A, 6A, 7A, 3B, 5B, 7B - chromosome pairing was analysed at metaphase I in PMCs with the C-banding method. The frequency of univalents of chromosome 1A was considerably higher than that of the other four wheat chromosomes of genome A (6A, 5A, 7A and 2A). Among chromosomes of genome B, the lowest mean frequency of univalents was observed for chromosome 5B. In monosomic lines, wheat chromosomes 1A, 2A, 5A, 6A, 7A and 5B paired with rye homoeologues most often in rod bivalents and in chain quadrivalents (also including 3B). The 47% pairing of 5B-5R chromosomes indicate that the rye genomes block the suppressor Ph1 gene activity. In monosomic plants with chromosomes 5A, 2A, 6A, 7A and 5B, a low frequency of rye univalents was observed. It was also found that the wheat chromosomes influenced the pairing of rye genome chromosomes, as well as the frequency of ring and rod bivalents and tri- and quadrivalents. However, the highest number of terminal chiasmata per chromosome occurred in the presence of chromosomes 5A and 2A, and the lowest - in the presence of chromosomes 3B and 7B. In the presence of chromosome 5B, the highest frequency of bivalents was observed. The results of the present study show that the rye genome is closer related to the wheat genome A of than to genome B. The high pairing of wheat-rye chromosomes, which occurs in tetraploid rye with substitution wheat chromosomes, indicates that there is a high probability of incorporating wheat chromosome segments into rye chromosomes.     

The origin of multiple sex chromosomes in the gerbil Gerbillus gerbillus (Rodentia: Gerbillinae)

The sex chromosomes of the partly sympatric species of gerbils Gerbillus pyramidum and G. gerbillus (Mammalia: Gerbillinae) were investigated by a variety of light- and electron-microscope methods, including DNA replication banding and synaptonemal complex (SC) techniques. The sex-chromosome mechanism of G. pyramidum is of the maleXY:femaleXX type, whereas that of G. gerbillus is of the less common maleXY1Y2:femaleXX system. The results include the demonstration that the X chromosomes of both species are compound. One segment is added to the X chromosome of G. pyramidum, leading to an increase in length from the standard 5% to approximately 7.3%, whereas two different extra segments increase the length of the X chromosome of G. gerbillus to approximately 11% of the length of the haploid genome. In both cases the extra material is autosomal and is also represented in the respective Y chromosomes. Classifying heterochromatin by the variation in staining quality was helpful in elucidating the possible origin of the different chromosome segments, including the pericentromeric regions. Observations on meiotic chromosome pairing and chiasma formation have confirmed the homologies established by band comparisons. The occurrence of chiasmata between the sex chromosomes supports the autosomal origin of the pairing segments. These and other findings have been interpreted in the framework of a multistep evolutionary model. This sequence starts from a hypothetical pair of sex chromosomes, the X element of which amounts to 5% of the haploid genome, and leads through three translocations involving two pairs of autosomes and one pericentric inversion to the most complex situation of this series, manifested in G. gerbillus. The adaptive value, if any, of autosome incorporation into the sex chromosomes repeatedly occurring here is unknown. It is, however, a remarkable fact that in one species, G. gerbillus, the complex sex-chromosome constitution is conserved over vast geographic distances, and in the other, G. pyramidum, the compound X and Y chromosomes withstand change in the face of extreme autosome restructuring.     

Functional analysis of maize RAD51 in meiosis and double-strand break repair

In Saccharomyces cerevisiae, Rad51p plays a central role in homologous recombination and the repair of double-strand breaks (DSBs). Double mutants of the two Zea mays L. (maize) rad51 homologs are viable and develop well under normal conditions, but are male sterile and have substantially reduced seed set. Light microscopic analyses of male meiosis in these plants reveal reduced homologous pairing, synapsis of nonhomologous chromosomes, reduced bivalents at diakinesis, numerous chromosome breaks at anaphase I, and that >33% of quartets carry cells that either lack an organized nucleolus or have two nucleoli. This indicates that RAD51 is required for efficient chromosome pairing and its absence results in nonhomologous pairing and synapsis. These phenotypes differ from those of an Arabidopsis rad51 mutant that exhibits completely disrupted chromosome pairing and synapsis during meiosis. Unexpectedly, surviving female gametes produced by maize rad51 double mutants are euploid and exhibit near-normal rates of meiotic crossovers. The finding that maize rad51 double mutant embryos are extremely susceptible to radiation-induced DSBs demonstrates a conserved role for RAD51 in the repair of mitotic DSBs in plants, vertebrates, and yeast.     

A genomewide survey argues that every zygotic gene product is dispensable for the initiation of somatic homolog pairing in Drosophila

Studies from diverse organisms show that distinct interchromosomal interactions are associated with many developmental events. Despite recent advances in uncovering such phenomena, our understanding of how interchromosomal interactions are initiated and regulated is incomplete. During the maternal-to-zygotic transition (MZT) of Drosophila embryogenesis, stable interchromosomal contacts form between maternal and paternal homologous chromosomes, a phenomenon known as somatic homolog pairing. To better understand the events that initiate pairing, we performed a genomewide assessment of the zygotic contribution to this process. Specifically, we took advantage of the segregational properties of compound chromosomes to generate embryos lacking entire chromosome arms and, thus, all zygotic gene products derived from those arms. Using DNA fluorescence in situ hybridization (FISH) to assess the initiation of pairing at five separate loci, this approach allowed us to survey the entire zygotic genome using just a handful of crosses. Remarkably, we found no defect in pairing in embryos lacking any chromosome arm, indicating that no zygotic gene product is essential for pairing to initiate. From these data, we conclude that the initiation of pairing can occur independently of zygotic control and may therefore be part of the developmental program encoded by the maternal genome.     

The influence of the Robertsonian translocation Rb(X.2)2Ad on anaphase I non-disjunction in male laboratory mice

A Robertsonian translocation in the mouse between the X chromosome and chromosome 2 is described. The male and female carriers of the Rb(X.2)2Ad were fertile. A homozygous/hemizygous line was maintained. The influence of the X-autosomal Robertsonian translocation on anaphase I non-disjunction in male mice was studied by chromosome counts in cells at metaphase II of meiosis and by assessment of aneuploid progeny. The results conclusively show that the inclusion of Rb2Ad in the male genome induces non-disjunction at the first meoitic division. In second metaphase cells the frequency of sex-chromosomal aneuploidy was 10.8%, and secondary spermatocytes containing two or no sex chromosome were equally frequent. The Rb2Ad males sired 3.9% sex-chromosome aneuploid progeny. The difference in aneuploidy frequencies in the germ cells and among the progeny suggests that the viability of XO and XXY individuals is reduced. The pairing configurations of chromosomes 2, Rb2Ad and Y were studied during meiotic prophase by light and electron microscopy. Trivalent pairing was seen in all well spread nuclei. Complete pairing of the acrocentric autosome 2 with the corresponding segment of the Rb2Ad chromosome was only seen in 3.2% of the cells analysed in the electron microscope. The pairing between the X and Y chromosome in the Rb2Ad males corresponded to that in males with normal karyotype. Reasons for sex-chromosomal non-disjunction despite the normal pairing pattern between the sex chromosomes may be seen in the terminal chiasma location coupled with the asynchronous separation of the sex chromosomes and the autosomes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Improper chromosome synapsis is associated with elongated RAD51 structures in the maize<Emphasis Type="Italic"> desynaptic2</Emphasis> mutant

The RecA homolog, RAD51, performs a central role in catalyzing the DNA strand exchange event of meiotic recombination. During meiosis, RAD51 complexes develop on pairing chromosomes and then most disappear upon synapsis. In the maize meiotic mutant desynaptic2 (dsy2), homologous chromosome pairing and recombination are reduced by ~70% in male meiosis. Fluorescent in situ hybridization studies demonstrate that a normal telomere bouquet develops but the pairing of a representative gene locus is still only 25%. Chromosome synapsis is aberrant as exemplified by unsynapsed regions of the chromosomes. In the mutant, we observed unusual RAD51 structures during chromosome pairing. Instead of spherical single and double RAD51 structures, we saw long thin filaments that extended along or around a single chromosome or stretched between two widely separated chromosomes. Mapping with simple sequence repeat (SSR) markers places the dsy2 gene to near the centromere on chromosome 5, therefore it is not an allele of rad51. Thus, the normal dsy2 gene product is required for both homologous chromosome synapsis and proper RAD51 filament behavior when chromosomes pair. Edited by: P. Moens     

Physical and genetic structure of the maize genome reflects its complex evolutionary history

Maize (Zea mays L.) is one of the most important cereal crops and a model for the study of genetics, evolution, and domestication. To better understand maize genome organization and to build a framework for genome sequencing, we constructed a sequence-ready fingerprinted contig-based physical map that covers 93.5% of the genome, of which 86.1% is aligned to the genetic map. The fingerprinted contig map contains 25,908 genic markers that enabled us to align nearly 73% of the anchored maize genome to the rice genome. The distribution pattern of expressed sequence tags correlates to that of recombination. In collinear regions, 1 kb in rice corresponds to an average of 3.2 kb in maize, yet maize has a 6-fold genome size expansion. This can be explained by the fact that most rice regions correspond to two regions in maize as a result of its recent polyploid origin. Inversions account for the majority of chromosome structural variations during subsequent maize diploidization. We also find clear evidence of ancient genome duplication predating the divergence of the progenitors of maize and rice. Reconstructing the paleoethnobotany of the maize genome indicates that the progenitors of modern maize contained ten chromosomes.