Allelic compositions at polymorphic gene loci in maize inbred and hybrid lines and their associations with levels of heterosis

We analyzed polymorphism in the parental lines GK26 and Mo17 and testers Od221MV, Od308MV, and Od329 using SSR-analysis. Recombinant inbred lines (RILs) from populations F₄ and F₆ were genotyped at ten polymorphic loci. Allelic compositions and allele frequencies at microsatellite loci were investigated in parental lines and testers, and the best highly heterotic hybrids and their molecular genetic formulae were derived. The allelic composition of microsatellites were investigated in RILs and high-yield hybrids for the best combining ability parameters.     

Comparative analysis of the spectrum of radiation-induced structural mutations in diploid and tetraploid cells of mouse bone marrow

A comparative analysis of the spectrum of radiation-induced mutations was carried out in diploid mouse bone marrow cells and in tetraploid cells obtained from the latter by means of colchicine treatment. It is established that solitary chromatid deletions prevail among aberrations induced at the G2--S stage. This is the difference of the spectrum of induced chromatid rearrangements in animal cells from that in plant cells. It is assumed that the differences observed are caused by the specificity of supramolecular organization of chromatin in animal and plant cells.     

Induction of tetraploid derivatives of maize inbred lines by nitrous oxide gas treatment

Maize (Zea mays L.) is a model organism for various genetic and physiological studies. Induction of autotetraploid lines from elite inbred lines is valuable for investigating gene dosage effects on the molecular level. We applied nitrous oxide gas at the time of fertilization (30-36 h after pollination) for 20 h on maize inbred line Oh43. The nitrous oxide gas treatment between pressures of 600-1000 kPa proved to be effective in inducing tetraploids. The treatment also significantly increased the rates of germless and shriveled kernels. Twelve inbred lines were treated with nitrous oxide gas for 20 h at either 800 or 900 kPa pressures, 30 or 36 h after pollination. Although tetraploid or tetraploid class aneuploid plants from 9 of 12 inbreds tested were successfully generated, only six genotypes produced progenies. The successful tetraploid inbred lines were from the A188, B73, H99, Oh43, Stock 6, and W22 genetic backgrounds. Aneuploids, plants with broken chromosomes and chimeras, were also found among the treated materials.     

The levels of DNA repair in the cells of diploid and tetraploid cell cultures]

Levels of unscheduled DNA synthesis after UV-irradiation were investigated in addition to the activities of DNA-repair enzyme uracil-DNA glycosilase in cells of both diploid (XC-D) and tetraploid (XC-T) cell cultures. It was shown that repair levels were increasing directly and proportionally to the cell ploidy.     

RFLP variation in diploid and tetraploid alfalfa

Summary Alfalfa (Medicago sativa L.) is a major forage crop throughout the world. Although alfalfa has many desirable traits, continued breeding is required to incorporate pest resistances and other traits. We conducted this study to determine the amount of restriction fragment length polymorphism (RFLP) variability present within and between diploid and tetraploid alfalfa populations, and whether or not this variability is sufficient for construction of an RFLP map. Diploid plants from M. sativa ssp. falcata, ssp. coerulea, and ssp. sativa and tetraploid spp. sativa cultivars Apollo, Florida 77, and Spredor 2 were included. A total of 19 cDNA clones was probed onto genomic Southern blots containing DNA digested by EcoRI, HindIII, or BamHI. Phylogenetic trees were produced, based on parsimony analysis of shared restriction fragments. Evidence for extensive gene duplication was found; most probes detected complex patterns of restriction fragments. Large amounts of variation are present within all diploid subspecies. M. sativa ssp. falcata plants formed clusters distinct from ssp. sativa or ssp. coerulea plants, which were not distinctly clustered. Some M. sativa ssp. falcata plants were more similar to the other groups than to other plants within ssp. falcata. Variation among tetraploid cultivars showed that Florida 77 and Apollo had more similarities than either showed with Spredor 2. All three cultivars showed large within-population variation, with Apollo being the most diverse and Spredor 2 the least. Based on these results, development of an RFLP map at the diploid level appears possible. Also, differentiation of cultivars, particularly ones of divergent origin, seems possible based on RFLP patterns.     

Desynapsis in diploid and tetraploid clones of ryegrass

Desynapsis is described in diploid and tetraploid plants of perennial ryegrass. The plants were derived by repeated cloning of a single mixoploid (2n=14 and 28) detected among colchicine-treated seedlings. The diploid and tetraploid clones varied in degree of desynapsis, chiasma number, and fertility. The variation among the clones was probably environmental. The progeny of the mixoploid parent included dipoids, tetraploids, and an aneuploid. One diploid and the aneuploid were desynaptic and originated perhaps by selfing. Apparently a single recessive gene determined desynapsis. The role of synaptic genes in controlling the chemical structure and function of nucleoprotein macromolecules is discussed.     

Isozyme gene duplication in diploid and tetraploid potatoes

Summary Isozyme techniques allow the study of gene redundancy in different ploidy levels of potato (Solanum tuberosum). In tetraploid potatoes all isozyme loci are duplicated. No sign of structural or regulatory divergence was found, as is expected due to their tetrasomic inheritance patterns. In addition to this genetic redundancy, produced by a relatively recent polyploidization event, some additional redundancy was found for at least three enzymes even in diploid groups and species. These older duplicate genes show structural and regulatory divergence, indicating they appeared by a separate polyploidization event far in the past. Their common origin is still recognizable by both their expression in the same subcellular compartment and by the dimerizing ability of the isozymes they encode. To account for the present chromosome number x = 12 of the Solanaceae family, the most frequently found among the species, a hypothetical polyploidization event is proposed.     

Pollen release by diploid and tetraploid sugar-beet plants

Pollen released during three summers at Broom's Barn, Suffolk, by diploid and tetraploid multigerm sugar-beet plants was caught by automatic volumetric spore traps and by glass funnels situated in the crop. Tetraploids produced on average 66% as much pollen as diploids, their pollen grains were larger and they needed drier air to release pollen, the concentration of which increased more slowly early in the day and was particularly small on dull, damp days. Pollen catches from diploids and tetraploids decreased at a similar rate with increasing distance from the pollinators. The germination of fruits harvested from male-sterile plants among the pollinators was consistently less with a tetraploid than with a diploid pollinator, but was not affected by distance of the male-sterile plants from either pollinator.     

Correspondence of trichome mutations in diploid and tetraploid cottons

Quantitative variation for leaf trichome number is observed within and among Gossypium species, varying from glabrous to densely pubescent phenotypes. Moreover, economically important cotton lint fibers are modified trichomes. Earlier studies have mapped quantitative trait loci (QTLs) affecting leaf pubescence in Gossypium using allotetraploids. In this study, we mapped genes responsible for leaf trichome density in a diploid A genome cross. We were able to map 3 QTLs affecting leaf pubescence based on trichome counts obtained from young leaves (YL) and mature leaves (ML). When the F(2) progeny were classified as pubescent versus glabrous, their ratio did not deviate significantly from a 3:1 model, suggesting that glabrousness is inherited in a simple Mendelian fashion. The glabrous mutation mapped to linkage group A3 at the position of major QTL YL1 and ML1 and appeared orthologous to the t1 locus of the allotetraploids. Interestingly, a fiber mutation, sma-4(ha), observed in the same F(2) population cosegregated with the glabrous marker, which indicates either close linkage or common genetic control of lint fiber and leaf trichomes. Studies of A genome diploids may help to clarify the genetic control of trichomes and fiber in both diploid and tetraploid cottons.     

Quantitative genetic studies at the tetraploid and hexaploid levels in alfalfa

Summary Two sets of diallel crosses involving seven tetraploid and five hexaploid alfalfa lines, previously selected for seed and forage yield, are studied. On the basis of combining ability and heritability estimates the following conclusion is drawn. The dominant and epistatic interactions (heterozygosity) are the major components in the expression of variability for yield and characters associated with yield in tetraploid populations. In hexaploid populations, however, there is an appreciable amount of additive variance, and the additive effect of genes along with interaction of various forms may account for the expressed variability for all the characters studied. Better genes and desirable interactions (possibly through heterozygosity) are the two essential genetic components of yield. The selection of better genes is feasible, but by itself will not produce the desired results. Future improvement may therefore depend on the utilization of heterosis and other forms of interactions. There is, in general, a very strong genetic correlation between yield and its components, and therefore, when a direct selection for yield is not feasible, the selection program may rely on a number of closely related characters.     

On the tetraploid origin of the maize genome

Data from cytological and genetic mapping studies suggest that maize arose as a tetraploid. Two previous studies investigating the most likely mode of maize origin arrived at different conclusions. Gaut and Doebley [7] proposed a segmental allotetraploid origin of the maize genome and estimated that the two maize progenitors diverged at 20.5 million years ago (mya). In a similar study, using larger data set, Brendel and colleagues (quoted in [8]) suggested a single genome duplication at 16 mya. One of the key components of such analyses is to examine sequence divergence among strictly orthologous genes. In order to identify such genes, Lai and colleagues [10] sequenced five duplicated chromosomal regions from the maize genome and the orthologous counterparts from the sorghum genome. They also identified the orthologous regions in rice. Using positional information of genetic components, they identified 11 orthologous genes across the two duplicated regions of maize, and the sorghum and rice regions. Swigonova et al. [12] analyzed the 11 orthologues, and showed that all five maize chromosomal regions duplicated at the same time, supporting a tetraploid origin of maize, and that the two maize progenitors diverged from each other at about the same time as each of them diverged from sorghum, about 11.9 mya.     

Gene dosage effects in human diploid and tetraploid fibroblasts

The effects of cell ploidy on the biochemical characteristics of cultured cells were compared using human diploid vs tetraploid fibroblasts isolated with a non-selective method. Their DNA replication was compared by thymidine incorporation, and DNA content by Feulgen staining and quantitative analysis. Their RNA and protein content, cell sizes and the specific activities of glucose-6-phosphate dehydrogenase (G-6-PD) and 6-phosphogluconate dehydrogenase (6-PGD) were assayed quantitatively. With the exception of RNA content, all other parameters demonstrated a 2-fold increase reflecting the increase in cell ploidy. These direct gene dosage effects on the genetic material and functional expression of the human genome were in contrast to previous observations in other species and validate the use of human intraspecific euploid hybrids for biochemical and genetic studies.     

Meiotic prophase in diploid and tetraploid strains ofAllomyces macrogynus

Summary Three-dimensional serial section reconstructions of meiotic prophase nuclei ofAllomyces macrogynus (Chytridiomycetes, Blastocladiales) have been carried out. Serial section reconstructions of pachytene nuclei have revealed that the fungus when grown at 23 °C is an autotetraploid and is a diploid when grown at 35 °C for at least 6 months or on a substrate containing para-fluorophenylalanine for 1–2 weeks.Studies of the duplication and migration of the centrioles during the first stages of prophase revealed the existence of four centrioles in the 23 °C strain after centriole duplication and two centrioles in the 35 °C strain after duplication. It is observed that a bivalent attaches to the nuclear envelope at a site where a centriole is situated. It is proposed that the presence of the four centrioles in the 23 °C strain is due to the fact that each bivalent is represented twice in the strain.     

Comparative analysis of callus formation and regeneration on cultured immature maize embryos of the inbred lines A188 and A632

Induction, maintenance, differentiation and embryogenic capacity of callus obtained from immature embryos by culture on induction medium, proliferation medium, maturation medium and regeneration medium, respectively, were compared for two inbred lines of maize, i.e. A188 and A632. The callus of inbred line A188 was embryogenic and maintained embryogenic capacity for at least 1 year. Immature embryos of inbred line A632 formed callus that was not embryogenic. It only produced roots. When sucrose was replaced by sorbitol to induce or improve embryogenesis, again only A188 formed embryogenic callus. Subculture of this callus, however, allowed 4 week intervals in stead of 2 week intervals without loss of embryogenic capacity. When A188 was pollinated with A632 pollen, embryogenic callus was obtained from cultured immature "F₁" embryos, showing that embryogenic capacity was inherited, maternally. The callus did not differ from the embryogenic callus generated on selfed A188 embryos. When A632 was pollinated with A188 pollen, embryogenic callus was obtained too, showing that embryogenic capacity was also inherited paternally, though the embryogenic capacity diminished quickly, and the stability of the callus was lower than in the reciprocal cross. This revised version was published online in June 2006 with corrections to the Cover Date.     

RAPD analysis of diploid and tetraploid populations of Aronia points to different reproductive strategies within the genus

Plant breeding in black chokeberry (Aronia melanocarpa) is based mainly on seedlings derived from domesticated Russian plants. Previous studies have, however, demonstrated very low levels of phenotypic variation within this gene pool. The present investigation was undertaken in order to study the genetic structure in native populations growing in North America. Random amplified polymorphic DNA (RAPD) marker variation was analysed in eight populations (three or five mother plants/population and five seedlings/mother plant) and compared with the variation in four cultivars and 15 seedlings derived from a Russian plantation. The four cultivars and all the Russian seedlings turned out to have identical RAPD profiles. In the native plant material, there were two types of mother plants: diploid plants that produced very heterogeneous offspring and tetraploid plants that produced homogeneous offspring. Partitioning of variability based on Shannon's diversity index attributed approx. 22% of the variation to the among-population level in diploids, compared to approx. 55% in the tetraploids. However, the diploid populations and the tetraploid populations did not differ significantly in within-population variation. These results prompted a second set of RAPD analyses, which were carried out on offspring obtained through open pollination of the initially examined material when growing in an experimental field. The analyses showed that tetraploid plants produced tetraploid offspring that, with few exceptions, were identical, indicating apomixis, whereas offspring of diploid plants were diploid or triploid, and highly heterogeneous, indicating outcrossing. Presumably, the tetraploid form of Aronia is an allopolyploid, with A. melanocarpa as one of the parents.