Triploid Endosperm Research Articles

ADVENTIVE EMBRYOGENESIS IN CITRUS AND ITS RELATION TO POLLINATION AND FERTILIZATION

Cytological and histological studies of seeds from three facultative apomictic Citrus cultivars show that adventive embryos develop, as a rule, from the first few cell layers of the nucellus adjacent to the embryo sac in the micropylar half and occasionally from the chalazal end. The adventive embryos initiated in nucellar tissue away from the embryo sac and most of those initiated from the chalazal end of the nucellus do not develop beyond the one‐celled stage. When two or more embryos are developing in the same seed, the successful development of a given embryo depends on its location in relation to access to nutrients from the endosperm. The presence of a zygote and triploid endosperm in seeds with adventive embryos, the abortion of seed when endosperm degenerates, and the lack of seed set without pollination indicate that pollination and fertilization are essential for in vivo adventive embryogenesis.

An ultrastructural study of early endosperm development and synergid changes in unfertilized cotton ovules

Excised, unfertilized cotton (Gossypium hirsutum L.) ovules were cultured for 1-5 days postanthesis and embryo-sac development was studied with the electron microscope. In some ovules the two polar nuclei fuse and the diploid endosperm nucleus goes through a limited number of free nuclear divisions after 2-3 days in culture. Each nucleus has two nucleoli, in contrast to nuclei of fertilized triploid endosperm which have three nucleoli. Precocious cell walls form between the endosperm nuclei on the 3rd day in culture. The morphology of the plastids, mitochondria, rough endoplasmic reticulum (RER), dictyosomes and microbodies, and the amount of starch and lipid in the diploid cellular endosperm are similar to those of the central cell. A few large helical polysomes appear close to plastids and mitochondria. After 2 days in culture, one of the two synergids in the unfertilized cultured ovules shows degenerative changes which in fertilized ovules are associated with the presence of the pollen tube, i.e., increase in electron density, collapse of vacuoles, irregular darkening and thickening of mitochondrial and plastid membranes, disappearance of the plasmalemma and the membranes of the plasmalemma and the membranes of the RER. The second synergid remains unchanged in appearance. The egg cell does not shrink or divide or show structural changes characteristic of the cotton zygote. Embryo-sac development is arrested on the 4th and 5th days in culture. The nucellus continues growth and at 14 days crushes the degenerate embryo sac.

Effects of homozygosity and heterozygosity on certain properties of genetically defined electrophoretic variants of alcohol dehydrogenase isozymes in maize

Genetically defined alcohol dehydrogenase isozymes have been studied. The faster electrophoretically migrating forms of both Adh-1 and Adh-2 have greater specific activity and are more heat stable than the alternative slow forms of the two isozymes. In the heterozygous state, it is confirmed by genetic dosage studies that the Adh-2 F/F homodimer is more catalytically active than the S/S homodimer. The heat stability relationship between the two homodimers is maintained in the heterozygous condition. Adh-1 also has the same heat stability when taken from the heterozygous as well as the homozygous background. The heterodimeric, hybrid molecules (F/S and S/F for Adh-2) have less catalytic activity than the F/F homodimer but more than the homodimer S/S enzyme. This is concluded from studies in the triploid endosperm where the effects of genetic dosing can be investigated. The hybrid enzymes have heat stability similar to the F/F homodimer. The finding that allelic forms of an enzyme can show altered properties of possible physiological advantage to the organism is discussed in relation to fitness of the alternative forms.

GENETIC CONTROL OF VARIATION IN A SELECTED POPULATION OF LOLIUM PERENNE L.

Progenies from a five genotype diallel cross in Lolium perenne L. were studied to assess the pattern of genetic control for seed weight, a number of seedling characters, and two adult plant characters.Variation in seed weight was largely determined by the maternal parent, but some control by the pollen parent was apparent. Gene action was additive, but some specific genetic effects were also present, suggesting possible interaction between the haploid pollen genome with a diploid ovular genome in the triploid endosperm, or alternatively between the developing seed and the female parent plant.Of the seedling characters (total germination, rate of germination, rate of leaf and tiller production, length of fifth leaf, and seedling dry matter production) only length of fifth leaf showed any additive variation which might be easily exploited.Adult plant production showed considerable variation which interacts with the season of assessment. However, the relative order of merit remained the same and only the magnitude of the differences in productivity between the families altered. This response was itself under simple genetic control.

DEPENDENCE OF THE R-MOTTLED ALEURONE PHENOTYPE IN MAIZE ON MODE OF SEXUAL TRANSMISSION

YPICAL R-mottling, the irregular distribution of anthocyanin in the aleurone Tlayer of maize endosperm, occurs upon crossing appropriate R R plants with r r (colorless) only when R R is the pollen parent (EMERSON 1918; KEMPTON 1919). When R R is used in the mating as female, the aleurone is solidly colored. From a conventional point of view the mottled and solidly colored phenotypes suggest the dosage effects associated with the unequal numbers of chromosome sets contributed to the triploid endosperm, two from the female and one from the male parent. This explapation assumes equal pigmenting action of R from the two origins. Furthermore, if parental source were to affect R expression, such an influence ordinarily would be confounded with the effect of dosage. ROMAN’S (1947) discovery that a given chromosome segment translocated to the centromeric portion of an accessory B chromosome in maize is transmitted by the male in duplicate provides an opportunity to vary the dosage of genes in such a segment independently of parental origin. Accordingly, his translocation B-loa, with the point of interchange in chromosome-10 between R and the centromere, permits introduction of two R factors into the endosperm via pollen. Unexpectedly on the R dosage interpretation, R R from the male when combined with rr from the female gave mottled, rather than solidly colored, kernels (ROMAN, cited by SCHWARTZ 1965; KERMICLE 1963). This observation called into question sufficiency of the dosage explanation and prompted further investigation. The evidence presented here shows the basis of the disparity in aleurone phenotype following reciprocal crosses between R R and r r plants is a pronounced difference in level of action at which R is transmitted to the endosperm through male and female gametophytes. MATERIALS AND METHODS The various alleles of R involved affect anthocyanin pigmentation of the aleurone and vegetative parts of the plant as follows: RV-Pigmented aleurone, red seedling and anthers; representative of those alleles which give darkly mottled aleurone in crosses to rgrg as female but solidly colored aleurone when rgrg is male. The particular Rr employed in experiments involving translocation B-loa was that present in the TB-loa stock obtained from the Maize Genetics Stock Center, University of Illinois, Urbana. A second source of Rr, that designated standard Rr in the Wisconsin collection, was 1Paper No. 1357 from the Laboratory of Genetics. Supported by AEC contract AT(ll-1)-1300 and NIH grant GM 15422-01.

The isolation of highly polymerized deoxyribonucleic acid from maize

1. The object of the study was the isolation of highly polymerized DNA from a plant source. 2. Maize kernels in the milk stage were chosen as a source because of the free nucleate stage of the triploid endosperm and the absence of chlorophyll. 3. A method, based on phenol treatment in the presence of a NaCl-sodium dodecylsulphate-Tris-EDTA buffer (pH 9.0) is described for isolating DNA. 4. Ultracentrifugation and viscosity show that the DNA can be isolated in different sizes, depending on the extraction procedure. 5. The sedimentation coefficient, at zero concentration, of the largest molecules agree well with those of certain bacteria and bacteriophages.

The loops and ultrastructure of the nucleolus of Ipheion uniflorum.

1. The nucleolus of Ipheion uniflorum was studied in root tips and endosperm in respect of structure of internal loops made visible with phase-contrast by means of the detergent “Tween 80”, and with the electron microscope in ultrathin sections of root tips, in respect of ultrastructure. 2. Two loops were observed in each large fused nucleolus in the diploid root tissue and three in those of the triploid endosperm. 3. The loops in this species, which are confined for the most part toward the centre of the nucleolus, attain a length of about 20 μ in nucleoli 15 μ in diameter, and irrespective of nucleolar size a thickness of about 1.4 μ during interphase. 4. Strands, containing DNA as shown by application of DNase to ultrathin sections, and whose staining behaviour with “electron stains” was similar to chromatin, were observed with the electron microscope in a channel of irregular width which evidently runs throughout each loop axis. 5. These strands, which were sometimes seen in large clusters up to 0.13 μ across, appeared to have a common diameter of about 200 A, derived from coiling of a fibre slightly less than 25 A. 6. The strands were flanked by two other loop elements: a unit 20 A-wide coiled singly in strands 100 A in diameter and another of duplex character 50 A-wide consisting of loosely coiled 20 A subunits; the duplex units were often seen running in closely parallel groups. 7. Nucleolar vacuoles, common to all species, appeared spasmodically in the loop mass, seemingly caught between the loops. 8. Particles, averaging 250 A in diameter and containing a tightly coiled fibril slightly thicker than 20 A, were seen outside the channel and vacuoles in the loop mass, in all or most of the nucleoli of sections of root tips from two fixations. 9. In sections of most fixations, particles (nucleolar ribosomes) averaging 150 A in diameter were seen in association with 100 A-wide threads which merged with the loop elements at the periphery of the nucleolus; both the particles and threads contained fibres 15–20 A in diameter, coiled to give their respective structures. No clear evidence was found in support of the premise that these nucleolar particles represent segments of the threads with which they are associated. 10. In sections of root tips which had been RNased between fixation and dehydration, the 150 A peripheral particles and 50 A duplex loop elements were not seen. It was not certain whether some other major elements were affected.

THE WAXY LOCUS IN MAIZE. I. INTRALOCUS RECOMBINATION FREQUENCY ESTIMATES BY POLLEN AND BY CONVENTIONAL ANALYSES

THE waxy (wx) locus in maize conditions the type of starch which is synthesized in the triploid endosperm and in the haploid pollen grain. Further, the phenotype of a pollen grain is determined by its own genotype and not by that of the plant which produces the pollen grain (BRINK and MACGILLIVRAY 1924) and (DEMEREC 1924). This makes it possible to use the pollen grain as the unit of observation in genetical investigations on the wx locus. The Wx pollen grains stain blue with an iodine-potassium iodide stain while wx pollen grains stain a reddish-brown color. Since a maize plant produces millions of pollen grains, large populations can be obtained and scored easily, thus markedly enhancing genetic resolution. The investigation of low frequency events is more easily carried out with this system than with any other locus in a higher organism