A molecular cytogenetic analysis of chromosome behavior in Lilium hybrids

Lily (Lilium) has become one of the top bulbous crops for the cut flower industry in the past two decades. The genus Lilium comprises of approximately 80 species, which have been classified into seven sections. Each section possesses distinctive phenotypic characters, such as flower color, flower shape and resistances to diseases and pests. Crosses between species in the same section are relatively easy and the resulting hybrids are in general fertile, while interspecific crosses between species from different sections are rather difficult and the resulting hybrids are in general sterile. As a result, different hybrid groups have been bred in the 20th century. Within these different hybrid groups, Longiflorum (L), Asiatic (A) and Oriental (O), which are derived from the section Leucolirion, Sinomartagon and Archelirion respectively, are of commercial importance and hence, are the most widely cultivated lilies worldwide. Lily hybrids provide an ideal model for molecular cytogenetic research. With the development of techniques of overcoming pre- and post- crossing barriers of interspecific crosses, as well as the application of asexual and sexual polyploidization to restore the fertility of F1 lily hybrids, combining of desirable traits from different hybrid groups has become feasible. As a result, interspecific hybridization and polyploidization have been widely used in the breeding of new cultivars of lily. These cultivars, as well as other breeding materials from interspecific crosses, facilitate the application of molecular cytogenetic analysis due to three reasons: 1) the chromosomes of lily are large enough for cytological observations; 2) genomes of different hybrid groups are homoeologous; and 3) these homoeologous genomes can be simultaneously distinguished by DNA in situ hybridization. Using these lily hybrids combined with genomic in situ hybridization (GISH) and florescence in situ hybridization (FISH), the interaction of homoeologous genomes can be studied though meiotic observation of the F1 hybrids. Meanwhile, chromosome sequential variation with relevance to crossover and chromosome rearrangements can also be observed. For this purpose, interspecific crosses between the Lilium longiflorum cultivar ‘White Fox’ and the Asiatic cultivar ‘Connecticut King’ were made, and some of these F1 hybrids, which show a relatively high fertility with the production of unreduced gametes, were backcrossed with an Asiatic cultivar . The meiosis of the interspecific hybrids, as well as the sexual polyploidized progenies, were analysed by GISH and FISH. In addition, one population of sexual polyploidized AOA hybrids was also analysed for the genome composition. Results showed that there was no evidence that lily allopolyploids possess any noticeable chromosome rearrangements. The equal segregation of reciprocal and non-reciprocal recombinant product showed that the intergenomic recombination in the sexual polyploidized progenies was indeed from a natural process-chiasmata formation and crossovers and hence, should not be considered as translocations as was suggested in literature for intergenomic recombination. This conclusion was further confirmed by meiotic observation of the interspecific F1 hybrids. Detailed meiotic observations were carried out in interspecific hybrids between Longiflorum × Asiatic groups of lilies (Lilium) which were used as parents to generate sexual polyploids with intergenomic recombination. Bivalents involving two homoeologous chromosomes, as well as unpaired univalents were the main configurations at metaphase I. However, in two genotypes, multivalents and bivalents both involving non-homologous pairing of two Asiatic chromosomes were observed. This indicated the presence of a duplication which was common to two non-homologous chromosomes in the hybrids. It is deduced that there was a reciprocal translocation in the Asiatic parent cv. ‘Connecticut King’ and these two genotypes resulted from duplication-deficiency gametes. Results from Anaphase I showed that chiasma formation involving non-sister chromatids gave rise to two strand single, two strand double, three strand double, four strand double and multiple exchanges. It is also noticeable that there was a high frequency of multiple crossovers in the genotypes with duplication, indicating a reduced crossover interference in multivalents. Beside the normal crossovers, also chromosome bridges at anaphase I of meiosis were observed. GISH and FISH painting showed that these bridges involve not only non-sister chromatids but also sister-chromatids. The bridges, without any differentiation along their length, were always accompanied by fragments with a variable size. These results indicated that the bridges, together with the accompanying fragments, were derived from U-type exchanges. Other than homologous recombination (HR), nonhomologous end joining (NHEJ) probably led to the production of bridges when repairing the double strand breaks (DSBs) during meiosis. Progenies from unilateral polyploidization of crosses between LA hybrids and Asiatic cultivars were predominant triploids. However, three exceptional plants, which possessed 35 normal chromosomes and a small aberrant chromosome instead of the expected normal number of 36, were observed. In all three cases the small aberrant chromosomes were isochromosomes which had obviously originated during the first backcross generation, and the length of the arms of these aberrant chromosomes were always related with the length of the short arm of the missing chromosome. Furthermore, one of these three chromosomes possessed 45S rDNA hybridization sites in the proximal positions, which resembles the short arm of the missing chromosome (chromosome 4 of L genome). Combined with the results of chromosome breakage during meiosis, centric breakage and fusion is a putative mechanism of the production of these isochromosomes. Meanwhile, two small, supernumerary or B chromosomes were detected as extra chromosomes in a tetraploid plant derived from chromosome doubling of an intersectional hybrid (2n=2x=24) between a cultivar of the Longiflorum (L) and the Trumpet (T) group. When this tetraploid LLTT hybrid was crossed with a triploid LLO hybrid (O=Oriental), the B chromosome was transmitted to 73.4% of the progenies. Based on GISH and FISH characterization it was shown that the B chromosome found consisted of two identical arms, with 5S rDNA hybridizing to the majority of it, which were flanked by normal telomeres, suggesting that this is an isochromosome. The results of current investigations are of practical implication for a number of reasons. Firstly, the behavior of homoeologous chromosomes during meiotic processes in lily hybrids was studied in detail, and it can be used to explain the profound genetic changes in the early generations during hybrid speciation. Secondly, some problems that go unnoticed in genetic mapping can be predicted and well explained by the occurrence of chromosome rearrangements in the parents which are used to produce the segregation population and thirdly, the discovery of U-type exchanges during meiosis and de novo isochromosomes in the backcross progenies supplies an alternative mechanism for the origin of B chromosomes.</p

Hund's Rule for Composite Fermions

We consider the ``fractional quantum Hall atom" in the vanishing Zeeman energy limit, and investigate the validity of Hund's maximum-spin rule for interacting electrons in various Landau levels. While it is not valid for {\em electrons} in the lowest Landau level, there are regions of filling factors where it predicts the ground state spin correctly {\em provided it is applied to composite fermions}. The composite fermion theory also reveals a ``self-similar" structure in the filling factor range $4/3>\nu>2/3$.Comment: 10 pages, revte

Skyrmion Excitations in Quantum Hall Systems

Using finite size calculations on the surface of a sphere we study the topological (skyrmion) excitation in quantum Hall system with spin degree of freedom at filling factors around $\nu=1$. In the absence of Zeeman energy, we find, in systems with one quasi-particle or one quasi-hole, the lowest energy band consists of states with $L=S$, where $L$ and $S$ are the total orbital and spin angular momentum. These different spin states are almost degenerate in the thermodynamic limit and their symmetry-breaking ground state is the state with one skyrmion of infinite size. In the presence of Zeeman energy, the skyrmion size is determined by the interplay of the Zeeman energy and electron-electron interaction and the skyrmion shrinks to a spin texture of finite size. We have calculated the energy gap of the system at infinite wave vector limit as a function of the Zeeman energy and find there are kinks in the energy gap associated with the shrinking of the size of the skyrmion. breaking ground state is the state with one skyrmion of infinite size. In the presence of Zeeman energy, the skyrmion size is determined by the interplay of the Zeeman energy and electron-electronComment: 4 pages, 5 postscript figures available upon reques

Skyrmion Dynamics and NMR Line Shapes in QHE Ferromagnets

The low energy charged excitations in quantum Hall ferromagnets are topological defects in the spin orientation known as skyrmions. Recent experimental studies on nuclear magnetic resonance spectral line shapes in quantum well heterostructures show a transition from a motionally narrowed to a broader `frozen' line shape as the temperature is lowered at fixed filling factor. We present a skyrmion diffusion model that describes the experimental observations qualitatively and shows a time scale of $\sim 50 \mu{\rm sec}$ for the transport relaxation time of the skyrmions. The transition is characterized by an intermediate time regime that we demonstrate is weakly sensitive to the dynamics of the charged spin texture excitations and the sub-band electronic wave functions within our model. We also show that the spectral line shape is very sensitive to the nuclear polarization profile along the z-axis obtained through the optical pumping technique.Comment: 6 pages, 4 figure

Hartree-Fock Theory of Skyrmions in Quantum Hall Ferromagnets

We report on a study of the charged-skyrmion or spin-texture excitations which occur in quantum Hall ferromagnets near odd Landau level filling factors. Particle-hole symmetry is used to relate the spin-quantum numbers of charged particle and hole excitations and neutral particle-hole pair excitations. Hartree-Fock theory is used to provide quantitative estimates of the energies of these excitations and their dependence on Zeeman coupling strength, Landau level quantum numbers, and the thicknesses of the two-dimensional electron layers. For the case of $\nu$ near three we suggest the possibility of first order phase transitions with increasing Zeeman coupling strength from a many skyrmion state to one with many maximally spin-polarized quasiparticles.Comment: 26 pages, 10 figure

The Effects of Disorder on the ν=1\nu=1 Quantum Hall State

A disorder-averaged Hartree-Fock treatment is used to compute the density of single particle states for quantum Hall systems at filling factor $\nu=1$. It is found that transport and spin polarization experiments can be simultaneously explained by a model of mostly short-range effective disorder. The slope of the transport gap (due to quasiparticles) in parallel field emerges as a result of the interplay between disorder-induced broadening and exchange, and has implications for skyrmion localization.Comment: 4 pages, 3 eps figure

Energy, interaction, and photoluminescence of spin-reversed quasielectrons in fractional quantum Hall systems

The energy and photoluminescence spectra of a two-dimensional electron gas in the fractional quantum Hall regime are studied. The single-particle properties of reversed-spin quasielectrons (QE$_{\rm R}$'s) as well as the pseudopotentials of their interaction with one another and with Laughlin quasielectrons (QE's) and quasiholes (QH's) are calculated. Based on the short-range character of the QE$_{\rm R}$--QE$_{\rm R}$ and QE$_{\rm R}$--QE repulsion, the partially unpolarized incompressible states at the filling factors $\nu={4\over11}$ and ${5\over13}$ are postulated within Haldane's hierarchy scheme. To describe photoluminescence, the family of bound $h($QE$_{\rm R})_n$ states of a valence hole $h$ and $n$ QE$_{\rm R}$'s are predicted in analogy to the found earlier fractionally charged excitons $h$QE$_n$. The binding energy and optical selection rules for both families are compared. The $h$QE$_{\rm R}$ is found radiative in contrast to the dark $h$QE, and the $h($QE$_{\rm R})_2$ is found non-radiative in contrast to the bright $h$QE$_2$.Comment: 9 pages, 6 figure

Three-dimensional Optical-resolution Photoacoustic Microscopy

Optical microscopy, providing valuable insights at the cellular and organelle levels, has been widely recognized as an enabling biomedical technology. As the mainstays of in vivo three-dimensional (3-D) optical microscopy, single-/multi-photon fluorescence microscopy and optical coherence tomography (OCT) have demonstrated their extraordinary sensitivities to fluorescence and optical scattering contrasts, respectively. However, the optical absorption contrast of biological tissues, which encodes essential physiological/pathological information, has not yet been assessable. The emergence of biomedical photoacoustics has led to a new branch of optical microscopy optical-resolution photoacoustic microscopy (OR-PAM), where the optical irradiation is focused to the diffraction limit to achieve cellular1 or even subcellular level lateral resolution. As a valuable complement to existing optical microscopy technologies, OR-PAM brings in at least two novelties. First and most importantly, OR-PAM detects optical absorption contrasts with extraordinary sensitivity (i.e., 100%). Combining OR-PAM with fluorescence microscopy or with optical-scattering-based OCT (or with both) provides comprehensive optical properties of biological tissues. Second, OR-PAM encodes optical absorption into acoustic waves, in contrast to the pure optical processes in fluorescence microscopy and OCT, and provides background-free detection. The acoustic detection in OR-PAM mitigates the impacts of optical scattering on signal degradation and naturally eliminates possible interferences (i.e., crosstalks) between excitation and detection, which is a common problem in fluorescence microscopy due to the overlap between the excitation and fluorescence spectra. Unique for optical absorption imaging, OR-PAM has demonstrated broad biomedical applications since its invention, including, but not limited to, neurology, ophthalmology, vascular biology, and dermatology. In this video, we teach the system configuration and alignment of OR-PAM as well as the experimental procedures for in vivo functional microvascular imaging

Partial wave analysis of J/\psi \to \gamma \phi \phi

Using $5.8 \times 10^7 J/\psi$ events collected in the BESII detector, the radiative decay $J/\psi \to \gamma \phi \phi \to \gamma K^+ K^- K^0_S K^0_L$ is studied. The $\phi\phi$ invariant mass distribution exhibits a near-threshold enhancement that peaks around 2.24 GeV/$c^{2}$. A partial wave analysis shows that the structure is dominated by a $0^{-+}$ state ($\eta(2225)$) with a mass of $2.24^{+0.03}_{-0.02}{}^{+0.03}_{-0.02}$ GeV/$c^{2}$ and a width of $0.19 \pm 0.03^{+0.06}_{-0.04}$ GeV/$c^{2}$. The product branching fraction is: $Br(J/\psi \to \gamma \eta(2225))\cdot Br(\eta(2225)\to \phi\phi) = (4.4 \pm 0.4 \pm 0.8)\times 10^{-4}$.Comment: 11 pages, 4 figures. corrected proof for journa