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Abstract The supernumerary B chromosome of maize has a drive mechanism to maintain itself in a population despite being dispensible. This involves nondisjunction of the B centromere at the second pollen mitosis that produces the two sperm followed by preferential fertilization of the egg by the B containing sperm during double fertilization. During an introgression of the supernumerary B chromosome into the inbred line B73, an unusually high frequency of trisomies for A chromosomes was observed. Due to parallels to the High Loss phenomenon in which three or more B chromosomes in a specific genetic background cause chromosomal breakage at heterochromatic knob sites during the second pollen mitosis as well as ploidy changes, this phenomenon was revisited. Examination of pollen of the High Loss line revealed a high frequency of single sperm in the presence of the B chromosomes, which was previously not realized. Crosses to tetraploid females confirmed that the single sperm were diploid and functional but also revealed the presence of diploids with their A chromosomes derived solely from the tetraploid parent indicating a “diploid induction”. Collectively, the results reveal two backgrounds in which the B drive mechanism is not confined to this chromosome causing detrimental effects by adherence of heterochromatic knobs and apparently A centromeres at the mitosis preceding sperm development. In most genetic backgrounds this process is restricted to the B chromosome but in B73 and the High Loss line, there is spillover to the normal chromosomes in distinct ways. 

Abstract The prompt emission mechanism of gamma-ray bursts (GRBs) is still unclear, and the time-resolved spectral analysis of GRBs is a powerful tool for studying their underlying physical processes. We performed a detailed time-resolved spectral analysis of 78 bright long GRB samples detected by Fermi/Gamma-ray Burst Monitor. A total of 1490 spectra were obtained and their properties were studied using a typical Band-shape model. First, the parameter distributions of the time-resolved spectrum are given as follows: the low-energy spectral indexα∼ − 0.72, high-energy spectral indexβ∼ − 2.42, the peak energyE_p∼ 221.69 keV, and the energy fluxF∼ 7.49 × 10⁻⁶erg cm⁻²s⁻¹. More than 80% of the bursts exhibit the hardest low-energy spectral index ${\alpha }_{\max }$exceeding the synchrotron limit (−2/3). Second, the evolution patterns ofαandE_pwere statistically analyzed. The results show that for multi-pulse GRBs the intensity-tracking pattern is more common than the hard-to-soft pattern in the evolution of bothE_pandα. The hard-to-soft pattern is generally shown in single-pulse GRBs or in the initial pulse of multi-pulse GRBs. Finally, we found a significant positive correlation betweenFandE_p, with half of the samples exhibiting a positive correlation betweenFandα. We discussed the spectral evolution of different radiation models. The diversity of spectral evolution patterns indicates that there may be more than one radiation mechanism occurring in the GRB radiation process, including photospheric radiation and synchrotron radiation. However, it may also involve only one radiation mechanism, but more complicated physical details need to be considered. 

Abstract The B chromosome in maize is a supernumerary chromosome that due to its dispensability is present in only some lines of maize. Over its evolution, the B chromosome has developed a two-part drive mechanism that ensures its continued presence in maize populations. Its drive mechanism involves nondisjunction at the second pollen mitosis in which two sperm cells are produced and preferential fertilization by the sperm with the two B chromosomes more often joining with the egg as opposed to the central cell in the process of double fertilization. Previous work had suggested some lines of maize exhibit a different response and that this was controlled by the female parent. We sought to examine the variation for this trait by testing a wide spectrum of characterized maize lines. Most inbred lines exhibit the canonical preference for the egg cell, some appear to have random fertilization, and one inbred line (B73) shows a preference for the B containing sperm to fertilize the central cell. 

Abstract Barbara McClintock recognized transposable elements originally by the movement of a site of chromosomal breakage, a genetic element calledDissociation(Ds) that was induced to break or transpose by another element she calledActivator. The chromosome breaking version, when analyzed on the molecular level was one transposon inside another. It is now known that transposition involving transposon termini in non-standard orientation with reference to each other results in chromosomal breakage. Here we used engineered transposon ends together with a phenotypic marker to cause targeted chromosomal breaks. The results indicate that engineered direct orientation of the naturally inverted repeats ofDissociationcan cause chromosomal breakage at the transgenic sites of insertion.