Fanconi anaemia pathway induces chromosome shattering
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A wide range of chromosome numbers result from unreduced gamete production in Brassica juncea × B. napus (AABC) interspecific hybrids
The establishment of successful interspecies hybrids requires restoration of a stable “2n” chromosome complement which can produce viable “n” gametes. This may occur (rarely) via recombination between non-homologous chromosomes, or more commonly is associated with a doubling of parental chromosome number to produce new homologous pairing partners in the hybrid. The production of unreduced “2n” gametes (gametes with the somatic chromosome number) may therefore be evolutionarily useful by serving as a key pathway for the formation of new polyploid hybrids, as might specific mechanisms permitting recombination between non-homologous chromosomes. Here, we investigated chromosome complements and fertility in third generation interspecific hybrids (AABC) resulting from a cross between allopolyploids Brassica juncea (AABB) × B. napus (AACC) followed by self-pollination for two generations. Chromosome numbers ranged from 2n = 48–74 in the experimental population (35 plants), with 9–16 B genome chromosomes and up to 4 copies of A genome chromosomes. Unreduced gamete production leading to a putative genome structure of approximately AAAABBCC was hence predicted to explain the high chromosome numbers observed. Additionally, the estimation of nuclei number in post-meiotic sporads revealed a higher frequency of unreduced gametes (0.04–5.21%) in the third generation AABC interspecific hybrids compared to the parental Brassica juncea (0.07%) and B. napus (0.13%). Our results suggest that unreduced gamete production in the subsequent generations following interspecific hybridization events may play a critical role in restoration of more stable, fertile chromosome complements.
FISH mapping in Xenopus pygmaeus refines understanding of genomic rearrangements and reveals jumping NORs in African clawed frogs
Chromosomal rearrangements are fundamental evolutionary drivers leading to genomic diversification. African clawed frogs (genus Xenopus, subgenera Silurana and Xenopus) represent an allopolyploid model system with conserved chromosome numbers in species with the same ploidy within each subgenus. Two significant interchromosomal rearrangements have been identified: a translocation between chromosomes 9 and 2, found in subgenus Silurana, and a fusion between chromosomes 9 and 10, probably widespread in subgenus Xenopus. Here, we study the allotetraploid Xenopus pygmaeus (subgenus Xenopus) based on in-depth karyotype analysis using chromosome measurements and fluorescent in situ hybridization (FISH). We designed FISH probes for genes associated with translocation and fusion to test for the presence of the two main types of rearrangements. We also examined the locations of 5S and 28S ribosomal tandem repeats, with the former often associated with telomeric regions and the latter with nucleolus organizer regions (NORs). The translocation-associated gene mapping did not detect the translocation in X. pygmaeus, supporting the hypothesis that the translocation is restricted to Silurana, but instead identified a pericentromeric inversion on chromosome 2S. The fusion-associated gene mapping confirmed the fusion of chromosomes 9 and 10, supporting this fusion as an ancestral state in subgenus Xenopus. As expected, the 5S repeats were found predominantly in telomere regions on almost all chromosomes. The nucleolar 28S repeats were localized on chromosome 6S, a position previously found only in the closely related species X. parafraseri, whereas other, phylogenetically more distant species have NORs located on different chromosomes. We therefore hypothesize that a jumping mechanism could explain the relatively frequent changes in the location of NORs during Xenopus evolution.
Addressing the Gaps in the Vitamin B12 Deficiency 2024 NICE Guidelines: Highlighting the Need for Better Recognition, Diagnosis, and Management of Pernicious Anaemia
The 2024 NICE guidelines on vitamin B12 deficiency have significant implications for the diagnosis and management of pernicious anaemia (PA), the commonest non-dietary cause of such deficiency. This perspective discusses the guidelines in relation to PA itself, suggests that clearer diagnostic protocols are required, and calls for clinician education to improve the patient journey for those with PA.
Horizontal transfer of accessory chromosomes in fungi – a regulated process for exchange of genetic material?
Horizontal transfer of entire chromosomes has been reported in several fungal pathogens, often significantly impacting the fitness of the recipient fungus. All documented instances of horizontal chromosome transfers (HCTs) showed a marked propensity for accessory chromosomes, consistently involving the transfer of an accessory chromosome while other chromosomes were seldom, if ever, co-transferred. The mechanisms underlying HCTs, as well as the factors regulating the specificity of HCTs for accessory chromosomes, remain unclear. In this perspective, we provide an overview of the observed propensity in reported cases of horizontal chromosome transfers. We hypothesize the existence of a signal that distinguishes mobile, i.e., horizontally transferred, accessory chromosomes from the rest of the donor genome. Recent findings in Metarhizium robertsii and Magnaporthe oryzae, suggest that a mobile accessory chromosome may contain putative histones and/or histone modifiers, which could generate such a signal. Based on this, we propose that mobile accessory chromosomes may encode the machinery required for their own horizontal transmission, implying that HCT could be a regulated process. Finally, we present evidence of substantial differences in codon usage bias between core and accessory chromosomes in 14 out of 19 analysed fungal species and strains. Such differences in codon usage bias could indicate past horizontal transfers of these accessory chromosomes. Interestingly, HCT was previously unknown for many of these species, suggesting that the horizontal transfer of accessory chromosomes may be more widespread than previously thought, and therefore an important factor in fungal genome evolution.
Cytoplasmic flow is a cell size sensor that scales anaphase
During early embryogenesis, fast mitotic cycles without interphase lead to a decrease in cell size, while scaling mechanisms must keep cellular structures proportional to cell size. For instance, as cells become smaller, if the position of nuclear envelope reformation (NER) did not adapt, NER would have to occur beyond the cell boundary. Here we found that NER position in anaphase scales with cell size via changes in chromosome motility, mediated by cytoplasmic flows that themselves scale with cell size. Flows are a consequence of friction between viscous cytoplasm and bulky cargo transported by dynein on astral microtubules. As an emerging property, confinement in cells of different sizes yields scaling of cytoplasmic flows. Thus, flows behave like a cell geometry sensor: astral microtubules approach the boundary causing flow velocity changes, which then affect the velocity of chromosome separation, thus scaling NER.
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