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1. Gene-Level, but Not Chromosome-Wide, Divergence between a Very Young House Fly Proto-Y Chromosome and Its Homologous Proto-X Chromosome

   https://doi.org/10.1093/molbev/msaa250  

   Son, Jae Hak; Meisel, Richard P (September 2020, Molecular Biology and Evolution)

   null (Ed.)

   Abstract X and Y chromosomes are usually derived from a pair of homologous autosomes, which then diverge from each other over time. Although Y-specific features have been characterized in sex chromosomes of various ages, the earliest stages of Y chromosome evolution remain elusive. In particular, we do not know whether early stages of Y chromosome evolution consist of changes to individual genes or happen via chromosome-scale divergence from the X. To address this question, we quantified divergence between young proto-X and proto-Y chromosomes in the house fly, Musca domestica. We compared proto-sex chromosome sequence and gene expression between genotypic (XY) and sex-reversed (XX) males. We find evidence for sequence divergence between genes on the proto-X and proto-Y, including five genes with mitochondrial functions. There is also an excess of genes with divergent expression between the proto-X and proto-Y, but the number of genes is small. This suggests that individual proto-Y genes, but not the entire proto-Y chromosome, have diverged from the proto-X. We identified one gene, encoding an axonemal dynein assembly factor (which functions in sperm motility), that has higher expression in XY males than XX males because of a disproportionate contribution of the proto-Y allele to gene expression. The upregulation of the proto-Y allele may be favored in males because of this gene’s function in spermatogenesis. The evolutionary divergence between proto-X and proto-Y copies of this gene, as well as the mitochondrial genes, is consistent with selection in males affecting the evolution of individual genes during early Y chromosome evolution. 

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2. Assembly of the threespine stickleback Y chromosome reveals convergent signatures of sex chromosome evolution

   https://doi.org/10.1186/s13059-020-02097-x  

   Peichel, Catherine L.; McCann, Shaugnessy R.; Ross, Joseph A.; Naftaly, Alice F.; Urton, James R.; Cech, Jennifer N.; Grimwood, Jane; Schmutz, Jeremy; Myers, Richard M.; Kingsley, David M.; et al (December 2020, Genome Biology)

   null (Ed.)
3. Moving and stopping: Regulation of chromosome movement to promote meiotic chromosome pairing and synapsis

   https://doi.org/10.1080/19491034.2017.1358329  

   Alleva, Benjamin; Smolikove, Sarit (September 2017, Nucleus)
4. Chromosome number, sex determination, and meiotic chromosome behavior in the praying mantid Hierodula membranacea

   https://doi.org/10.1371/journal.pone.0272978  

   Paliulis, Leocadia V.; Stowe, Emily L.; Hashemi, Leila; Pedraza-Aguado, Noemi; Striese, Cynthia; Tulok, Silke; Müller-Reichert, Thomas; Fabig, Gunar (August 2022, PLOS ONE)

   Cimini, Daniela (Ed.)

   Praying mantids are important models for studying a wide range of chromosome behaviors, yet few species of mantids have been characterized chromosomally. Here we show that the praying mantid Hierodula membranacea has a chromosome number of 2n = 27, and X 1 X 1 X 2 X 2 (female): X 1 X 2 Y (male) sex determination. In male meiosis I, the X 1 , X 2 , and Y chromosomes of H . membranacea form a sex trivalent, with the Y chromosome associating with one spindle pole and the X 1 and X 2 chromosomes facing the opposite spindle pole. While it is possible that such a sex trivalent could experience different spindle forces on each side of the trivalent, in H . membranacea the sex trivalent aligns at the spindle equator with all of the autosomes, and then the sex chromosomes separate in anaphase I simultaneously with the autosomes. With this observation, H . membranacea can be used as a model system to study the balance of forces acting on a trivalent during meiosis I and analyze the functional importance of chromosome alignment in metaphase as a preparatory step for subsequent correct chromosome segregation. 

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5. Mechanical Mechanisms of Chromosome Segregation

   https://doi.org/10.3390/cells10020465  

   Anjur-Dietrich, Maya I.; Kelleher, Colm P.; Needleman, Daniel J. (February 2021, Cells)

   Chromosome segregation—the partitioning of genetic material into two daughter cells—is one of the most crucial processes in cell division. In all Eukaryotes, chromosome segregation is driven by the spindle, a microtubule-based, self-organizing subcellular structure. Extensive research performed over the past 150 years has identified numerous commonalities and contrasts between spindles in different systems. In this review, we use simple coarse-grained models to organize and integrate previous studies of chromosome segregation. We discuss sites of force generation in spindles and fundamental mechanical principles that any understanding of chromosome segregation must be based upon. We argue that conserved sites of force generation may interact differently in different spindles, leading to distinct mechanical mechanisms of chromosome segregation. We suggest experiments to determine which mechanical mechanism is operative in a particular spindle under study. Finally, we propose that combining biophysical experiments, coarse-grained theories, and evolutionary genetics will be a productive approach to enhance our understanding of chromosome segregation in the future. 

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