Sex fuse

Chromosomal fusion plays a recurring role in the evolution of adaptations and reproductive isolation among species, yet little is known of the evolutionary drivers of chromosomal fusions. Because sex chromosomes X and Y in male heterogametic systems, Z and W in female heterogametic systems differ in their selective, mutational, and demographic environments, those differences provide a unique opportunity to dissect the evolutionary forces that drive chromosomal fusions. We estimate the rate at which fusions between sex chromosomes and autosomes become established across the phylogenies of both fishes and squamate reptiles. Both the incidence among extant species and the establishment rate of Y-autosome fusions is much higher than for X-autosome, Z-autosome, or W-autosome fusions.
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Sexual reproduction

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The Benefits of Sex - Molecular Biology of the Cell - NCBI Bookshelf

NCBI Bookshelf. Molecular Biology of the Cell. New York: Garland Science; In the sexual reproductive cycle, haploid generations of cells, each carrying a single set of chromosomes, alternate with diploid generations of cells, each carrying a double set of chromosomes Figure Genomes mix when two haploid cells fuse to form a diploid cell. Later, new haploid cells are generated when a descendant of this diploid cell divides by the process of meiosis.
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Y Fuse? Sex Chromosome Fusions in Fishes and Reptiles

Chromosomal fusion plays a recurring role in the evolution of adaptations and reproductive isolation among species, yet little is known of the evolutionary drivers of chromosomal fusions. Because sex chromosomes X and Y in male heterogametic systems, Z and W in female heterogametic systems differ in their selective, mutational, and demographic environments, those differences provide a unique opportunity to dissect the evolutionary forces that drive chromosomal fusions. We estimate the rate at which fusions between sex chromosomes and autosomes become established across the phylogenies of both fishes and squamate reptiles. Both the incidence among extant species and the establishment rate of Y-autosome fusions is much higher than for X-autosome, Z-autosome, or W-autosome fusions. Using population genetic models, we show that this pattern cannot be reconciled with many standard explanations for the spread of fusions.
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Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete such as a sperm or egg cell with a single set of chromosomes haploid combines with another to produce an organism composed of cells with two sets of chromosomes diploid. Sexual reproduction does not occur in prokaryotes organisms without cell nuclei , but they have processes with similar effects such as bacterial conjugation , transformation and transduction , which may have been precursors to sexual reproduction in early eukaryotes. In the production of sex cells in eukaryotes, diploid mother cells divide to produce haploid cells known as gametes in a process called meiosis that involves genetic recombination. The homologous chromosomes pair up so that their DNA sequences are aligned with each other, and this is followed by exchange of genetic information between them. Two rounds of cell division then produce four haploid gametes, each with half the number of chromosomes from each parent cell, but with the genetic information in the parental chromosomes recombined.
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