![]() We found that in naturally and experimentally produced F1 hybrids asexuality is achieved by genome endoreplication, which occurs in gonocytes just before entering meiosis or, rarely, one or a few divisions before meiosis. Here, we investigated meiotic and premeiotic cells of diploid and triploid hybrids of spined loaches (Cypriniformes: Cobitis) that reproduce by gynogenesis. ![]() However, it is still not clear when endoreplication occurs, how many gonial cells it affects and whether its rate differs among clonal lineages. The gametogenesis of many hybrid asexuals involves premeiotic genome endoreplication leading to bypass hybrid sterility and forming clonal gametes. The transition from sexual reproduction to asexuality is often triggered by hybridization. In a t(14 21) Robertsonian translocation, the translocated chromosome and the two normal counterparts form a trivalent. In t(11 22) reciprocal translocation, two translocated chromosomes in concert with two normal counterparts form a quadrivalent to complete synapsis. (C) Pachytene configuration in balanced carriers of chromosomal translocation. These Holliday junctions are resolved in one of two ways, crossover (right) or non-crossover (left), with the Holliday junction resolvase (green wedges). DNA synthesis seals the DSBs and the double Holliday junctions emerge. Next, a single-stranded DNA end invades the homologous duplex DNA, forming a D-loop structure. With the aid of RAD51 or DMC1, the DNA ends produce nucleofilament complexes (blue circles) that facilitate genome-wide homology scanning in order to find homologous chromosomes (red lines). The 5′-ends of the DSBs are then resected and the 3′-protruding single-stranded ends are generated. Initially, programmed DSBs are induced by SPO11 endonuclease (pink circles). (B) Schematic representation of meiotic recombination. After the disassembly of SC, two homologous chromosomes are pulled in opposite directions to the two spindle poles (gray). HR results in the production of at least one obligatory chiasma via crossover. Then, two homologous chromosomes undergo pairing, and a synaptonemal complex (pink ladder) is established between them. During the pre-meiotic S-phase, both maternal (red) and paternal (green) chromosomes are replicated and tightly connected to cohesin complexes (purple circles). (A) The critical steps that affect chromosomal segregation in meiosis I. Schematic representation of the prophase of meiosis I.
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