Meiotic drivers are genetic variants that selfishly manipulate the production of gametes to increase their own rate of transmission, often to the detriment of the rest of the genome and the individual that carries them. This genomic conflict potentially occurs whenever a diploid organism produces a haploid stage, and can have profound evolutionary impacts on gametogenesis, fertility, individual behaviour, mating system, population survival, and reproductive isolation. Multiple research teams are developing artificial drive systems for pest control, utilising the transmission advantage of drive to alter or exterminate target species. Here, we review current knowledge of how natural drive systems function, how drivers spread through natural populations, and the factors that limit their invasion. Trends Box  Both naturally occurring and synthetic "meiotic drivers" violate Mendel's law of equal segregation and can rapidly spread through populations even when they reduce the fitness of individuals carrying them. Synthetic drivers are being developed to spread desirable genes in natural populations of target species. How ecology influences the population dynamics of meiotic drivers is important for predicting the success of synthetic drive elements. An enduring puzzle concerns why some meiotic drivers persist at stable, intermediate frequencies rather than sweeping to fixation. Drivers can have a wide range of consequences from extinction to changes in mating system. preferentially associating with and moving toward the egg pole at Meiosis I) will be 75 transmitted to more than half of the maturing eggs. Although this bias does not necessarily 76 reduce the production of eggs (as only one egg matures per meiosis), the fitness of other 77 alleles at the same locus, that do not bias transmission, and alleles linked to them, is 78 reduced. Such meiotic drivers could reduce the fitness of individuals that carry them, if the 79 driving variant is genetically linked to deleterious mutations or has deleterious pleiotropic 80 effects. 81Male meiotic drive takes multiple forms -some at least partially meiotic, some entirely 82 post-meiotic -but all involve a driving element that prevents maturation or function of 83 sperm that do not contain it. Because haploid sperm within a single ejaculate compete to 84 fertilize the same pool of eggs, disabling non-carrier sperm results in transmission of the 85 driving element to more than half of the functional gametes and resulting offspring ([5], Box 86 1). However, disabling non-carrier sperm often reduces fertility [6]. 87Spore drive in fungi, in which the products of meiosis are packaged together in an ascus, 88 operates via similar mechanisms. Spores with one haploid genotype will kill or disable 89 spores of the alternative haplotype ([7], Box 1). If spores disperse long distances sibling 90 spores are unlikely to compete and killing them will not increase the killer's fitness. 91However, spore killing can be beneficial if there is local resource competition. 92Excit...

The mitotic and meiotic chromosomes of four male East-Asiatic mice, Apodemus peninsulae, having three to seven chromosomes in addition to the standard karyotype (2n = 48), were investigated. B-chromosomes were represented by medium-sized metacentric and dotlike chromosomes. Mosaicism of bone marrow cells due to a numerical variation of accessory chromosomes was established for the males examined. Capacity of B-chromosomes to- form axial elements and synaptonemal cornplexes in meiotic prophase I was revealed by electron microscopy. The occurrence of univalents of different morphology, bivalents, and multivalents, corresponding to B-chromosomes, was demonstrated. An increase in the number of B-chromosomes was found in spermatocytes at zygotene-pachytene relative to the number in bone marrow cells, which may be evidence of B-chromosome accumulation in the germ cell line of the East-Asiatic mouse.

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