The Origin and Evolution of Meiosis

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Abstract
The evolution of meiosis from mitosis was studied in the chromosomes of protozoa from the wood-feeding roach, Cryptocercus punctulatus. Two processes occurring in these protozoa, namely, permanent mitotic di-ploidy and meiosis without gametogenesis, lie between fertilization and mitosis and are significant. In the genus Holomastigotoides, the haploid chromosome number is 2, but in many spp. diploids and polyploids are present and are derived from the basic 2-chromosome form. Every division of the polyploids is exactly like the first division in meiosis. The diploidy is permanent since the centrioles are not duplicated until the chromosomes have also been duplicated. In the 2d process called endomitosis, meiosis occurs without nuclear fission. The process is descr. in Barbulanympha, where the centrioles are so large that the mechanism which makes the chromosome reduction possible is the accelerated duplication and function of the centrioles without duplication of the chromosomes. The centrioles are duplicated betw. the 1st and 2d meiotic divisions while the chromosomes are not and the centrioles make up for the generation which was lost earlier. Sometimes the centrioles produce an achromatic figure and nuclear division occurs. The daughter nuclei fuse. This process is autogamy. Polyploidy is always relieved by meiosis in Barbulanympha and no degeneration results. In gametic meiosis before fertilization and in zygotic meiosis following fertilization, centrioles are duplicated without chromosome duplication and the extra generation makes up for the one at fertilization by one of the gametes. In changing from mitosis to meiosis the centrioles gain a generation on the chromosomes, and in changing back to mitosis, they lose a generation. In the transition from mitosis to meiosis the first step is diploidy and anything which prevents the production or function of an achromatic figure can cause a change from haploidy to diploidy. Barbulanympha has evolved a method for changing from haploidy to diploidy and vice versa. In the next step, the loss of a generation of centrioles does not occur until after the nucleus divides. Cytoplasmic division does not occur, the nuclei fuse, the chromosomes are duplicated, and two meiotic divisions change them to haploids. In the final step the cytoplasm divides and produces gametes. Trichonympha goes through all the stages that the other organisms do and, in addition, produces gametes.

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