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Once the germ cells have migrated to the gonad, they can begin meiosis. Meiosis is perhaps the most revolutionary invention of eukaryotes. It is difficult now to appreciate how startling this concept was for biologists at the end of the nineteenth century. The discovery of meiosis signaled the critical breakthrough for the investigation of inheritance. Van Beneden’s 1883 observations that the divisions of germ cells caused the resulting gametes to contain half the diploid number of chromosomes “demonstrated that the chromosomes of the offspring are derived in equal numbers from the nuclei of the two conjugating germ-cells and hence equally from the two parents” (Wilson 1924). All subsequent theories of heredity, including the Sutton-Boveri model that united Mendelism with cell biology, are based on meiosis as the mechanism for sexual reproduction and the transmission of genes from one generation to the next. Meiosis completes the cycle of life. The body decays and dies; but the gametes formed by meiosis
In some species, meiosis is modified to allow for the production of progeny without sexual recombination. Some animal species consist entirely of females; such species are said to be parthenogenetic (Greek, “virgin birth”). In these species, meiosis is modified such that the resulting gamete is diploid and need not be fertilized to develop. In the fly Drosophila mangabeirai, one of the polar bodies (a meiotic cell having very little cytoplasm) acts as a sperm and “fertilizes” the oocyte after the second meiotic division. In some other insects and in the lizard Cnemidophorus uniparens, the oogonia further double their chromosome number before meiosis, so that the halving of the chromosomes restores the diploid number. The germ cells of the grasshopper Pycnoscelus surinamensis dispense with meiosis altogether, forming diploid ova by two mitotic divisions (Swanson et al. 1981). About 80 species are known to be exclusively parthenogenetic, and several other species have been observed to occasionally produce offspring this way (Booth et al. 2012).
In other species, haploid parthenogenesis is widely used not only as a means of reproduction but also as a mechanism of sex determination. In the Hymenoptera (bees, wasps, and ants), unfertilized haploid eggs develop into males, whereas fertilized eggs are diploid and develop into females. The haploid males are able to produce sperm by abandoning the first meiotic division, thereby forming two sperm cells through second meiosis. There are even several modifications of these schemes in different species of Hymenoptera.