Hypothesis: Germline rejuvenation during meiosis underlies animal oogenesis

Animal oogenesis utilizes features shared among diverse phylogenetic groups, whose functional roles in promoting progeny development have remained unclear. However, germ cells not only produce the next generation, they also maintain long term species integrity by fully restoring acquired damage that deviates from genomic specifications and by controlling parasitic elements that pose a multi-generational threat. Here we discuss how oogenesis "rejuvenates" the germline to sustain the effective immortality needed for species to survive, adapt and evolve. We argue that animal oocytes actively rebuild critical cellular components including organelles during meiosis, usually within an ancient syncytial format, the germline cyst, that enhances renewal mechanisms long inherent to meiosis in single-celled eukaryotes. The Balbiani body accumulates rejuvenated materials and connects them with germ cell inducers relatively early in oogenesis, to ensure genome-quality constituents are inherited by germ cells of the next generation. This strategy explains the existence of germ plasm and why the GV-Bb axis presages the embryo's animal-vegetal axis in diverse species. The advent of a more powerful rejuvenation system may have enabled the evolution of animals.