FIGURE 1.

Schematic representation of the aging C. elegans adult hermaphrodite reproductive system. (A) Early adult gonad anatomy. The germline progenitor zone is located at the distal end of the gonad. As progenitors proliferate, they are displaced and move proximally (“germline flux”), progressing through oogenic prophase of meiosis I until they become arrested in diakinesis at the proximal end of the oviduct, awaiting maturation-inducing signals from sperm in the spermatheca. (B) Simplified schematic of a days 3–4 adult gonad, depicting distal stem/progenitor pool and proximal arrested oocytes, sperm in the spermatheca and embryos in the uterus. (C) Schematics representing aged (days 5–6) sperm-depleted and sperm-replete conditions. (D) Schematic representing a post-reproductive day 12 gonad. Regardless of sperm availability, reproduction has ceased, and the progenitor zone is severely reduced. Diagrams in (A–D) represent wild-type worms. Additional notes: The progenitor zone of aged hermaphrodites contains fewer cells than at adult day 1 peak; in mated hermaphrodites (as yet untested in the other conditions) SYGL-1- and LST-1- positive pools also decline. Sperm-depleted hermaphrodites accumulate unfertilized oocytes in the oviduct, whereas sperm-replete hermaphrodites do not. The PZ mitotic cell cycle also slows, as does the rate of meiotic entry and the rate of progression through prophase of meiosis I. These age-related changes in bulk germline flux contribute to the reduction in progeny production with age. The numbers of oocytes are schematized here to emphasize flux vs. stacking; Kocsisova et al. (2019) report an average of ∼14 oocytes are in diplotene and diakinesis at days 1 and 3 and an average of ∼9 at day 5 in mated hermaphrodites. See text for details and additional references.