Making Young from Old: How is Sex Designed to Help?

The Danaid Theory of Aging

“…Indeed, Turke ( Turke, 2013 ) argues that a germ-soma distinction need not be between cells . In unicellular organisms, there can be regions that are insulated from adversities, which serve as the germ regions for next generations, while other regions take a soma role, and are pared away through asymmetrical cell division.…”

“…A promising mechanistic line of thought in aging—that aging is due to an extension of development, and to constraints arising from developmental processes and cell differentiation ( Magalhães, 2012 )—may also have relevance for the evolution of aging. Building on the idea of the germ-soma distinction, it has been suggested that the very developmental process in multicellular organisms, during which cells differentiate away from totipotency, is sufficient to explain why aging occurs, as this limits repair ( Turke, 2013 ). This idea is supported by the observation that induced neurons generated directly from human fibroblasts have an ‘old’ epigenetic and metabolic profile, whereas induced neurons from fibroblast-derived induced pluripotent stem cells are “young” ( Mertens et al, 2015 ).…”

The deteriorating soma and the indispensable germline: gamete senescence and offspring fitness

“…These are termed the induction and inheritance modes. The induction mode, typical of mammals, is apparently the more prevalent mode and is thought to be the ancestral condition; here, PGC formation is induced by cell signalling pathways activated by the zygote genome [36,37]. In contrast, in the inheritance mode, PGCs arise from specialized germplasm already present in the oocyte cytoplasm.…”

“…This germplasm contains specific proteins and RNAs needed for PGC formation. The inheritance mode occurs in many taxa including birds and, interestingly, in the non-mammalian species typically used in developmental studies such as Drosophila, Xenopus, and zebrafish [36]; both modes are found in insects and amphibians. It has been suggested that the rate of evolution within taxa might relate to the mode of PGC formation, but this has been shown not to be the case [38].…”

“…The PGCs form at the blastoderm stage in the inheritance mode, but shortly before gastrulation (i.e. slightly later) in the induction mode [36]. This could potentially influence age-related effects on germ cells, since the number of cell divisions before PGC formation is higher in the induction mode [37].…”