Constant mortality and fertility over age inHydra

Abstract

Senescence, the increase in mortality and decline in fertility with age after maturity, was thought to be inevitable for all multicellular species capable of repeated breeding. Recent theoretical advances and compilations of data suggest that mortality and fertility trajectories can go up or down, or remain constant with age, but the data are scanty and problematic. Here, we present compelling evidence for constant age-specific death and reproduction rates in Hydra, a basal metazoan, in a set of experiments comprising more than 3.9 million days of observations of individual Hydra. Our data show that 2,256 Hydra from two closely related species in two laboratories in 12 cohorts, with cohort age ranging from 0 to more than 41 y, have extremely low, constant rates of mortality. Fertility rates for Hydra did not systematically decline with advancing age. This falsifies the universality of the theories of the evolution of aging that posit that all species deteriorate with age after maturity. The nonsenescent life history of Hydra implies levels of maintenance and repair that are sufficient to prevent the accumulation of damage for at least decades after maturity, far longer than the short life expectancy of Hydra in the wild. A high proportion of stem cells, constant and rapid cell turnover, few cell types, a simple body plan, and the fact that the germ line is not segregated from the soma are characteristics of Hydra that may make nonsenescence feasible. Nonsenescence may be optimal because lifetime reproduction may be enhanced more by extending adult life spans than by increasing daily fertility.T he classic genetic theories of the evolution of aging formulated by Medawar (1) and Williams (2), mathematically specified by Hamilton (3), and further explained by Charlesworth and Williamson (4) predict increasing mortality and decreasing fertility from maturity for iteroparous multicellular species. As Hamilton (3) put it, senescence starting at maturity is inevitable. Subsequent theoretical advances by Kirkwood (5, 6) and others (7-9) allow a more nuanced range of possibilities. Hamilton (3), however, continues to be widely cited, usually uncritically and as dogma.

Do reef corals age?

“…When this occurs, death is usually preceded by functional Hydra (Martinez, 1998;Schaible et al, 2015), although similar demographic data have been reported for other species (Finch, 2009;Jones et al, 2014). The classical definition of ageing applies most readily to species where the individual is easily defined, and much of the current uncertainty about ageing in corals derives from questions about the nature of the individual.…”

Resilience to aging in the regeneration‐capable flatworm Macrostomum lignano

“…The lack of a clearly increasing mortality rate with advancing age and the age‐independent regeneration capacity of M. lignano reminds of Hydra , which is shown to be practically immortal (Martínez, 1998; Schaible et al., 2015). As Hydra , flatworms possess stem cells, including a pluripotent subpopulation (Hobmayer et al., 2012; Wagner, Wang & Reddien, 2014), which can replace damaged and old cells and therefore attenuate aging.…”

“…Studying species with aging profiles different from those of the established aging models, such as Caenorhabditis elegans , Drosophila melanogaster , and mice, offers insight into naturally occurring mechanisms to delay or suppress senescence and age‐related phenotypes. Particularly interesting are the few animals which are claimed to be immortal, such as Hydra , for which compelling evidence of a nonsenescent life trajectory exists (Martínez, 1998; Schaible et al., 2015) and certain flatworms (Haranghy & Balázs, 1964; Lange, 1968; Tan et al., 2012). …”

Animal and human models to understand ageing

“…Though ageing is a widespread phenomenon in biology, it is by no means universal: lobsters [13] and hydra [14] show no loss of tissue integrity or reproductive capacity with increasing chronological time, while the North Atlantic Quahog (a hard shelled clam) can live to at least 400 years with no obvious signs of 'ageing' [15]. Salamanders retain regenerative capacity over long periods of chronological time [16] and acute cell senescence is required during regeneration [17]; it is possible that senescence may initially have arisen as a developmental and/or wound healing mechanism that has only recently in evolutionary time been co-opted as a tumour suppressor mechanism with ageing as a side-effect [18].…”