Senescence in duckweed: age‐related declines in survival, reproduction and offspring quality

Abstract

Summary1. As they grow old, most organisms experience progressive physiological deterioration resulting in declining rates of survival and reproduction -a seemingly maladaptive phenomenon known as senescence. 2. Although senescence is usually defined with respect only to survival and reproduction, a third component of fitness, offspring quality, may also decline with age. Few studies, however, have assessed age-related changes in offspring quality using measures that truly reflect fitness. 3. In a controlled environment, we tested for age-related declines in three demographic components of fitness (survival, reproduction and offspring quality) in Lemna minor, a small aquatic plant in the subfamily Lemnoideae (the duckweeds) with a short life span and rapid rate of asexual reproduction. Our primary measure of offspring quality, the intrinsic rate of increase, more closely approximates fitness than measures used in previous studies such as size, life span and total reproductive output. 4. We observed strong age-related declines in all three components of fitness: old plants had lower rates of survival and reproduction, and produced lower-quality offspring than younger plants. 5. Theoretical and empirical research on the evolutionary biology of senescence should devote more attention to offspring quality. This often unrecognized component of fitness may change with age -as we have shown in L. minor -and may be shaped by, and feed back into, the same evolutionary forces that give rise to senescence.

Sex-specific pathways of parental age effects on offspring lifetime reproductive success in a long-lived seabird

“…). Offspring LRS, as well as intrinsic rate of increase, declined curvilinearly with parental age in asexually reproducing duckweed (Barks and Laird ). Finally, LRS linearly declined with maternal age in female song sparrows and with paternal age in male song sparrows (Schroeder et al.…”

“…) or altered age‐specific (Bouwhuis et al. a) reproductive success, a reduced intrinsic rate of increase (Barks and Laird ), or a reduced life span (e.g., Priest et al. ; Carnes et al.…”

“…As recently pointed out (Barks and Laird ), classical theories for the evolution of senescence implicitly assume offspring fitness to be independent of parental age (e.g., Hamilton ). If this is not the case, then estimates of age‐specific selection pressures can be over‐ or underestimated, depending on whether offspring quality improves or declines with parental age, respectively, (Barks and Laird ), although care must be taken when assigning measures of offspring fitness to parents in evolutionary models (Wolf and Wade ). In cases of sex‐specific effects, the potential evolutionary implications are even wider, because the optimality of life‐history strategies will additionally depend on processes of age‐assortative mating and adaptive sex allocation (Trivers and Willard ; Uller ).…”

Evolution of maternal effect senescence

“…Similar studies of age-specific maternal IGEs are lacking from the literature. However, as discussed above, there is good evidence to suggest maternal effects and IGEs are widespread and important in nature (29)(30)(31) and that maternal age is a powerful predictor of a range of offspring traits (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19). Given that age-specific maternal effects have been detected and estimated in a wild mammal system (59), the estimation of age-specific IGEs in other experimental and observational studies should be feasible.…”

“…Amid the many studies of senescing traits, a clear pattern is emerging that relates maternal age to the health, fitness, and lifespan of their progeny across widely diverse taxa including humans (8), birds (9, 10), mammals (11,12), Drosophila species (13)(14)(15) and other arthropods (16)(17)(18), and plants (19). Although observations suggest that maternal effect senescence may not proceed at the same rate as fertility senescence (12,(20)(21)(22)(23)(24), there is no evolutionary theory yet capable of explaining maternal effect senescence or its distinction from fertility senescence.…”