Parental age effects and the evolution of senescence in Lemna minor

Senescence is characterized by age-related deterioration within individual organisms and a resultant decline in rates of survival or reproduction. Such declines seem inherently maladaptive, but occur nonetheless in a wide range of species. My thesis contributes to the questions of (i) why senescence is common in nature, and (ii) why patterns of senescence sometimes vary markedly both within and among species. With respect to why senescence is common, most evolutionary theory on senescence makes the simplifying assumption that all offspring are of equal quality. I show that this assumption does not hold in the aquatic plant Lemna minor, and develop a theoretical model to investigate how age-related declines in offspring quality influence the ‘force’ of natural selection. With respect to variation in patterns of senescence, I describe a common garden experiment demonstrating a high degree of among-population consistency in life expectancy and rates of senescence in L. minor

A multigenerational effect of parental age on offspring size but not fitness in common duckweed (Lemna minor)

Sherpa Romeo yellow journal (pre-print only; accepted for publication)Classic theories on the evolution of senescence make the simplifying assumption that all offspring are of equal quality, so that demographic senescence only manifests through declining rates of survival or fecundity. However, there is now evidence that, in addition to declining rates of survival and fecundity, many organisms are subject to age-related declines in the quality of offspring produced (i.e. parental age effects). Recent modelling approaches allow for the incorporation of parental age effects into classic demographic analyses, assuming that such effects are limited to a single generation. Does this ‘single generation’ assumption hold? To find out, we conducted a laboratory study with the aquatic plant Lemna minor, a species for which parental age effects have been demonstrated previously. We compared the size and fitness of 423 lab-cultured plants (asexually-derived ramets) representing various birth orders, and ancestral ‘birth-order genealogies’. We found that offspring size and fitness both declined with increasing ‘immediate’ birth order (i.e. birth order with respect to the immediate parent), but only offspring size was affected by ancestral birth order. Thus, the assumption that parental age effects on offspring fitness are limited to a single generation does in fact hold for L. minor. This result will guide theorists aiming to refine and generalise modelling approaches that incorporate parental age effects into evolutionary theory on senescence

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

Sherpa Romeo yellow journal (pre-print only, accepted for publication)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. 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. 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 lifespan 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, lifespan, and total reproductive output. 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. 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

Data from: A multigenerational effect of parental age on offspring size but not fitness in common duckweed (Lemna minor)

Classic theories on the evolution of senescence make the simplifying assumption that all offspring are of equal quality, so that demographic senescence only manifests through declining rates of survival or fecundity. However, there is now evidence that, in addition to declining rates of survival and fecundity, many organisms are subject to age-related declines in the quality of offspring produced (i.e. parental age effects). Recent modelling approaches allow for the incorporation of parental age effects into classic demographic analyses, assuming that such effects are limited to a single generation. Does this ‘single-generation’ assumption hold? To find out, we conducted a laboratory study with the aquatic plant Lemna minor, a species for which parental age effects have been demonstrated previously. We compared the size and fitness of 423 lab-cultured plants (asexually-derived ramets) representing various birth orders, and ancestral ‘birth-order genealogies’. We found that offspring size and fitness both declined with increasing ‘immediate’ birth order (i.e. birth order with respect to the immediate parent), but only offspring size was affected by ancestral birth order. Thus, the assumption that parental age effects on offspring fitness are limited to a single generation does in fact hold for L. minor. This result will guide theorists aiming to refine and generalise modelling approaches that incorporate parental age effects into evolutionary theory on senescence

Data from: Senescence in duckweed: age-related declines in survival, reproduction, and offspring quality

1. 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 lifespan 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, lifespan, 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

Offspring Quality vs. Parental Age (Phase 2)

Reproduction schedules and fitness measures for all of the offspring (n = 542) detached from 41 parental fronds of Lemna minor in Phase 2 of our study

R Code for Phase 2

R code for Phase 2 to analyze changes in offspring quality as a function of parental age

R Code for Phase 1

R code for Phase 1 to analyze changes in rates of survival and reproduction with age

Survival and Reproduction vs. Age (Phase 1)

Lifespans and reproduction schedules for the 214 fronds of Lemna minor from Phase 1 of our study

R Code for Graphics

R code used to produce the figures in our paper