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

Caloric restriction extends lifespan in a clonal plant

When subjected to dietary caloric restriction (CR), individual animals often outlive well-fed conspecifics. Here, we address whether CR also extends lifespan in plants. Whereas caloric intake in animals comes from ingestion, in plants it derives from photosynthesis. Thus, factors that reduce photosynthesis, such as reduced light intensity, can induce CR. In two lab experiments investigating the aquatic macrophyte Lemna minor, we tracked hundreds of individuals longitudinally, with light intensity-and hence, CR-manipulated using neutral-density filters. In both experiments, CR dramatically increased lifespan through a process of temporal scaling. Moreover, the magnitude of lifespan extension accorded with the assumptions that (a) light intensity positively relates to photosynthesis following Michaelis-Menten kinetics, and (b) photosynthesis negatively relates to lifespan via a power law. Our results emphasize that CR-mediated lifespan extension applies to autotrophs as well as heterotrophs, and suggest that variation in light intensity has quantitatively predictable effects on plant aging trajectories.

Evidence of maternal and paternal age effects on speed in thoroughbred racehorses

Effects of parental age on offspring viability have been reported in a wide range of species. However, to what extent parental age influences offspring traits beyond viability remains unclear. Moreover, previous research has primarily focused on maternal age effects. The purpose of this study was to test for paternal and maternal age effects on offspring speed in thoroughbred racehorses. We analysed over 900 000 race performances by over 100 000 horses on British racecourses between 1996 and 2019. With knowledge of the age of all 41 107 dams and 2 887 sires at offspring conception, we jointly modelled maternal and paternal age effects using a 'within-individual centring' approach. Within-parents, we identified a significant effect of maternal age on offspring speed of -0.017 yards s^-1 yr^-1 and a corresponding paternal age effect of -0.011 yards s^-1 yr^-1. Although maternal age effects were stronger (more negative), the existence and magnitude of paternal effects is particularly noteworthy, given thoroughbred sires have no involvement in parental care. Our results also suggest that the selective disappearance of both sires and dams is ongoing. These findings could potentially be used to optimize thoroughbred racehorse breeding decisions, and more generally, add to the increasing body of evidence that both maternal and paternal age affect a range of offspring characteristics.

Evidence of maternal and paternal age effects on speed in thoroughbred racehorses

Effects of parental age on offspring viability have been reported in a wide range of species. However, to what extent parental age influences offspring traits beyond viability remains unclear. Moreover, previous research has primarily focused on maternal age effects. The purpose of this study was to test for paternal and maternal age effects on offspring speed in thoroughbred racehorses. We analysed over 900 000 race performances by over 100 000 horses on British racecourses between 1996 and 2019. With knowledge of the age of all 41 107 dams and 2 887 sires at offspring conception, we jointly modelled maternal and paternal age effects using a 'within-individual centring' approach. Within-parents, we identified a significant effect of maternal age on offspring speed of -0.017 yards s^-1 yr^-1 and a corresponding paternal age effect of -0.011 yards s^-1 yr^-1. Although maternal age effects were stronger (more negative), the existence and magnitude of paternal effects is particularly noteworthy, given thoroughbred sires have no involvement in parental care. Our results also suggest that the selective disappearance of both sires and dams is ongoing. These findings could potentially be used to optimize thoroughbred racehorse breeding decisions, and more generally, add to the increasing body of evidence that both maternal and paternal age affect a range of offspring characteristics.

Evidence of maternal and paternal age effects on speed in thoroughbred racehorses

Effects of parental age on offspring viability have been reported in a wide range of species. However, to what extent parental age influences offspring traits beyond viability remains unclear. Moreover, previous research has primarily focused on maternal age effects. The purpose of this study was to test for paternal and maternal age effects on offspring speed in thoroughbred racehorses. We analysed over 900 000 race performances by over 100 000 horses on British racecourses between 1996 and 2019. With knowledge of the age of all 41 107 dams and 2 887 sires at offspring conception, we jointly modelled maternal and paternal age effects using a 'within-individual centring' approach. Within-parents, we identified a significant effect of maternal age on offspring speed of -0.017 yards s^-1 yr^-1 and a corresponding paternal age effect of -0.011 yards s^-1 yr^-1. Although maternal age effects were stronger (more negative), the existence and magnitude of paternal effects is particularly noteworthy, given thoroughbred sires have no involvement in parental care. Our results also suggest that the selective disappearance of both sires and dams is ongoing. These findings could potentially be used to optimize thoroughbred racehorse breeding decisions, and more generally, add to the increasing body of evidence that both maternal and paternal age affect a range of offspring characteristics.

Pace and shape of senescence in three species of duckweed

Senescence is progressive bodily deterioration associated with declines in survival and fecundity in older age classes. There is great diversity in patterns of senescence across species, but these patterns can be difficult to compare formally due to variation in the absolute time scales in which species live and die: members of some species live for a matter of days, others for millennia. To address this issue, the “pace‐shape” approach was developed to decouple absolute time from analyses and instead standardize life history traits in terms of average life expectancy, facilitating intra‐ and interspecific comparisons. Here, we use this approach to distinguish the generic form of demographic trajectories (shape) from the time scale on which the trajectories occurred (pace) in three species of tiny, free‐floating aquatic plants known as duckweeds (Lemna gibba L., L. minor L., and L. turionifera Landolt), which have mean lifespans of less than a month under typical lab conditions, and exhibit age‐related declines in survivorship and reproduction. Using a randomized block design in which we tracked a final total of 430 individuals, we report differences in pace and shape among the three species. Specifically, the largest, least‐fecund, and typically longest‐lived species, L. gibba, tended to exhibit more rapid decreases in time‐standardized survivorship and fecundity compared with the other two species. This study emphasizes variation in aging patterns that can be found among plant species, including those in the same genus, and provides further validation for the utility of applying the pace and shape approach in interspecific comparisons.

Incorporating effects of age on energy dynamics predicts nonlinear maternal allocation patterns in iteroparous animals

Iteroparous parents face a trade-off between allocating current resources to reproduction versus maximizing survival to produce further offspring. Parental allocation varies across age and follows a hump-shaped pattern across diverse taxa, including mammals, birds and invertebrates. This nonlinear allocation pattern lacks a general theoretical explanation, potentially because most studies focus on offspring number rather than quality and do not incorporate uncertainty or age-dependence in energy intake or costs. Here, we develop a life-history model of maternal allocation in iteroparous animals. We identify the optimal allocation strategy in response to stochasticity when energetic costs, feeding success, energy intake and environmentally driven mortality risk are age-dependent. As a case study, we use tsetse, a viviparous insect that produces one offspring per reproductive attempt and relies on an uncertain food supply of vertebrate blood. Diverse scenarios generate a hump-shaped allocation when energetic costs and energy intake increase with age and also when energy intake decreases and energetic costs increase or decrease. Feeding success and environmentally driven mortality risk have little influence on age-dependence in allocation. We conclude that ubiquitous evidence for age-dependence in these influential traits can explain the prevalence of nonlinear maternal allocation across diverse taxonomic groups.

Flowering and Seed Production across the Lemnaceae

Plants in the family Lemnaceae are aquatic monocots and the smallest, simplest, and fastest growing angiosperms. Their small size, the smallest family member is 0.5 mm and the largest is 2.0 cm, as well as their diverse morphologies make these plants ideal for laboratory studies. Their rapid growth rate is partially due to the family's neotenous lifestyle, where instead of maturing and producing flowers, the plants remain in a juvenile state and continuously bud asexually. Maturation and flowering in the wild are rare in most family members. To promote further research on these unique plants, we have optimized laboratory flowering protocols for 3 of the 5 genera: Spirodela; Lemna; and Wolffia in the Lemnaceae. Duckweeds were widely used in the past for research on flowering, hormone and amino acid biosynthesis, the photosynthetic apparatus, and phytoremediation due to their aqueous lifestyle and ease of aseptic culture. There is a recent renaissance in interest in growing these plants as non-lignified biomass sources for fuel production, and as a resource-efficient complete protein source. The genome sequences of several Lemnaceae family members have become available, providing a foundation for genetic improvement of these plants as crops. The protocols for maximizing flowering described herein are based on screens testing daylength, a variety of media, supplementation with salicylic acid or ethylenediamine-N,N'-bis(2-hydroxyphenylacetic acid) (EDDHA), as well as various culture vessels for effects on flowering of verified Lemnaceae strains available from the Rutgers Duckweed Stock Cooperative.

All that glitters is litter? Ecological impacts of conventional versus biodegradable glitter in a freshwater habitat

Biodegradable plastics are becoming increasingly popular due to global concerns about plastic pollution. In this study, the impacts of glitter manufactured of conventional, non-biodegradable polyethylene terephthalate (PET) versus glitter of alternative materials (modified regenerated cellulose (MRC), mica or synthetic mica) on the biodiversity and ecosystem functioning of freshwater, lotic habitats were compared using a semi-natural mesocosm experiment. After 36 days, there was no effect of glitter on overall assemblage structure or diversity indices, however there was a two-fold increase in the abundance of New Zealand mud snails (Potamopyrgus antipodarum) in response to MRC glitter. In addition, the root length of common duckweed (Lemna minor) and phytoplankton biomass (based on chlorophyll content) were significantly reduced by exposure to any type of glitter. On the contrary, the chlorophyll content in the sediment (indicating microphytobenthos biomass) was significantly greater in those exposed to synthetic mica glitter. Organic matter content of sediment did not differ amongst any of the treatments. However initially, on days 8 and 15, NO₃^2- concentration in the control treatment were significantly greater than in all glitter treatments, but this observation disappeared over time. Overall, results indicate that both conventional and alternative glitters can cause ecological impacts in aquatic ecosystems.

Birth order as a source of within-genotype diversification in the clonal duckweed, Spirodela polyrhiza (Araceae: Lemnoideae)

Organismal persistence attests to adaptive responses to environmental variation. Diversification bet hedging, in which risk is reduced at the cost of expected fitness, is increasingly recognized as an adaptive response, yet mechanisms by which a single genotype generates diversification remain obscure. The clonal greater duckweed, Spirodela polyrhiza (L.), facultatively expresses a seed-like but vegetative form, the ‘turion’, that allows survival through otherwise lethal conditions. Turion reactivation phenology is a key fitness component, yet little is known about turion reactivation phenology in the field, or sources of variation. Here, using floating traps deployed in the field, we found a remarkable extent of variation in natural reactivation phenology that could not be explained solely by spring cues, occurring over a period of ≥ 200 days. In controlled laboratory conditions, we found support for the hypothesis that turion phenology is influenced jointly by phenotypic plasticity to temperature and diversification within clones. Turion ‘birth order’ consistently accounted for a difference in reactivation time of 46 days at temperatures between 10 and 18 °C, with turions early in birth order reactivating more rapidly than turions late in birth order. These results should motivate future work to evaluate the variance in turion phenology formally as a bet-hedging trait.

Intergenerational Transfer of Ageing: Parental Age and Offspring Lifespan

The extent to which the age of parents at reproduction can affect offspring lifespan and other fitness-related traits is important in our understanding of the selective forces shaping life history evolution. In this article, the widely reported negative effects of parental age on offspring lifespan (the 'Lansing effect') is examined. Outlined herein are the potential routes whereby a Lansing effect can occur, whether effects might accumulate across multiple generations, and how the Lansing effect should be viewed as part of a broader framework, considering how parental age affects offspring fitness. The robustness of the evidence for a Lansing effect produced so far, potential confounding variables, and how the underlying mechanisms might best be unravelled through carefully designed experimental studies are discussed.

Defining features of age-specific fertility and seed quality in senescing indeterminate annuals

PREMISE OF THE STUDY

A trade-off between fertility and offspring viability underpins plant reproductive response to sub-optimal environmental conditions. Senescence involves internal resource limitation, and it is a sub-optimal body condition. We tested if senescence affects age-specific fertility and seed viability (quality) in indeterminate annuals.

METHODS

Fertility in individual pods on the monopodial indeterminate inflorescence of Arabidopsis thaliana and its big-seeded relative Brassica nigra was quantified. The reproductive phase was divided into three phases: (1) early-senescence (initial flowers) (2) mid-senescence and (3) late-senescence (wilting leaves). Seed-viability probability as a function of pod position on the inflorescence (a proxy for parent's age) and seed position within pod was verified by germination tests in Brassica and then analysed using a binomial logistic regression model.

KEY RESULTS

Age-specific fertility increased gradually, peaked, and then declined significantly during senescence in Arabidopsis and Brassica. Acropetal size distribution of rosette leaves was similar to that of pods (age-specific fertility) in Arabidopsis. Seeds positioned closest to stigma tended to be heavier and more viable than others in highly fertile pods, characteristic of mid-senescence phase in Brassica. Pod position (parent's age) was a significant predictor of seed-viability probability or seed quality, which improved in old and senescing Brassica.

CONCLUSIONS

High viability probability of seeds produced in low-fertility pods during late-senescence phase suggests weakening of maternal control over seed-size optimization (bigger, fewer, and better seeds) in internally resource-depleted older parent plants. Proximity to stigma can increase seed quality. The unexpected increase in fertility and seed viability during early-senescence phase is likely due to highly conserved developmental constraints on leaf and pod phenotype. Indeterminate annuals can shed light on fertility, offspring quality and senescence relationships in all plants that reproduce sexually and indeterminately.

Senescence in growth and reproductive allocation in a bunchgrass

Senescence is a puzzling phenomenon. Few convincing studies of senescence in perennial herbaceous plants exist. While ramets are known to senesce, whether senescence of bunchgrasses actually occurs is not clear. In this study, we grew a set of plants of Elymus excelsus, a bunchgrass, to examine plant size, sexual reproduction and bud formation in individual plants in relation to their gradual ageing, in order to determine whether E. excelsus experiences senescence. We collected data in two consecutive years (2009 and 2010) from field samples of plants from 1 to 5 years old. Using regression models, we performed age-related analyses of growth and reproduction parameters. Our results showed that individual plant size (diameter, individual biomass), total biomass of ramets, number and biomass of reproductive ramets, percentage of ramets that were reproductive, reproductive allocation, over-wintering buds and juvenile ramets all declined with age. However, vegetative growth (number and biomass of vegetative ramets) did not decrease with age. Those plants that survived, dwindled in size as they aged. However, no plants shifted their resource allocation between growth and reproduction as they aged, so the shift in allocation did not account for the fall in size.

Maternal age alters offspring lifespan, fitness, and lifespan extension under caloric restriction

Maternal age has a negative effect on offspring lifespan in a range of taxa and is hypothesized to influence the evolution of aging. However, the mechanisms of maternal age effects are unknown, and it remains unclear if maternal age alters offspring response to therapeutic interventions to aging. Here, we evaluate maternal age effects on offspring lifespan, reproduction, and the response to caloric restriction, and investigate maternal investment as a source of maternal age effects using the rotifer, Brachionus manjavacas, an aquatic invertebrate. We found that offspring lifespan and fecundity decline with increasing maternal age. Caloric restriction increases lifespan in all offspring, but the magnitude of lifespan extension is greater in the offspring from older mothers. The trade-off between reproduction and lifespan extension under low food conditions expected by life history theory is observed in young-mother offspring, but not in old-mother offspring. Age-related changes in maternal resource allocation to reproduction do not drive changes in offspring fitness or plasticity under caloric restriction in B. manjavacas. Our results suggest that the declines in reproduction in old-mother offspring negate the evolutionary fitness benefits of lifespan extension under caloric restriction.

Retrospective analyses of archive phytotoxicity test data can help in assessing internal dynamics and stability of growth in laboratory duckweed cultures

High growth potential of duckweed species (Lemnaceae family) has been utilized in wide range of research and practical applications. Based on literature data, however, it can be assumed that duckweed populations maintain constant growth rates only when short periods are considered but can vary over longer time scales. This intrinsic instability in growth can affect the interpretation of growth data. Duckweed phytotoxicity tests are usually performed according to highly standardized protocols. Therefore the archive data provide an opportunity for retrospective comparisons. In the present study we collected growth (frond number- and frond area-based relative growth rates) and morphology (average frond and colony sizes) data from control treatments of phytotoxicity tests. All the analyzed tests were carried out with the same Spirodela polyrhiza (L.) Schleid. (giant duckweed) clone (RDSC ID No. 5501) under the same experimental conditions over more than four years. We aimed to assess the overall variability of the above parameters and to test if intrinsic growth patterns affect growth data in short-term. In general, the results reflected high stability of the measured parameters in long term but also indicated that some temporal variability is inevitable which can bias the comparability of growth tests. The frond area-based relative growth rate resulted in smaller coefficient of variation than the usually preferred frond number-based one. The results also revealed a negative correlation between mean growth rates and their coefficients of variation. Therefore, it would be advisable to introduce higher minimal growth rates and/or maximized tolerable coefficients of variation for control cultures into the standard duckweed growth inhibition tests. Analyses of growth data aggregated on seasonal basis indicated faster growth and larger mean frond size in laboratory duckweed cultures from mid-autumn till mid-spring than during summer and early autumn. But, in shorter term (∼50 days) we did not observe distinct trends in growth suggesting that the successive frond generations have no effect on growth traits within this time-scale. Our results point to the importance of assessing intrinsic growth dynamics in duckweed cultures and also to the re-usability of the already collected phytotoxicity data in addressing new research questions.

Aberrant clones: Birth order generates life history diversity in Greater Duckweed, Spirodela polyrhiza

Environmental unpredictability is known to result in the evolution of bet-hedging traits. Variable dormancy enhances survival through harsh conditions, and is widely cited as a diversification bet-hedging trait. The floating aquatic plant, Spirodela polyrhiza (Greater Duckweed), provides an opportunity to study diversification because although partially reliable seasonal cues exist, its growing season is subject to an unpredictable and literally "hard" termination when the surface water freezes, and overwinter survival depends on a switch from production of normal daughter fronds to production of dense, sinking "turions" prior to freeze-over. The problem for S. polyrhiza is that diversified dormancy behavior must be generated among clonally produced, genetically identical offspring. Variation in phenology has been observed in the field, but its sources are unknown. Here, we investigate sources of phenological variation in turion production, and test the hypothesis that diversification in turion phenology is generated within genetic lineages through effects of parental birth order. As expected, phenotypic plasticity to temperature is expressed along a thermal gradient; more interestingly, parental birth order was found to have a significant and strong effect on turion phenology: Turions are produced earlier by late birth-order parents. These results hold regardless of whether turion phenology is measured as first turion birth order, time to first turion, or turion frequency. This study addresses a question of current interest on potential mechanisms generating diversification, and suggests that consistent phenotypic differences across birth orders generate life history variation.

Offspring of older parents are smaller-but no less bilaterally symmetrical-than offspring of younger parents in the aquatic plant Lemna turionifera

Offspring quality decreases with parental age in many taxa, with offspring of older parents exhibiting reduced life span, reproductive capacity, and fitness, compared to offspring of younger parents. These "parental age effects," whose consequences arise in the next generation, can be considered as manifestations of parental senescence, in addition to the more familiar age-related declines in parent-generation survival and reproduction. Parental age effects are important because they may have feedback effects on the evolution of demographic trajectories and longevity. In addition to altering the timing of offspring life-history milestones, parental age effects can also have a negative impact on offspring size, with offspring of older parents being smaller than offspring of younger parents. Here, we consider the effects of advancing parental age on a different aspect of offspring morphology, body symmetry. In this study, we followed all 403 offspring of 30 parents of a bilaterally symmetrical, clonally reproducing aquatic plant species, Lemna turionifera, to test the hypothesis that successive offspring become less symmetrical as their parent ages, using the "Continuous Symmetry Measure" as an index. Although successive offspring of aging parents older than one week became smaller and smaller, we found scant evidence for any reduction in bilateral symmetry.

Aging parasites produce offspring with poor fitness prospects

Senescing individuals have poor survival prospects and low fecundity. They can also produce offspring with reduced survival and reproductive success. We tested the effect of parental age on the performance of descendants in the nematode Heligmosomoides polygyrus, an intestinal parasite of rodents. We found that offspring of senescing worms had reduced within-host survival and reduced egg shedding over the first month post-infection compared with offspring produced by young parents. These results suggest that declining offspring quality is a component of senescence in parasitic nematodes and might have evolutionary consequences for the optimal schedule of age-dependent investment into reproductive effort.

The effect of injury on whole-plant senescence: an experiment with two root-sprouting Barbarea species

BACKGROUND AND AIMS

Senescence is the process of losing fitness when growing old, and is shaped by the trade-off between maintenance and reproduction that makes reproduction more unsure and maintenance more costly with age. In repeatedly reproducing plants, reductions in growth and fertility are signs of senescence. Disturbance, however, provides an opportunity to reset the ageing clock and consequently potentially ameliorate senescence.

METHODS

To test the effects of disturbance on traits closely related to fitness and thus to senescence, a long-term garden experiment was established with two short-lived perennial congeners,Barbarea vulgaris and Barbarea stricta, that differ in their ability to resprout after injury. In the experiment, five damage treatments were applied to plants in four different phenophases.

KEY RESULTS

It was found that damage to the plant body significantly prolonged life span in B. vulgaris but decreased whole-life seed production in both species. High concentration of seed production in one growing season characterized short life spans. Both more severe damage and a more advanced phenological phase at the time of damage caused reproduction to be spread over more than one growing season and equalized per-season seed production. In terms of seed quality, average weight of a single seed decreased and seed germination rate increased with age regardless of damage.

CONCLUSIONS

Although disturbance is able to reset the ageing clock of plants, it is so harmful to plant fitness that resprouting serves, at best, only to alleviate slightly the signs of senescence. Thus, in terms of whole-life seed production, injured plants were not more successful than uninjured ones in the two studied species. Indeed, in these species, injury only slightly postponed or decelerated senescence and did not cause effective rejuvenation.

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 laboratory-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 generalize modelling approaches that incorporate parental age effects into evolutionary theory on senescence.

Evolution of maternal effect senescence

Increased maternal age at reproduction is often associated with decreased offspring performance in numerous species of plants and animals (including humans). Current evolutionary theory considers such maternal effect senescence as part of a unified process of reproductive senescence, which is under identical age-specific selective pressures to fertility. We offer a novel theoretical perspective by combining William Hamilton's evolutionary model for aging with a quantitative genetic model of indirect genetic effects. We demonstrate that fertility and maternal effect senescence are likely to experience different patterns of age-specific selection and thus can evolve to take divergent forms. Applied to neonatal survival, we find that selection for maternal effects is the product of age-specific fertility and Hamilton's age-specific force of selection for fertility. Population genetic models show that senescence for these maternal effects can evolve in the absence of reproductive or actuarial senescence; this implies that maternal effect aging is a fundamentally distinct demographic manifestation of the evolution of aging. However, brief periods of increasingly beneficial maternal effects can evolve when fertility increases with age faster than cumulative survival declines. This is most likely to occur early in life. Our integration of theory provides a general framework with which to model, measure, and compare the evolutionary determinants of the social manifestations of aging. Extension of our maternal effects model to other ecological and social contexts could provide important insights into the drivers of the astonishing diversity of lifespans and aging patterns observed among species.