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

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.

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.

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.

Reproductive senescence and parental effects in an indeterminate grower

Reproductive senescence is the decrease of reproductive performance with increasing age and can potentially include trans-generational effects as the offspring produced by old parents might have a lower fitness than those produced by young parents. This negative effect may be caused either by the age of the father, mother or the interaction between the ages of both parents. Using the common woodlouse Armadillidium vulgare, an indeterminate grower, as a biological model, we tested for the existence of a deleterious effect of parental age on fitness components. Contrary to previous findings reported from vertebrate studies, old parents produced both a higher number and larger offspring than young parents. However, their offspring had lower fitness components (by surviving less, producing a smaller number of clutches or not reproducing at all) than offspring born to young parents. Our findings strongly support the existence of trans-generational senescence in woodlice and contradict the belief that old individuals in indeterminate growers contribute the most to recruitment and correspond thereby to the key life stage for population dynamics. Our work also provides rare evidence that the trans-generational effect of senescence can be stronger than direct reproductive senescence in indeterminate growers.

Latitudinal variation in norms of reaction of phenology in the greater duckweed Spirodela polyrhiza

Variable environments may result in the evolution of adaptive phenotypic plasticity when cues reliably indicate an appropriate phenotype-environment match. Although adaptive plasticity is well established for phenological traits expressed across environments, local differentiation in norms of reaction is less well studied. The switch from the production of regular fronds to overwintering 'turions' in the greater duckweed Spirodela polyrhiza is vital to fitness and is expressed as a norm of reaction induced by falling temperatures associated with the onset of winter. However, the optimal norm of reaction to temperature is expected to differ across latitudes. Here, we test the hypothesis that a gradient in the length and predictability of growing seasons across latitudes results in the evolution of reaction norms characterized by earlier turion production at higher latitudes. We test this by collecting S. polyrhiza from replicate populations across seven latitudes from Ontario to Florida and then assessing differentiation in thermal reaction norms of turion production along a common temperature gradient. As predicted, northern populations produce turions at a lower birth order and earlier; a significant latitude-by-temperature interaction suggests that reaction norm differentiation has occurred. Our results provide evidence of differentiation in reaction norms across latitudes in a phenological trait, and we discuss how the adaptive significance of this plasticity might be further tested.

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.

Testing hypotheses for maternal effects in Daphnia magna

Maternal effects are widely observed, but their adaptive nature remains difficult to describe and interpret. We investigated adaptive maternal effects in a clone of the crustacean Daphnia magna, experimentally varying both maternal age and maternal food and subsequently varying food available to offspring. We had two main predictions: that offspring in a food environment matched to their mothers should fare better than offspring in unmatched environments, and that offspring of older mothers would fare better in low food environments. We detected numerous maternal effects, for example offspring of poorly fed mothers were large, whereas offspring of older mothers were both large and showed an earlier age at first reproduction. However, these maternal effects did not clearly translate into the predicted differences in reproduction. Thus, our predictions about adaptive maternal effects in response to food variation were not met in this genotype of Daphnia magna.

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.