- ItemOffspring of older parents are smaller-but no less bilaterally symmetrical-than offspring of younger parents in the aquatic plant Lemna turionifera(Wiley, 2018) Ankutowicz, Eric J.; Laird, Robert A.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
- ItemA multigenerational effect of parental age on offspring size but not fitness in common duckweed (Lemna minor)(2016-06-15) Barks, Patrick M.; Laird, Robert A.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.
- ItemSenescence in duckweed: age-related declines in survival, reproduction, and offspring quality(2016-06-14) Barks, Patrick M.; Laird, Robert A.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.
- ItemFitness declines toward range limits and local adaptation to climate affect dispersal evolution during climate-induced range shifts(2016-06-14) Hargreaves, A.L.; Bailey, S.F.; Laird, Robert A.Dispersal ability will largely determine whether species track their climatic niches during climate change, a process especially important for populations at contracting (low-latitude/low-elevation) range limits that otherwise risk extinction. We investigate whether dispersal evolution at contracting range limits is facilitated by two processes that potentially enable edge populations to experience and adjust to the effects of climate deterioration before they cause extinction: a) climate-‐induced fitness declines toward range limits, and b) local adaptation to a shifting climate gradient. We simulate a species distributed continuously along a temperature gradient using a spatially explicit, individual-‐ based model. We compare range-‐wide dispersal evolution during climate stability vs. directional climate change, with uniform fitness vs. fitness that declines toward range limits (RLs), and for a single climate genotype vs. multiple genotypes locally adapted to temperature. Dispersal decreased toward stable RLs when range-‐wide fitness was uniform, but increased when fitness declined toward RLs, due to highly dispersive genotypes maintaining sink populations at RLs, increased kin selection in smaller populations, and an emergent fitness asymmetry that favoured dispersal in low-‐quality habitat. However, this initial dispersal advantage at low-‐fitness RLs did not facilitate climate tracking, as it was outweighed by an increased probability of extinction. Locally-‐adapted genotypes benefited from staying close to their climate optima; this selected against dispersal under stable climates but for increased dispersal throughout shifting ranges, compared to cases without local adaptation. Dispersal increased at expanding RLs in most scenarios, but only increased at the range centre and contracting RLs given local adaptation to climate.
- ItemA review of diapause and tolerance to extreme temperatures to dermestids (Coleoptera)(2016-06-14) Wilches, D.M.; Laird, Robert A.; Floate, Kevin D.; Fields, P.G.Numerous species in Family Dermestidae (Coleoptera) are important economic pests of stored goods of animal and vegetal origin, and museum specimens. Reliance on chemical methods for of control has led to the development of pesticide resistance and contamination of treated products with insecticide residues. To assess its practicality as an alternate method of control, we review the literature on the tolerance of dermestids to extreme hot and cold temperatures. The information for dermestid beetles on temperature tolerance is fragmentary, experimental methods are not standardized across studies, and most studies do not consider the role of acclimation and diapause. Difficulties in determining the diapause status of dermestid larvae may explain the lack of studies. The few studies that do examine these factors show that they can greatly increase tolerance to cold temperatures. The use of extreme temperatures will need to target the most tolerant life stage, which for dermestids at cold temperatures will potentially be the cold-acclimated individuals in diapause. The development of effective protocols will be facilitated by studies that clearly and completely describe experimental and statistical methods, consider factors (life-stage, acclimation, diapause) that increase tolerance to extreme temperatures, and assess the mortality at various temperatures to develop mathematical models.