Deborah A. Roach

Multigenerational effects of flowering and fruiting phenology in Plantago lanceolata

Phenological patterns of flowering and fruiting can be influenced by the effects of reproductive time on seed production. We propose here that these patterns are also influenced by phenological effects on offspring quality. Furthermore, we hypothesize that there are cross-generational trade-offs between parental and offspring components of parental fitness influencing the evolution of reproductive phenology. To test our hypothesis, we examined the multigenerational effects of flowering and fruiting phenology in Plantago lanceolata. Offspring of 30 families were transplanted into field plots to measure the effects of onsets of flowering and fruiting, duration of fruiting, percentage fungal infection, and damage by grasshoppers on total seed production, our measure of the within-generational component of parental fitness. To gather information about cross-generational contributions to parental fitness, we assessed the quality of off- spring produced at different times in terms of seed mass and germination. Families significantly differed in flowering and fruiting onsets. Larger plants began flowering earlier, and earlier flowering plants matured fruits earlier and produced fruits for a longer time. Significant family-mean correlations among these traits suggest that selection on any one trait will change all three traits. A negative family-mean correlation between fruiting onset and seed production suggests that we can expect an antagonistic trade-off in response to selection on these two traits. Early fruiting significantly reduced seed predation by grasshoppers and increased seed production. In contrast, late-maturing seeds were sig- nificantly heavier and germinated more rapidly than did early-maturing seeds produced by the same plants. The directions of the multigenerational effects support the hypothesis that there are cross-generational trade-offs between parental and offspring components of pa- rental fitness. The experiments indicate that multigenerational fitness effects should be considered in future studies addressing the evolution of flowering and fruiting phenology.

Life History Variation in Geranium carolinianum. 1. Covariation between Characters at Different Stages of the Life Cycle

Seeds of Geranium carolinianum were collected according to common female parent from three populations, and then planted back with identification labels in a randomized block design into one of the original populations. Seven individuals per family were destructively harvested at three stages of the life cycle-early juvenile, late juvenile, and adult-and the same traits were measured at each life stage. The genetic correlations measured between early juvenile and adult traits were negative, and the genetic correlations of age-specific characters with fecundity changed during the life cycle and became positive later in life. Positive phenotypic correlations were found between these same characters. The implications for life history evolution of these genetic correlations between different life stages are discussed.

Mating Frequency and Inclusive Fitness in Drosophila melanogaster

In many species, increased mating frequency reduces maternal survival and reproduction. In order to understand the evolution of mating frequency, we need to determine the consequences of increased mating frequency for offspring. We conducted an experiment in Drosophila melanogaster in which we manipulated the mating frequency of mothers and examined the survival and fecundity of the mothers and their daughters. We found that mothers with the highest mating frequency had accelerated mortality and more rapid reproductive senescence. On average, they had 50% shorter lives and 30% lower lifetime reproductive success (LRS) than did mothers with the lowest mating frequency. However, mothers with the highest mating frequency produced daughters with 28% greater LRS. This finding implies that frequent mating stimulates cross‐generational fitness trade‐offs such that maternal fitness is reduced while offspring fitness is enhanced. We evaluate these results using a demographic metric of inclusive fitness. We show that the costs and benefits of mating frequency depend on the growth rate of the population. In an inclusive fitness context, there was no evidence that increased mating frequency results in fitness costs for mothers. These results indicate that cross‐generational fitness trade‐offs have an important role in sexual selection and life‐history evolution.

Buried seed and standing vegetation in two adjacent tundra habitats, northern Alaska

SummaryA comparison is made of the species of the above ground vegetation and the buried seeds in a tussock tundra meadow, northern Alaska. Vegetation types are compared across a boundary between two habitats which have distinct differences in above ground vegetation. Sixteen species germinated from the soil cores. There were distinct differences between the species of buried seed and the species in the standing vegetation in the two tundra habitats. There was a positive correlation between the diversity of standing vegetation and the diversity of buried seeds. The significance of buried seeds and viable rhizomes in the soil cores is discussed with respect to seed reproduction in the arctic and revegetation.

Cross-generational fitness benefits of mating and male seminal fluid.

In many species, the physical act of mating and exposure to accessory gland proteins (Acps) in male seminal fluid reduces female survival and offspring production. It is not clear what males gain from harming their sexual partners or why females mate frequently despite being harmed. Using sterile strains of Drosophila melanogaster that differ in their production of Acps, we found that both the physical act of mating and exposure to male seminal fluid in mothers increase the fitness of daughters. We show that the changes in daughter fitness are mediated by parental effects, not by sexual selection involving good genes or owing to variation in maternal egg production. These results support the idea that male harm of females might partly evolve through cross-generational fitness benefits.

Timing of Seed Production and Dispersal Geranium Carolinianum: Effects on Fitness

The importance of the timing of seed production and of seed dispersal to the fitness of an annual plant was examined using Geranium carolinianum in the piedmont of North Carolina. Mature seeds were collected from naturally growing plants on four dates in May and June and were then sown back into the population on five dates in late May, June, and July. The mean mass of seed produced varied during the growing season and this had important consequences for fitness. Seeds produced early were heavier, but lighter seeds germinated earlier. Seedlings that emerged earlier developed into larger plants with relatively higher reproductive output. Timing of seed dispersal had no effect on date of emergence, plant size, or fecundity.

Age-specific demography in Plantago: Uncovering age-dependent mortality in a natural population

Accurate measures of age‐dependent mortality are critical to life‐history analysis and measures of fitness, yet these measures are difficult to obtain in natural populations. Age‐dependent mortality patterns can be obscured not only by seasonal variation in environmental conditions and reproduction but also by changes in the heterogeneity among individuals in the population over time due to selection. This study of Plantago lanceolata uses longitudinal data from a field study with a large number of individuals to develop a model to estimate the shape of the baseline hazard function that represents the age‐dependent risk of mortality. The model developed here uses both constant (genetics, spatial location) and time‐varying (temperature, rainfall, reproduction, size) covariates not only to estimate the underlying mortality pattern but also to demonstrate that the risk of mortality associated with fitness components can change with time/age. Moreover, this analysis suggests that increasing size after reproductive maturity may allow this plant species to escape from demographic senescence.

Variation in Seed and Seedling Size in Anthoxanthum odoratum

Success at the juvenile stage of the life cycle may have important consequences for the size-hierarchies and distribution patterns of adult plants. This study was designed to evaluate the factors contributing to variation in seed and seedling size in Anthoxanthum ordoratum. In experiments in the greenhouse and within the natural habitat of this species, significant differences in seed weight were found among inflorescences and maternal half-sib families, and marginal differences were found among populations. Larger seeds had a higher probability of germinating and individuals from them had a longer leaf length as juveniles. Covariate analysis showed that differences among populations in leaf length were caused by population differences in seed weight. Root growth was studied using plexiglass root boxes. Individuals from larger seeds tended to produce longer roots, and there were no differences among populations, maternal families, or inflorescences for root length.

Longitudinal analysis of Plantago: Age-by-environment interactions reveal aging

We know very little about aging (senescence) in natural populations, and even less about plant aging. Demographic aging is identified by an increasing rate of mortality following reproductive maturity. In natural populations, quantifying aging is often confounded because changes in mortality may be influenced by both short- and long-term environmental fluctuations as well as age-dependent changes in performance. Plants can be easily marked and monitored longitudinally in natural populations yet the age-dependent dynamics of mortality are not known. This study was designed to determine whether a plant species, Plantago lanceolata, shows demographic aging in its natural environment. A large, multiple-cohort design was used to separate age-independent and age-dependent processes. Seven years of results show environmental influences on mortality as evidenced by synchronous changes in mortality across four cohorts over time. Age-dependent mortality was found through an age-by-environment interaction when the oldest cohorts had significantly higher mortality relative to the younger cohorts during times of stress. Neither size nor quantity of reproduction could explain this variation in mortality across cohorts. These results demonstrate demographic senescence in a natural population of plants.

AGE-SPECIFIC DEMOGRAPHY IN PLANTAGO: VARIATION AMONG COHORTS IN A NATURAL PLANT POPULATION

The major starting point to life history analysis is the schedule of repro- duction and mortality; hence, knowledge of age-specific demographic dynamics is needed. The key ingredients to studies on age-specific demography must include large cohorts of individuals of known age, an accurate accounting of all individuals, and an experimental design to facilitate a separation of age-dependent and age-independent dynamics. In this study with Plantago lanceolata, multiple, large cohorts were planted over four successive years, and the individuals were censused monthly for nearly five years. Lon- gitudinal analysis showed seasonal variation in demography that was correlated with max- imum temperature and cumulative precipitation. Cross-sectional analysis of the different cohorts showed variation across cohorts in age-specific demography. The cohort with the lowest juvenile mortality had the highest adult mortality and the lowest fecundity, suggesting that there is an interdependence of demographic patterns across life stages and that the history of mortality within a cohort may be critical to late-age demographic patterns.