Effects of Different Photoperiods and Chilling Treatments on Growth and Reproduction of Cardamine flexuosa With. and Its Ecological Implications

Abstract Phenotypic variability of Cardamine flexuosa (Cruciferae) was examined in response to different lengths of exposure to low temperature (5°C) at the juvenile stage, and to two photoperiod regimes (8 and 16 hrs day-lengths) in the subsequent growth period. The results indicated that this species had a facultative chilling or long-day requirement for flowering. The long-day and chilling treatments both caused an earlier onset of stem internode elongation. Longer chilling treatments reduced the number of nodes and increased internode length under both long and short photoperiod regimes. Prolonged chilling treatments followed by long-day photoperiod remarkably induced more numerous basal branches and inflorescences, as a result producing more siliques. Remarkable rosette leaves Were formed at the base of the main stem when partial chilling and the short-day treatment suppressed stem internode elongation. The responses to chilling and long-day treatment closely resembled the synchronized flowering of this species in spring in the field. Delayed flowering under the short-day treatment resembled size- or age-dependent flowering in late summer to autumn in the field populations.     

Variability in life history traits of the aphid,Acyrthosiphon pisum (Harris), from sexual and asexual populations

Many aphid species have shown remarkable adaptability by invading new habitats and agricultural crops, although they are parthenogenetic and might be expected to show limited genetic variation. To determine if the mode of reproduction limits the level of genetic variation in adaptively important traits, we assess variation in 15 life history traits of the pea aphid, Acyrhosiphon pisum (Harris), for five populations sampled along a north-south transect in central North America, and for three traits for three populations from eastern Australia. The traits are developmental times and rates as affected by temperature, body weights as affected by temperature, fecundity, measures of migratory tendency, and photoperiodic responses. The most southerly population from North America is shown to be obligately parthenogenetic, as are the Australian populations, and the four more northerly North American populations are facultatively parthenogenetic with the number of parthenogenetic generations per year increasing from north to south. The broad-sense heritabilities of life history traits varied from 0.36 to 0.71 for nine quantitive traits based on a comparison of within-and between-lineage variances. Using these traits, 7–13 distinct genotypes (i.e. clones) were identified among each of the 18 lines sampled from the North American populations, but the number did not differ significantly among populations. The level of genetic variation differed from trait to trait. For 4 of 12 quantitative traits, the level of variation in the obligately parthenogenetic population from North America was lowest, but significantly lower than all the sexual populations for only 1 trait. The obligately parthenogenetic population had the highest level of genetic variation for two traits, and had intermediate levels for the others. The most northerly population, which was sexual and had relatively few parthenogenetic generations each year, had the lowest level of variation for 5 of 12 traits and the highest level of variation for 2 traits. There was no decline in variability from north to south correlated with the increase in the annual number of parthenogenetic generations. The Australian populations showed no less variation than the North American populations for two of three traits, although the pea aphid was introduced to Australia only 5 years prior to the study, whereas the aphid has been in North America for at least 100 years. The mode of reproduction has not had a substantial impact on the level of genetic variation in life history traits of the pea aphid, but there are population-specific factors that effect the level of variation in certain traits.     

Life-history traits of the lizard Sceloporus undulatus from two populations raised in a common laboratory environment

Hatchling Sceloporus undulatus elongatus from Washington Co., Utah and S. u. garmani from Woods Co., Oklahoma were raised to maturity and reproduction under identical laboratory conditions with ad libitum food availability. Growth, allometry, age and size of maturity, clutch size and egg mass were compared among lab-raised cohorts from the two populations, among lab-raised and field-caught animals (including their field-caught mothers) and, for growth, with values obtained by previously published field studies on the same or nearby populations. For all traits population differences observed in previous field studies and current field samples resulted from both a plastic response to proximate environmental conditions and intrinsic (possibly genetic) difference. The most plastic traits were growth and age of maturity. Cohorts from both populations expressed the ability to mature in less than 6 months in the laboratory but only the S.u. garmani express early maturity in the field. Allometric differences generated during growth in the lab were not observed in field samples but may reflect an adaptive physiological difference. The least plastic trait was egg mass. The only trait for which the rank order of the difference in the field was reversed in the lab was growth rate. S.u. elongatus grew significantly faster than S.u. garmani in the lab but much slower in the field. The tendency of S.u. garmani females to breed at minimum size of maturity may be greater than that of S.u. elongatus.     

The evolution of reaction norms: simple models for age and size at maturity

This paper presents a simple model for the evolution of reaction norms for age and size at maturity that predicts reaction norms with a variety of shapes. Using realistic parameter values the model predicts reaction norms close to those observed in Drosophila. The major assumptions of the model are: 1) that net reproductive rate is maximized, 2) that growth is determinate, and 3) that mortality rates are independent of age and size at maturity. If, additionally, juvenile mortality is uncorrelated with a growth coefficient, k, the model predicts that selection favors maturation later at a smaller size when k is reduced by environmental factors and that decreased juvenile mortality leads to delayed maturity. These two predictions conform with those found by previous models using other measures of fitness. Correlations between k and juvenile mortality can change the shape of the predicted reaction norm. Depending on the precise form of the correlation, the model can predict done- or bowl-shaped reaction norms and can predict delayed or earlier maturity as k decreases. These shapes are qualitatively different from those predicted by previous models that used different fitness measures. Systematic estimates of the parameter values for this and for related models are required to determine the appropriate fitness measure for models of reaction norms.     

Population studies on a South African carrageenophyte: Iridaea capensis (Gigartinaceae,Rhodophyta)

An intertidal population of Iridaea capensis in the south-western Cape Province of South Africa has been studied in detail for a year. The species is generally restricted to sand-inundated rocky shores, which appears not to be the case for Iridaea species in the Pacific. In early winter (June) biomass, plant density and plant size were at a minimum. A small increase in these parameters occurred in August, but there was a massive increase in plant numbers and biomass in October. The largest plants were recorded in summer (February). The population was over 70% sporophytes in April, but switched to almost 80% gametophytes following heavy recruitment between August and October, and remained significantly gametophyte-dominated through April of the following year. Carrageenan levels were high (ca 42% of dry weight) in late winter-spring, and lowest (ca 30–35%) in summer-autumn. Strong offshore wind conditions in early summer caused a reduction in biomass, and it is hypothesized that the associated upwelling of nutrient rich water may be implicated in the lower carrageenan levels at this time.     

Latitudinal clines in life-history variables of anadromous brown trout in Europe

Based on published data, we reviewed clinal variations in life-history characteristics of anadromous brown trout Salmo trutta from 102 European rivers at latitudes between 54 and 70° N. Growth rate in fresh water, mean smolt age, mean sea age at first sexual maturity, proportion of repeal spawners among adults, longevity, and length of adult life span exhibited latitudinal clines. Brown trout grew faster in fresh water, smolted and matured younger, lived fewer years but spawned more times in the south than in the north. The life-history traits studied were often correlated. Longevity, smolt age and sea age at maturity were negatively and smolt length and proportion of repeat spawners among adults were positively correlated with growth rate in fresh water. Longevity was positively correlated with smolt age and sea age at maturity. The latter also increased with increasing smolt age. None of these significant correlations among life history variables, except for those between smolt age and parr growth and proportion of repeat spawners and parr growth, are latitudinal effects. We do not know to what extent the latitudinal variation in life–history traits reflects phenotypic plasticity and to what extent it is caused by inherited differences among populations.     

Is there a threshold size regulating seaward migration of brown trout and Atlantic salmon?

We investigated the influence of variation in body size and growth rate on age of smolting in Atlantic salmon and brown trout in four different Norwegian rivers. In Atlantic salmon smolt ages varied between 2 and 6 years, and in brown trout between 2 and 7 years. Smolt age was negatively correlated with parr growth, and positively correlated with smolt size. Age at smolting was more variable in the two northern than the two southern rivers. Smolt sizes and ages were also more variable in brown trout than in Atlantic salmon. Based on the observed variation in smolt size and age, we reject the hypothesis that a threshold size alone regulates age at smolting. Within populations smolt age depends on growth rate so that fast-growing parr smolted younger and smaller than slow-growing parr. We hypothesize that smolt size and age is a trade-off between expected benefits and costs imposed by differences in individual growth rate.     

GENETICS OF HOST-CACTUS RESPONSE AND LIFE-HISTORY EVOLUTION AMONG ANCESTRAL AND DERIVED POPULATIONS OF CACTOPHILIC DROSOPHILA MOJAVENSIS

The extent of host-specific genetic variation for two life-history traits, egg to adult developmental time and viability, and one morphological trait closely tied to fitness, adult thorax size, was exposed by employing a nested half-sib/full-sib breeding design with Baja and mainland populations of Drosophila mojavensis recently extracted from nature. This study was motivated by the presence of substantial variation in life histories among populations of D. mojavensis that use the fermenting tissues of particular species of columnar cacti for feeding and breeding in the Sonoran Desert. Full-sib progeny from all sire-dam crosses were split into cultures of agria cactus, Stenocereus gummosus, and organ pipe cactus, S. thurberi, to examine patterns of genotype-by-environment interaction for these fitness components. Baja flies expressed shorter egg-to-adult developmental times, higher viabilities, and smaller body sizes than mainland flies consistent with previous studies. Significant sire and dam components of variance were exposed for developmental time and thorax size. Genotype-by-environment interactions were significant at the level of dams for developmental time and nearly significant for viability (P = 0.09). Narrow- and broad-sense heritabilities were influenced by host cactus, sex, and population. No strong pattern of genetic correlation emerged among fitness components suggesting that host-range expansion has not been accompanied by formation of coadapted life histories, yet the ability to estimate genetic correlations and their standard errors was compromised by the unbalanced nature of the data set. Genetic correlations in performance across cacti were slightly positive, evidence for ecological generalism among populations explaining the observed pattern of multiple host cactus use within the species range of D. mojavensis.     

QUANTITATIVE GENETICS OF SIZE AND PHENOLOGY OF LIFE-HISTORY TRAITS IN CHAMAECRISTA FASCICULATA

Despite numerous adaptive scenarios concerning the evolution of plant life-history phenologies few studies have examined the heritable basis for and genetic correlations among these phenologies. Documentation of genetic variation for and covariation among reproductive phenologies is important because it is this variation/covariation that will determine the potential for response to evolutionary forces. To address this problem, I conducted a breeding experiment to determine narrow-sense heritabilities for and genetic correlations among the phenologies of life-history events and plant size in Chamaecristafasciculata, a temperate summer annual plant species. Paternal families showed no evidence of heritable variation for two estimates of plant size, six measures of reproductive phenology or two fitness components. Similarly, paternal estimates of genetic correlations among these traits were low or zero. In contrast, maternal estimates of heritability suggested the influence of maternal parent on one estimate of plant size and four phenological traits. Likewise, maternal effects influenced maternal estimates of genetic correlations. These maternal effects can arise from three sources: endosperm nuclear, cytoplasmic genetic and/or maternal phenotypic. The degree to which the phenology of one life-history trait acts as a constraint on the evolution of other phenological traits depends on the source of the maternal influence in this species.     

Evolutionary senescence in plants

Senescence is a decline in age-specific survival and reproduction with advancing age. Studies of evolutionary plant senescence are designed to explain this decline in life history components within the context of natural selection. A review of studies of plant demography reveals senescent declines in both annual and perennial plants, but also suggests that there are some plant species which may not be expected to show senescence. Thus, future comparative studies of closely related species, with and without senescence, should be possible. The assumptions of the major evolutionary theories of senescence are evaluated for their validity with respect to plants. Different plant species violate one or more of the assumptions of the theories, yet the consequences of violating these assumptions have never been investigated. Whereas, to date, evolutionary senescence has been studied only indirectly in plants, it is concluded that plants provide good experimental systems for clarifying our understanding of senescence in natural populations.