Life-cycle Lengths Research Articles

Early or late? The role of genotype phenology in determining wheat response to drought under future high atmospheric CO2 levels.

The combination of a future rise in atmospheric carbon dioxide concentration ([CO2 ]) and drought will significantly impact wheat production and quality. Genotype phenology is likely to play an essential role in such an effect. Yet, its response to elevated [CO2 ] and drought has not been studied before. Here we conducted a temperature-controlled glasshouse [CO2 ] enrichment experiment in which two wheat cultivars with differing maturity timings and life cycle lengths were grown under ambient (aCO2 approximately 400 μmol mol-1 ) and elevated (eCO2 approximately 550 μmol mol-1 ) [CO2 ]. The two cultivars, bred under dry and warm Mediterranean conditions, were well-watered or exposed to drought at 40% pot holding capacity. We aimed to explore water × [CO2 ] × genotype interaction in terms of phenology, physiology, and agronomic trait response. Our results show that eCO2 had a significant effect on plants grown under drought. eCO2 boosted the booting stage of the late-maturing genotype (cv. Ruta), thereby prolonging its booting-to-anthesis period byapproximately 3 days (p < 0.05) while unaffecting the phenological timing of the early-maturing genotype (cv. Zahir). The prolonged period resulted in a much higher carbon assimilation rate, particularly during pre-anthesis (+87% for Ruta vs. +22% for Zahir under eCO2 ). Surprisingly, there was no eCO2 effect on transpiration rate and grain protein content in both cultivars and under both water conditions. The higher photosynthesis (and transpiration efficiency) of Ruta was not translated into higher aboveground biomass or grain yield, whereas both cultivars showed a similar increase of approximately 20% in these two traits at eCO2 under drought. Overall, Zahir, the cultivar that responded the least to eCO2, had a more efficient source-to-sink balance with a lower sink limitation than Ruta. The complex water × [CO2 ] × genotype interaction found in this study implies that future projections should account for multifactor interactive effects in modeling wheat response to future climate.

Product Life Cycle Lengths and Real Activities Management

We examine how short product life cycle lengths constrain real activities management among a group of manufacturing firms. Companies with short-cycle products face fast product obsolescence and constant pressure to develop and market new products. Fast product obsolescence increases the cost of overproduction, while constant pressure for new product development increases the cost of opportunistic spending cuts. We predict and find evidence that in general, short-cycle firms do not beat earnings thresholds through overproduction and discretionary spending cuts as extensively as long-cycle firms do. However, since the Sarbanes-Oxley Act came into effect, short-cycle firms have increasingly overproduced, and there is no longer a difference in overproduction between short- and long-cycle firms. Moreover, the presence of major customers attenuates the relationship between product life cycle lengths and real activities management, while a large market share and high fixed-assets intensity both accentuate the relationship. Finally, short-cycle firms are also less likely to use LIFO (last-in, first out) liquidation to inflate earnings than long-cycle firms.

Biologi dan statistik demografi kutudaun Rhopalosiphum rufiabdominale (Sasaki) dan Tetraneura nigriabdominalis (Sasaki) (Hemiptera: Aphididae) di akar padi

Rhopalosiphum rufiabdominale (Sasaki) dan Tetraneura nigriabdominalis (Sasaki) merupakan spesies kutudaun yang ditemukan hidup di akar tanaman padi. Informasi tentang kisaran inang dan biologi R. rufiabdominale relatif lebih banyak diketahui dibandingkan dengan T. nigriabdominalis. Penelitian ini bertujuan mengetahui biologi dan statistik demografi kutudaun R. rufiabdominalis dan T. nigriabdominalis yang hidup di akar tanaman padi. Kutudaun yang diperoleh dari akar padi sawah di Leuwiliang, Kabupaten Bogor dipelihara dan diperbanyak pada akar padi varietas Ciherang di laboratorium. Setiap nimfa instar satu kutudaun cohort yang sama diinfestasikan ke akar padi di dalam 60 wadah plastik untuk pengamatan biologi dan peubah statistik demografi. Peluang hidup (lx), keperidian harian (mx), dan rata-rata jumlah nimfa kutudaun yang dilahirkan oleh imago setiap harinya pada umur (x) digunakan untuk menghitung parameter statistik demografi. Hasil penelitian menunjukkan bahwa kedua spesies kutudaun tersebut masing-masing memiliki 4 stadia nimfa. Siklus hidup dan lama hidup R. rufiabdominale ialah 4,98 hari dan 15,94 hari dengan keperidian 67,44; Sedangkan siklus hidup dan lama hidup T. nigriabdominalis ialah 5,25 hari dan 18,11 hari dengan keperidian 11,11. Kutu daun R. rufiabdominale memiliki tingkat pertumbuhan intrinsik (r) 0,46 hari dan waktu berlipat ganda (DT) 1,50 hari; Sedangkan T. nigriabdominalis memiliki tingkat pertumbuhan intrinsik 0,14 hari dan waktu berlipat ganda 4,99 hari. Hasil penelitian ini menunjukkan bahwa meskipun lama siklus hidup kedua spesies kutudaun hampir sama, namun populasi R. rufiabdominale berkembang 3,5 kali lebih cepat dibandingkan dengan T. nigriabdominalis dan oleh karenanya berpotensi menjadi hama penting pada tanaman padi.

Multimodal Life-Cycle Variation in 13- and 17-Year Periodical Cicadas (Hemiptera: Cicadidae:Magicicada)

Abstract Periodical cicadas, known for strict life-cycle lengths of 13 or 17 yr, actually exhibit developmental plasticity in cycle length. This variation tends to occur in 1- and 4-yr increments for both life-cycle types, with the largest events involving 4-yr accelerations of the 17-yr species. The pattern has stimulated hypotheses to explain brood formation and life-cycle evolution, but most of the evidence is anecdotal. We present the first quantitative evidence confirming that a site with a 4-yr acceleration does not necessarily experience comparable emergence in the following year, and we provide the first density estimate for a 4-yr early emergence of 17-yr cicadas (1.28/m2, within the range of published estimates for on-schedule emergences). We also document a 4-yr early 13-yr emergence—cicadas apparently emerging in 9 yr. Multimodal life-cycle patterns spanning −4 to +4 yr are evident in both 13- and 17-yr cicadas.

Minimization of the Expected Total Net Loss in a Stationary Multistate Flow Network System

In the present paper, a three-component, stationary, multistate flow network system is studied. Detailed costs and incomes are specified. The aim is to minimize the expected total net loss with respect to the expected times the components spend in each state. This represents a novelty in that we connect the expected component times spent in each state to the minimal total net loss of the system, without first finding the component importance. This is of interest in the design phase where one may tune the components to minimize the expected total net loss. Due to the complex nature of the problem, we first study a simplified version. There the expected times spent in each state are assumed equal for each component. Then a modified version of the full model is presented. The optimization in this model is completed in two steps. First the optimization is carried out for a set of pre-chosen fixed expected life cycle lengths. Then the overall minimum is identified by varying these expectations. Both the simplified and the modified optimization problems are nonlinear. The setup used in this article is such that it can easily be modified to represent other flow network systems and cost functions. The challenge lies in the optimization of real life systems.

Patent Laws, Product Life-Cycle Lengths, and Multinational Activity

Do intellectual property rights influence multinationals' manufacturing location decisions? My theoretical model indicates that countries with strong patent laws attract multinational activity, but only in sectors with relatively long product life cycles. By contrast, firms with short life-cycle technologies are insensitive, because offshore imitation is less likely to succeed before obsolescence. I document strong empirical regularities consistent with the model using a panel dataset on the global operations of US-based multinational firms and a new measure of product obsolescence. Moreover, my identification strategy allows me to isolate the causal effect of patent laws on multinational activity. (JEL D92, F23, K11, L60, O34, R32)

Patent Laws, Product Lifecycle Lengths, and the Global Sourcing Decisions of U.S. Multinationals

This paper investigates the impact of patent laws on firms’ global sourcing decisions. I develop a theoretical model of multinational firms’ location and production decisions in the presence of cross-country differences in intellectual property rights and cross-sector differences in the length of product lifecycles. I show that patent reforms are irrelevant to firms’ sourcing decisions in industries with rapid product turnover. By contrast, strong patent laws attract affiliate activity in industries with longer product lifecycles, because products in these industries are more likely to be imitated prior to obsolescence and are thus more reliant on patent enforcement to protect revenues. These effects are more pronounced for less-productive firms. Using comprehensive panel data on the sales, assets, and employment of U.S. multinationals and their affiliates abroad and a new measure of product obsolescence, I find robust empirical support for these predictions. Effects are significant along all margins of multinational activity, including multinational presence by country and sector, total affiliate sales conditional on presence, the number of affiliates, and affiliate-level sales. In addition, I find that stronger patent rights tilt the balance of cross-border activity away from exports and toward multinational activity. Finally, my identification strategy allows me to isolate the causal effect of patent reforms on multinational operations, which the prior literature has struggled to establish because of concurrent policy reforms.

SELECTION FOR PRIME-NUMBER INTERVALS IN A NUMERICAL MODEL OF PERIODICAL CICADA EVOLUTION

Periodical cicadas are known for unusually long and prime-numbered life cycles (13 and 17 years) for insects. To explain the evolution of prime-numbered reproductive intervals (life cycles), the hybridization hypothesis claims that prime numbers greatly reduce the chance of hybridization with other life cycles. We investigate the hybridization hypothesis using a simulation model. This model is a deterministic, discrete population model with three parameters: larval survival per year, clutch size, and emergence success. Reproductive intervals from 10 years to 20 years compete for survival in the simulations. The model makes three key assumptions: a Mendelian genetic system, random mating among broods of different life-cycle lengths, and integer population sizes. Longer life cycles have larger clutch sizes but suffer higher total mortality than shorter life cycles. Our results show that (1) nonprime-numbered reproductive intervals disappear rapidly in comparison to the selection among the various prime-numbered life cycles, (2) the selection of prime-numbered intervals happens only when populations are at the verge of extinction, and (3) the 13- and 17-year prime phenotypes evolve under certain conditions of the model and may coexist. The hybridization hypothesis is discussed in light of other hypotheses for the evolution of periodical cicada life cycles.

Effects of intraspecific competition on the life cycle of the stonefly, Nemurella pictetii (Plecoptera: Nemouridae)

BackgroundConsiderable variation of life cycle duration in given insect species has been frequently recorded. Splitting of populations into cohorts with different life cycle lengths may occur, sometimes even between siblings from the same batch. Larval populations of the stonefly Nemurella pictetii in central Europe regularly split into a very fast developing and a normal univoltine cohort, leading to partial multivoltinism. The causes for such variation remain unknown but presumably act on the larval stage in which most of the life cycle is spent. We therefore studied possible effects of intraspecific competition on growth and development of larvae in the laboratory.ResultsIntraspecific competition had important influence on growth and development of the larvae. High larval densities led to reduced growth and retarded development through interference, not through exploitative competition. All specimens were negatively affected by frequent encounters and the resulting disturbance. There were no dominant individuals able to grow and develop faster than the rest, at the expense of the others.ConclusionDifferences in life cycle length of Nemurella pictetii may result from different larval densities in different microhabitats and resultant different degrees of interference competition. Although competition alone probably does not cause splitting of populations into cohorts with different life cycle duration differences in size and development caused by other factors are certainly enhanced by intraspecific competition.

A COMMENT ON GENE INTROGRESSION VERSUS EN MASSE CYCLE SWITCHING IN THE EVOLUTION OF 13-YEAR AND 17-YEAR LIFE CYCLES IN PERIODICAL CICADAS

In companion papers, Simon et al. (2000) and Marshall and Cooley (2000) discuss geographic distributions of phenotypic characters and genetic markers of Magicicada decim spp. in the lower Mississippi valley and vicinity. Periodical cicadas (Magicicada spp.) of eastern North America have spurred many debates concerning the evolutionary origin of their unusual 13-year and 17-year life cycles, and instantaneous speciation through en masse cycle switching has been a recurring hypothesis. En masse switching both from 13to 17-year cycles and from 17to 13-year cycles are components of the cycle-switching model of Magicicada evolution (e.g., Lloyd and Dybas 1966a, 1966b; Lloyd and White 1976; Simon 1979, 1988; Martin and Simon 1988, 1990; Williams and Simon 1995). The new field data of Marshall and Cooley (2000) document character displacement between 13-year cicadas with genotypes similar to northern 17-year populations, and sympatric 13-year cicadas with genotypes similar to southern 13-year populations. Simon et al. (2000) interpret this character displacement and other new genetic data as support for the hypothesis of instantaneous speciation by en masse cycle switching from 17to 13-year cycles. However, it is our view that these authors do not adequately explore conventional gene flow explanations of the observed phenotypic and genetic variation that do not require en masse cycle switching. Evaluation of these competing hypotheses can benefit from an interdisciplinary perspective incorporating paleoecological data. We (Cox and Carlton 1988, 1991, 1998; Cox 1992) previously proposed an alternative selection-gene introgression model for Magicicada evolution incorporating paleoecological reconstructions that also explains these new genetic and phenotypic observations of Simon et al. (2000) and Marshall and Cooley (2000). Our hypothesis offers an important caveat about the use of mtDNA in certain circumstances to interpret the magnitude of gene flow between populations and evolutionary processes. We hypothesize unidirectional gene flow via dispersing male cicadas that transferred limited nuclear DNA and no mtDNA to the recipient population. In brief, our model presented in Cox and Carlton (1988) proposes that life-cycle timing differences between cicada populations arose not from en masse cycle switching, but from stochastic selection processes involving Magicicada temperature thresholds, Pleistocene climate characteristics, and hybridization frequencies between populations having a spectrum of life-cycle lengths. Heath (1967, 1968) reports that periodical cicadas emerge from their burrows when soil temperature is 64.3?F and that the air temperature must reach 68?F for flight and mating. In our quantitative model, during glacial maximum it was much more common than it is now for summers to remain too cold for ancestral periodical cicadas to emerge and/or mate, and the ancestors of periodical cicada populations must have failed to reproduce during those years. This seasonal capriciousness gave rise to periodicity (years without cicadas) and selection of long life cycles (conferring a reduced chance of emerging on a cool summer). By the parameters of our model, selected life cycles would be ? 12 years and would be longer in colder climates. This aspect of our model explains the climatically correlated zonation of Magicicada life cycles (Fig. 1). Once periodicity and long life cycles were established, satiation of predators by synchronized emergence became an important strategy for periodical cicadas. Predator satiation works best with precisely synchronized periodicity, and cycle synchronization is diminished by increased heterozygosity. Thus, cicadas that are hybrids of two populations with different cycle lengths will suffer greater predation losses, as many may emerge on years before or after the main population. Cicadas with prime-numbered cycles (13 years and 17 years) will hybridize significantly less frequently than cicadas with non-prime (composite) cycles and thus will have larger emergences and a greater advantage of predator satiation. See Cox and Carlton (1988) for a full discussion of this topic. Marshall and Cooley (2000) studied M. decim spp. of southern 13-year Brood XIX (each brood consists of all synchronized individuals) within the contact zone with more northern 17-year broods (M. septendecim). These authors document character displacement of call pitch between male 13year cicadas with abdominal coloration like cicadas of contiguous 17-year populations (new 13-year cicadas, M. neotredecim) and sympatric male 13-year cicadas with coloration like contiguous Brood XIX populations in the lower Mississippi valley (old 13-year cicadas, M. tredecim). Marshall and Cooley (2000, see their Figs. 4 and 9) argue that M. tredecim have never been present within the range M. neotredecim north of the contact zone and that these two species have hybridized only along the contact zone at the southern margin of the M. neotredecim range. The steep gradient in call pitch frequency along the southern margin of M. neo-

The Resident Strikes Back: Invader-Induced Switching of Resident Attractor

The aim of this paper is two-fold: (a) by way of example, we elucidate the phenomenon of invader-induced switches in a resident attractor; (b) we expose in detail how resonance and phase have a strong impact when semelparous organisms (as, e.g. Pacific salmon) with different life-cycle lengths compete in a self-induced periodically fluctuating environment. We analyse a simple model for the competition between annuals and biennials, focusing on the situation that the annual population in isolation converges to a two-cycle. Well-timed biennial mutants sample the periodically varying environment more efficiently than the annual resident. They can invade successfully even when they are inferior to the resident, in the sense that they have lower viability and/or fertility. Successful invasion can lead to resonance-mediated coexistence if the invader is rather inferior to the resident. Remarkably, for mutants that are less inferior to the resident, successful invasion by a mutant strategy will inevitably be followed by the extinction of the former invader and concurrent re-establishment of the resident. The expulsion of the invader is brought about by an invasion-induced phase shift or attractor switch. We call this phenomenon “the resident strikes back” and say that the resident strategy is invasible, yet invincible. After the resident has struck back, other mutants can successfully invade again. On a longer time-scale, this might lead to an intermittent occurrence of ultimately inferior strategies. The results show that even in a deterministic setting, successful invasion does not necessarily lead to establishment and that mutual invasibility is not always sufficient for coexistence.

Anomalous distribution of nuclear and mitochondrial DNA markers in periodical cicadas

Ecological genetics theory identifies the age at first reproduction and life-cycle length as important components of life-history strategies, and predicts that variation in these traits can have significant evolutionary consequences1. Changes in the timing of reproduction can affect the fitness of an organism2, disrupt gene flow3 and lead to speciation4. The existence of two life-cycle lengths differing by four years in periodical cicadas of the genus Magicicada provides an excellent opportunity to investigate both the control and evolutionary consequences of changes in the timing of reproduction. In Magicicada, life-cycle length has been assumed to be a genetically fixed species-specific trait5. Alternatively, however, periodical cicadas could switch between the two life-cycle lengths in response to either genetic6 or environmental7 factors possibly providing a mechanism for the generation of the extant year classes of cicadas8, called broods9. Here we describe a survey of populations of 13- and 17-year periodical cicadas which suggests that 13-year cicadas in a large region of central North America are descendents of 17-year cicadas that switched their life-cycle length to 13 years, providing the first strong genetic evidence that life-cycle length is a plastic trait. The change in maturation time and the resultant anomalous distribution of cytoplasmic and nuclear gene markers implies that these cicadas became temporally isolated from the parent brood, joined an already existing brood, and thus brought together two independently evolving gene pools.

Paleoclimatic Influences in the Evolution of Periodical Cicadas (Insecta: Homoptera: Cicadidae: Magicicada spp.)

We present a paleoclimatic model of Magicicada evolution that predicts increased probabilities of survival for broods having longer cycle lengths and periodi- cal emergences. This model assumes a hypothetical probability that summer maxi- mum temperatures in deciduous forest refuges during Pleistocene glacial stades failed to sustain threshold temperatures required for cicada flight and copulation. If hybridization of individuals of differing cycle lengths produced adverse effects on the timing accuracy of the cycle lengths of the offspring, then less frequently hy- bridizing prime-valued cycle lengths would have been characterized by more precise periodicity and higher population densities. If 13-yr and 17-yr life cycle forms are the only prime-valued forms in a sequence of life cycle lengths, the sequence will ulti- mately be reduced to only these life cycle lengths. Hybridization of 13-yr and 17-yr broods can produce new broods temporally re- moved from the parental brood by 4 years. Environmentally triggered 4-yr accelera- tions and decelerations of sympatric, temporally offset broods may have developed due to the selective advantage of high population densities during emergences of con- specifics. This paleoclimatic model predicts the climatically zoned distribution of the two

Does Pontoporeia affinis (Amphipoda) Optimize Age at Reproduction in the Gulf of Bothnia?

Current life-history theory was applied to explain the variation in life-cycle length of the amphipod Pontoporeia affinis Lindstrom in the Gulf of Bothnia. The life-cycles of P. affinis at 9 localities (depths) were evaluated on the basis of a maximization of the per capita growth rate. The per capita growth rates of the observed life-cycles were compared with those of mutants with alternative life-cycles. The observed life-cycle lengths were two years at 5 and 210 m depth, three years between 50 and 174 m, and both three and four years at 84 m. All the observed two year life-cycles were optimal, and at 5 of the 9 localities the observed life-cycles coincided with the optimal on one or several occasions. About 40% of the observed life-cycles coincided with the calculated optima. Two alternative proximate mechanisms regulating the age at reproduction were evaluated with respect to their ability to contribute to an optimization of the life-cycle length. The life-cycle length was suggested to be either directly, or indirectly determined by the growth rate. The latter mechanism is linked to size rather than growth rate, i.e. such that the life-cycle length is correlated to the time needed to reach a certain threshold size. Both mechanisms were shown to have the possibility to account for the optimal solution. We argue in favour of the latter mechanism.

Littoral chironomid communities in an arctic Alaskan lake

The soft‐sediment littoral zone of arctic Toolik Lake (68°N) is composed of patches of macrophytes and bare sediments, which support distinct communities of chironomid larvae. Macrophytes were typically dominated by Stictochironomus rosenschoeldi (Zett.) but also had persistent populations of Heterotrissocladius maeaeri Brund., Paratanytarsus spp., Ablabesmyia sp., as well as several other less common taxa. Bare sediments were dominated by H. maeaeri, Parakiefferiella sp., and Zalutschia zalutschicola Lip. S. rosenschoeldi was common in bare sediments hut far less so than in macrophytes. Few taxa were restricted to either habitat. Life cycle lengths, estimated for some species, ranged from 1 to 4 yr. Analysis of indicator species suggested that Toolik is oligotrophic to ultra‐oligotrophic, which is consistent with the available primary production estimate for Toolik Lake. Littoral chironomid communities, in addition to profundal communities, may be useful in lake typology.

Life Histories of Alpine Tundra Arachnida in Colorado

Pitfall jars were used to collect 16,342 specimens of spiders, phalangids, erythraeid mites, and ground beetles at 3500-4270 m (11,500-14,000 ft) elevation. Most alpine tundra arachnid species have biennial life cycles, but some are annuals. This biennial pattern also predominates near sea level at high latitudes (50?N+). Annual cycles are more typical of mid-latitude (40?N), low-elevation regions. Various species of alpine tundra Pardosa wolf spiders have either twoor one-year cycles; the length of life appears to be directly proportional to the net primary productivity of the various species habitats. Mating and egg-laying in Pardosa tend to occur later in the year in the alpine tundra than at low elevations. No significant differences in the number of eggs per cocoon in Pardosa were noted when tundra and low-altitude data were compared. The overwintering stages of alpine tundra spiders (young and penultimate juveniles) are the same as for sea level, except that no tundra species overwinter as adults, whereas many low-elevation species do. Pardosa concinna spends its first winter within the cocoon, a novel behavior for a wolf spider. Dramatic temporal changes in the abundance or activity of some carabid ground beetles were recorded from incidental data obtained on this group. INTRODUCTION Relatively little is known about the life histories of arachnids or carabid ground beetles in the alpine tundra. This is particularly true for North America. Since data concerning life histories at lower elevations and in the arctic tundra are relatively more extensive, more research at high altitudes is desirable. In particular, the effects of the seemingly less hospitable alpine tundra environment on life cycle lengths are worth investigating. METHODS AND MATERIALS Nearly all of this research was done between tree line (ca. 3500 m or 11,500 ft) and 4270 m (14,000 ft) elevation in the Front Range of the Colorado Rockies. The principal study areas were Mount Evans and Rocky Mountain National Park. These two areas are about 77 km (48 miles) apart in a north-south direction. Most of the collecting was done during 6 June 15 November 1967, but some was also done during 1966 and 1968. The 1967 frost-free season at 3750 m was 61 days long (fairly typical) with snow prevalent except in June to October. During the 1967 season pitfall trap jars, 53 mm inside diameter and 55 mm deep, were placed in the ground, their rims flush to the surface. Ethylene glycol (10 mm deep) was added as a preservative 1 Based on part of a thesis submitted in partial fulfillment of the requirements for the Ph.D. degree at the University of Colorado.