Host Age Response in the Parasitoid Wasp Spalangia cameroni (Hymenoptera: Pteromalidae)

Female Spalangia cameroni produced more offspring from younger house fly pupae, both when given a choice of host ages and when not given a choice. Host age did not affect offspring survivorship. Offspring were larger when they had developed on younger hosts and the effect was independent of offspring sex. Having previously parasitized old hosts versus young hosts did not reduce a female's production of offspring in subsequent hosts. Females distinguished between young and old hosts both in the light and in the dark. Females did not distinguish between host ages prior to physical contact with the host but could distinguish by the time they first began exploring a host by tapping it with their antennae; thus, they could distinguish before drilling into a host.     

Phenotypic plasticity in fluctuating environments: consequences of the lack of individual optimization

I consider the problem of characterizing the optimal plasticresponse when there are large-scale fluctuations in the environmentaffecting all population members. Individuals differ in theirstate, and each makes a reproductive decision before the environmentalconditions are known. An individual's state, its decision, andenvironmental conditions together determine the number of descendantsleft at the next decision epoch. I restrict attention to thesimple problem in which the state of the descendants left atthis epoch does not depend on these three factors. Because theenvironment is fluctuating, there is no individual optimization;instead the best action in one state implicitly depends on thebest action in other states. I characterize an optimal state-dependentstrategy, give a method of computation, and show how behaviorof each individual following the optimal strategy may be reinterpretedas a form of "individual optimization." Concepts are illustratedwith an example of optimal dutch size as a function of territoryquality.     

EVOLUTIONARY ADAPTATION TO TEMPERATURE. IV. ADAPTATION OF ESCHERICHIA COLI AT A NICHE BOUNDARY

Following an environmental change, the course of a population's adaptive evolution may be influenced by environmental factors, such as the degree of marginality of the new environment relative to the organism's potential range, and by genetic factors, including constraints that may have arisen during its past history. Experimental populations of bacteria were used to address these issues in the context of evolutionary adaptation to the thermal environment. Six replicate lines of Escherichia coli (20°C group), founded from a common ancestor, were propagated for 2000 generations at 20°C, a novel temperature that is very near the lower thermal limit at which it can maintain a stable population size in a daily serial transfer (100-fold dilution) regime. Four additional groups (32/20, 37/20, 42/20, and 32–42/20°C groups) of six lines, each with 2000 generation selection histories at different temperatures (32, 37, 42, and daily alternation of 32 and 42°C), were moved to the same 20°C environment and propagated in parallel to ascertain whether selection histories influence the adaptive response in this novel environment. Adaptation was measured by improvement in fitness relative to the common ancestor in direct competition experiments conducted at 20°C. All five groups showed improvement in relative fitness in this environment; the mean fitness of the 20°C group after 2000 generations increased by about 8%. Selection history had no discernible effect on the rate or final magnitude of the fitness responses of the four groups with different histories after 2000 generations. The correlated fitness responses of the 20°C group were measured across the entire thermal niche. There were significant tradeoffs in fitness at higher temperatures; for example, at 40°C the average fitness of the 20°C group was reduced by almost 20% relative to the common ancestor. We also observed a downward shift of 1–2°C in both the upper and lower thermal niche limits for the 20°C selected group. These observations are contrasted with previous observations of a markedly greater rate of adaptation to growth near the upper thermal limit (42°C) and a lack of trade-off in fitness at lower temperatures for lines adapted to that high temperature. The evolutionary implications of this asymmetry are discussed.     

A TEST FOR SEXUAL SELECTION ON HYBRIDS OF TWO SYMPATRIC STICKLEBACKS

In this study we assessed whether sexual selection against hybrids contributes to reproductive isolation between two sympatric stickleback species. The species are recently diverged and possibly in the final stages of speciation. Our aim was to find whether mating discrimination of the parental species selects against F₁ hybrids, and what conditions are necessary for such sexual selection to operate. We used conservative no-choice laboratory trials with reproductively naive, lab-reared fish to measure female mating preferences. Females exhibited ranked preferences, preferring in order: conspecific, hybrid, then heterospecific males. However, intermediate attractiveness does not necessarily imply selection against hybrids: two-way ANOVAs suggested that limnetic, benthic, and hybrid males were statistically equivalent when averaged across females. Thus, this experiment found no evidence for a hybrid mating disadvantage. Our interpretation is that if sexual selection against hybrids is present in the wild, then some factor that biases encounter rates between hybrids and parental species (e.g., habitat selection) is necessary to produce it.     

Individual adaptations in stochastic environments

Summary Many natural populations undergo radical and unpredictable fluctuations, associated with stochastic environmental conditions. Under such circumstances, fitness of a genotype (or strategy) is defined as the geometric mean of the intergenerational genotypic population growth ratel(t). Unfortunately, this population-level criterion has proved difficult to apply at the level of individual organisms.After developing a formula for the variance ofl as the sum of developmental and environmental variance, we discuss several models of individual adaptations, involving clutch size, progeny size and number, and foraging behaviour under risk of predation, based on the geometric-mean fitness concept. We then show how the method of dynamic programming can be extended to deal with facultative behaviour in stochastic environments. Finally we discuss the concept of an evolutionarily stable strategy in a stochastic environment.Our analysis suggests several novel interpretations of field and laboratory observations. Under the geometric mean criterion behaviour may be determined primarily by the worst likely environment; behaviour may appear suboptimal if observed only under normal or average conditions. For example,except under extreme environmental conditions, avian clutches larger than those that are observed might result in increased fecundity, with little if any cost of reproduction in terms of parental survival; however, in unusually bad years such large clutches might be disastrous, in terms of parental survival. This consideration may help explain some recently reported experimental clutch-size manipulation results. Similarly, our analysis indicates that the known phenomenon of seasonal reduction in seed size may constitute a double bet-hedging strategy, determined by parental mortality risk and future seed survival probability. We also discuss circumstances in which phenotypic polymorphism is an adaptation to environmental uncertainty. Thus almost any individual life history or behavioural adaptation may be affected by environmental stochasticity.     

Seasonal changes in components of male and female reproductive success in Raphanus sativus L. (Brassicaceae)

Summary To document seasonal changes in the reproductive behavior of the perfect-flowered, self-incompatible mustard, Raphanus sativus L., we monitored individual survival, flower and fruit production among 58 individuals in a California population over six census dates (cohorts). Population size declined dramatically and mean individual levels of fruit set changed significantly between cohorts. The frequency distribution of flower and fruit production became increasingly skewed over the first four cohorts. The phenotypic maleness of individuals, a standardized measure of phenotypic gender, oscillated during the reproductive season, peaking in the third and fourth cohorts. We calculated a simple estimate of expected male reproductive success of each plant (the number of fruits sired on conspecifics); this estimate was a function of an individual's flower production and the fruit production of its potential mates in our sampled population. Mean expected male success did not differ significantly among cohorts; expected male success per flower did, however, change significantly among cohorts. Among individuals within each cohort, maternal fruit production and expected male success were both positively correlated with flower production throughout the season. Spearman rank correlation coefficients indicate that the strength of these associations, however, changed during the season. Linear regressions of transformed variables indicated that the shape of several fitness functions also changed over time. In addition, the amount of variation in maternal or expected paternal success explained by flower production declined over the first four cohorts. If typical of wild populations, these temporal changes in these functions suggest that measurements of the intensity of phenotypic selection on flower production will depend on when and how fitness is measured in natural populations.     

Fitness cost of parasitoid avoidance behavior in the arrowhead scale, Unaspis yanonensis Kuwana

We examined trade‐offs between fitness components and the parasitoid avoidance behavior of the arrowhead scale, Unaspis yanonensis Kuwana (Homoptera: Diaspididae). We examined the performance of two settling modes: singleton and double settling with the burrower under the burrowee. Each of these modes differs in vulnerability to parasitism by Aphytis yanonensis DeBach et Rosen and Coccobius fulvus Compere et Annecke (Hymenoptera: Aphelinidae), and performance. Field and laboratory observations revealed smaller body size and lower parasitism of burrower scales, while both singletons and burrowees showed the opposite tendency (larger body size and a greater parasitism rate). Laboratory observations under parasitoid‐free conditions revealed greater mortality and reduced growth rate for simulated burrowers than for singletons during the 40‐day nymphal period. This suggests that competition for food resources reduces performance. The results also suggest trade‐offs between parasitoid avoidance and resource acquisition ability in U. yanonensis. The trade‐offs also explain our previous findings, that introduction of the parasitoids increased the proportion of burrowers, but that within seven years equilibrium was reached at fairly low levels (ca. 5%) in the field, even under intense parasitism.     

Subandrodioecy and male fitness in Sagittaria lancifolia subsp. lancifolia (Alismataceae)

Sagittaria lancifolia subsp. lancifolia is described as cosexual (monoecious), but the study population consisted of 84% cosexuals that typically had 35% pistillate buds and 16% predominant males that typically had 0-2% pistillate buds. Hand-pollinations showed that pistillate flowers required pollination to set seed, and pollen from both male and cosexual plants was potent. No gender switching was seen in the field or greenhouse. From 24 experimental crosses, 890 offspring were grown to maturity. Among these, all offspring of cosexual sires were cosexual, but approximately half the offspring of male sires were male, implying that maleness was inherited as a single, dominant allele. These results indicate that S. lancifolia is subandrodioecious, a very rare breeding system. It is rare, in part because its maintenance requires a large male-fitness differential between male and cosexual plants. In the study population, this condition was met by the differential survival of staminate buds on male racemes. Larvae of the weevil Listronotus appendiculatus killed many staminate buds. They did so in a vertical gradient, with buds lower on racemes safer. Male plants have replaced pistillate with staminate buds at these safer positions and thereby enjoy disproportionally higher male fitness.     

Fecundity, phenology, and seed dormancy of F1 wild-crop hybrids in Sunflower (Helianthus annuus, Asteraceae)

Crop-to-wild hybridization has the potential to introduce beneficial traits into wild populations. Gene flow from genetically engineered crops, in particular, can transfer genes coding for traits such as resistance to herbicides, insect herbivores, disease, and environmental stress into wild plants. Cultivated sunflower (Helianthus annuus) hybridizes spontaneously with wild/weedy populations (also H. annuus), but little is known about the relative fitness of F1 hybrids. In order to assess the ease with which crop-to-wild introgression can proceed, we compared characteristics of F1 wild-crop progeny with those of purely wild genotypes. Two nontransgenic, cultivated varieties were crossed with wild plants from three different regions-Texas, Kansas, and North Dakota. Seed burial experiments in the region of origin showed that wild-crop seeds had somewhat higher germination rates (less dormancy) than wild seeds from Kansas and North Dakota, while no differences were seen in seeds from Texas. Progeny from each type of cross were grown in outdoor pots in Ohio and in a weedy field in Kansas to quantify lifetime fecundity and flowering phenology. Flowering periods of hybrid and wild progeny overlapped considerably, especially in plants from North Dakota and Texas, suggesting that these hybrids are very likely to backcross with wild plants. In general, hybrid plants had fewer branches, flower heads, and seeds than wild plants, but in two crosses the fecundity of hybrids was not significantly different from that of purely wild plants. In Ohio, wild-crop hybrids from North Dakota appeared to be resistant to a rust that infected 53% of the purely wild progeny, indicating a possible benefit of "traditional" crop genes. In summary, our results suggest that F1 wild-crop hybrids had lower fitness than wild genotypes, especially when grown under favorable conditions, but the F1 barrier to the introgression of crop genes is quite permeable.     

The identification and fitness of virulent potato cystnematode populations (Globodera pallida) selected on resistant Solarium vernei hybrids for up to eleven generations

Selection of Globodera pallida populations on resistant Solanum vernei-hybrids resulted in distinct virulent strains after eleven generations. Some of these virulent populations were assessed on their environmental fitness under field-type conditions. All reproduced less well in unsterilised soil, but virulent populations were less affected by environmental variation than their avirulent counterparts. Evaluation of their reproductive ability could not equate virulence to overall enhanced or reduced genotypic fitness compared with their avirulent counterparts. These populations were shown to be genetically distinct from their unselected counterparts using isoelectric focusing and specific enzyme staining. The control and management of virulent G. pallida populations is discussed.