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One of the major issues in modeling expert systems (ESs) for enhanced reusability is capturing a high-level view of their operations. Principled knowledge representation schemes have been used to model components of complex software systems. However, the potential for applying these principled modeling techniques to explicitly capture the functional requirements of ESs has not been fully explored. This paper investigates issues and provides solutions to the use of an AI knowledge representation scheme for developing an ontology of the software components to facilitate their classification and retrieval. Its benefits are demonstrated using two real world knowledge-based systems. >

The diversity of known strategies for parasitic lifestyles is truly astonishing. Many species of parasitic worms, for example, utilize only one host species, while others cycle between two or more (as many as four) different species of hosts. Some parasites are highly virulent, seriously debilitating or even killing their hosts, while others cause only minor damage. Some parasites (such as viruses) are very small relative to their hosts and have the capacity for explosive reproduction. Others are almost as large as their hosts, and have relatively slow generation times. Therefore, parasites are difficult to categorize. Here, I use parasite to refer to organisms that have an obligate association with, and a negative effect on, another organism (the host). Host strategies for dealing with parasites are equally complex. Vertebrates have highly specialized immune systems that can rapidly respond to infection and then store information that can be used to mount future responses to the same type of infection. Invertebrates lack the memory cells of true immune systems, but they do have complex self-nonself recognition systems for recognizing and killing foreign tissues. Plants also have highly specialized defenses against pathogens, and the genetic basis of these defenses is especially well known due to the work of plant pathologists on crop plants. The myriad of details involved in the interactions between hosts and their parasites is overwhelming, but there are some shared, general aspects of these interactions that are of particular interest to evolutionary ecologists. First, parasites may attack in a frequency-dependent way. In other words, the probability of infection for a particular host genotype is expected to be, at least in part, a function of the frequency of that host genotype. This expectation has implications for sexual selection and the evolutionary maintenance of cross-fertilization (Sakai, this volume; Savalli, this volume). Second, parasites may affect the population density of their hosts, and host density may feed back to affect the numerical dynamics of the parasite. Host density may also affect natural selection on the reproductive rates of parasites, which in turn is likely to affect host fitness and host dynamics.

Parasites should be better at infecting hosts from sympatric populations than allopatric populations most of the time (parasite local adaptation). In a previous study of a population of snail parasites (Microphallus sp.) from Lake Alexandrina, New Zealand, we found that Microphallus was more infective to snails (Potamopyrgus antipodarum) in shallow water but not in deep water. Here, we repeated the original study and also monitored the development of the parasite. We found that parasites from shallow water were more infective to hosts from shallow water and developed more rapidly in these hosts. In contrast, parasites from deep water were not more infective to hosts from deep water and did not develop more rapidly in them. These results suggest clinal variation in the susceptibility of these snails, with shallow-water snails more susceptible than deep-water snails. We offer 2 possible explanations for these results. First, gene flow in the Microphallus population is primarily from shallow to deep water, leading to an asymmetric pattern of local adaptation. Alternatively, snails from shallow water may be more susceptible for reasons independent of gene flow, perhaps due to differences in host condition between habitats.

According to the Red Queen hypothesis--which states that interactions among species (such as hosts and parasites) lead to constant natural selection for adaptation and counter-adaptation--the disproportionate evolutionary success of parasites on common host genotypes leads to correlated selection for sexual reproduction and local adaptation by the parasite population. Here we determined whether local adaptation is due to disproportionate infection of common host genotypes, and, if so, whether infection of common host genotypes is due to commonness per se, or some other aspect of these genotypes. In a reciprocal cross-inoculation experiment parasites occupying the same geographical area (sympatric) infected locally common host genotypes significantly more often than rare host genotypes, whereas parasites occupying separate geographical areas (allopatric) showed no such significant difference. A mixed source of parasites (containing F1 hybrids) also showed no difference in infection between rare and common host genotypes. These results show that local adaptation results from parasite tracking of locally common host genotypes, and, as such, a necessary condition of the Red Queen hypothesis is met.

Quadrupole moment measurements of the 1/2{sup +}[660] (i{sub 13/2}) bands in {sup 133}Nd (N=73) and {sup 135}Nd (N=75) were performed using the Doppler-shift attenuation method. These results, coupled with the previously measured Q{sub 0} for the same configuration in {sup 137}Nd (N=77), clearly demonstrate a trend of decreasing quadrupole deformation with increasing neutron number. The larger quadrupole moment in {sup 133}Nd compared with that in {sup 135}Nd and {sup 137}Nd offers evidence for the role played by the large shell gap at N=72 for {beta}{sub 2}{approximately}0.35{endash}0.40 in stabilizing the shape at enhanced deformation. The comparison of results from gating below and above the level of interest provides information on the time scale of the sidefeeding contributions to highly deformed structures in the A{approximately}130 region. {copyright} {ital 1999} {ital The American Physical Society}

THE Red Queen hypothesis for the maintenance of biparental sexual reproduction suggests that, for species locked in revolutionary struggles with biological enemies, the production of variable progeny compensates for the genetic or ecological disadvantages of sex1–7. The advantage of sex and recombination under this hypothesis stems from the production of rare phenotypes, which are expected to be more likely to escape infection or predation by coevolved biological enemies. Like many evolutionary hypotheses, the Red Queen hypothesis is difficult to test directly, but its assumptions and predictions can be evaluated7–18. The most critical assumption is that biological enemies will disproportionately attack the most common phenotype19, 20. In this study of parasite loads of coexisting sexual and clonal fish, we find empirical support for this assumption.

The quadrupole moments for variety of configurations, involving the 9/2+[404](g9/2) proton, 1/2+[660](i13/2) and 1/2−[541](f7/2,h9/2) neutron orbitals, were measured using the Doppler-shift attenuation method in a wide range of nuclei in the mass∼130 region. While the involvement of the first two orbitals leads to quadrupole deformations that are comparable to those observed for the so-called superdeformed bands in this mass region, the β2 values for structures that include the 1/2−[541] neutron are found to lie intermediate between those observed for normally deformed and highly deformed bands.

On theoretical grounds, coevolutionary interactions with parasites can select for cross-fertilization, even when there is a twofold advantage gained by reproducing through uniparental means. The suspected advantage of cross-fertilization stems from the production of genetically rare offspring, which are expected to be more likely to escape infection by coevolving enemies. In the present study, we consider the effects that parasites have on parthenogenetic mutants in obligately sexual, dioecious populations. Computer simulations show that repeated mutation to parthenogenesis can lead to the accumulation of clones with different resistance genotypes, and that a moderately diverse set of clones could competitively exclude the ancestral sexual subpopulation. The simulations also show that, when there are reasonable rates of deleterious mutation, Muller’s ratchet combined with coevolutionary interactions with parasites can lead to the evolutionary stability of cross-fertilization. In addition, we consider the effects that parasites can have on the evolution of uniparental reproduction in cosexual populations. Strategy models show that parasites and inbreeding depression could interact to select for evolutionarily stable reproductive strategies that involve mixtures of selfed and outcrossed progeny.

We review the spatial and temporal correlates of uniparental and biparental reproduction in three species of freshwater snails as they pertain to the ecological hypotheses for the maintenance of biparental sex. The biogeographic evidence from two species (Potamopyrgus antipodarum and Bulinus truncatus) presently supports the Red Queen hypothesis that biparental reproduction is selected as a way to reduce the risk to progeny of parasite attack. Uniparental reproduction in these species is associated with low levels of infection by parasites (castrating digenetic trematodes), suggesting that parthenogenesis or self-fertilization can replace cross-fertilization when the risk of infection is low. In addition, in B. truncatus, the timing of cross-fertilization coincides with the season in which parasite attack is highest. In a third species (Campeloma decisum), parthenogenetic reproduction is correlated with latitude and the presence of a non-castrating trematode that may prevent cross-fertilization; these patterns suggest that parthenogenesis has been selected as a mechanism to assure reproduction. We also discuss the taxonomic distribution of parthenogenesis in aquatic invertebrates, and suggest that brooding may be an exaptation for the evolution of parthenogenetic reproduction in these animals.

The Frozen Niche-Variation hypothesis (FNV) suggests that clones randomly sample and "freeze" the genotypes of their ancestral sexual populations. Hence, each clone expresses only a fraction of the total niche-use variation observed in the sexual population, which may lead to selection for ecological specialization and coexistence of clones. A generalized form of the FNV model suggests that the same is true for life-history (as well as other) traits that have important fitness consequences, but do not relate directly to niche use. We refer to the general form of the model as the Frozen Phenotypic Variation (FPV) model. A mixed population of sexual and parthenogenetic snails (Potamopyrgus antipodarum) in a New Zealand lake allowed us to examine the phenotypic variation expressed by coexisting clones in two benthic habitats, and to compare that variation to the sexual population. Three clones were found primarily in an aquatic macrophyte zone composed of Isoetes kirkii (1.5-3.0 m deep), and three additional clones were found in a deeper macrophyte zone composed of Elodea canadensis (4.0-6.0 m deep). These clones showed significant variation between habitats, which mirrored that observed in the sexual population. Specifically, clones and sexuals from the deeper habitat matured at a larger size and had larger broods. There was also significant among-clone variation within habitats; and as expected under the FPV model, the within-clone coefficients of variation for size at maturity were low in both habitats when compared to the sexual population. In addition, we found four clones that were common in both macrophyte zones. The reaction norms of these clones were flat across habitats, suggesting little phenotypic plasticity for morphology or life-history traits. Flat reaction norms, high among-clone variation, and low coefficients of variation (relative to the sexual population) are in accordance with the FPV model for the origin of clonal lineages. We also measured the prevalence of infection by trematode larvae to determine whether clones are inherently more or less infectable, or whether they are freezing phenotypic variation for resistance from the sexual population. We did this in the deep habitats of the lake where recycling of the parasite by the vertebrate host is unlikely, thereby reducing the complications raised by frequency-dependent responses of parasites to host genotypes. We found no indication that clones are either more or less infectable than the resident sexual population. Taken together, our results suggest that phenotypic variation for both life-history traits and resistance to parasites is frozen by clones from the local sexual population.

Gene flow and the genetic structure of host and parasite populations are critical to the coevolutionary process, including the conditions under which antagonistic coevolution favors sexual reproduction. Here we compare the genetic structures of different populations of a freshwater New Zealand snail (Potamopyrgus antipodarum) with its trematode parasite (Microphallus sp.) using allozyme frequency data. Allozyme variation among snail populations was found to be highly structured among lakes; but for the parasite there was little allozyme structure among lake populations, suggesting much higher levels of parasite gene flow. The overall pattern of variation was confirmed with principal component analysis, which also showed that the organization of genetic differentiation for the snail (but not the parasite) was strongly related to the geographic arrangement of lakes. Some snail populations from different sides of the Alps near mountain passes were more similar to each other than to other snail populations on the same side of the Alps. Furthermore, genetic distances among parasite populations were correlated with the genetic distances among host populations, and genetic distances among both host and parasite populations were correlated with "stepping-stone" distances among lakes. Hence, the host snail and its trematode parasite seem to be dispersing to adjacent lakes in a stepping-stone fashion, although parasite dispersal among lakes is clearly greater. High parasite gene flow should help to continuously reintroduce genetic diversity within local populations where strong selection might otherwise isolate "host races." Parasite gene flow can thereby facilitate the coevolutionary (Red Queen) dynamics that confer an advantage to sexual reproduction by restoring lost genetic variation.

On theoretical grounds, coevolutionary interactions with parasites can select for cross-fertilization, even when there is a twofold advantage gained by reproducing through uniparental means. The suspected advantage of cross-fertilization stems from the production of genetically rare offspring, which are expected to be more likely to escape infection by coevolving enemies. In the present study, we consider the effects that parasites have on parthenogenetic mutants in obligately sexual, dioecious populations. Computer simulations show that repeated mutation to parthenogenesis can lead to the accumulation of clones with different resistance genotypes, and that a moderately diverse set of clones could competitively exclude the ancestral sexual subpopulation. The simulations also show that, when there are reasonable rates of deleterious mutation, Muller’s ratchet combined with coevolutionary interactions with parasites can lead to the evolutionary stability of cross-fertilization. In addition, we consider the effects that parasites can have on the evolution of uniparental reproduction in cosexual populations. Strategy models show that parasites and inbreeding depression could interact to select for evolutionarily stable reproductive strategies that involve mixtures of selfed and outcrossed progeny.

Two classes of models attempt to explain why obligate parthenogenesis only rarely replaces sexual reproduction in natural populations, in spite of the apparent reproductive advantage that parthenogens gain by producing only female offspring1. The mutation-accumulation models suggest that sex is adaptive because it purges the genome of harmful recurrent mutations2,3. The ecological genetic models postulate that sex is adaptive in variable environments, particularly when the relevant variation is generated by revolutionary interactions with parasites4–7. Both of these models have considerable merit, but would seem to have limitations. The mutation-accumulation models require high rates of mutation3,8; the coevolutionary models require that parasites have severe fitness effects on their hosts9. In addition, parasites could select for clonal diversity and thereby erode any advantage that sex gains by producing variable progeny10. Here we consider the interaction between mutation accumulation and host–parasite coevolution. The results suggest that even moderate effects by parasites combined with reasonable rates of mutation could render sex evolutionary stable against repeated invasion by clones.

Asexual lineages are thought to be subject to rapid extinction because they cannot generate recombinant offspring. Accordingly, extant asexual lineages are expected to be of recent derivation from sexual individuals. We examined this prediction by using mitochondrial DNA sequence data to estimate asexual lineage age in populations of a freshwater snail (Potamopyrgus antipodarum) native to New Zealand and characterized by varying frequency of sexual and asexual individuals. We found considerable variation in the amount of genetic divergence of asexual lineages from sexual relatives, pointing to a wide range of asexual lineage ages. Most asexual lineages had close genetic ties (approximately 0.1% sequence divergence) to haplotypes found in sexual representatives, indicating a recent origin from sexual progenitors. There were, however, two asexual clades that were quite genetically distinct (1.2% sequence divergence) from sexual lineages and may have diverged from sexual progenitors more than 500,000 years ago. These two clades were found in lakes that had a significantly lower frequency of sexual individuals than lakes without the old clades, suggesting that the conditions that favor sex might select against ancient asexuality. Our results also emphasize the need for large sample sizes and spatially representative sampling when hypotheses for the age of asexual lineages are tested to adequately deal with potential biases in age estimates.

Background Female mate choice may be adaptive when males exhibit heritable genetic variation at loci encoding resistance to infectious disease. The Hamilton-Zuk hypothesis predicts that females should assess the genetic quality of males by monitoring traits that indicate health and vigor (condition-dependent choice, or CD). Alternatively, some females may employ a more direct method of screening and select mates based on the dissimilarity of alleles at the major histocompatibility loci (we refer to this as opposites-attract, or OA). Empirical studies suggest that both forms of mate choice exist, but little is known about the potential for natural selection to shape the two strategies in nature.

The Comment by Woolhouse and Webster (this issue) makes several valid points. For example, the authors indicate that, because of time lags, the most common host genotypes might sometimes be overinfected and sometimes be underinfected. We agree 1xHost–parasite interactions: infection of common clones in natural populations of a freshwater snail (Potamopyrgus antipodarum). Dybdahl, M.F. and Lively, C.M. Proc. R. Soc. London B Biol. Sci. 1995; 260: 99–103CrossrefSee all References. Our awareness of the complications imposed by time lags is the reason we focused on the host population (in Lake Poerua, New Zealand) for which we knew the identity of clones over time, thereby allowing us to pick out the clones that had been common in the recent past 2xParasite adaptation to locally common host genotypes. Lively, C.M. and Dybdahl, M.F. Nature. 2000; 405: 679–681Crossref | PubMed | Scopus (316)See all References. We were interested primarily in whether these recently common clones (RCCs) would become more infected in our experiment than would rare clones, independent of parasite source (the trade-off hypothesis); or whether overinfection of RCCs would be restricted to the sympatric source of parasites (the co-evolution hypothesis).Woolhouse and Webster suggest that our statistical analysis of this experiment was inadequate, because we did not test for an interaction between host genotype and parasite origin. The interaction they seek, between infection, host genotype (common vs rare), and parasite source (sympatric, allopatric and mixed), was highly significant (Likelihood Ratio χ2=10.637; df=3; p=0.0049). Underlying this effect is our result that common clones were more infected than were rare clones by the sympatric source of parasites, but there were no overall differences in infection for rare vs common clones in either the allopatric or the mixed sources of parasites 2xParasite adaptation to locally common host genotypes. Lively, C.M. and Dybdahl, M.F. Nature. 2000; 405: 679–681Crossref | PubMed | Scopus (316)See all References. One isolated exception to the general pattern was RCC 22, which was more infected than were the rare clones by the allopatric parasite source (χ2=7.18, df=1; p=0.007); but the same clone was not more infected in the mixed source of parasites (χ2=0.78, df=1; p=0.378). Hence, there is no evidence that this particular common clone is inherently more susceptible to infection. Similarly, there is no evidence that any of the other three RCCs were inherently more susceptible to infection than were the rare clones, as required by the trade-off hypothesis 2xParasite adaptation to locally common host genotypes. Lively, C.M. and Dybdahl, M.F. Nature. 2000; 405: 679–681Crossref | PubMed | Scopus (316)See all References. This finding does not eliminate the possibility of costs of resistance; but it does suggest that these costs (if present) do not account for the overinfection of common clones by the local parasite source 2xParasite adaptation to locally common host genotypes. Lively, C.M. and Dybdahl, M.F. Nature. 2000; 405: 679–681Crossref | PubMed | Scopus (316)See all References.Woolhouse and Webster suggest that our design could, at best, provide only weak evidence for local adaptation, because we used only one source of target snails. However, our evidence for local adaptation comes from three reciprocal crossinfection experiments: two experiments involving four populations in 1987 (3xAdaptation by a parasitic trematode to local populations of its snail host. Lively, C.M. Evolution. 1989; 43: 1663–1671CrossrefSee all References), and one experiment involving two populations in 1997 (2xParasite adaptation to locally common host genotypes. Lively, C.M. and Dybdahl, M.F. Nature. 2000; 405: 679–681Crossref | PubMed | Scopus (316)See all References). In all six populations, the parasites were more infective to local hosts. Nonetheless, we accept that we had only host source in our experimental test of the trade-off and co-evolution models. More experiments of this kind would be welcome, but we note that our experiment could have easily falsified the co-evolution model. All that was required was either: (1) no difference in infection between RCCs and rare clones in the sympatric parasite source; or (2) RCCs were more infected, independent of parasite source.Along these lines, Woolhouse and Webster suggest that the Red Queen hypothesis would be hard to test by experiments that investigate only geographic patterns. Our feeling is that it would be impossible to prove the existence of the Red Queen using only reciprocal crossinfection experiments, but easy to falsify. We would have abandoned the idea if these parasites had shown no local adaptation on some spatial scale. But we agree that the search for the Red Queen is far from over. Hopefully, many creative tests and exciting ideas will emerge during this search, whether or not the Red Queen eventually proves to be a useful model.We thank Mark Woolhouse, Andy Peters, Jukka Jokela and Joanne Webster for constructive comments, and Lynda Delph for advice concerning log-linear models.

The mutation accumulation hypothesis predicts that sex functions to reduce the population mutational load, while the Red Queen hypothesis holds that sex is adaptive as a defense against coevolving pathogens. We used computer simulations to examine the combined and separate effects of selection against deleterious mutations and host-parasite coevolution on the spread of a clone into an outcrossing sexual population. The results suggest that the two processes operating simultaneously may select for sex independent of the exact shape of the function that maps mutation number onto host fitness.

Nature 367, 554-557 (1994) DURING the production process, the contrast in shading in Figs 1 and 2 of this letter was lost; in addition, parts a and b of Fig. 3 were transposed. These figures should have appeared as they are shown here. The first sentence of each legend is given for guidance. FIG. 1 Results from computer simulations in which sexual populations were challenged by asexual lineages in the presence of coevolving parasites.

In our paper on experiments conducted at multiple sites, we suggested that there are two options for statistical analysis (McKone and Lively 1993). One option is the standard ANOVA where sites are crossed with treatment, and the F-ratio for testing treatment effects is calculated using the treatment-by-site interaction. This analysis is obviously preferable when the number of sites is large, because it has the advantage of being able to test for treatment effects averaged over all sites. However, when the number of sites is small, there is very little power for testing for a treatment effect in this way. We therefore recommended that treatment be analyzed as nested within site when there are few sites. Nesting allows for the assessment of treatment effects within individual sites, but has the drawback that the effect of treatment cannot be generalized to the population of sites. Greenwood (1994) thinks that our recommendation is mistaken, precisely because one cannot generalize using the nested approach. But he does not address what we see as the real problem. We agree unconditionally that it would always be preferable to generalize. Our main point is that it will often be counterproductive to attempt generalization from a handful of sites. The question here is not which is better in a perfect world with a large random sample of sites. The answer to that is clear. The question is: what should we do when only a few sites are manageable, due to unavoidable constraints? We see three options: (1) recognize the futility of generalizing from these few sites, and simply not conduct the experiment; (2) analyze the data using a factorial ANOVA, and hope that the interaction term is not significant and therefore possibly combinable with the error term (Sokal and Rohlf 1981), or (3) use the nested approach to analyze site effects and treatment effects within sites. Nothing is learned from the first approach; the second approach lacks statistical power, and has a high risk of type II error. The third (nested) approach is not generalizable, but something is learned. In our example of the nested method, herbivores had significant effects at all three sites. This is an important finding for these sites, whether or not the result is later shown to be general. We stick by our recommendation.

It is generally recognized that biparental reproduction experiences a "cost" in comparison with uniparental reproduction by either asexual means or self-fertilization. However, less agreement exists on the nature and size of the cost. The cost of cross-fertilization (or less formally, the cost of sex) may arise from two different sources. Williams (1971) proposed that crossfertilizing individuals are at a disadvantage because they contribute only one set of genes to each offspring, whereas uniparentally reproducing individuals contribute two sets. If the genetic cost is measured as the relative loss of fitness from cross-fertilization compared with uniparental reproduction, the gene halving constitutes a 50% loss of fitness, the "cost of meiosis." In the same publication, Maynard Smith (1971a) postulated that biparental organisms are disadvantaged because they spend resources on producing males that do not contribute towards raising young. This allocation cost, the "cost of males," is also precisely 50% if parents invest equally in male and female offspring. The alternative costs of sex apply in different circumstances, depending on the order in which decisions are made on investing in male and female offspring (or gametes) and on reproducing uniparentally or biparentally (Lively and Lloyd 1990). When the decision on how much to invest in the sexes is made first, the subsequent mode of reproduction does not affect the previous sex allocations, and there is no cost of males. A 50% cost of meiosis is then experienced. The cost of meiosis applies to asexual reproduction via embryos in cosexes (parthenogenesis in animals, agamospermy in seed plants), and to open flower (chasmogamous) self-fertilization in plants. However, when the decision to reproduce uniparentally is made first, the cost of meiosis is fully retrieved because the double contribution to the offspring is exactly canceled by the loss of the outcrossing male contribution. The cost of males is experienced unless males contribute to parental care (Maynard Smith 197 lb). The cost of males applies to somatic asexuality (vegetative reproduction in plants, budding in animals), embryonic asexuality by females of dioecious populations, and cleistogamous self-fertilization in plants. The cross-fertilizing components of reproduction may not occur randomly among all members of a population. One source of nonrandom mating is biparental inbreeding through the mating of relatives. In many flowering plants, for example, both seeds and pollen are dispersed very locally so that adjacent relatives mate disproportionately (Handel 1983; Levin 1988). A proportion, r, of the genes of cross-fertilizing relatives are identical by descent from a common ancestor. Several authors therefore have suggested that a gene in an individual that crosses with a relative obtains an indirect contribution to its inclusive fitness in addition to the direct contribution to offspring. Hence, the cost of meiosis is reduced in proportion to the relatedness of the inbreeding relatives, from 1/2 to 1/2(1 r) (Barash 1976; Maynard Smith 1978; Solbrig 1979; Lloyd 1980; Williams 1980; Uyenoyama 1986, 1987; Yahara 1992). It is this conclusion that is reexamined here. We concentrate our attention on self-fertilization within open flowers, which Yahara (1992) considered, with some changes to the symbols. Consider a population in which individuals with

Ecological phenomena are often spatially variable, and hence it is desirable to simultaneously conduct experiments at multiple sites. However, the number of sites is often constrained to be few, resulting in many more replicates within sites than there are sites. Such experiments are commonly analyzed by mixed model ANOVAs (where site is the random effect), which are very powerful at detecting differences among sites and siteby-treatment interactions. But, since few sites are sampled, there is little statistical power to make inferences concerning the treatment effects themselves. This lack of power is unfortunate whenever the primary reason for conducting the experiment is to test for differences among treatments, as is true for most ecological field experiments. In this note we suggest that, when there are few sites, the treatment effect should be examined only within sites using a nested ANOVA.

Two alternative (but not mutually exclusive) hypotheses were contrasted for their abilities to explain the distribution of parthenogenesis in the freshwater snail Potamopyrgus antipodarum: the reproductive assurance hypothesis, which predicts that parthenogenesis will be favored in sparse populations where mates are difficult to find, and the Red Queen hypothesis, which predicts that parthenogenesis will be favored in populations that have a low risk of parasitism. The results were inconsistent with the prediction of the reproductive assurance hypothesis; male frequency was not significantly or positively correlated with snail density. Thus, there was no support for any of the hypotheses for the maintenance of sex that rely on selection for reproductive assurance to explain the distribution of parthenogenesis (e.g., recombinational repair). The results, however, were consistent with the Red Queen hypothesis; male frequency was positively and significantly correlated with the frequency of individuals infected by trematodes. This correlation suggests that parthenogenetic females have replaced sexual females in populations where parasites are rare, and that sexual females have persisted in populations where parasites are common.

When more pollinator visits are required to remove pollen than are required to fertilize all of the ovules, male-functioning plants are expected under theory to allocate more resources to secondary floral structures and products. We found that for the gynodioecious shrub, Hebe stricta, there were no differences between the perfect-flowered («male») morph and the male-sterile («female») morph in many important aspects of their displays (number fo flowering shoots, number of flowers per inflorescence, timing of floral opening, size of corolla), but the flowers on the male morph had significantly larger anthers that extended a greater distance out of the floral tube (...)

Studies were conducted to investigate the effect of reduced food intake on pituitary gonadotropin-releasing hormone (GnRH) sensitivity as a possible link in the delay of puberty by underfeeding. Immature female rats (30-32 days old at the start of each experiment) were used throughout the study. Rats on reduced food intake (R) received 60% of normal food intake beginning at the age of 21 days, whereas control (C) animals were fed ad libitum. In ovariectomized (OVX), estradiol benzoate-progesterone-treated R rats, low doses (less than 5 ng/100 g body wt) of GnRH injected intravenously produced higher serum luteinizing hormone (LH) concentration than in comparable C rats. No difference was found at 5 ng GnRH but response to high doses (15 and 45 ng/100 g body wt) was sharply reduced in R rats. No difference in serum follicle-stimulating hormone concentrations was found. In contrast to the OVX-steroid treated animals, in intact R rats, GnRH evoked higher serum LH concentrations at both low and high doses. These studies indicate that reduced caloric intake can affect pituitary GnRH sensitivity and that the effect depends on both the dose of GnRH and the internal, gonadal steroid hormone milieu.

A model is presented for the evolution of developmental control in a spatially variable environment. Individuals are assumed to disperse at random into one of two patches (one harsh and the other benign) and use one of three developmental strategies for the production of one of two discrete morphological types. Two of the strategies are unconditional (develop as either the stress-tolerant or the nontolerant morph), and the third strategy depends on the environment. Invasion criteria are used to determine the conditions under which each of the three pure strategies are evolutionarily stable and the conditions under which the population is expected to contain some mixture of these strategies at equilibrium. The results demonstrate that environmental control of development requires a cost to the stress-tolerant morphology, and that the average probability of making the right choice is greater than 50%. The range of patch frequencies over which environmental control is stable increases with increases in these...

Field experiments were conducted in order to determine the potential for desiccation and predation to mediate the effect of mussels (Brachidontes semilaevis) on barnacles (Chthamalus anisopoma) in the highly seasonal northern Gulf of California. We did this by removing both mussels and a common mussel predator (Morula ferruginosa: Gastropoda) and by spraying selected sites with sea water during summertime spring low tides. We also determined the effect of crowding on resistance to desiccation in barnacles, and the effect of barnacles on colonization by mussels. The mussel-barnacle community was not affected by keeping experimental quadrats damp during daytime low tides throughout the summer. Exposure to summertime low tides, however, did affect the survivorship of isolated, but not crowded, barnacles; and barnacle clumps enhanced the recruitment of mussels. Hence crowding in barnacles had a positive effect on both barnacle survivorship and mussel recruitment. Morula had a negative effect on mussel density, and mussels had a negative effect on barnacle density. The effect of Morula on barnacle density was positive, presumably due to its selective removal of mussels. These results suggest an indirect mutualism between barnacles and the gastropod predator, because barnacles attract settlement or enhance the survival of mussels, and the predator reduces the competitive effect of mussels on barnacles.

Two discrete shell morphs of the acorn barnacle Chthamalus anisopoma coexist on rocky intertidal shores in the northern Gulf of California. One morph (called "conic") has the conical shape characteristic of acorn barnacles, while the other morph (called "bent") has the rim of the barnacle's aperture oriented perpendicular (rather than parallel) to its base. The results of a companion study show that development of the bent morph is induced by a carnivorous gastropod (Acanthina angelica), that bents are more resistant than conics to attack by this predator, and that the risk of predation is spatially heterogeneous. The purpose of the present study was to determine if bents are less fit than conics in the absence of predation by Acanthina, a necessary condition for the predator- induced developmental switch to bents to be resistant to invasion and replacement by the unconditional strategy "always grow bent over." Four alternative hypotheses were tested: bents (1) are poorer competitors for space, (2) grow slower, (3) reproduce later, and (4) are less fecund than conics. The results of field experiments gave no evidence to suggest that bents are inferior competitors for primary space. Bents, however, were found to be both less fecund and slower growing than conics. Because of the multiplicative effect of growth and fecundity on fitness, the cost of being bent is expected to be high, and a necessary condition for maintenance of the barnacle dimorphism is met.

The Application Display Generator (ADG) is a graphical environment for the design and implementation of embedded system user interfaces. It is a major component of the Graphical Specification Subsystem (GSS) in Lockheed's Express knowledge-based software development environment. ADG gives non-programmers simple and flexible methods for graphically specifying the presentation and behavior of embedded system user interfaces. In the ADG methodology arbitrary presentations are attached to abstract object behaviors. This approach makes it possible to provide unconstrained presentations, intelligent user support, rapid prototyping, and flexible facilities for composing complex objects.

Acanthina angelica shells were experimentally manipulated in order to determine the effect of shell mass, external surface topography, and depth for withdrawal on shell selection by the hermit crab, Clibanarius digueti Bouvier. Reducing shell mass by up to 30% and the removal of shell knobs on the outer surface of shells had no effect on shell preference by this hermit crab. The plugging of shells 1.25 whorls back from the aperture, however, resulted in the hermit crabs selecting significantly larger shells. These results in combination with other studies suggest that hermit crabs may select their preferred shells in order to maximize the room for withdrawal into the shell, subject to a constraint imposed by the shell's mass.

Phenotypic models of selection are used to determine the effect of facultative parthenogenesis on the production of males in a spatially variable environment when (i) sex determination is under strict genetic control, and (ii) when sex may be environmentally determined. The results show that when sex is under strict genetic control and there is some chance of maturing in isolation, selection favors a female-biased sex ratio. When sex can be environmentally induced by cues which indicate high density, selection favors a mixture of genetic and environmental control, such that half the individuals always become female and the other half become females when isolated and become males when not isolated.

Imparting a basic understanding of evolutionary principles to students in an active, engaging fashion can be troublesome because the logistics involved in designing experiments where students pose their own questions and use the data to test alternative hypotheses often outstrip time and financial constraints. In recent years, educators have begun publishing exercises that teach evolution using innovative, in-class experiments. This article adds to this growing forum by describing a classroom exercise that introduces the concept of evolution by natural selection in a hypothesis-driven, experimental fashion, using a deck of cards. Our standard exercise is suitable for upper-level high school and introductory biology students at the college level. In this paper, we discuss the exercise in detail and give several examples that illustrate how our games provide accessible bridges to the primary literature. Finally, we discuss how extensions of our basic exercise can be used to effectively teach advanced evolutionary concepts.

The widespread occurrence of sexual reproduction is an important problem in evolutionary theory, because of the cost of producing males in dioecious organisms and the allocation of limited resources in cosexes1–4. Here I compare the predictions of the major ecological hypotheses on the maintenance of sex by examining male frequency in Potamopyrgus antipodarum, a dioecious freshwater snail with both sexual and parthenogenetic populations5. The results do not-support the view that sex is maintained by a variable physical environment6–9, but they are consistent with the idea that sex is favoured by selection resulting from host–parasite interactions10–14.

Every year advances in computer systems and technology cause increased usage of the computer as a tool to further improve health care delivery. This session endeavors to present some of the more recent trends of computer applications in the field of medicine. Papers in this session will cover physiologic research systems, diagnostic system and medical computer program information centers.

Rapid development in microprocessor technology points the way to significant impacts on a number of industries. One such industry is that of health care delivery in which the micro-miniaturization and almost zero cost of the microprocessor itself will lead to increasingly widespread applications.The assessment of the impact of this new technology is of paramount importance for component and medical device manufacturers in delineating new product directions and for the prioritization of research funding.It is necessary to make a thorough survey of current and projected microprocessor devices and applications through literature searches, research and manufacturer surveys, a special conference and a workshop. The results of these efforts would be incorporated into a report containing the background information and recommendations for further research and development. Failure to collect and disseminate technological information of this type as widely as possible retards the ultimate technological advances that may be derived. Enlightened technological application forecasting and direction emphasis become a key to effective and efficient future developments in health care delivery.

Field experiments were conducted in order to determine the nature of shell dimorphism in the acorn barnacle Chthamalus anisopoma and the adaptive significance of the atypical form. The typical morph has the conical shape which is characteristic of acorn barnacles, while the atypical morph appears bent over, with the rim of its aperture oriented perpendicular to its base. The experiments showed that: 1) the bent-over morphology is an environmentally-induced de- velopmental response to the presence of a carnivorous gastropod (Acanthina angelica) and 2) that "bents" are more resistant than "conics" to specialized predation by this snail. The results also showed that predation by A. angelica is patchy and heaviest in the near vicinity of cracks and crevices, which it uses as refuges during periods of tidal inundation. Because predation is patchy and bents are less fecund and grow slower than conics, the conditional developmental strategy is likely to be favored over strict genetical control of shell morphology.

Field experiments were used to determine the effect of a common intertidal snail (Nerita funiculata) on the use of space for foraging by the hermit crab Clibanarius digueti. Removals of Nerita resulted in an increased density of foraging Clibanarius, while additions of the gastropod had the opposite effect. The observed negative effect of the gastropod on individual hermit crabs appears to be food-related. Field surveys, however, suggested that the hermit crab population is limited by shell number, rather than food. Because Nerita contributes to the shell resource, its effect on the hermit crab population is positive. Nerita, therefore, has a negative effect on the distribution of foraging hermit crabs, but a positive effect on their abundance. Such decouplings of distribution and abundance effects are rare.

Heuristic liver modeling and learning program techniques have been utilized to develop a method for augmenting presurgical guidance in anatomic biliary tree obstruction and hepatitis in neonates. A liver model programmed for evaluation of clinical data leads to one of four categories: (1) hepatitis, (2) atresia, (3) normal, (4) no diagnosis possible. Using a simple set of data (serum glutamate-oxalacetate transaminase and direct Van den Bergh bilirubin, a defined disease duration, and Rose Bengal fecal excretion), 36 test patients whose diagnoses were unknown were tested. Comparison with actual subsequent diagnoses in these patients confirmed the method as a reliable aid in medical diagnosis and management decisions.

Previous studies have shown that primary cleavages in nascent picornavirus precursors are accomplished by cellular proteases. This study has characterized the enzyme in infected cells that produces the capsid polypeptides by secondary cleavages of viral precursors. The kinetics of the production of protease activity correlate with the time course of virus protein synthesis, and the new enzyme has characteristic pH and temperature optima. Guanidine and cycloheximide, which are inhibitors of virus RNA and protein synthesis, prevent production of the protease. As determined by introduction of amino acid analogs into the protease or inhibition by a leucyl chloromethyl ketone, the enzyme is synthesized at a time of infection when host cell proteins are not produced, and the enzyme copurified with a 40,000-dalton virus polypeptide present in the cytoplasm of infected cells. Wild-type levels of protease activity are produced by viral mutants that are defective in coat protein synthesis. The conclusion is that a non-structural poliovirus gene product participates in protein cleavages that produce the viral coat proteins.