Parasitism of the grasshopper Melanoplus sanguinipes by a sarcophagid fly, Blaesoxipha atlanis: influence of solitary and gregarious development on host and parasitoid

Blaesoxipha atlanis (Aldrich) (Diptera: Sarcophagidae) is a common parasitoid of the grasshopper Melanoplus sanguinipes (F.) (Orthoptera: Acrididae) in western Canada. We tested the hypothesis that B. atlanis can develop as either a solitary or a gregarious parasitoid, and assessed the influence of parasitism on the growth and survival of infected grasshoppers. Males and females of M. sanguinipes were parasitized manually with one, two, or three first-instar larvae of B. atlanis in the laboratory. Parasitism was more deleterious to males than females of M. sanguinipes; females are larger than males. Host survival and longevity declined with the number of larvae per host in a sex-specific manner. In females, 39%, 24%, and 8% of hosts containing, respectively, one, two, and three sarcophagid larvae survived parasitism. Although 41% of single-parasitized males survived, all males containing more than one larva died. Variations in host quality as measured by dry mass explained much of the response to parasitism in male, but not female, hosts. Parasitoid larvae, apparently, did not cause significant physical damage to host organs and tissues but instead functioned as a metabolic sink. The greater metabolic activity associated with egg production could account for the relatively higher tolerance to parasitism of female, as opposed to male, grasshoppers. Developmental time, adult size, and percentage survival of B. atlanis declined with the intensity of parasitism, especially in parasitoids developing in male hosts. Females developing gregariously contained fewer ovarioles at eclosion than counterparts developing as solitary larvae. The mean body size of field-collected B. atlanis did not differ from that of laboratory-reared parasitoids developing singly in a host. Gregarious development is an alternative strategy to solitary development that may enable B. atlanis to maintain population numbers during periods of grasshopper scarcity.     

Parasitoid-plant mutualism: parasitoid attack of herbivore increases plant reproduction

We tested whether a plant's life time seed production is increased by parasitization of herbivores in a tritrophic system, Arabidopsis thaliana (Brassicaceae) plants, Pieris rapae (Lepidoptera: Pieridae) caterpillars and the solitary endoparasitoid Cotesia rubecula (Hymenoptera: Braconidae). We established seed production for intact A. thaliana plants, plants that were mechanically damaged, plants fed upon by parasitized caterpillars and plants fed upon by unparasitized caterpillars. In the first experiment, with ecotype Landsberg (erecta mutant), herbivory by unparasitized P. rapae caterpillars resulted in a strongly reduced seed production compared to undamaged plants. In contrast, damage by P. rapae caterpillars that had been parasitized by C. rubecula did not result in a significant reduction in seed production. For the second experiment with the ecotype Columbia, the results were identical. Plants damaged by unparasitized caterpillars only produced seeds on regrown shoots. Seed production of plants that had been mechanically damaged was statistically similar to that of undamaged plants. Production of the first ripe siliques by plants fed upon by unparasitized caterpillars was delayed by 18–22 days for Landsberg and 9–10 days for Columbia. We conclude that parasitization of P. rapae by C. rubecula potentially confers a considerable fitness benefit for A. thaliana plants when compared to plants exposed to feeding damage by unparasitized P. rapae larvae. Plants that attract parasitoids and parasitoids that respond to herbivore-induced plant volatiles will both experience selective advantage, justifying the use of the term mutualism for this parasitoid-plant interaction. This type of mutualism is undoubtedly very common in nature.     

Effectiveness of ants as pollinators of<Emphasis Type="Italic"> Lobularia maritima</Emphasis>: effects on main sequential fitness components of the host plant

The effectiveness of ants as pollinators of Lobularia maritima (Cruciferae) was experimentally analyzed by assessing (1) their quantitative importance at flowers; (2) their effect on host plant seed production; (3) their effect on the performance of host plant progeny, estimated as seed germination, seedling emergence, seedling survival to flowering, and (4) the overall effect of ants on a cumulative, more realistic measure of plant fitness related to the recruitment probability. Flowers of L. maritima were visited during the 2 years of study (1996 and 1997) by more than 50 pollinator species belonging to about 30 families of disparate taxonomic affiliation, notably ants and flies. There was significant seasonal variability in insect abundance and type. Ants, especially Camponotus micans (Formicidae), visited the flowers of L. maritima in summer, representing 81.2% of the visits during this season. This ant species acted as a pollinator of L. maritima, with flowers visited exclusively by ants producing significantly more seeds than flowers from which all pollinators were excluded, whereas flowers visited by only winged insects did not differ from self-pollination. Ant-pollinated flowers produced seeds with a germination rate comparable to the other treatments. Moreover, seedlings from these seeds emerged as fast, and survived at the same rate as controls. Consequently, both ants and winged insects had similar overall effects on host plant recruitment probability (0.6 and 0.7% of initial ovules produced flowering offspring, respectively), a result similar to that of open-pollinated flowers (1.0%). This study demonstrated that the overall effectiveness of the ant C. micans as a summer pollinator of L. maritima was as high as that of other winged insects, contributing not only to the seed production of this crucifer but also to the recruitment of new flowering offspring. Received: 17 February 1999 / Accepted: 26 August 1999     

Simulating the extinction of parental lineages from introgressive hybridization: the effects of fitness,initial proportions of parental taxa,and mate choice

Genomic introgression among divergent taxa following human-mediated secondary contact is a growing concern for the management and conservation of aquatic biodiversity. We simulated the composition of taxa following admixture and hybridization by independently altering three variables: (1) initial proportion of parental taxa following secondary contact; (2) fitness gradients among parental and introgressant taxa; and, (3) strength of assortative mating among these taxa. Ultimately, we established that parental taxa will trend toward extinction as introgression proceeds in spite of even a heavy fitness penalty for the hybrids. Also, the number of generations required (rate) to reach an arbitrarily determined threshold of extinction (< 5.0%) was inversely related to the strength of the relative fitness gradients among parental and derivative hybridized lineages. Moreover, the rates of extinction for parental taxa depended on the initial relative proportions in the admixture with rare taxa going extinct more rapidly than abundant taxa. Finally, the strength of assortative mating (as an evolved or reinforced mechanism of pre-mating isolation) will affect the rate of extinction. Introgressive hybridization, therefore, emerges as an important risk to structural biodiversity wherever divergent, yet reproductively compatible, taxa come together naturally or are brought together through human activities.     

用超显性模型估计遗传多态平衡的种群参数

基于超显性模型,采用数学方法阐述了种群遗传多样性的延续机制及平衡条件,从双等位基因导出的定律有较大的局限性,在许多教科书中常用的估计平衡态基因频数的定律只适用于双等位基因,本文用另一种方法导出一些公式,将其扩展到多等位基因,讨论了种群中基因数ｎ,遗传负载荷（Ｌ）,杂合频ＹＸ（Ｈｅ）和纯合频率（Ｈｏｍ）之间的关系。     

Male parental care, female reproductive success, and extrapair paternity

Birds differ considerably in the degree of male parental care,and it has been suggested that interspecific variation in extrapairpaternity is determined by the relative importance of benefitsto females from male parental care and good genes from extrapairsires. I estimated the relationship between extrapair paternityand the importance of male parental care for female reproductivesuccess mainly based on male removal studies, using a comparativeapproach. The reduction in female reproductive success causedby the absence of a male mate was positively correlated withthe male contribution to feeding offspring. The frequency ofextrapair paternity was negatively related to the reductionin female reproductive success caused by the absence of a mate.This was also the case when potentially confounding variablessuch as developmental mode of offspring and sexual dichromatismwere considered. A high frequency of extrapair paternity occursparticularly in bird species in which males play a minor rolein offspring provisioning and in which attractive males providerelatively little parental care. Bird species with frequentextrapair paternity thus appear to be those in which direct fitness benefits from male care are small, females can readilycompensate for the absence of male care, and indirect fitnessbenefits from extrapair sires are important.     

The evolution of plant response to herbivory: simultaneously considering resistance and tolerance in Brassica rapa

Although the evolution of plant response to herbivory can involve either resistance (a decrease in susceptibility to herbivore damage) or tolerance (a decrease in the per unit effect of herbivory on plant fitness), until recently few studies have explicitly incorporated both of these characters. Moreover, theory suggests these characters do not evolve independently, and also that the pattern of natural selection acting on resistance and tolerance depends on their costs and benefits. In a genotypic selection analysis on an experimental population of Brassica rapa (Brassicaceae) I found a complex set of correlational selection gradients acting on resistance and tolerance of damage by flea beetles (Phyllotreta cruciferae: Chrysomelidae) and weevils (Ceutorhynchus assimilis: Curculionidae), as well as directional and stabilizing selection on resistance to attack by weevils. Evolution of response to flea beetle attack is constrained by a strong allocation cost of tolerance, and this allocation cost may be caused by a complex correlation among weevil resistance, weevil tolerance, flea beetle resistance, and flea beetle tolerance. Thus, one important conclusion of this study is that ecological costs may involve complex correlations among multiple characters, and for this reason these costs may not be detectable by simple pairwise correlations between characters. The evolution of response to weevil attack is probably constrained by a series of correlations between weevil resistance, weevil tolerance, and fitness in the absence of weevil damage, and possibly by a cost of tolerance of weevil damage. However, the nature of these constraints is complicated by apparent overcompensation for weevil damage. Because damage by both flea beetles and weevils had non-linear effects on plant fitness, standard measures of tolerance were not appropriate. Thus, a second important contribution of this study is the use of the area under the curve defined by the regression of fitness on damage and damage-squared as a measure of tolerance. This revised version was published online in July 2006 with corrections to the Cover Date.     

沈阳市城市森林结构与效益分析

采用样方调查方法对沈阳市243km^2范围的树木进行每木实地调查,在地理信息系统应用软件Arc View3．2平台上建立了基于CITYgreen模型的城市森林信息管理系统,应用该信息系统分析了沈阳市土地利用及树木分布的格局、树种构成、树木密度、树高、胸径、健康状况等反映城市森林结构特征的指标结果表明,2001年沈阳市有树136种,1 914 500株树木,森林覆盖率为9．77％,25种主要树木占树木总数的84．78％,胸径小于0．25m(一类胸径)和大于0．5m(三类胸径)的树木占82．8％;幼龄树27％、中龄树58％、老龄树15％;84％的树木健康状况为中等以上,城市森林信息管理系统计算表明,沈阳市城市森林每年生态效益折合经济价值为2．65千万美元。