Complex food webs prevent competitive exclusion among producer species

Herbivorous top-down forces and bottom-up competition for nutrients determine the coexistence and relative biomass patterns of producer species. Combining models of predator-prey and producer-nutrient interactions with a structural model of complex food webs, I investigated these two aspects in a dynamic food-web model. While competitive exclusion leads to persistence of only one producer species in 99.7% of the simulated simple producer communities without consumers, embedding the same producer communities in complex food webs generally yields producer coexistence. In simple producer communities, the producers with the most efficient nutrient-intake rates increase in biomass until they competitively exclude inferior producers. In food webs, herbivory predominantly reduces the biomass density of those producers that dominated in producer communities, which yields a more even biomass distribution. In contrast to prior analyses of simple modules, this facilitation of producer coexistence by herbivory does not require a trade-off between the nutrient-intake efficiency and the resistance to herbivory. The local network structure of food webs (top-down effects of the number of herbivores and the herbivores' maximum consumption rates) and the nutrient supply (bottom-up effect) interactively determine the relative biomass densities of the producer species. A strong negative feedback loop emerges in food webs: factors that increase producer biomasses also increase herbivory, which reduces producer biomasses. This negative feedback loop regulates the coexistence and biomass patterns of the producers by balancing biomass increases of producers and biomass fluxes to herbivores, which prevents competitive exclusion.     

DPKQDFMRFamide expression is similar in two distantly related Drosophila species

Drosophila melanogaster DPKQDFMRFamide was isolated and its expression reported. Distribution of DPKQDFMRFamide immunoreactivity is now described in Drosophila virilis. DPKQDFMRFamide antibody stained a cell in the subesophageal ganglion in embryo. DPKQDFMRFamide antibody stained cells in the superior protocerebrum, subesophageal ganglion, thoracic ganglia, and an abdominal ganglion in larva, pupa, and adult. DPKQDFMRFamide antibody stained an additional pair of cells in the optic lobe and a cell in the lateral protocerebrum in adult. Structure identity and similar distribution of DPKQDFMRFamide in D. virilis and D. melanogaster, two distantly related Drosophila species, suggests an important and conserved activity for the peptide.     

Predator-released compounds, ambient temperature and competitive exclusion among differently sized Daphnia species

1. We studied the effects of fish water and temperature on mechanisms of competitive exclusion among two Daphnia species in flow‐through microcosms. The large‐bodied D. pulicaria outcompeted the medium sized D. galeata × hyalina in fish water, but not in the control treatment. Daphnia galeata × hyalina was competitively displaced 36 days earlier at 18 °C than at 12 °C. 2. It is likely that the high phosphorus content of fish water increased the nutritional value of detrital seston particles by stimulating bacterial growth. Daphnia pulicaria was presumably better able to use these as food and hence showed a more rapid somatic growth than its competitor. This led to very high density of D. pulicaria in fish water, but not in the controls. The elevated D. pulicaria density coincided with high mortality and reduced fecundity in D. galeata × hyalina, resulting in competitive displacement of the hybrid. 3. It is clear that the daphnids competed for a limiting resource, as grazing caused a strong decrease in their seston food concentration. However, interference may also have played a role, as earlier studies have shown larger Daphnia species to be dominant in this respect. The high density of large‐bodied D. pulicaria in fish water may have had an allelopathic effect on the hybrid. Our data are inconclusive with respect to whether the reached seston concentration was below the threshold resource level (R*) of the hybrid, where population growth rate and mortality exactly balance, as it would be set in the absence of interference, or whether interference actually raised the hybrid's R* to a value above this equilibrium particle concentration. 4. Our results do clearly show that fish‐released compounds mediated competitive exclusion among zooplankton species and that such displacement occurred at a greatly enhanced rate at an elevated temperature. Fish may thus not only structure zooplankton communities directly through size‐selective predation, but also indirectly through the compounds they release.     

Heat-shock DNA homology in distantly related species of Drosophila

Polytene chromosomes of D. melanogaster and D. virilis were hybridized in situ with ¹²⁵I labeled mRNA isolated from polysomes of D. melanogaster tissue culture cells incubated at 37° C. ¹²⁵I mRNA hybridized preferentially with subdivisions 87A and 87Cl of the D. melanogaster 3R chromosome; grains were also observed at regions 93D, 95D and over the chromocenter. A considerable cross hybridization of this mRNA with D. virilis polytene chromosomes was observed. The 29C region of the D. virilis second chromosome was the main site of hybridization. Significant grain numbers also appeared in region 20F of the same chromosome. The two regions mentioned belong to heat shock loci in the latter species. Based on label intensity we conclude that region 29C of D. virilis contains DNA sequences retaining molecular homology with those at subdivisions 87A and 87Cl of D. melanogaster. SDS-polyacrylamide gel electrophoresis revealed similar distributions of heat shock proteins in the two species studied.     

Convergent evolution of levee building behavior among distantly related ant species in a floodplain ant assemblage

Flooding impacts ground nesting ant colonies by destroying the infrastructure housing and organizing societal function. Here, we report the convergent evolution in distantly related ant species of a behavioral trait that minimizes costs of flooding: the construction of earthen levees around nest entrances. In a South American floodplain ecosystem, we observed five ant species constructing prominent earthen berms encircling nest entrances shortly after large rainfall events. In four of these species, experimental flooding of nests demonstrated that earthen berms sufficed to prevent floodwaters from entering the below ground portions of the nest. Additional manipulations revealed that levee breaching caused, pronounced, and extended reductions in food collection for two distantly related species. Foraging was preempted by the allocation of workers to repair the internal structure of the nest. These findings represent convergent evolution of a functionally important nest construction behavior in response to comparable selective forces.     

Unusually similar patterns of antibody V segment diversity in distantly related marsupials

A pattern of coevolution between the V gene segments of Ig H and L chains has been noted previously by several investigators. Species with restricted germline V(H) diversity tend to have limited germline V(L) diversity, whereas species with high levels of germline V(H) diversity have more diverse V(L) gene segments. Evidence for a limited pool of V(H) but diverse V(L) gene segments in a South American opossum, Monodelphis domestica, is consistent with this marsupial being an exception to the pattern. To determine whether M. domestica is unique or the norm for marsupials, the V(H) and V(L) of an Australian possum, Trichosurus vulpecula, were characterized. The Ig repertoire in T. vulpecula is also derived from a restricted V(H) pool but a diverse V(L) pool. The V(L) gene segments of T. vulpecula are highly complex and contain lineages that predate the separation of marsupials and placental mammals. Thus, neither marsupial follows a pattern of coevolution of V(H) and V(L) gene segments observed in other mammals. Rather, marsupial V(H) and V(L) complexity appears to be evolving divergently, retaining diversity in V(L) perhaps to compensate for limited V(H) diversity. There is a high degree of similarity between the V(H) and V(L) in M. domestica and T. vulpecula, with the majority of V(L) families being shared between both species. All marsupial V(H) sequences isolated so far form a common clade of closely related sequences, and in contrast to the V(L) genes, the V(H) likely underwent a major loss of diversity early in marsupial evolution.     

Mobilizable narrow host range plasmids as natural suicide vectors enabling horizontal gene transfer among distantly related bacterial species

Klebsiella pneumoniae 287-w carries three small narrow host range (NHR) plasmids (pIGMS31, pIGMS32, and pIGRK), which could be maintained in several closely related species of Gammaproteobacteria, but not in Alphaproteobacteria. The plasmids contain different mobilization systems (MOB), whose activity in Escherichia coli was demonstrated in the presence of the helper transfer system originating from plasmid RK2. The MOBs of pIGMS31 and pIGMS32 are highly conserved in many bacterial plasmids (members of the MOB family), while the predicted MOB of pIGRK has a unique structure, encoding a protein similar to phage-related integrases. The MOBs of pIGMS31 and pIGMS32 enabled the transfer of heterologous replicons from E. coli into both gammaproteobacterial and alphaproteobacterial hosts, which suggests that these NHR plasmids contain broad host range MOB systems. Such plasmids therefore represent efficient carrier molecules, which may act as natural suicide vectors promoting the spread of diverse genetic information (including other types of mobile elements, e.g. resistance transposons) among evolutionarily distinct bacterial species. Thus, mobilizable NHR plasmids may play a much more important role in horizontal gene transfer than previously thought.     

Incompatibility and competitive exclusion of genomic segments between sibling Drosophila species

The extent and nature of genetic incompatibilities between incipient races and sibling species is of fundamental importance to our view of speciation. However, with the exception of hybrid inviability and sterility factors, little is known about the extent of other, more subtle genetic incompatibilities between incipient species. Here we experimentally demonstrate the prevalence of such genetic incompatibilities between two young allopatric sibling species, Drosophila simulans and D. sechellia. Our experiments took advantage of 12 introgression lines that carried random introgressed D. sechellia segments in different parts of the D. simulans genome. First, we found that these introgression lines did not show any measurable sterility or inviability effects. To study if these sechellia introgressions in a simulans background contained other fitness consequences, we competed and genetically tracked the marked alleles within each introgression against the wild-type alleles for 20 generations. Strikingly, all marked D. sechellia introgression alleles rapidly decreased in frequency in only 6 to 7 generations. We then developed computer simulations to model our competition results. These simulations indicated that selection against D. sechellia introgression alleles was high (average s = 0.43) and that the marker alleles and the incompatible alleles did not separate in 78% of the introgressions. The latter result likely implies that most introgressions contain multiple genetic incompatibilities. Thus, this study reveals that, even at early stages of speciation, many parts of the genome diverge to a point where introducing foreign elements has detrimental fitness consequences, but which cannot be seen using standard sterility and inviability assays.     

Variation in juvenile survival and related physiological traits among dipterocarp species co‐existing in a Bornean forest

Question: What environmental factors and physiological traits determine the juvenile survival rates of 11 dipterocarp species (Dipterocarpaceae) co-occurring in a Bornean tropical rain forest? Location: Tropical northwestern Borneo. Methods: In 248 quadrats, distributed over 12 ha, the juvenile (height< 2.0 m) survival of 11 shade-tolerant dipterocarp species was monitored for one year. The probability of survival for each species was expressed as a logistic function of plant height and three environmental factors: canopy openness, soil water potential and conspecific individual density. The correlation between survival under shaded conditions and several physiological traits of juveniles was tested by normal Pearson correlation and a phylogeny based manner using independent contrasts. Results: Seven species had increased survival under more open canopies, one species had increased and two species had decreased survival at drier sites. Six species had higher survival as the density of conspecifics increased, whereas two species had lower survival. The magnitudes of the effects for the three environmental factors were similar. However, the interspecific difference in survival was largely determined by a dependence of small seedlings on the light environment. Survival under low light was correlated positively with root dry mass and negatively with total leaf area of juveniles. Conclusions: Responses of seedlings to the fine-scale gradient of light availability were considerably different among the 11 species. Species that suffered increased mortality under low light had a set of structural traits that would potentially allow rapid growth under bright conditions. Differential performance along the light gradient may contribute to the stable coexistence of these species.     

Comparison of MHC genes among distantly related members of the genus Mus

The genomic content of class I and class II MHC DNA sequences in a variety of wild mice has been analyzed. The panel of mice includes members of three subgenera of the genus Mus. By genomic hybridization with the use of a variety of DNA probes, both class I and class II DNA sequences appear to be conserved in all of the species examined. However, the number of class I DNA sequences differs among the species. Furthermore, this variation appears to result from differential increases within subsets of class I genes. These data suggest that the class I multigene family is dynamic and changing over short periods of evolutionary time. In contrast, none of the class II genes appears to vary in copy number. More extensive polymorphism was noted amongst the class II beta genes than the alpha genes. Interestingly, the genomic sequence corresponding to E beta 2 is highly conserved, leading to the prediction that it is a genetically functional sequence.     

The actin loci in the genus Drosophila: establishment of chromosomal homologies among distantly related species by in situ hybridization

We have mapped by in situ hybridization the actin genes in selected, distantly related Drosophila species, using the 5C actin gene of D. melanogaster as a probe. In all species six dispersed actin loci were observed, probably corresponding to six genes, and they were similarly distributed among the chromosomes. In conjunction with previously available genetic and cytogenetic evidence, this consistent pattern of actin gene distribution reinforces the hypothesis that the chromosomal elements have maintained their essential identities throughout Drosophila evolution, and permits identification of these elements in very diverse species. Conservation of the actin loci also offers fixed points for the analysis of chromosomal inversions and other rearrangements.     

Vitellogenin genes and their products in closely and distantly related species of Xenopus

Plasma vitellogenins from two closely related species of Xenopus, X. laevis and X. borealis, and a more ancient species, X. tropicalis, exhibited the same size on gel electrophoresis and were immunologically related. Partial peptide maps of 125I-labelled plasma vitellogenins, however, revealed marked differences in th structure and organisation of vitellogenin in the three Xenopus species. Northern blot hybridisation of liver RNA from oestrogen-treated males and females, probed with cloned vitellogenin cDNA, revealed the presence of mRNA of the same size in the three species of Xenopus, which was absent in untreated male liver. Cell-free translation of total liver RNA showed the presence of functional mRNA coding for vitellogenin subunit of the same size (Mr congruent to 210,000). Restriction endonuclease digestion patterns of genomic DNA from the three Xenopus species, using cloned X. laevis vitellogenin cDNA as the hybridisation probe, revealed significant differences in the organisation of these genes, which occur at a higher multiplicity in X. laevis and X. borealis than in X. tropicalis. Thus, despite a high degree of conservation of size, overall sequence and immunological identity of vitellogenin genes and their products in the three species of Xenopus, there is a substantial structural rearrangement during evolution of Xenopus within this multigene family.     

Constancy of RAPD primer amplification strength among distantly related taxa of flowering plants

A survey of 480 10-mer primers for RAPD markers revealed general consistency in primer amplification strength among the flowering plant generaDatisca, Helianthus andYucca. Six characteristics of primer base sequences were analyzed: total (G+C) content; the amounts of G, A, C, and T taken separately; and the (G+C) content in the last four bases of the 3′ end. Of these, total (G+C) content showed the most value in predicting primer amplification strength. Since the consistency of amplification strength is true only globally, there are still many primers showing high variation in amplification strength among genera, most probably due to DNA sequence differences, but perhaps also resulting from experimental artifact. Nevertheless, we suggest that this survey be used as a rough guide for prioritizing primer deployment in RAPD studies involving plants in the hope of improving efficiency during the search for adequate levels of polymorphism, with the understanding that taxon-specific differences in primer amplification strength are bound to occur.     

Functional conservation of the malaria vaccine antigen MSP-119across distantly related Plasmodium species

The C-terminal region of Plasmodium falciparum merozoite surface protein 1 (MSP-119) is at present a leading malaria vaccine candidate. Antibodies against the epidermal growth factor-like domains of MSP-1 19are associated with immunity to P. falciparum and active immunization with recombinant forms of the molecule protect against malaria challenge in various experimental systems. These findings, with the knowledge that epidermal growth factor-like domains in other molecules have essential binding functions, indicate the importance of this protein in merozoite invasion of red blood cells. Despite extensive molecular epidemiological investigations, only limited sequence polymorphism has been identified in P. falciparum MSP-119 (refs. 9-11). This indicates its sequence is functionally constrained, and is used in support of the use of MSP-119 as a vaccine. Here, we have successfully complemented the function of most of P. falciparum MSP-119 with the corresponding but highly divergent sequence from the rodent parasite P. chabaudi. The results indicate that the role of MSP-119 in red blood cell invasion is conserved across distantly related Plasmodium species and show that the sequence of P. falciparum MSP-119 is not constrained by function.