Evolution of insect-plant relationships: chemical constraints, coadaptation, and concordance of insect/plant traits

Co-adaptations, co-evolution, and co-speciation between herbivores and their host plants have been topics of interest for several decades. Difficulties in deciphering these relationships as well as physiological, biochemical, and ecological adaptations of herbivorous insects themselves are discussed here in relation to biotic and abiotic environmental factors that create temporal as well as spatial mosaics of genetic variation. Hybridization was shown in swallowtail butterflies (Papilio) (Lepidoptera: Papilionidae) to produce some trait concordance, but mostly independent geographic trait clines (physiological, biochemical, and morphological). Strong and extensive genetic introgression of Liriodendron tulipifera detoxification abilities was documented northward across the hybrid zone, presumably as a result of regional climate warming only during the last 3–4 years. These and other genetic novelties produced by hybridization may be important in speciation processes, and they also emphasize the difficulties identifying appropriate taxonomic classifications for discussing any species concept. Host plant detoxification abilities (as `key innovations') are shown capable of rapid movement between different polyphagous herbivore species independently of the host plant availability and well beyond the insect species geographical range distribution. Part of the difficulty associated with ecologically categorizing herbivore species and identifying affiliated adaptations for host plant use may be related to independent movement of various `species-diagnostic' traits. Climate-driven local selection regimes could help generate the dynamic variation observed for co-adapted, co-evolved, or non-adapted genotypes, and may produce the confusing and changing patterns of geographic mosaics seen within and among closely related herbivores. Experimental analyses of several factors that could explain the asymmetrical shapes of trait clines across the hybrid zone for tiger swallowtail butterflies are discussed.     

Major surface protein 1a effects tick infection and transmission of Anaplasma marginale

Anaplasma marginale, an ehrlichial pathogen of cattle and wild ruminants, is transmitted biologically by ticks. A developmental cycle of A. marginale occurs in a tick that begins in gut cells followed by infection of salivary glands, which are the site of transmission to cattle. Geographic isolates of A. marginale vary in their ability to be transmitted by ticks. In these experiments we studied transmission of two recent field isolates of A. marginale, an Oklahoma isolate from Wetumka, OK, and a Florida isolate from Okeechobee, FL, by two populations of Dermacentor variabilis males obtained from the same regions. The Florida and Oklahoma tick populations transmitted the Oklahoma isolate, while both tick populations failed to transmit the Florida isolate. Gut and salivary gland infections of A. marginale, as determined by quantitative PCR and microscopy, were detected in ticks exposed to the Oklahoma isolate, while these tissues were not infected in ticks exposed to the Florida isolate. An adhesion-recovery assay was used to study adhesion of the A. marginale major surface protein (MSP) 1a to gut cells from both tick populations and cultured tick cells. We demonstrated that recombinant Escherichia coli expressing Oklahoma MSP1a adhered to cultured and native D. variabilis gut cells, while recombinant E. coli expressing the Florida MSP1a were not adherent to either tick cell population. The MSP1a of the Florida isolate of A. marginale, therefore, was unable to mediate attachment to tick gut cells, thus inhibiting salivary gland infection and transmission to cattle. This is the first report of MSP1a being responsible for effecting infection and transmission of A. marginale by Dermacentor spp. ticks. The mechanism of tick infection and transmission of A. marginale is important in formulating control strategies and development of improved vaccines for anaplasmosis.     

Habitat-associated morphological divergence in two Neotropical fish species

We examined intraspecific morphological diversification between river channel and lagoon habitats for two Neotropical fish ( Bryconops caudomaculatus , Characidae; Biotodoma wavrini , Cichlidae). We hypothesized that differences between habitats (e.g. flow regime, foraging opportunities) might create selective pressures resulting in morphological divergence between conspecific populations. We collected fish from four channel-lagoon habitat pairs in the Río Cinaruco, Venezuela, and compared body morphology using geometric morphometrics. There were two aspects of divergence in both species: (1) placement of maximum body depth and (2) orientation of the mouth. For both species, maximum body depth was positioned more anteriorly (i.e. fusiform) in the river channel than in lagoons. Both species exhibited a relatively terminal mouth in lagoons compared to the channel. The mouth of B. caudomaculatus was relatively upturned, whereas the mouth of B. wavrini was relatively subterminal, in channel habitats. Observed morphological patterns are consistent with functional morphological principles suggesting adaptive divergence. We also show that spatial distance between habitats, presumably reflecting rates of population mixing, appears to have constrained diversification. For both species, morphological divergence increased with distance between habitats. Thus morphological divergence between channel and lagoon habitats apparently reflects a balance between diversification driven by natural selection, and homogenization driven by population mixing.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 80 , 689–698.     

Root-zone constraints and plant-based solutions for dryland salinity

Limitations to agricultural productivity imposed by the root-zone constraints in Australian dryland soils are severe and need redemption to improve the yields of grain crops and thereby meet world demand. Physical, chemical and biological constraints in soil horizons impose a stress on the plant and restrict plant growth and development. Hardsetting, crusting, compaction, salinity, sodicity, acidity, alkalinity, nutrient deficiencies and toxicities due to boron, carbonates and aluminium are the major factors that cause these constraints. Further, subsoils in agricultural regions in Australia have very low organic matter and biological activity. Dryland salinity is currently given wide attention in the public debate and government policies in Australia, but they only focus on salinity induced by shallow groundwater. However, the occurrence of transient salinity in root-zone layers in the regions where water tables are deep is an important issue with potential for larger economic loss than water table-induced seepage salinity. Root-zone constraints pose a challenge for salinity mitigation in recharge as well as discharge zones. In recharge zones, reduced water movement in sodic horizons results in salt accumulation in the root zone resulting in chemical and physical constraints that reduce transpiration that, in turn, upsets salt balance and plant growth. High salinity in soil and groundwater restricts the ability of plants to reduce water table in discharge zones. Thus plant-based strategies must address different kinds of limitations in soil profiles, both in recharge and discharge zones. In this paper we give an overview of plant response to root-zone constraints but with an emphasis on the processes of salt accumulation in the root-zone of soils. We also examine physical and chemical methods to overcome subsoil limitations, the ability of plants to adapt to and ameliorate these constraints, soil modification by management of agricultural and forestry ecosystems, the use of biological activity, and plant breeding for resistance to the soil constraints. We emphasise that soil scientists in cooperation with agronomists and plant breeders should design site-specific strategies to overcome multiple soil constraints, with vertical and lateral variations, and to develop plant-based solutions for dryland salinity.     

Selective Pressures on RNA Hairpins In Vivo and In Vitro

Comparison of the most stable potential hairpins in the sequences of natural ribozymes with those in the randomized sequences has revealed that the hairpin loop energies are lower than expected by chance. Although these hairpins are not necessarily parts of functional structures, there is a selective pressure to diminish the destabilizing free energies of the hairpin loops. In contrast, no significant bias is observed in the stacking values of the most stable stems. In the ribozymes isolated in vitro the loops of potential hairpins are closer to random values, which can result in less efficient folding rates. Furthermore, the effects of kinetic traps seem to be more significant in the folding pathways of the in vitro isolates due to a potential to form stable stacks incompatible with the functional folds. Similarly to natural ribozyme sequences, the untranslated regions of viral RNAs also form hairpins with relatively low loop free energies. These evolutionary trends suggest ways for efficient engineering of improved RNA constructs on the basis of analysis of in vitro isolates and approaches for the search of regions coding for functional RNA structures in large genome sequences. Received: 12 January 2001 / Accepted: 21 May 2001     

Artificial selection on the shape of reaction norms for eyespot size in the butterfly Bicyclus anynana: direct and correlated responses

The tropical butterfly Bicyclus anynana shows phenotypic plasticity in its ventral wing pattern as an adaptive response to wet‐dry seasonality. Wet season form individuals have large eyespots, whereas individuals of the dry season generation have small eyespots. In the laboratory these forms can be obtained by rearing larvae at high and low temperatures, respectively. To study the extent to which the shape of the nearly linear reaction norms for eyespot size can be changed we applied four generations of sib selection by rearing full‐sib families across three temperatures. In addition, we measured ecdysteroid titres shortly after pupation in the final generation. Although phenotypic variation in shape was present initially, the experiment yielded lines with reaction norms with similar shapes but different elevations. High, positive genetic correlation across temperatures can explain this lack of response. Differences in ecdysteroid titres did not readily relate to differences in eyespot size.     

Are Nectar Sugar Composition and Corolla Tube Length Related to the Diversity of Insects that Visit Asteraceae Flowers?

Abstract: In this work, we analysed interspecific variation in nectar sugar composition, corolla tube length, and the diversity of floral visitors of 35 Asteraceae species. The potential correlations between these variables could arise either as a result of selection to improve pollinator attractiveness or simply as a consequence of phylogenetic constraints. Samples of nectar and flowers, and data on floral visitors, were obtained from living plants in natural populations from Argentina. Asteraceae species showed a large variability in corolla tube length. Nectar of most species presented a larger proportion of hexoses than sucrose. All species were visited by numerous insects belonging to 2 different orders. Results showed that floral traits are not significantly correlated with the diversity of floral visitors. These characters seem to be linked to the phylogeny of the species. Early branching species (species phylogenetically close to the root of the Asteraceae tree) tend to have longer corollas, higher sucrose proportions and lesser diversity of floral visitors than late branching species. Considering that longer corolla tubes and higher nectar sucrose percentages may indicate some specialization in the pollination system, we suggest that there is an evolutionary tendency toward generalist pollination systems within the family.     

Why are there no really big bony fishes? A point-of-view on maximum body size in teleosts and elasmobranchs

The most massive teleost, the ocean sunfish(Mola mola), is an order of magnitude smaller than the largest cartilaginous fish,the whale shark (Rhincodon typus), and issignificantly smaller than several other extantelasmobranch species. Possible reasons for this discrepancy in maximum size include:anatomical, physiological, ecological, and life-history/ontogenetic constraints. Weexamined life-history traits and growth ratesas the most likely constraints on maximum teleostsize. For pelagic fishes there appear to be two life-history strategies: producing few,large, live young or many, tiny eggs. We propose that this dichotomy is an evolutionaryvestige of the freshwater origins of teleosts, and is the basis of the limitation onmaximal body size in teleosts.     

The reproductive flight phenology of a neotropical ant assemblage

1. Alate flights reflect an ant colony's investment in sexual reproduction and dispersal yet little is known about community‐wide patterns of alate phenology. Two Malaise traps (for 2 years) and two light traps (for 1 year) were used to explore the flight phenologies of 22 common neotropical species from Barro Colorado Island, Panama. 2. The traps caught 23 182 individuals and 286 species/morphospecies. The two trap methods shared only 18 species. Samples also differed in sexual composition: light trap samples were 80% female, Malaise trap samples were 2.6% female. 3. Of 22 common species, all but one flew over half the year, with about half flying every month of the year. These data, combined with a literature review, suggest a latitudinal gradient in alate flight season: one north temperate assemblage (42°N) averaged 1.6 lunar months per species. The ever‐warm tropical year provides a larger flight window that allows a diversity of phenologies, from continuous to strongly pulsed. 4. Rainfall was correlated with alate flights in one‐third of the species. Quantile regression suggested that high weekly rainfall was necessary but not sufficient to produce alate flights in about a quarter of the species. 5. By decreasing the number of nests releasing alates on a given day, long flight seasons may lower the probability of finding a mate. At the same time, long flight seasons may increase the opportunity of finding vacant nest sites. High population densities and high incidence of nest disturbance in this community may ameliorate the first cost while enhancing the second benefit.     

Determinants of Land Degradation and Fragmentation in Semiarid Vegetation at Landscape Scale

The objective of this paper was to investigate the sensitivity to degradation of semiarid plant communities in terms of plant cover and fragmentation, quantifying relationships between landscape characteristic (physical, socio-economical and historical) and vegetation degradation. The degradation of vegetation was measured as the degree of coverage of the two dominant vegetation types (i.e. tall arid brush and tall grass steppe), while fragmentation was measured as patch size and isolation. Data were obtained using GIS tools, and analyzed by logistic regression and linear multivariate regression. Results showed denser coverage at more elevated, gradual slopes that were not sea-oriented. Historical elements of the landscape had a significant effect on current natural vegetation. According to the fragmentation patterns, the vegetation is in fairly good condition (medium coverage had the largest patches but dense coverage was less isolated) but attention must be given to preserve vegetation, due to the relationships between fragmentation and human activities. Moreover, the protection plan under way in the area appeared to favour denser vegetation cover, while human activities had a measurable effect on vegetation degradation.