Structural changes of vegetation and its association with microclimate in a successional gradient of low thorn forest in northeastern Mexico

Understanding how microclimate and vegetation are associated during secondary succession is of primary importance for plant conservation in the face of the increasing land cover modification. However, these patterns are still unstudied for many plant communities. This study aimed to evaluate the structure (species richness, Shannon's diversity index, Simpson´s dominance index, abundance of each species, average height of species, species cover (%), species composition, and indicator values) of a low thorn forest fragment and to analyze its relation with microclimate along a successional gradient. Four stages of succession were delimited by the analysis of Landsat images, in the state of Tamaulipas, northeast Mexico. Statistical models incorporated species richness, diversity indices, abundance, height, and cover, as variables for searching differences between stages, or to evaluate microclimate associations. A total of 70 species, 54 genera, and 27 families were determined. Height of tree layer was the most important variable for discrimination of the successional stages. Conserved areas differed floristically from other stages, associated mainly with the lowest values of wind speed originated by tree layer characteristics. A significant association between species and microclimate was found, being wind speed and relative humidity the most important variables. Some species, due to their high importance values and their patterns of association with microclimate, may be considered as key taxa for low thorn forest, which is a threatened semitropical community in northeast Mexico. Conserved and late successional areas account for climatic regulation of this plant community, and the importance of these forest patches may be considered when establishing biodiversity protection areas.

     

An Electrochemical Study of the Factors Responsible for Modulating the Reduction Potential of Putidaredoxin

 The gene coding for putidaredoxin has been synthesized using a combination of chemical and enzymatic methods and subsequently expressed in Escherichia coli. The recombinant protein characterized by electronic spectroscopy, mass spectrometry, and electrochemistry was found to be identical to putidaredoxin obtained from Pseudomonas putida. Polylysine was found to promote the fast and reversible electrochemistry of putidaredoxin at negatively charged electrodes such as indium-doped tin oxide or gold surfaces modified with mercaptoalkanoate groups. The value of the heterogeneous electron transfer rate constant obtained from solutions containing a mixture of putidaredoxin and polylysine (k _s =1.3×10^–3 cm/s) is one order of magnitude larger than the values reported previously at gold electrodes modified with mercaptoethylamine or at antimony-doped tin oxide semiconductor electrodes. It was observed that when the reduction potential of putidaredoxin is measured by cyclic voltammetry, the resultant value is consistently more positive (64 mV) than the reduction potential measured with potentiometric titrations. A comparison between the electrochemical responses of putidaredoxin and spinach ferredoxin, combined with the examination of their corresponding three-dimensional structures, indicates that the positive shift in the reduction potential of putidaredoxin originates from the formation of a transient complex between putidaredoxin and polylysine at the electrode surface. The formation of this transient complex modulates the reduction potential of putidaredoxin by lowering the value of the dielectric constant around its iron-sulfur cluster microenvironment, specifically by neutralizing negative charges surrounding the active site and by excluding water from the solvent exposed iron sulfur cluster. The observed positive shift in E°′, which is induced by complexation with polylysine at the electrode-surface, suggests that similar factors are likely to contribute to the anodic shift in the E°′ of cytochrome P450_cam-bound putidaredoxin (+44 mV) with respect to the E°′ measured for free putidaredoxin. Received: 14 June 1999 / Accepted: 6 August 1999     

Sequence-based species delineation and molecular phylogenetics of the transitional Nearctic–Neotropical grasshopper genus Taeniopoda (Orthoptera,Romaleidae)

Taeniopoda is a genus of grasshoppers currently represented by 12 species distributed from southern USA to Panama, with most of them occurring along the transitional Nearctic–Neotropical region in central and southern Mexico. Despite being a small group of conspicuous, colourful species, the systematics of Taeniopoda has been largely neglected, including its phylogenetic affinity with the morphologically similar, monotypic genus Romalea. Here, we assessed the species limits in 11 of the species of Teniopoda based on two mitochondrial (mt) markers (COI, cyt b). Phylogenetic relationships were reconstructed adding two nuclear gene markers (28S, H3). A relaxed molecular clock analysis was performed based on the mt markers. We detected nuclear mt paralogues (numts) and the probable introgression of T. tamaulipensis mtDNA in specimens of T. eques from central Mexico. Between six and 14 species of Taeniopoda were delimited by the sequence-based approaches performed (COI divergence with thresholds of 1 and 2%; General Mixed Yule-Coalescent (GMYC) model). The GMYC and 1% threshold analyses with COI were more congruent with the currently recognized morphology-based taxonomy with 10 and 11 putative species, respectively. Four of these species were regarded as ‘stable’, since they were supported by at least one of the molecular analyses and by diagnostic morphological features. The species-based phylogeny recovered Taeniopoda as paraphyletic with respect to the monotypic genus Romalea. Three morphologically and geographically congruent major clades were recovered, two with species having a considerably elevated pronotal crest and one with its members having it less elevated. The origin and subsequent diversification of Taeniopoda were estimated to occur from the mid and late Miocene to Pliocene, respectively. The current species diversity in Taeniopoda was estimated to occur during the Pleistocene, which was probably influenced by the climatic oscillations that occurred during this period and the uplift of mountain ranges in Central America.     

A comparative study of two Mexican isolates with an Australian isolate of Metarhizium anisopliae var. acridum – strain characterisation, temperature profile and virulence for wingless grasshopper, Phaulacridium vittatum

Three isolates of Metarhiziumanisopliae var. acridum, FI-985,from the spur-throated locust, Austracrisguttulosa from Australia, and QF-01 and QF-02both from Schistocerca piceifrons inMexico were compared. FI-985 had much largerconidia than the other two isolates and alsohad a different colony appearance. The twoMexican isolates showed small differences inconidial size and colony morphology. Over arange of 6 primers, RAPDs patterns were verysimilar in that the two Mexican isolates being identical, and FI-985 having some unique bandsonly with 2 primers. The 3 isolates were alsosimilar in growth profiles on agar plates, andat high temperatures (36°C), QF-01 didnot grow while the other two isolates grewslowly. All three isolates were similar invirulence for wingless grasshoppers, Phaulacridium vittatum, at 20–30°C but at 35°C, FI-985 was the mostvirulent and QF-01 the least over a range of 3doses. At 15°C, FI-985 was also the mostvirulent with QF-02 being least virulent. At30°C, the LD₅₀ at 17 days rangedfrom 248 conidia for QF-02 to 501 conidia forFI-985, however the differences were notsignificant (p > 0.05). It is suggested thatconsideration should be given to using anexotic isolate such as FI-985 as a commerciallocust biopesticide in countries such asMexico, since the isolate is cheap to massproduce, more effective at higher temperaturescommon in tropical to subtropical Mexico, andrelatively resistant to UV.     

FUNCTIONAL EQUIVALENCE OF GRASPING CERCI AND NUPTIAL FOOD GIFTS IN PROMOTING EJACULATE TRANSFER IN KATYDIDS

The function of nuptial gifts has generated longstanding debate. Nuptial gifts consumed during ejaculate transfer may allow males to transfer more ejaculate than is optimal for females. However, gifts may simultaneously represent male investment in offspring. Evolutionary loss of nuptial gifts can help elucidate pressures driving their evolution. In most katydids (Orthoptera: Tettigoniidae), males transfer a spermatophore comprising two parts: the ejaculate‐containing ampulla and the spermatophylax—a gelatinous gift that females eat during ejaculate transfer. Many species, however, have reduced or no spermatophylaces and many have prolonged copulation. Across 44 katydid species, we tested whether spermatophylaces and prolonged copulation following spermatophore transfer are alternative adaptations to protect the ejaculate. We also tested whether prolonged copulation was associated with (i) male cercal adaptations, helping prevent female disengagement, and (ii) female resistance behavior. As predicted, prolonged copulation following (but not before) spermatophore transfer was associated with reduced nuptial gifts, differences in the functional morphology of male cerci, and behavioral resistance by females during copulation. Furthermore, longer copulation following spermatophore transfer was associated with larger ejaculates, across species with reduced nuptial gifts. Our results demonstrate that nuptial gifts and the use of grasping cerci to prolong ejaculate transfer are functionally equivalent.