Species Of Columnar Cacti Research Articles

Spatial Associations, Size–Distance Relationships and Population Structure of Two Dominant Life Forms in a Semiarid Enclave of the Venezuelan Andes

The role of three thorny legume species as nurse plants and competitive relationships with columnar cacti were evaluated in a semiarid enclave of the Venezuelan Andes. Abundance and size of three columnar cacti species (Stenocereus griseus, Cereus repandus and Pilosocereus tillianus) under isolated shrubs of three thorny legumes species (Prosopis juliflora, Acacia farnesiana and A. macracantha) were recorded and compared with open areas. Using size–distance data we inferred the intensity of intra- and interspecific competition between both life forms in a “cardonal” (xeric zone) and “espinar” (mesic zone) of the enclave. Sixty-one columnar cacti were found beneath the three thorny legume species (92%), while only 5 cactus individuals were found in open areas (8%). Comparison of observed and expected number of cactus individuals shows that S. griseus and C. repandus are significantly associated with isolated shrubs of A. farnesiana and A. macracantha. Although P. juliflora contributed more than 40% of the total legume plant cover, the number of columnar cacti under its isolated shrubs was significantly lower than would be expected by chance. In all, 19 of 21 possible plant–plant combinations between and within columnar cacti and legume shrubs were recorded (espinar: 18, cardonal: 13, common combinations: 12). In both zones, intra- and interspecific combinations among columnar cacti species were relatively high (positive correlation between the sum of neighbour plant sizes and the distances separating them). Our results strongly suggest, at least for case of S. griseus and C. repandus, that these columnar cacti species require nurse plants for their establishment. The results on interference need still support from further research. We discuss the effects of plant–plant positive interactions on natural regeneration of these columnar cacti.

Analysis of the factors that affect the distribution and abundance of three Neobuxbaumia species (Cactaceae) that differ in their degree of rarity

We studied three species of columnar cacti in the genus Neobuxbaumia which differ in their degree of rarity: Neobuxbaumia macrocephala (the rarest), Neobuxbaumia tetetzo (intermediate), and Neobuxbaumia mezcalaensis (the most common). To investigate the ecological factors that limit their distribution and abundance, we surveyed 80 localities within the region of Tehuacan-Cuicatlan, in Central Mexico. At each locality we measured several environmental variables, and the density of the Neobuxbaumia populations present. We used a principal component analysis (PCA) to identify the factors that are associated to the presence/absence of each species. Additionally, we carried out multiple regressions between environmental variables and population density to test whether the variation in these variables was related to changes in abundance. The results show that factors significantly affecting the distribution of these species are mean annual temperature, altitude, rainfall, and soil properties such as texture and organic matter content. N. mezcalaensis reaches maximum population densities of 14,740 plants per ha (average density = 3943 plants per ha) and is associated with localities with relatively abundant rainfall. N. tetetzo shows maximum population densities of 14,060 plants per ha (average = 3070 plants per ha), and is associated with sites located at high latitudes and with high phosphorous content in the soil. The rarest species, N. macrocephala, shows maximum densities of 1180 plants per ha (average = 607 plants per ha) and is associated with localities with high soil calcium content. The distribution of this species is limited to sites with specific values of the environmental variables recorded, conferring it a high habitat specificity which accounts for its rarity.

Clonal Diversity and Allelic Relationships between Two Closely Related Species of Columnar Cacti from the Sonoran Desert:Stenocereus erucaandStenocereus gummosus

Stenocereus eruca and Stenocereus gummosus are two closely related columnar cacti of the Sonoran Desert that are thought to be highly clonal. In addition, it has been suggested that S. eruca has recently been derived from an S. gummosus–like ancestor. We used allozymes analysis to test these hypotheses and to describe levels of genotypic diversity in populations of both taxa. Allelic composition in S. eruca was a subset of the composition found in S. gummosus, while polymorphism and expected heterozygosity were greater in S. gummosus, supporting the hypothesis that S. eruca is a recently derived species. The proportion of unique genotypes ($$G/N$$) and genotype diversity (D) varied from 0.42 to 0.68 and from 0.78 to 0.98, respectively, among populations of S. eruca and from 0.64 to 0.96 and from 0.96 to 1.00, respectively, among populations of S. gummosus. The evidence on clonal diversity indicates that both species combine clonal growth and sexual recruitment as mechanisms of regeneration and that S. eruca seems to rely more on clonal propagation than does S. gummosus. We hypothesize that the procumbent growth habit of S. eruca in the plains of Magdalena may have evolved in response to aeolian dune formation during the Pleistocene.

Stem surface injuries of Neobuxbaumia tetetzo and Neobuxbaumia mezcalaensis of the Tehuacan Valley ofcentral Mexico

Evans, L. S. (Laboratory of Plant Morphogenesis, Biological Sciences Research Laboratories, Manhattan College, the Bronx, NY 10471). Stem surface injuries of Neobuxbaumia tetetzo and Neobuxbaumia mezcalaensis of the Tehuacan Valley of central Mexico. J. Torrey Bot. Soc. 132: 33–37. 2005.—Identical scale and bark stem surface injuries are present on surfaces of fourteen species of long-lived columnar cacti in both North and South America. The purpose of this research was to determine the amounts of injuries on south-, east-, north-, and west-facing stem trough surfaces for two species of columnar cacti, Neobuxbaumia tetetzo and N. mezcalaensis, from the Tehuacan Valley (18° N latitude) of central Mexico. Stem surface injury levels on south (equatorial)-facing surfaces were twice that of north (polar)-facing stem surfaces for both cactus species. Calculations indicate that twice as much total annual direct sunlight occurs to south-facing cactus surfaces than on north-facing surfaces at 18° N latitude. In general, surface injury levels for east-facing and west-facing plant surfaces were usually intermediate between the high levels of injury for south-facing surfaces and the low levels of injury for north-facing surfaces. It should be noted that published results from columnar cacti growing at 32 °N latitude have about four times more stem surface injuries and four times more direct sunlight exposure on equatorial-facing surfaces than on polar surfaces. So, for columnar cacti at 18° and 32° latitude, the ratios of injuires were coincident with ratios of sunlight on equatorial and polar surfaces. Overall, the results of these studies are consistent with the hypothesis that sunlight, possibly UV-B, cause injuries to stems of long-lived, columnar cacti.

Introducing Cereus into an Arid Region as a New Fruit Crop

One of the major steps in responding to imminent water shortages in the Middle East is improving water use efficiency. Drought-resistant crops would be an effective technology to curb rising demands of water. Columnar Cactus species characteristics fit with most of the requirements of a drought tolerant crop with very high water-use efficiency. Cereus cacti have physiological and morphological methods of exploiting environments that would soon desiccate other plants. Four Cereus species were introduced into UAE deserts and could be ideal for establishing crop plantations in the arid environment. The introduced fruiting cacti are Cereus hexagonus, C. pachanoi, C. peruvianus, and C. validus. Plants were propagated by cuttings in the greenhouse. Cuttings developed roots within 2*&8211;4 weeks of planting. The propagated plants were acclimatized and transplanted into the field in the desert. C. peruvianus was the most promising in the new environment in terms of its high adaptability and healthy growth in the new environment. C. pachanoi grew very fast, averaging up to a fifteen centimeter a month of new growth. C. pachanoi was recommended as a rootstock for other species. C. validus could not survive the new environment.

Demography of the columnar cactus Neobuxbaumia macrocephala: a comparative approach using population projection matrices

Neobuxbaumia macrocephala is a long-lived columnar cactus endemic to the Tehuacan-Cuicatlan Valley in south-central Mexico. This plant has a very restricted distribution and few recruitment events have been detected in its populations. In this study, we analyze the N. macrocephala demographic pattern using a projection matrix in order to determine the main limiting factors of this species. To accomplish this goal, we compare our results with those obtained for another species of the same genus, N. tetetzo. Considering that both species inhabit the same valley, we believe that this comparative study will offer insights into the main demographic limitations of N. macrocephala. Results showed that these species of columnar cacti have similar demographic patterns in which survival is the process with the highest relative contribution to λ, followed by growth and reproduction. Of all the life cycle stages, seeds and seedlings have the lowest survival probabilities due to a high mortality caused by seed predation and effects of direct solar radiation on germinated seeds. The estimated growth rates indicate that populations of these species of Neobuxbaumia are in a numerical equilibrium. With respect to reproduction, N. macrocephala produce a lower number of seeds per plant than N. tetetzo. This low level of sexual reproduction may decrease the probability of establishment of new individuals in N. macrocephala populations. It is suggested that pollen limitation and pre-dispersal seed predation could be some factors that limit the distribution and abundance of this columnar cactus.

Effect of ingestion by bats and birds on seed germination of Stenocereus griseus and Subpilocereus repandus (Cactaceae)

Effects of seed passage through digestive tracts of principal animal dispersers (bats and birds) on seed germination were investigated for two columnar cactus species, Stenocereus griseus and Subpilocereus repandus. In general, seeds ingested showed more rapid germination times and higher overall germination rates than untreated seeds. The bat Glossophaga longirostris, and the birds Mimus gilvus and Tachyphonus rufus seem to be the most efficient cacti dispersers in terms of quality of treatment. The increment of germination speed and particularly the decrease of imbibition time, could favour the probability of germination in dry areas such as arid and semi-arid environments.

In Vitro Propagation of Three Species of Columnar Cacti from the Sonoran Desert

In vitro propagation systems were developed for Carnegiea gigantea (Engelm.) Britt & Rose, Pachycereus pringlei (Berger) Britt & Rose and Stenocereus thurberi (Engelm.) Buxb, three North American species of columnar cacti. In vitro germinated seedlings were used as a source of explants. Multiple shoot formation from areoles was achieved for three types of explants (apical, lateral, and transverse) cultured on Murashige and Skoog (MS) basal media supplemented with 3% sucrose, 10 g·L-1 agar and various treatments with growth regulators. The highest shoot production efficiency for C. gigantea was obtained on transverse explants cultured on a medium with 2 mg·μmL-1 (8.87 μm) BA, where 5.3 shoots per explant were obtained. In P. pringlei and S. thurberi the best response was obtained using transverse explants on medium with 1 mg·L-1 (4.44 μm) BA (3.8 and 4.3 shoots per explant, respectively). Rooting of the in vitro generated shoots was achieved most efficiently on MS basal media with 3% sucrose, 10 g·L-1 agar and 1 mg·L-1 (4.9 μm) indole-3-butyric acid. Rooting frequencies were 92%, 88%, and 96% for C. gigantea, P. pringlei and S. thurberi, respectively, and the frequency of survival of the plants once transferred to soil was 86% on average. Chemical name used: benzyladenine (BA).

On the Allometry of Biomass Partitioning and Light Harvesting for Plants with Leafless Stems

Prior explicit allometric models are extended to predict the scaling relationship between the ability of plants with leafless stems to harvest sunlight H and total standing plant biomass M T (which equals the sum of standing stem and root biomass, M S and M R ). Provided that H scales in a directly proportional manner (isometrically) with respect to either stem surface area (i.e. H∝SA S ) or total stem biomass (i.e. H∝ M S ), the allometric model presented here predicts that SA S ∝M T 3/4 or M S ∝M T 3/4 , respectively. These alternative predictions are tested empirically using data for standing stem and root biomass gathered for the large columnar cactus species Pachycereus pringlei. Statistical comparisons between observed and predicted scaling relationships indicate that SA S ∝ M T 3/4, whereas M S ∝ M T 3/4 is mathematically inconsistent with the observation that stem biomass scales nearly isometrically with respect to root biomass. The contention that the H of leafless stems scales isometrically with respect to stem surface area is thus reasonable both theoretically and empirically.

The biomechanics of Pachycereus pringlei root systems

We report on the root system of the large columnar cactus species Pachycereus pringlei to explore the hypothesis that increasing plant size decreases the ability to resist wind-throw but increases the capacity to absorb and store nutrients in roots (i.e., plant size limits the performance of these functions and may shift the performance of one function in favor of another as size increases). Based on 18 plants differing in size, the root system is characterized by a broad and deep bayonet-like root central to a shallow and extensive lateral system of root elements bearing sinker roots near the stem base. All root types have a living secondary cortex and contain wood with a large volume fraction of ray tissues that increases toward the stem base. Wood stiffness and tensile strength are correlated negatively with the ray tissue volume fraction and thus decrease toward the stem base in lateral and bayonet roots. Calculations show that the ability of the bayonet and proximal lateral root elements to resist wind-throw decreases with increasing plant size, whereas the nutrient absorption/storage capacity of the total root system increases with plant size (i.e., a size-dependent shift between these two root functions occurs).

Proximate Composition, Protein Quality and Oil Composition in Seeds of Columnar Cacti from the Sonoran Desert

Fruits of columnar cacti are an important food resource from the arid lands of México. However, little is known about their nutritional value. In this paper, seeds of five species of columnar cacti from the Sonoran desert were studied to determine their protein and oil composition. The studied species were: Pachycereus pringlei, Pachycereus pecten-aboriginum, Carnegiea gigantea, Stenocereus thurberi and Stenocereus gummosus. Protein content varied from 20.3 to 22.0% among species. These proteins showed a high content of sulfur containing amino acids but were low in lysine. In vitro digestibility ranged from 77 to 86% among species and increased with the thermal treatment. Protein from P. pringlei was assessed in vivo and showed PER values of 2.8% and digestibility of 91.6%. Oil content ranged from 28.4 to 30.7% among species. Most oils showed a high degree of unsaturation. Linoleic (50–53%) and oleic (35–37%) acid were the most important fatty acids present in all oils. The results are compared with other studies on the seeds of columnar cacti. Future studies aimed at evaluating the nutritional value of the fruits are discussed.

Sonoran Desert Columnar Cacti and the Evolution of Generalized Pollination Systems

We studied variation in flowering phenology, fruit and seed set, and the abundance of the pollinators of four species of night-blooming Sonoran Desert columnar cacti for up to eight years at one site in Mexico and one year at one site in Arizona. We determined how spatiotemporal variation in plant–pollinator interactions affects the evolution of generalized pollination systems. We conducted pollinator exclusion and hand pollination experiments to document annual variability in pollinator reliability and to determine whether pollination systems were redundant (different species are partially or totally substitutable) or complementary (different species have an additive effect on fruit set). The cacti we studied included three species with generalized pollination systems involving bats, birds, and bees (cardon, Pachycereus pringlei; saguaro, Carnegiea gigantea; and organ pipe, Stenocereus thurberi) and one specialized moth-pollinated species (senita, Lophocereus schottii). We predicted that the migratory lesser long-nosed bat, Leptonycteris curasoae, is a less reliable pollinator than birds and bees, and that cacti with generalized pollination systems have more variable flowering phenologies than the specialized species. Annual time of peak flowering and mean size of flower crops were relatively invariant in saguaro and organ pipe. Time of peak flowering in cardon varied by as much as six weeks, and mean flower crop size varied three-fold over six years. In senita, peak flowering varied by as much as 5–8 wk among years. Peak numbers of the nectar bat L. curasoae varied among years, and bat density (0.9/ha) was an order of magnitude lower than that of cactus-visiting birds at both study sites. The abundance of migratory hummingbirds was also highly variable among years. Pollinator exclusion experiments indicated that bats were major pollinators of cardon, whereas diurnal visitors accounted for most fruit set in saguaro (except in 1995 when bats were most important) and organ pipe at our Mexican site; honeybees accounted for 64–87% of diurnal fruit set in these species. Annual variation in the contribution to fruit set by bats was substantially higher than that of diurnal pollinators in saguaro and organ pipe, but not in cardon. There was little geographic variation in the relative importance of nocturnal vs. diurnal pollinators in saguaro and senita, but bats were much more important for fruit set in organ pipe in Arizona than in Mexico. We generally detected no effect of different pollinators on number of seeds per fruit in any species. Annual variation in fruit set was lowest in saguaro, the species with the most diurnal pollination system, and highest in organ pipe, the species with the most generalized pollination system. Fruit set was strongly pollen limited only in females of cardon (a trioecious species) and in organ pipe (at both sites). The “missing” pollinators in both species are likely Leptonycteris bats. The pollination systems of saguaro and cardon were partially redundant, whereas that of organ pipe was complementary. The four species of cactus that we studied occur at the northern geographic limits of Mexican columnar cacti where many vertebrate pollinators are seasonal migrants. In the Sonoran Desert, variation in rainfall and spring temperatures affects timing of flowering and the extent of competition between cacti for pollinator visits and causes the relative importance of particular pollinators, especially Leptonycteris bats, for fruit set to vary annually. Under such conditions, selection has favored generalized pollination systems (as seen in organ pipe) or shifts from reliance primarily on nocturnal pollinators (as seen in cardon) to reliance primarily on diurnal pollinators (as seen in saguaro). Nonetheless, as exemplified by the senita–senita moth system, highly specialized pollination mutualisms can also evolve in this habitat in plants that rely on sedentary insects rather than migratory bats and birds for pollination.

SONORAN DESERT COLUMNAR CACTI AND THE EVOLUTION OF GENERALIZED POLLINATION SYSTEMS

We studied variation in flowering phenology, fruit and seed set, and the abundance of the pollinators of four species of night-blooming Sonoran Desert columnar cacti for up to eight years at one site in Mexico and one year at one site in Arizona. We determined how spatiotemporal variation in plant–pollinator interactions affects the evolution of generalized pollination systems. We conducted pollinator exclusion and hand pollination experiments to document annual variability in pollinator reliability and to determine whether pollination systems were redundant (different species are partially or totally substitutable) or complementary (different species have an additive effect on fruit set). The cacti we studied included three species with generalized pollination systems involving bats, birds, and bees (cardon, Pachycereus pringlei; saguaro, Carnegiea gigantea; and organ pipe, Stenocereus thurberi) and one specialized moth-pollinated species (senita, Lophocereus schottii). We predicted that the migratory lesser long-nosed bat, Leptonycteris curasoae, is a less reliable pollinator than birds and bees, and that cacti with generalized pollination systems have more variable flowering phenologies than the specialized species. Annual time of peak flowering and mean size of flower crops were relatively invariant in saguaro and organ pipe. Time of peak flowering in cardon varied by as much as six weeks, and mean flower crop size varied three-fold over six years. In senita, peak flowering varied by as much as 5–8 wk among years. Peak numbers of the nectar bat L. curasoae varied among years, and bat density (0.9/ha) was an order of magnitude lower than that of cactus-visiting birds at both study sites. The abundance of migratory hummingbirds was also highly variable among years. Pollinator exclusion experiments indicated that bats were major pollinators of cardon, whereas diurnal visitors accounted for most fruit set in saguaro (except in 1995 when bats were most important) and organ pipe at our Mexican site; honeybees accounted for 64–87% of diurnal fruit set in these species. Annual variation in the contribution to fruit set by bats was substantially higher than that of diurnal pollinators in saguaro and organ pipe, but not in cardon. There was little geographic variation in the relative importance of nocturnal vs. diurnal pollinators in saguaro and senita, but bats were much more important for fruit set in organ pipe in Arizona than in Mexico. We generally detected no effect of different pollinators on number of seeds per fruit in any species. Annual variation in fruit set was lowest in saguaro, the species with the most diurnal pollination system, and highest in organ pipe, the species with the most generalized pollination system. Fruit set was strongly pollen limited only in females of cardon (a trioecious species) and in organ pipe (at both sites). The “missing” pollinators in both species are likely Leptonycteris bats. The pollination systems of saguaro and cardon were partially redundant, whereas that of organ pipe was complementary. The four species of cactus that we studied occur at the northern geographic limits of Mexican columnar cacti where many vertebrate pollinators are seasonal migrants. In the Sonoran Desert, variation in rainfall and spring temperatures affects timing of flowering and the extent of competition between cacti for pollinator visits and causes the relative importance of particular pollinators, especially Leptonycteris bats, for fruit set to vary annually. Under such conditions, selection has favored generalized pollination systems (as seen in organ pipe) or shifts from reliance primarily on nocturnal pollinators (as seen in cardon) to reliance primarily on diurnal pollinators (as seen in saguaro). Nonetheless, as exemplified by the senita–senita moth system, highly specialized pollination mutualisms can also evolve in this habitat in plants that rely on sedentary insects rather than migratory bats and birds for pollination.

The reproductive phenology of three sympatric species of columnar cacti on Curaçao

The phenological response by three sympatric species of columnar cacti of the semi-arid island of Curaçao (Stenocereus griseus, Subpilocereus repandus, and Pilosocereus lanuginosus) to recent rainfall was investigated during 21 months. Rainfall is the climatic cue most likely to affect bud formation on Curaçao, and each species of cactus responded differently to it. Pilosocereus lanuginosus started budding immediately after rainfall, whereas Stenocereus griseus responded negatively with abortions and cessation of bud production within 2–3 weeks after rain. Subpilocereus repandus showed no response to rain within one month. Despite a long period of temporal overlap (81% overlap) between the budding/flowering activity of Subpilocereus repandus and that of Stenocereus griseus, buds and/or flowers on Stenocereus griseus appeared more than a month earlier than on Subpilocereus repandus.The effect of plant size on phenology and how anthropogenic disturbance may affect cactus resource availability to nectar-feeders and frugivores were also examined. The larger the individual tree within a species, the more flowers were produced and the earlier the tree started to flower. Thus, the indiscriminate removal of columnar cacti for urban development can drastically affect the timing and availability of resources to threatened pollinators and other nectar-feeders, as well as to frugivores and omnivores.

Chemical Interactions in the Cactus-Microorganism-DrosophilaModel System of the Sonoran Desert

SYNOPSIS. The Cactus-Microorganism-Drosophila Model System of the Sonoran Desert represents an excellent paradigm of the role of chemistry in plant-animal interactions. In this system, four species of endemic Drosophila feed and reproduce in necrotic tissue of five species of columnar cacti. Studies over the past 35 yr have characterized a myriad of interactions between the three major components of the model system. The cacti contain a variety of allelochemicals which are primarily responsible for the highly specific pattern of host plant utilization exhibited by the desert Drosophila. Plant chemistry, through its effect on the microbially produced volatile patterns, is further involved in host specificity because the flies use the volatile pattern to cue in on necroses in the appropriate species of cactus. The metabolic activities of microorganisms (bacteria and yeasts) living in the necrosis can affect the substrate chemistry in both positive and negative ways (i.e., acting to increase or to decrease the toxicity of the substrate). Finally, cactus chemistry may affect drosophilid mating behavior since larval rearing substrate has been shown to influence adult hydrocarbon epicuticular composition. In D. mojavensis, adult hydrocarbon profile has been implicated as a determinant of mate choice leading to premating isolation between geographically isolated populations that use chemically different cactus substrates. Current research is focused on the evolution and regulation of genes whose products (cytochrome P450 enzymes) are involved in the specific insect-host plant relationships which exist between the Drosophila species and the cactus species. There are many reasons why investigators choose to focus their research efforts on what are referred to as ‘‘model systems.’’ Typically included among these would be the idea that model systems are easier to study because they are less complex than other scientific situations. At the same time, model systems should be representative of more complex, natural systems so that information that is obtained from their study is broadly applicable. For almost a century, the fruit fly, Drosophila melanogaster, has served as a model organism for the study of genetics. As a genetic paradigm, Drosophila is more tractable to scientific investigation than most organisms and has provided important insights into a wide variety of human maladies from alcohol abuse to neurological brain disorders (Bellen, 1998). Similarly, the interrelationships of the columnar cacti and the cactophilic Drosophila species of the Sonoran Desert have, for the past 35 yr, provided an excellent model system with which to study relevant questions in evolution, ecological genetics, and chemical ecology. The intent of this article is to briefly review and characterize the chemical interactions between the plants (cacti) and animals (Drosophila) of this model system, and, in addition, provide some thoughts on possible future directions for integrative approaches in this research area.

Chemical Interactions in the Cactus-Microorganism-Drosophila Model System of the Sonoran Desert1

The Cactus-Microorganism-Drosophila Model System of the Sonoran Desert represents an excellent paradigm of the role of chemistry in plant-animal interactions. In this system, four species of endemic Drosophila feed and reproduce in necrotic tissue of five species of columnar cacti. Studies over the past 35 yr have characterized a myriad of interactions between the three major components of the model system. The cacti contain a variety of allelochemicals which are primarily responsible for the highly specific pattern of host plant utilization exhibited by the desert Drosophila. Plant chemistry, through its effect on the microbially produced volatile patterns, is further involved in host specificity because the flies use the volatile pattern to cue in on necroses in the appropriate species of cactus. The metabolic activities of microorganisms (bacteria and yeasts) living in the necrosis can affect the substrate chemistry in both positive and negative ways (i.e., acting to increase or to decrease the toxicity of the substrate). Finally, cactus chemistry may affect drosophilid mating behavior since larval rearing substrate has been shown to influence adult hydrocarbon epicuticular composition. In D. mojavensis, adult hydrocarbon profile has been implicated as a determinant of mate choice leading to premating isolation between geographically isolated populations that use chemically different cactus substrates. Current research is focused on the evolution and regulation of genes whose products (cytochrome P450 enzymes) are involved in the specific insect-host plant relationships which exist between the Drosophila species and the cactus species.There are many reasons why investigators choose to focus their research efforts on what are referred to as “model systems.” Typically included among these would be the idea that model systems are easier to study because they are less complex than other scientific situations. At the same time, model systems should be representative of more complex, natural systems so that information that is obtained from their study is broadly applicable. For almost a century, the fruit fly, Drosophila melanogaster, has served as a model organism for the study of genetics. As a genetic paradigm, Drosophila is more tractable to scientific investigation than most organisms and has provided important insights into a wide variety of human maladies from alcohol abuse to neurological brain disorders (Bellen, 1998). Similarly, the interrelationships of the columnar cacti and the cactophilic Drosophila species of the Sonoran Desert have, for the past 35 yr, provided an excellent model system with which to study relevant questions in evolution, ecological genetics, and chemical ecology. The intent of this article is to briefly review and characterize the chemical interactions between the plants (cacti) and animals (Drosophila) of this model system, and, in addition, provide some thoughts on possible future directions for integrative approaches in this research area.

Arthropod diversity in necrotic tissue of three species of columnar cacti (Cactaceae)

AbstractWe compared the insect and arachnid species found in spring and summer samples of necrotic tissue of three species of columnar cacti, cardón [Pachycereus pringlei (S. Watson) Britten and Rose], organ-pipe (Stenocereus thurberi Buxb.), and senita [Lophocereus schottii (Engelm.) Britten and Rose] (all Cactaceae), endemic to the Sonoran Desert of North America. A total of 9380 arthropods belonging to 34 species, 23 families, 10 orders, and 2 classes were collected in 36 samples. Arthropod communities differed in composition among host cacti, as well as between seasons. These differences may be a function of variation in host characteristics, such as chemical composition and abiotic factors, such as water content or temperature.

Response of Drosophila melanogaster to selection for P450-mediated resistance to isoquinoline alkaloids

Several species of columnar cacti in the Sonoran Desert contain isoquinoline alkaloids that are toxic to all but the resident drosophilids that feed and breed in necrotic stems. Cytochrome P450 enzymes are known to be involved in the metabolic detoxification of these alkaloids by the desert Drosophila and are consequently responsible for their ability to utilize these substrates. D. melanogaster is not normally exposed to these xenobiotic compounds and cannot live in necrotic cactus tissue. However, a previous study found evidence of a phenobarbital-inducible P450 in adults of this species that is capable of metabolizing cactus alkaloids. The current investigation sought to determine whether D. melanogaster responds to selection for alkaloid resistance. Significant increases in larval viability and adult longevity as well as shorter larvae-to-adult development times were observed after 16 generations of selection on medium containing isoquinoline alkaloids. The selected lines that exhibited a positive response can now be used to assay for changes in gene regulation as a possible mechanism of their response. This information will contribute to the understanding of evolution of P450-mediated resistance in insects.

Ordenación de las poblaciones silvestres de pitayo y cardón en la Cuenca de Sayula, Jalisco

Se estudian las relaciones ecológicas existentes entre las poblaciones silvestres de "pitayo" y "cardón" e n la Cuenca de Sayula, Jalisco. Se utilizó un enfoque multivariable considerando tres conjuntos de variables explicatorias: físico-geográficas (FG), edáficas (E) y bióticas (B). Se detectó la presencia de tres especies de cactáceas columnares, Stenocereus dumortieri, Pachycereus grandis y Stenocereus queretarnensis. De acuerdo a un análisis de correspondencia desprovisto de tendencia (DCA), la longitud del gradiente fue de 3.59 Unidades de Desviación Estándar (DE), con lo cual se justifica el uso del modelo unimodal de respuesta de las especies. Posteriormente, mediante un análisis de correspondencia (AC), se manifiesta que el arreglo de los sitios y las especies se relacionan con un supuesto gradiente de humedad. Debido a la presencia de valores aberrantes en algunas variables, fue necesario declarar pasivos algunos sitios en el análisis de correspondencia canónica (ACC). Las variables seleccionadas fueron: a] físico-geográficas (FG): latitud, longitud, altitud y exposición y; b] edáficas (E): potasio, capacidad de intercambio catiónico y porcentaje de arena. De estos componentes, los primeros explican el 28.94% y los segundos el 23.66% de la variación de los datos de especies, y la interacción de ambos el 8.23%. Los árboles y arbustos acompa1iantes se relacionaron con las poblaciones de pitayo y cardón de dos formas: a] en tres poblaciones separadas, o b] como complejo: los atributos seleccionados fueron cobertura total y promedio de altura de los individuos del complejo, o bien de S. dumortieri y P. grandis, respectivamente, para la opción de especies separadas. Sin embargo, existe relación débil'para las especies (17.4%), con la variación de los árboles y arbustos acompañantes.

Stem biomechanics of three columnar cacti from the Sonoran Desert

The allometric relationship of stem length L with respect to mean stem diameter D was determined for 80 shoots of each of three columnar cactus species (Stenocereus thurberi, Lophocereus schottii, and S. gummosus) to determine whether this relationship accords with that predicted by each of three contending models purporting to describe the mechanical architecture of vertical shoots (i.e., geometric, stress, and elastic similitude, which predict L proportional to D(alpha), with alpha = 1/1, 1/2, and 2/3, respectively). In addition, anatomical, physical, and biomechanical stem properties were measured to determine how the stems of these three species maintain their elastic stability as they increase in size. Reduced major axis regression of L with respect to D showed that alpha = 2.82 ± 0.14 for S. thurberi, 2.32 ± 0.19 for L. schottii, and 4.21 ± 0.31 for S. gummosus. Thus, the scaling exponents for the allometry of L differed significantly from that predicted by each of the three biomechanical models. In contrast, these exponents were similar to that for the allometry previously reported for saguaro. Analyses of biomechanical data derived from bending tests performed on 30 stems selected from each of the three species indicated that the bulk stem tissue stiffness was roughly proportional to L2, while stem flexural rigidity (i.e., the ability to resist a bending force) scaled roughly as L3. Stem length was significantly and positively correlated with the volume fraction of wood, while regression analysis of the pooled data from the three species (i.e., 90 stems) indicated that bulk tissue stiffness scaled roughly as the 5/3-power of the volume fraction of wood in stems. These data were interpreted to indicate that wood served as the major stiffening agent in stems and that this tissue accumulates at a sufficient rate to afford unusually high scaling exponents tot stem length with respect to stem diameter (i.e., disproportionately large increments of stem length with respect to increments in stem diameter). Nevertheless, the safety factor against the elastic failure of stems (computed on the basis of the critical buckling height divided by actual stem length) decreased with increasing stem size tot each species, even though each species maintained an average safety factor equal to two. We speculate that the apparent upper limit to plant height calculated for each species may serve as a biomechanical mechanism for vegetative propagation and the establishment of dense plant colonies by means of extreme stem flexure and ultimate breakage, especially for S. gummosus.