Senita Research Articles

Enhanced mycorrhization in four species of columnar cacti under the Sonoran Desert tree <em>Olneya tesota</em>

Background: Columnar cacti survival in deserts occurs mainly under nurse plants, due to better conditions for germination and establishment, as greater humidity, and nutrient concentration. However, interactions like the association between arbuscular mycorrhizal fungi (AMF) and roots remain relatively unexplored. We evaluated the mycorrhizal status and soil rhizosphere properties of four columnar cacti species growing both under and outside their nurse plant. Hypotheses: Soil properties improvement under nurse plants has been correlated with the abundance of AMF. Therefore, a greater number of AMF structures and roots mycorrhization would be in facilitated plants, compared to the same species growing outside the nurse. Studied species: Nurse plant: Olneya tesota, facilitated species: Carnegiea gigantea, Pachycereus pringlei, Pachycereus schottii, and Stenocereus thurberi. Study site and dates: Central Gulf Coast subdivision of the Sonoran Desert, Sonora, Mexico; October 2020. Methods: Cacti roots and soil rhizosphere were collected under and outside the nurse plant. Roots were stained, and mycorrhizal colonization (MC), hairy roots (HR), visual density (VD), and number of spores were determined. Physicochemical soil properties were analyzed. Results: Values of MC, VD, spore number, organic matter (OM), nitrate, and total and assimilable phosphorous were higher under the nurse compared to bare areas. Conclusions: The hypothesis that mycorrhization parameters are greater in roots of cacti growing under the nurse was confirmed, besides previous knowledge about soil quality under nurse plants, suggesting that AMF play a significant role in the cacti facilitation process under nurses plants, and that nurse plant plays a significant role in AMF formation.

First Report of Pitaya Virus X Infecting Lophocereus schottii f. mieckleyanus (Thin-Stemmed Totem Pole Cactus) in the United States.

Lophocereus is a genus of three species of columnar cacti native to Arizona and Mexico (Lodi, 2015). These cacti produce several tall, ascending, columnar stems that branch at the base in a candelabra-like arrangement. The most common species, L. schottii is known as the senita cactus. Several unusual knobby-stemmed spineless forms of senita cactus have been found in nature in Baja California, Mexico, which are collectively known as totem pole cacti. The thin-stemmed totem pole cactus, L. schottii f. mieckleyanus is an important part of landscapes in southern Arizona. Cacti are clonally propagated which makes viral infections of economic importance in the ornamental/nursery industry. In February 2023, virus-like symptoms, such as mosaic and chlorotic spots were observed on the stems of L. schottii f. mieckleyanus grown in a nursery in Phoenix, AZ, USA. Total RNA was extracted from two symptomatic cacti (YPHC-61 A & B) following the protocol by Tzanetakis et al. (2007), and cDNA was synthesized using the Superscript IV Reverse Transcriptase (Invitrogen, Vilnius, Lithuania). Reverse transcription polymerase chain reaction (RT-PCR) performed with cactus virus X specific primers (Kim et al. 2016) targeting the coat protein (CP) gene failed to generate any amplicon, while potexvirus-replicase primers, Potex 2RC and Potex 5 (van der Vlugt and Berendsen 2002) targeting RNA-dependent RNA polymerase (RdRp) gene amplified an expected amplicon of ~580 bp from both the samples. One of the amplicons was Sanger sequenced and showed 90.7% nucleotide (nt) identity with pitaya virus X (PiVX) in the GenBank (MN982522). Sequence was submitted in the GenBank under the accession number OR425049. PiVX is a new species of the genus Potexvirus and is named after its origin from pitaya (Hylocereus spp.). Further, RT-PCR was conducted with PiVX-specific primers, CP 110F/CP 604R targeting CP gene (Bae and Park 2022) and RdRp gene (RdRp F 5' GCGTGGGCCCTGGAAAA-3'/RdRp R 5' CTAAGATTCATCAATTCACCTCTCC-3') (this study). Amplicons of ~500 and 1100 bp were obtained using primers, CP 110F/CP 604R and RdRp F/RdRp R, respectively. A BLAST search revealed 90.5% nt identity to PiVX CP sequences (OM802135 and OM802134) and 87.3% nt identity to RdRp sequences (MN982523 and LC654699) in the GenBank. Sequences of isolates YPHC-61A and YPHC-61B were submitted in the GenBank under accession numbers, OQ915350 and PP182358 (CP gene) and OQ915351 and PP209539 (RdRp gene). Phylogenetic analysis based on the combined sequence datasets of CP and RdRp genes also grouped YPHC-61A and YPHC-61B with PiVX isolates and separated from other potexviruses species. For a bioassay of the virus, sap extract from symptomatic cactus was mechanically inoculated onto indicator plant species, i.e., beans, alfalfa, and melon. Ten days post- inoculation, chlorotic lesions were observed on beans and alfalfa plants, while melon and mock-inoculated plants did not show any symptoms. Similarly, L. schottii f. mieckleyanus plants grafted with infected cactus showed chlorotic spots after 30 days post grafting. Mechanically inoculated beans, alfalfa, and cactus plants were found to be positive for PiVX based on RT-PCR and Sanger sequencing. PiVX has earlier been detected on Notocactus leninghausii f. cristatus (Park et al. 2018) and dragon fruit (Selenicereus undatus) plants in South Korea (Bae and Park 2022). To our knowledge, this is the first report of PiVX on L. schottii f. mieckleyanus in the United States and worldwide.

Chemical Constituents, Antimicrobial and Antibiofilm Activity of Lophocereus schottii Against Uropathogenic Escherichia coli

The objective of the present study was to determine the chemical profile, and the antimicrobial and antibiofilm effect of the Lophocereus schottii extract against uropathogenic Escherichia coli. A methanolic extract of L. schottii was analyzed by mass spectrometry (ESI-IT-MSn and APCI-IT-MSn) to determine the profile of bioactive compounds. The antimicrobial potential was assayed by the microdilution broth method. The effect of the extract on motility was also determined. Quorum sensing systems were evaluated using a biosensor with Chromobacterium violaceum. The prevention of biofilm formation and biofilm remotion were analyzed by the crystal violet method. The chemical analysis allows to identify 8 phenolic compounds, 2 alkaloids and 1 triterpene, most of these reported for the first time to this plant. L. schottii extract showed inhibitory and bactericidal activity against uropathogenic E. coli (MIC= 6.5 mg/mL and CMB= 13 mg/mL). In addition, the effect of L. schottii on motility was observed, since the concentration of 6.5 mg/mL reduced this process by 60%. With regard to the inhibition of the Quorum sensing system, L. schottii (6.5 mg/mL) reduce the production of violacein in 45%. The same concentration of the extract (6.5 mg/mL) resulted the most effective to prevent biofilm formation in 65%. Regarding the biofilm remotion the higher tested concentration (3 MIC) was the most effective, removing 84% of biomass. The results showed that L. schottii has a variety of bioactive compounds and antimicrobial and antibiofilm potential against uropathogenic E. coli.

In vitro Anticancer Activity of the Polar Fraction From the Lophocereus schottii Ethanolic Extract.

Cancer is an increasingly common disease and is considered one of the main causes of death in the world. Lophocereus schottii (L. schottii) is a cactus used in Mexico in traditional medicine for cancer treatment. This study aimed to determine the effect of the ethanolic extract and the polar and nonpolar fractions of L. schottii in murine L5178Y lymphoma cells in vitro, analyzing their effect on the proliferative activity of splenocytes, and establishing the effective concentration 50 (EC50) of the polar fraction. In addition, the secondary metabolites present in the extracts were determined by ultra-performance liquid chromatography-mass spectrometry (UPLC-MS). The study establishes that the three extracts of L. schottii have a cytotoxic effect on L5178Y cells and on the splenocytes stimulated with ConA. Additionally, the polar fraction has a significantly greater effect being three times more effective than cyclophosphamide on inhibiting the viability of L5178Y cells. Secondary metabolites present are mainly flavonoids and alkaloids, but there are also some terpenoids and sterols. Ultimately, polar fraction can be considered an anticancer substance, since its EC50 of 15 μg/mL is within the parameters established by the National Cancer Institute.

The genome of a thorny species: comparative genomic analysis among South and North American Cactaceae.

The first South American cactus nuclear genome assembly associated with comparative genomic analyses provides insights into nuclear and plastid genomic features, such as size, transposable elements, and metabolic processes related to cactus development. Here, we assembled the partial genome, plastome, and transcriptome of Cereus fernambucensis(Cereeae, Cactaceae), a representative species of the South American core Cactoideae. We accessed other genomes and transcriptomes available for cactus species to compare the heterozygosity level, genome size, transposable elements, orthologous genes, and plastome structure. These estimates were obtained from the literature or using the same pipeline adopted for C. fermabucensis. In addition to the C. fernambucensis plastome, we also performed de novo plastome assembly of Pachycereus pringlei, Stenocereus thurberi, and Pereskia humboldtii based on the sequences available in public databases. We estimated a genome size of ~ 1.58Gb for C. fernambucensis, the largest genome among the compared species. The genome heterozygosity was 0.88% in C. fernambucensis but ranged from 0.36 (Carnegiea gigantea) to 17.4% (Lophocereus schottii) in the other taxa. The genome lengths of the studied cacti are constituted by a high amount of transposable elements, ranging from ~ 57 to ~ 67%. Putative satellite DNAs are present in all species, excepting C. gigantea. The plastome of C. fernambucensis was ~ 104kb, showing events of translocation, inversion, and gene loss. We observed a low number of shared unique orthologs, which may suggest gene duplication events and the simultaneous expression of paralogous genes. We recovered 37 genes that have undergone positive selection along the Cereus branch that are associated with different metabolic processes, such as improving photosynthesis during drought stress and nutrient absorption, which may be related to the adaptation to xeric areas of the Neotropics.

Complete chloroplast genome sequence and phylogenetic analysis of dragon fruit (Selenicereus undatus (Haw.) D.R.Hunt)

Selenicereus undatus (Haw.) D.R.Hunt is a member of the family Cactaceae. The chloroplast genome of S. undatus was sequenced, assembled, and annotated in the present study. The chloroplast genome was 133,326 bp in length, consisting of a typical quadripartite circle: a large single-copy region of 68,256 bp, two inverted repeat regions of 21,677 bp, and a small single copy region of 21,716 bp. A total of 120 predicted genes were identified, and a maximum likelihood was constructed, placing S. undatus as the sister taxon of Lophocereus schottii and Carnegiea gigantea, other members of the family Cactaceae.

New Records of Cactaceae from Gran Canaria (Canary Islands, Spain)

Recent field work in Gran Canaria (Canary Islands, Spain), mostly in March and April 2017, yielded first records of 10 species of Cactaceae, all originally introduced as ornamentals. Cylindropuntia bigelovii, C. imbricata, C. prolifera, Disocactus speciosus, Opuntia lindheimeri and O. microdasys are locally naturalized or likely to become so, whereas Ferocactus herrerae, Harrisia tetracantha, Lophocereus schottii and Trichocereus spachianus are considered casuals. All these species are illustrated, georeferenced localities provided and other useful information, especially with regard to their identification and invasion status (in Gran Canaria as well as elsewhere in the world) are also presented. The presence of two further species, Opuntia monacantha and O. robusta, both often considered doubtful in the Canary Islands, is confirmed from Gran Canaria. New data are also provided for Opuntia leucotricha and O. pilifera, two species that were recently recorded for the first time for Gran Canaria, that seem to have increased.

Subdermal Woody Thorns with Secondary Growth May Support Shoots of Senita,Lophocereus schottii(Cactaceae)

Abstract: Lophocereus (Pachycereus) schottii has lignified thorns with secondary growth connecting areoles to the vascular cylinder, which may help support their massive highly parenchymatous shoots.

Diversification and coevolution in brood pollination mutualisms: Windows into the role of biotic interactions in generating biological diversity.

Brood pollination mutualisms-interactions in which specialized insects are both the pollinators (as adults) and seed predators (as larvae) of their host plants-have been influential study systems for coevolutionary biology. These mutualisms include those between figs and fig wasps, yuccas and yucca moths, leafflowers and leafflower moths, globeflowers and globeflower flies, Silene plants and Hadena and Perizoma moths, saxifrages and Greya moths, and senita cacti and senita moths. The high reciprocal diversity and species-specificity of some of these mutualisms have been cited as evidence that coevolution between plants and pollinators drives their mutual diversification. However, the mechanisms by which these mutualisms diversify have received less attention. In this paper, we review key hypotheses about how these mutualisms diversify and what role coevolution between plants and pollinators may play in this process. We find that most species-rich brood pollination mutualisms show significant phylogenetic congruence at high taxonomic scales, but there is limited evidence for the processes of both cospeciation and duplication, and there are no unambiguous examples known of strict-sense contemporaneous cospeciation. Allopatric speciation appears important across multiple systems, particularly in the insects. Host-shifts appear to be common, and widespread host-shifts by pollinators may displace other pollinator lineages. There is relatively little evidence for a "coevolution through cospeciation" model or that coevolution promotes speciation in these systems. Although we have made great progress in understanding the mechanisms by which brood pollination mutualisms diversify, many opportunities remain to use these intriguing symbioses to understand the role of biotic interactions in generating biological diversity.

Population genetics and phylogenetic relationships of beetles (Coleoptera: Histeridae and Staphylinidae) from the Sonoran Desert associated with rotting columnar cacti

Dozens of arthropod species are known to feed and breed in the necrotic tissues (rots) of columnar cacti in the Sonoran Desert. Because the necrotic patches are ephemeral, the associated arthropods must continually disperse to new cacti and therefore the populations of any given species are expected to show very little local genetic differentiation. While this has been found to be true for the cactophilic Drosophila, the evolutionary histories and characteristics of other arthropods inhabiting the same necrotic patches, especially the beetles, have yet to be examined. Here we used nucleotide sequence data from segments of the mitochondrial 16S rRNA and cytochrome c oxidase subunit I (COI) genes to examine population structure and demographic history of three sympatric beetle species (Coleoptera: Histeridae and Staphylinidae) collected on senita cactus (Lophocereus schottii) from six widely-separated localities on the Baja California peninsula of northwestern Mexico. Two histerids, Iliotona beyeri and Carcinops gilensis, and an unidentified staphylinid, Belonuchus sp., showed little or no population structure over a broad geographic area on the peninsula, consistent with the prediction that these beetles should show high dispersal ability. Demographic tests revealed varying levels of historical population expansion among the beetle species analyzed, which are discussed in light of their ecologies and concurrent biogeographic events. Additionally, phylogenetic analyses of COI sequences in Carcinops collected on a variety of columnar cacti from both peninsular and mainland Mexico localities revealed several species-level partitions, including a putative undescribed peninsular species that occurred sympatrically with C. gilensis on senita.

Hierarchical effects of rainfall, nurse plants, granivory and seed banks on cactus recruitment

AbstractAimsPlant facilitation is recognized as an important process, contributing not only to the ecology of individual species, but the diversity, organization and stability of communities as a whole. While decades of studies have enhanced our knowledge of the mechanisms contributing to plant facilitation, greater attention is now being given to the conditionality of the mechanisms, whereby multiple biotic and abiotic factors may act independently and jointly with one another in shaping plant–plant facilitation. Our objective was to evaluate the conditionality and hierarchy of mechanisms contributing to cactus germination and early seedling recruitment arising from plant–plant facilitation, including the effects of seed banks, rainfall, granivory and nurse plants.LocationBahia de Kina, Sonora, Mexico; Sonoran Desert of North America.MethodsWe employed descriptive analyses and a factorial experiment to examine the consequences of rainfall, nurse plants, granivory and seed banks for senita cactus recruitment (Pachycereus schottii) in two plant communities of the Sonoran Desert: one degraded of its key nurse plants and the other with its nurse plants intact.ResultsSeed banks did not occur in either of the plant communities, indicating that seed banks and seed dormancy do not affect senita recruitment and thereby do not modulate other mechanisms that contribute to nurse plant facilitation and cactus recruitment. Juvenile senita cacti were consistently associated with nurse plants in both plant communities. Yet, the density and species composition of nurse plants differed between the communities, resulting in reduced senita recruitment and shifts to alternative nurse plants in the community degraded of its key nurse plants compared to the intact community. In a factorial experiment manipulating rainfall, granivory and nurse plants, we found that rainfall and granivory had significant main factor effects, leading to increases and decreases in seedling recruitment, respectively. There was no main factor effect of nurse plants, but there was a significant nurse plant by granivory interaction, indicating that nurse plants increase seedling recruitment through escape from granivory conditional upon rainfall.ConclusionsTaken together, these results reveal a conditional hierarchy of mechanisms of plant–plant facilitation shaping senita cactus recruitment, namely the predominant effects of rainfall increasing recruitment and of granivory decreasing recruitment, for which nurse plants played a key role in increasing seedling recruitment with rainfall and by reducing granivory.

Mutations in the neverland gene turned Drosophila pachea into an obligate specialist species.

Most living species exploit a limited range of resources. However, little is known about how tight associations build up during evolution between such specialist species and the hosts they use. We examined the dependence of Drosophila pachea on its single host, the senita cactus. Several amino acid changes in the Neverland oxygenase rendered D. pachea unable to transform cholesterol into 7-dehydrocholesterol (the first reaction in the steroid hormone biosynthetic pathway in insects) and thus made D. pachea dependent on the uncommon sterols of its host plant. The neverland mutations increase survival on the cactus's unusual sterols and are in a genomic region that faced recent positive selection. This study illustrates how relatively few genetic changes in a single gene may restrict the ecological niche of a species.

Consequences of ants and extrafloral nectar for a pollinating seed‐consuming mutualism: ant satiation, floral distraction or plant defense?

Non‐pollinating consumers of floral resources, especially ants, can disrupt pollination and plant reproductive processes. As an alternative food resource to flowers, extrafloral nectar (EFN) may distract and satiate ants from flowers, thereby reducing their antagonistic effects on plants. Yet, EFN may actually attract and increase ant density on plants, thus increasing the disruption of pollination and/or their defense of plants. In this study, we tested the effects of ants and EFN on pollinating seed‐consuming interactions between senita cacti and senita moths in the Sonoran Desert. Prior study of senita showed that EFN can distract ants from flowers, but consequences for plant–pollinator interactions remain unstudied. In our current study, ant exclusion had no effect on pollination or oviposition when moths were abundant (>85% flower visitation). Yet, in an ant by EFN factorial experiment under lower moth abundance (<40% visitation), there was a significant effect of ant exclusion (but not EFN or an ant × EFN) on pollination and oviposition. In contrast with our predictions, ant presence increased rather than decreased pollination (and oviposition) by moths, indicating a beneficial effect of ants on plant reproduction. While ant density on plants showed a saturating response to continuous experimental variation in EFN, in support of ant satiation and distraction, the probability of pollination and oviposition increased and saturated with ant density, again showing a beneficial effect of ants on plant reproduction. Ants showed no significant effect on fruit set, fruit survival, or fruit production of oviposited flowers in the ant exclusion experiment. Ants did not affect the survival of moth larvae, but there was a marginally significant effect of ants in reducing wasp parasitism of moths. We suggest that EFN may not only distract ants from disrupting plant–pollinator interactions, but they may also enhance plant–pollinator interactions by increasing pollination and reducing wasp parasitism. Though often considered dichotomous hypotheses, ant distraction and plant defense may be synergistic, though the mechanism(s) for such positive ant effects on plant–pollinator interactions needs further study.

Temporal variation in extrafloral nectar secretion by reproductive tissues of the senita cactus, Pachycereus schottii (Cactaceae), in the Sonoran Desert of Mexico

Plant/ant interactions mediated by extrafloral nectar (EFN) are common in nature. EFN is produced by plant species of >330 genera across 25% of all angiosperm families. Despite natural history observations suggesting its widespread occurrence within the Cactaceae, few studies have quantified EFN production by cacti. In this study, we conducted ant-exclusion experiments to examine temporal variation in, and ant consumption of, EFN produced by buds and fruits of the senita cactus (Pachycereus schottii) in the Sonoran Desert. EFN production by both buds and fruits was greatest at night and nearly absent by day. EFN remaining on buds and fruits was lower (and nearly absent) with ants than without ants. These results suggest the need for further studies of senita and other cacti that examine the ability of EFN production to attract and reward, but not necessarily oversupply ant consumers that provide them with herbivore resistance.

Effects of Pollen Load and Donor Diversity on Seed and Fruit Mass in the Columnar Cactus, Pachycereus schottii (Cactaceae)

As seed and fruit size can influence the success of later life history stages, it is important to understand their sources of variation. In this study, we examined the effects of pollen load and donor diversity on variation in seed and fruit mass of outcrossing senita cacti (Pachycereus schottii) in the Sonoran Desert. By massing all individual seeds per fruit from pollen supplementation and donor diversity experiments, we were able to examine their effects on seed and fruit mass, compared with intra‐ and interplant variation. Seed and fruit mass showed up to 13‐ and 15‐fold variations, respectively. Pollen load did statistically increase seed and fruit mass, but it explained <6% of their variation. Pollen donor diversity did not affect seed or fruit mass. Variation in seed and fruit mass was explained by interplant variation (19%), intraplant variation among fruits (30%), and intrafruit variation (50%). These results indicate that intra‐ and interplant sources, excluding pollen load and donor diversity but possibly including environmental, architectural, and maternal effects, contribute to the substantial variation observed in seed and fruit mass of senita cacti. Such variation may prove useful for plant recruitment under the highly variable water‐ and nutrient‐stressed conditions of desert environments.

Optimal defence theory predicts investment in extrafloral nectar resources in an ant–plant mutualism

Summary 1. Nearly all mutualisms entail the production of resources by one species that attract and reward the species with which it interacts. As such resource production could otherwise be allocated to growth or reproduction, mutualists are predicted to minimize these investment costs. Here, we employ optimal defence theory for plant secondary compounds to evaluate plant production of extrafloral nectar (EFN) resources to attract and reward ants for resistance against herbivores. 2. Through ant exclusion and artificial herbivory experiments, we examined investment in EFN by both buds and fruits of Pachycereus schottii (senita cacti) in the Sonoran Desert of North America. We tested predictions of optimal defence theory that plants invest more in high value parts (fruits) through constitutive EFN, less in low value parts (buds) through induced EFN, and that for a given plant part (buds or fruits), constitutive and induced EFN are negatively correlated. 3. Constitutive levels of EFN were greater in fruits than in buds. Only buds showed induced EFN production following artificial herbivory. Constitutive EFN scaled positively with fruit size but not bud size. Induced EFN was negatively correlated with constitutive EFN for fruits (but not buds), suggesting a potential trade-off in these two herbivore resistance traits. 4. These results indicate that P. schottii minimizes its costs by investing in induced EFN for buds and constitutive EFN for fruits. Moreover, by shifting from induced to constitutive EFN with the morphogenesis of buds to fruits, our results show that investment in EFN can change as a particular plant tissue increases from a lower to a higher value. 5. Synthesis . Indirect defence through EFN production is in accord with that of direct defence through plant secondary compounds, thereby supporting optimal defence theory as a general framework with which to evaluate the costs of producing resources (i.e. the defence) by plants, lycaenids and homopterans to attract and reward ants for their resistance against natural enemies in protection mutualisms.

Molecular evolution and population genetics of two Drosophila mettleri cytochrome P450 genes involved in host plant utilization

Understanding the genetic basis of adaptation is one of the primary goals of evolutionary biology. The evolution of xenobiotic resistance in insects has proven to be an especially suitable arena for studying the genetics of adaptation, and resistant phenotypes are known to result from both coding and regulatory changes. In this study, we examine the evolutionary history and population genetics of two Drosophila mettleri cytochrome P450 genes that are putatively involved in the detoxification of alkaloids present in two of its cactus hosts: saguaro (Carnegiea gigantea) and senita (Lophocereus schottii). Previous studies demonstrated that Cyp28A1 was highly up-regulated following exposure to rotting senita tissue while Cyp4D10 was highly up-regulated following exposure to rotting saguaro tissue. Here, we show that a subset of sites in Cyp28A1 experienced adaptive evolution specifically in the D. mettleri lineage. Moreover, neutrality tests in several populations were also consistent with a history of selection on Cyp28A1. In contrast, we did not find evidence for positive selection on Cyp4D10, although this certainly does not preclude its involvement in host plant use. A surprising result that emerged from our population genetic analyses was the presence of significant genetic differentiation between flies collected from different host plant species (saguaro and senita) at Organ Pipe National Monument, Arizona, USA. This preliminary evidence suggests that D. mettleri may have evolved into distinctive host races that specialize on different hosts, a possibility that warrants further investigation.

DENSITY‐MEDIATED, CONTEXT‐DEPENDENT CONSUMER–RESOURCE INTERACTIONS BETWEEN ANTS AND EXTRAFLORAL NECTAR PLANTS

Interspecific interactions are often mediated by the interplay between resource supply and consumer density. The supply of a resource and a consumer's density response to it may in turn yield context-dependent use of other resources. Such consumer-resource interactions occur not only for predator-prey and competitive interactions, but for mutualistic ones as well. For example, consumer-resource interactions between ants and extrafloral nectar (EFN) plants are often mutualistic, as EFN resources attract and reward ants which protect plants from herbivory. Yet, ants also commonly exploit floral resources, leading to antagonistic consumer-resource interactions by disrupting pollination and plant reproduction. EFN resources associated with mutualistic ant-plant interactions may also mediate antagonistic ant-flower interactions through the aggregative density response of ants on plants, which could either exacerbate ant-flower interactions or alternatively satiate and distract ants from floral resources. In this study, we examined how EFN resources mediate the density response of ants on senita cacti in the Sonoran Desert and their context-dependent use of floral resources. Removal of EFN resources reduced the aggregative density of ants on plants, both on hourly and daily time scales. Yet, the increased aggregative ant density on plants with EFN resources decreased rather than increased ant use of floral resources, including contacts with and time spent in flowers. Behavioral assays showed no confounding effect of floral deterrents on ant-flower interactions. Thus, ant use of floral resources depends on the supply of EFN resources, which mediates the potential for both mutualistic and antagonistic interactions by increasing the aggregative density of ants protecting plants, while concurrently distracting ants from floral resources. Nevertheless, only certain years and populations of study showed an increase in plant reproduction through herbivore protection or ant distraction from floral resources. Despite pronounced effects of EFN resources mediating the aggregative density of ants on plants and their context-dependent use of floral resources, consumer-resource interactions remained largely commensalistic.

Vivipary in coastal cacti: a potential reproductive strategy in halophytic environments

Vivipary, the germination of seeds within the fruit prior to abscission from the maternal plant, is an important event in plants. Two main types of vivipary are known in vascular plants: true vivipary and pseudovivipary. In crop grasses, pseudovivipary is an undesirable character as it results in lower yields. To date, vivipary in the Cactaceae has been reported for less than 20 species, most of which are cultivated. Here, we report viviparous (cryptoviviparous-a subcategory of true vivipary) cacti in nature in members of the tribes Cacteae and Pachycereeae (subfamily Cactoideae). We present four species inhabiting coastal plains in areas subject to periodic flooding, namely, Ferocactus herrerae, Stenocereus alamosensis, S. thurberi, and Pachycereus schottii. These species from localities in northwestern Mexico had viviparous fruits and offspring in different stages of development. A potential trend in the data indicates that the overall proportion of viviparous plants is higher in coastal flooding areas relative to halophytic, nonflooding areas. In our view, vivipary is a reproductive strategy that has evolved to provide a more efficient mechanism favoring germination and new avenues for survival by contributing to population maintenance and short-distance dispersal on halophytic substrates.

ECOLOGICAL AND EVOLUTIONARY MECHANISMS FOR LOW SEED : OVULE RATIOS: NEED FOR A PLURALISTIC APPROACH?

Central to the ecology and evolution of a broad range of plants is understanding why they routinely have submaximal reproduction manifested as low seed : ovule and fruit : flower ratios. We know much less about the processes responsible for low seed : ovule ratios than we do for fruit : flower ratios. Current hypotheses for low seed : ovule ratios are largely drawn from those for fruit : flower ratios, including proximate (ecological) causes of pollen limitation, resource limitation, and pollen quality, as well as the ultimate (evolutionary) hypothesis of "bet hedging" on stochastic pollination. Yet, such mechanisms operating on fruit : flower ratios at the whole-plant level may not best explain low seed : ovule ratios at the individual-flower level. We tested each of these proximate and ultimate causes for low seed : ovule ratios using the specialized pollination mutualism between senita cacti (Pachycereus schottii) and senita moths (Upiga virescens). Seed : ovule ratios were consistently low (approximately 0.61). Such excess ovule production by senita likely has a strong genetic component given the significant differences among plants in ovule number and the consistency in ovule production by plants within and among flowering seasons. Excess ovule production and low seed : ovule ratios could not be explained by pollen limitation, resource limitation, pollen quality, or bet hedging. Nevertheless, phenotypic selection analyses did show significant selection gradients for increased ovule number, suggesting that other evolutionary processes may be responsible for excess ovule production and low seed : ovule ratios. In contrast, low fruit : flower ratios at the whole-plant level were explained by an apparent equilibrium between pollen and resource limitation. Thus, mechanisms responsible for low fruit : flower ratios at the whole-plant level are not necessarily in accord with those of low seed : ovule ratios at the individual-flower level. This suggests that we may need to adopt a more pluralistic approach to seed : ovule ratios and consider alternative hypotheses, including a greater array of proximate and ultimate causes. Initial results of this study suggest that floral allometry, selection on correlated floral traits, stigma clogging with pollen grains, and style clogging with pollen tubes may provide promising avenues for understanding low seed : ovule ratios.