Reproductive biology and relative male and female fitness in a trioecious cactus,Pachycereus pringlei (Cactaceae)

We describe the breeding system of an autotetraploid trioecious cactus, Pachycereus pringlei, provide estimates of the fitnesses of males and females relative to that of hermaphrodites, and discuss the role played by pollinators in the maintenance of three sexual morphs. Relatively high frequencies of females (45%) and males (26%) exist in coastal desert populations around Bahia Kino, Sonora, Mexico. They differ from hermaphrodites in flower size (females only), initiation of the flowering season, number of flowers produced per night and per season, sucrose content of nectar, and, in females, number of fruits produced per season under open pollination and in response to hand-pollination. Major similarities between the sex classes include overall plant size, nectar volume per flower, percent fruit set in open-pollinated flowers of females and hermaphrodites, seed mass and number of seeds per fruit, and pollen mass per flower in males and hermaphrodites. Hermaphrodites are self-compatible, and the selfing rate is high (65%). Levels of inbreeding depression in selfed fruits and seeds appear to be low. Fruit set is strongly pollinator-dependent in females but much less so in hermaphrodites. Relative fitness of males and females, as measured by annual production of pollen or seeds, is at least 1.5 times higher than that of the corresponding sex function in hermaphrodites. Given the high selfing rate and apparent lack of inbreeding depression, these fitness differences are insufficient to explain the occurrence of trioecy in this species.     

Irrigation magnifies CAM-photosynthesis in Opuntia basilaris (Cactaceae)

Summary Measurements of acid metabolism and gas exchange were carried out four times during a year to assess the relative importance of temperature and the accompanying seasonal change to the carbon metabolism of Opuntia basilaris Engelm. & Bigel. plants growing in situ under irrigated and natural (control) conditions. Our experiments showed that this cactus did not change its pattern of carbon assimilation when continuously irrigated under field conditions. Non-irrigated cacti had maximum acid accumulation after periods of precipitation. Maximum acid accumulation in irrigated cacti occurred when there was a large difference between day/night temperatures (i.e., 16°C), and when nighttime temperatures were moderate (14C). Irrigated cacti had greater duration of stomatal opening and lower resistance to ¹⁴CO₂ uptake. When temperatures were low, daytime stomatal resistance to ¹⁴CO₂ uptake decreased (to 20–40 s cm^-1), but never to the level of the nocturnal resistances (5–10 s cm^-1). During periods of drought, nonirrigated cycti changed to a pattern in which organic acids fluctuated. Irrigated cacti continued to have ¹⁴CO₂ uptake when nighttime temperatures were as high as 33°C. ¹³C/¹²C isotope composition ratios, determined after two years of irrigation, were near -12 in irrigated and non-irrigated plants. Therefore, under conditions of continual irrigation, seasonal and temperature changes affected the degree of dark CO₂ fixation and acid metabolism, but these cacti did not change from CAM to CO₂ fixation in the light.Abbreviations C₃ reductive pentose phosphate cycle - C₄ dicarboxylic acid cacle - CAE carbon assimilation efficiency - CAM Crassulacean acid metabolism - THO tritiated water - T/P transpiration ratio - vpd vapor pressure deficit - water potential This study was supported in part by National Science Foundation grant OIP 74-15673. Under the auspice of this grant a cooperative research project was carried out between the Australian National University, Canberra, and the Philip L. Boyd Deep Canyon Desert Research Center, University of California, Riverside. The studies involved a comparison of the photosynthetic pathways employed by succulents during growth in their native environment (the Sonoran Desert of southeastern California) and in a favorable introduced environment (Queensland and New South Wales, Australia). Studies carried out in Australia are under the direction of Dr. C.B. Osmond     

Opuntia fragilis (Cactaceae) verified in Illinois flora

Opuntia fragilis has been reported only once from Illinois. Recent workers have not been able to find a substantiating specimen and have excluded the species from the flora. The missing specimen has now been found and is referable to this species.     

Sporophytic inbreeding depression in mosses occurs in a species with separate sexes but not in a species with combined sexes

Inbreeding depression is a critical factor countering the evolution of inbreeding and thus potentially shaping the evolution of plant sexual systems. Current theory predicts that inbreeding depression could have important evolutionary consequences, even in haploid-dominant organisms. To date, no data have been reported on inbreeding depression in moss species. Here, we present data on the magnitude of inbreeding depression in sporophytic traits of moss species with contrasting breeding systems. In Ceratodon purpureus (Ditrichaceae), a moss species with separate sexes, self-fertilizations between sibling gametophytes (intergametophytic selfing) significantly reduced fitness in two of four traits quantified, with seta length and capsule length having inbreeding coefficients significantly different from zero, resulting in a cumulative inbreeding depression that was also significantly greater than zero (δ = 0.619 ± 0.076). In hermaphroditic Funaria hygrometrica (Funariaceae), there was no evidence of inbreeding depression in seta length, spore number, capsule mass, or capsule length resulting from sporophytes generated by self-fertilization within an individual (intragametophytic selfing), and cumulative inbreeding depression was also not different from zero (δ = 0.038 ± 0.022). These results provide evidence that, despite haploid dominance, inbreeding depression can be expressed at the diploid stage in mosses and may have implications for the evolution and maintenance of combined versus separate sexes in mosses.     

Intriguing thigmonastic (sensitive) stamens in the Plains Prickly Pear Opuntia polyacantha (Cactaceae)

The movement of sensitive stamens in flowers of the Plains Prickly Pear (Opuntia polyacantha) is described in detail along with the external and internal filament anatomy. The goals of this investigation were: (1) to provide a synthesis of floral phenology and determine whether this rather unique stamen movement is nastic or a tropism and (2) to conduct macro- and micro-morphological analyses of filaments to determine if there are anatomical traits associated with this movement. To better understand the internal and external structure in sensitive filaments of O. polyacantha, we performed comparative anatomical analyses in two additional species from the Opuntioideae with stamens lacking such sensitivity. The consistent unidirectional movement of stamens, independent of the area stimulated, indicates a thigmonastic response. This movement serves multiple purposes, from enhancing pollen presentation to facilitating cross-pollination, protecting pollen and preventing insects from robbing pollen. Anatomically, the sensitive and non-sensitive filaments exhibit different tissue organization. Cuticle thickness, presence of capsular structures, two layers of curved cells, and more and larger intercellular spaces are characteristic of sensitive filaments. A thin unicellular epidermal layer is characteristic in sensitive filaments versus 2–3 epidermal layers in non-sensitive filaments. Another striking feature in sensitive filaments is the presence of papillae and capsular structures. We believe that these elements are related to water mobility with subsequent contraction during the thigmonastic response. Capsular structures might have a role in fluid mobility according to the stimulus of the filaments. We hypothesize that the thigmonastic response is controlled by cells with elastic properties, as evidenced by the plasmolyzed curved and contracted cells in the filaments and the fact that the movement is activated by changes in cell turgor followed by contraction as a result of plasmolysis.     

Aborted fruits of Opuntia microdasys (Cactaceae): insurance against reproductive failure

New individuals in clonal populations arise through the recruitment of sexual or clonal offspring. The predominance of one type of regeneration over the other has been correlated with different selective environmental pressures. We compared the reproductive mode (sexual through seeds and vegetative through plantlets or detached cladodes) of Opuntia microdasys from three desert habitats of the Chihuahuan Desert: bajada (BH), hill-piedmont (HPH), and an interdune (IDH). Successful establishment and growth of plantlets were determined in two experiments: (1) the effect of light (three levels of photosynthetically active radiation [PAR]: full, low, and medium) and two levels of watering and (2) maternal effects and provenance of plantlets. Adult plant densities did not differ among habitats (639 individuals/ha), but the number of offspring and fruit production increased significantly at BH. Plantlets (94.3%) dominated the form of recruitment for all habitats, followed by cladodes (3.1%) and seedlings (2.6%). A higher proportion of plantlets established in the low and medium PAR treatments (76%) in comparison to full exposure (39%). Maternal factors affected survival and growth, but plantlet provenance did not. The high fruit abortion rate resulting from environmental and maternal effects provided suitable conditions for establishment of plantlets.     

Phylogeny in Echinocereus (Cactaceae) based on combined morphological and molecular evidence: taxonomic implications

Echinocereus is a morphologically diverse genus that includes 64 species grouped into eight taxonomic sections based on morphological traits. In previous molecular phylogenetic analyses, the relationships amongst Echinocereus species were not entirely revealed and useful characters to recognize clades were not provided. The inclusion of several sources of evidence in a phylogenetic analysis is likely to produce more supported hypotheses. Therefore, we performed a combined phylogenetic analysis with a set of 44 morphological characters and six chloroplast DNA sequences. Topologies from parsimony and Bayesian analyses were mostly congruent. However, the relationships of E. poselgeri were not consistent between analyses. A second Bayesian analysis using a long-branch extraction test resulted in a topology with the morphological position of E. poselgeri congruent with that in parsimony analysis. Parsimony and Bayesian analyses corroborated the monophyly of Echinocereus, which included eight monophyletic groups. The combined phylogeny integrated into different clades those taxa that were not determined in previous analyses and changed the relationships of some recognized clades. The clades did not recover the recent infrageneric classification. In the present study, a new sectional classification for Echinocereus is proposed based on the eight recovered clades, which is supported by a combination of morphological and molecular characters. An identification key for sections in the genus is included.     

Reproductive ecology and inbreeding depression in Opuntia rastrera (Cactaceae) in the Chihuahuan Desert: Why are sexually derived recruitments so rare?

We evaluated the influence of the reproductive ecology on low recruitment of sexually derived progeny observed in Opuntia rastrera Weber in the Southern Chihuahuan Desert, in two vegetation types. The flowers are diurnal, remaining open 9–10 hr. Pollen is released in the morning and at the same time the stigmas become receptive. Nectar is produced all day, but the production rate is higher at noon. The flowers are visited by insects, mainly solitary bees (Diadasia sp. and Lithurge sp.). Floral characteristics and the pollen/ovule ratio suggest that Opuntia rastrera is a facultative xenogamous species. Controlled pollinations indicate that it is not apomictic and pollinators are required to set fruit; it is also self-compatible, but there is strong inbreeding depression for fruit set. However, we were unable to demonstrate differences in reproductive characters between the populations in both vegetation types. The average density of adults per hectare was one order of magnitude higher in the nopalera (Opuntia-dominated scrublands) than in the grassland. However, the average density of plants that originated from seeds was one order of magnitude lower in the nopalera. Opuntia rastrera produces abundant flowers, fruits, and seeds in both vegetation types in natural conditions. The low success in the recruitment of new genets cannot be ascribed to the reproductive ecology.     

Programmed cell death promotes male sterility in the functional dioecious Opuntia stenopetala (Cactaceae)

Background and Aims

The sexual separation in dioecious species has interested biologists for decades; however, the cellular mechanism leading to unisexuality has been poorly understood. In this study, the cellular changes that lead to male sterility in the functionally dioecious cactus, Opuntia stenopetala, are described.

Methods

The spatial and temporal patterns of programmed cell death (PCD) were determined in the anthers of male and female flowers using scanning electron microscopy analysis and histological observations, focusing attention on the transition from bisexual to unisexual development. In addition, terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling assays were used as an indicator of DNA fragmentation to corroborate PCD.

Key results

PCD was detected in anthers of both female and male flowers, but their patterns differed in time and space. Functionally male individuals developed viable pollen, and normal development involved PCD on each layer of the anther wall, which occurred progressively from the inner (tapetum) to the outer layer (epidermis). Conversely, functional female individuals aborted anthers by premature and displaced PCD. In anthers of female flowers, the first signs of PCD, such as a nucleus with irregular shape, fragmented and condensed chromatin, high vacuolization and condensed cytoplasm, occurred at the microspore mother cell stage. Later these features were observed simultaneously in all anther wall layers, connective tissue and filament. Neither pollen formation nor anther dehiscence was detected in female flowers of O. stenopetala due to total anther disruption.

Conclusions

Temporal and spatial changes in the patterns of PCD are responsible for male sterility of female flowers in O. stenopetala. Male fertility requires the co-ordination of different events, which, when altered, can lead to male sterility and to functionally unisexual individuals. PCD could be a widespread mechanism in the determination of functionally dioecious species.     

Cladode development for Opuntia ficus-indica (Cactaceae) under current and doubled CO2 concentrations

Morphological and anatomical changes for first-order daughter cladodes (flattened stem segments) of a prickly pear cactus, Opuntia ficus-indica, were monitored to determine the effects of a doubled atmospheric CO₂ concentration on their development and mature form. For daughter cladodes developing in controlled environment chambers for 60 d, maximal elongation rates were similar under a photosynthetic photon flux density (PPFD) of 6 mol m⁻² d⁻¹ and a CO₂ concentration of 370 μl liter⁻¹, an increased PPFD (10 mol m⁻² d⁻¹), and an increased PPFD and a doubled CO₂ concentration. These maximal rates, however, occurred at 20, 15, and 12 d, respectively. The maximal relative growth rate under the doubled CO₂ concentration was about twice that under the other conditions. For cladodes at 60 d as well as after 4 and 16 mo in open-top chambers, doubling the CO₂ concentration had no effect on final length or width. At 4 mo, cladodes under doubled C0₂ were 27% thicker, perhaps allowing the earlier production of second-order daughter cladodes. The chlorenchyma was then 31% thicker and composed of longer cells. At 16 mo, the difference in cladode thickness diminished, but the chlorenchyma remained thicker under doubled CO₂, which may contribute to greater net CO₂ uptake for O. ficus-indica under elevated CO₂ concentrations. Two other persistent differences were a 20% lower stomatal frequency and a 30% thicker cuticle with more epicuticular wax for cladodes under doubled CO₂, both of which may help reduce transpirational water loss.     

Biomechanics and anatomy of cladode junctions for two Opuntia (Cactaceae) species and their hybrid

Hybridization between the introduced arborescent Opuntia ficus-indica and the native shrubby O. littoralis has led to populations, referred to as O. "occidentalis," which form thickets that can dominate hillsides of chaparral and that can survive fires. Because the thickets apparently develop via vegetative reproduction, O. "occidentalis" was hypothesized to have a greater ability than its parent species to reproduce vegetatively due to weaker cladode junctions. Of the three taxa, the junctions for O. "occidentalis" had the least amount of wood, despite having cladode masses and junction cross-sectional areas similar to those of O. littoralis. The cladodes of O. "occidentalis" resisted deflection about their junctions the least and their junctions required the least amount of applied mass and the smallest bending moment to fail mechanically. The junction wood for all three taxa consisted mostly of parenchyma, with lesser amounts of cells with thickened secondary cell walls, indicating that some junction strength depended on hydrostatic pressure, especially for terminal junctions. Libriform fibers, which contribute to support and resist bending moments, were about 80% less frequent in the sub-subterminal junctions of O. "occidentalis" than in O. ficus-indica and O. littoralis. Vascular tracheids, which probably reduced shear among cells in the wood, were 90% less frequent in the terminal and sub-subterminal junction wood of O. "occidentalis" compared to O. littoralis. Thus wood characteristics can account for the weaker junctions of O. "occidentalis" compared to those of O. ficus-indica and O. littoralis, which apparently increases the ability of the hybrid to reproduce vegetatively.     

The Opuntia (Cactaceae) and Dactylopius (Hemiptera: Dactylopiidae) in Mexico: a historical perspective of use, interaction and distribution

A historical perspective on the use and production of species of Dactylopius (Hemipetera: Dactylopiidae) and Opuntia (Cactaceae: Opuntioidae), information on their origin, diversity and distribution in Mexico are reviewed, and aspects of their conservation are discussed. The use and exploitation of both genera are part of Mexican cultures since prehistory. Opuntia species were among the main components of human diet during pre-agricultural times. Cochineal was used and probably cultivated at least from the Tenth century. During the colonial period, cochineal generated significant benefits to the Spaniard colonizers and Mexico was the world’s first producer of insects and dyes until the mid Nineteenth century. Currently, Mexico is the main producer of Opuntia cladodes and prickly pear, but cochineal cultivation is marginal and only maintained in traditional indigenous systems. Mexico is one of the main areas of diversity of Opuntia, having 83–104 out of nearly 200 species worldwide. More than 50 species are used mainly as food, fodder and medicine and 20 species are cultivated with different degrees of domestication. The genus Dactylopius includes nine species, with five of them naturally occurring in Mexico. Only D. coccus has been cultivated and domesticated but other wild species have been used throughout history. Arid and semiarid areas of Mexico are among the most important reservoirs of biological diversity for both genera, particularly for D. coccus. Specific measures for protection of such biodiversity and generic resources are required. Strategies for in situ conservation combined with re-established use and cochineal production may enhance conservation policies.     

A cytological and morphometric analysis of a triploid apomict,Opuntia × Kelvinensis (subgenus Cylindropuntia,Cactaceae)

A morphometric analysis ofOpuntia spinosior, O. fulgida, and their putative hybrid,O. × kelvinensis was supplemented with cytogenetic data and pollen stainability for all OTUs. The morphometric analysis supported the hypothesis for the hybrid origin ofO.×kelvinensis and indicated that limited backcrossing has occurred betweenO.×kelvinensis andO. spinosior. Almost all individuals investigated ofO.×kelvinensis are triploid, with 33 chromosomes, those ofOpuntia fulgida are mostly diploid, but in part triploid, and all ofO. spinosior investigated are diploid. The very high percentage of sterile seed produced by triploidO.×kelvinensis is almost certainly a consequence of unequal segregation of chromosomes in pollen mother cells during anaphase I. The ability ofO.×kelvinensis to reproduce vegetatively is attributable to itsO. fulgida parentage. It is hypothesized that the success ofO.×kelvinensis is a result of its particularly preadaptive genome isolated from infrequent backcrossing via meiotic irregularities of odd-polyploidy and its ability to reproduce vegetatively.     

Functional Gynodioecy in Opuntia quimilo (Cactaceae), a Tree Cactus Pollinated by Bees and Hummingbirds

Abstract: The tree cactus Opuntia quimilo is one of three known gynodioecious cacti. Its flowers deviate from most Opuntias in features that are attributable to ornithophily: petals are shiny red in colour, and fleshy in consistency, a nectar chamber is present, and stamen seismonasty is lacking. Pollinators include large matinal bees (predominantly Ptilothrix tricolor and Megachile sp.) and hummingbirds ( Chlorostilbon aureoventris and Heliomaster furcifer ). Hummingbirds rarely visit other local Opuntias. Hummingbirds, which are more common in the afternoon, prefer female flowers whereas bees prefer hermaphroditic flowers. Female flowers have more dilute nectar than hermaphroditic flowers. Under experimental conditions female fertility is as high as that of hermaphrodites, however, seeds from females always result from cross-pollination and from more severe ovule selection (ovule number is higher in female flowers). Under natural conditions female plants are reproductively more successful than hermaphrodites. Known cases of bird pollination in Opuntia and the incidence of ornithophilic features in Opuntia and related genera are discussed.     

Moving to mate: the evolution of separate and combined sexes in multicellular organisms

Which conditions favour the evolution of hermaphroditism or separate sexes? One classical hypothesis states that an organism’s mode of locomotion (if any) when searching for a mate should influence breeding system evolution. We used published phylogenies to reconstruct evolutionary changes in adult mate‐search efficiency and breeding systems among multicellular organisms. Employing maximum‐likelihood analyses, we found that changes in adult mate‐search efficiency are significantly correlated with changes in breeding system, and this result is robust to uncertainties in the phylogenies. These data provide the first statistical support, across a broad range of taxa, for the hypothesis that breeding systems and mate‐search efficiency did not evolve independently. We discuss our results in context with other causal factors, such as inbreeding avoidance and sexual specialization, likely to affect breeding system evolution.     

On the possible role of nonreproductive traits for the evolution of unisexuality: Life‐history variation among males,females, and hermaphrodites in Opuntia robusta (Cactaceae)

In angiosperms, dioecy has arisen in 871–5,000 independent events, distributed in approximately 43% of the flowering families. The reproductive superiority of unisexuals has been the favorite explanation for the evolution of separate sexes. However, in several instances, the observed reproductive performance of unisexuals, if any, does not seem to compensate for the loss of one of the sex functions. The involvement of fitness components not directly associated with reproduction is a plausible hypothesis that has received little attention. Life‐history traits recently recognized as predictors of plant performance were compared among males, females, and hermaphrodites of a rare trioecious Opuntia robusta population in the field, using the cladode as the study unit. Cladode mortality by domestic herbivores was common and higher in females and hermaphrodites than in males. Males, females, or both displayed lower shrinkage and higher rates of survival, growth, and reproductive frequency than hermaphrodites. Unisexuals simultaneously outperformed hermaphrodites in demographic traits known to compete for common limiting resources, such as the acceleration of reproductive maturation (progenesis) and survival. A meta‐analysis combining the outcomes of each of the analyzed life‐history traits revealed a tendency of males (d₊₊ = 1.03) and females (d₊₊ = 0.93) to outperform hermaphrodites in presumably costly demographic options. Clonality is induced by human or domestic animal plant sectioning; and males and females highly exceeded hermaphrodites in their clonality potential by a factor of 8.3 and 5.3, respectively. The performances of unisexuals in the analyzed life‐history traits may enhance their reproductive potential in the long run and their clonality potential and could explain the observed increase of unisexuality in the population. Life‐history traits can be crucial for the evolution of unisexuality, but their impact appears to be habitat specific and may involve broad ontogenetic changes.     

Reproductive biology of Myrtillocactus geometrizans (Cactaceae) flowers with contrasting orientation

In columnar cacti, a higher production of reproductive structures on branches oriented towards the Equator has been explained by their higher interception of photosynthetic active radiation (PAR) as well as resource availability. The goal of this study was to evaluate the effect of orientation on diverse aspects of the reproductive biology of Myrtillocactus geometrizans. Phenology was studied in north- and south-facing branches. Floral cycle events, floral visitors, reproductive traits associated with sexual and attraction functions, and reproductive success were estimated from reproductive structures with contrasting orientation. Pollination experiments were conducted to evaluate the effect of orientation on mating system. Our results showed that south-facing branches had a longer duration of the mature fruit phenophase. Moreover, flower synchrony, production of reproductive structures, and floral traits associated with the male (number of anthers and pollen grains per floral bud), female (number and size of ovules and dimensions of both ovary and ovary cavity), and attraction (petal size) functions had higher values in south-facing flowers. The beginning and ending of the male function and the end of flower anthesis occurred earlier in south-facing flowers. Diversity of floral visitors was similar between orientations, except for beetles whose abundance was greater in flowers oriented towards the south. North- and south-facing flowers had a mixed mating system, with similar reproductive success. Our results showed strong differences in the reproductive biology of an intertropical columnar cactus, probably in response to the uneven PAR interception and resource availability in branches and flowers with contrasting orientation.     

Self-fertilization in mosses: a comparison of heterozygote deficiency between species with combined versus separate sexes

Self-fertilization is a key difference of adaptive significance between species with combined versus separate sexes. In haploid-dominant species such as mosses and ferns, species with either combined or separate sexes (monoicous and dioicous, respectively) have the potential to self-fertilize (intergametophytic selfing), but being monoicous allows an additional mode of selfing (intragametophytic selfing). We used allozyme electrophoresis to estimate deviations from expected levels of heterozygosity under Hardy-Weinberg equilibrium to infer selfing rates in 10 moss species from 36 New Zealand populations. We found that while there were deficiencies of heterozygotes compared to expectation in both monoicous and dioicous mosses, monoicous species had significantly higher levels of heterozygote deficiency than dioicous species (F(IS)=0.89+/-0.12 and 0.41+/-0.11, respectively). Estimated selfing rates suggest that selfing occurs frequently in monoicous populations, and rarely in dioicous populations. However, in two dioicous species (Polytrichadelphus magellanicus and Breutelia pendula), we found significant indications of mixed mating or biparental inbreeding in a handful of populations. These data provide the first analysis of heterozygote deficiency and selfing among haploid-dominant species with breeding system variation, and we discuss our results with respect to the consequences of inbreeding depression and the evolution of breeding systems.