Francisco Molina-Freaner

The pollination biology of two paniculate agaves (agavaceae) from Northwestern Mexico: Contrasting roles of bats as pollinators

Paniculate agaves from tropical deserts depend on nectar-feeding bats for their reproduction, while species from extratropical areas near the northern limit of Leptonycteris curasoae are pollinated by diurnal and nocturnal insects and birds. Agave angustifolia is a paniculate agave with a broad distribution in Mexico whose range coincides with the distribution of L. curasoae, while A. subsimplex has a narrow distribution in Sonora within the range of nectar-feeding bats. We studied the pollination biology of A. angustifolia and A. subsimplex in northwestern Mexico to evaluate the relative importance of bat pollination in a region where L. curasoae is seasonal. Flower visitors included a wide variety of animals, with bats having greater visitation rates in A. angustifolia. A pollinator-exclusion experiment revealed that bats were responsible for most of the control fruit set in A. angustifolia, whereas for A. subsimplex, diurnal and nocturnal visitors were equally effective. Overall, our data indicate that in central Sonora, A. angustifolia depends on nectar-feeding bats for its sexual reproductive success, while A. subsimplex relies on both diurnal and nocturnal pollinators. Given the contrasting distribution ranges of paniculate agaves, our results seem to support the trend from specialization to generalization along the continuum of tropical to extratropical deserts.

Clonal diversity and distribution in Stenocereus eruca (Cactaceae), a narrow endemic cactus of the Sonoran Desert

Stenocereus eruca (Cactaceae), a prostrate cactus endemic to the Sonoran Desert, is thought to be highly clonal. We examined its clonal diversity and distribution: (1) at the population level, in four distinct populations along its distribution range; and (2) at a micro scale level, within a single population. Our objective was to evaluate the importance of sexual versus clonal recruitment through the use of RAPD markers. Contrary to previous field observations, clonal diversity was relatively high across the distribution range. This finding suggests that sexual recruitment is an important regeneration mechanism. The proportions of distinguishable genotypes (G/N = 0.83) and genotypic diversity (D = 0.987) were greater than in other clonal cacti, suggesting that clonal propagation is not the major regeneration mechanism. Autocorrelation analyses revealed a spatial genetic structure that may be the result of restricted gene flow (via pollen or seeds) and clonal propagation. A molecular variance analysis (AMOVA) indicated that most of the variation (66.3%) was found within and not across populations. Future studies on pollen and seed dispersal are needed to understand the role of the clonal habit in the mating system of S. eruca.

The evolution of distyly from tristyly in populations of Oxalis alpina (Oxalidaceae) in the Sky Islands of the Sonoran Desert

The evolution of distyly from tristyly was investigated in populations of Oxalis alpina at high elevations throughout the Sky Islands of the Sonoran Desert. Incompatibility systems in tristylous populations, where self-incompatible short-, mid-, and long-styled morphs occur in populations, vary from those typical of tristylous species in which each morph is equally capable of fertilizing ovules of the other two morphs, to breeding systems in which incompatibility relationships are asymmetric. In these populations, selection against the allele controlling expression of the mid-styled morph is likely. The degree of modification of incompatibility in the short- and long-styled morphs in 10 populations was strongly associated with fewer mid-styled morphs, supporting models predicting the effect of these modifications of incompatibility on frequency of the mid-styled morph. Self-compatibility of the mid-styled morph may be important for maintaining the frequency of this morph, depending on the level of self-pollination, self-fertilization, and the extent of inbreeding depression. Modifications of incompatibility in tristylous populations and the distribution of distylous populations of O. alpina in the Sky Island region have similar geographic components, indicating the potential importance of historical factors in the evolution of distyly from tristyly.

Pollination biology of the columnar cactus Pachycereus pecten-aboriginum in north-western México

Columnar cacti in tropical deserts depend on nectar-feeding bats for their reproduction while species from extra-tropical deserts show a relatively generalized pollination system with both nocturnal and diurnal pollinators. Pachycereus pecten-aboriginum is a columnar cactus with a broad distribution along the Pacific coast of Mexico, from Oaxaca to Sonora. Along its distribution, the nectar-feeding bat, Leptonycteris curasoae, changes from resident within the tropics to migratory in the Sonoran desert. If bat unpredictability has been an important force in the evolution of pollination systems in columnar cacti, P. pecten-aboriginum is expected to show a relatively generalized system in northern populations. We studied the pollination biology of P. pecten-aboriginum in two northern populations in the state of Sonora. Hand pollination experiments showed that this species has a self-incompatible, hermaphroditic breeding system. Although flowers open at night, they remain open and continue secreting nectar during the morning, allowing visitation by both nocturnal and diurnal pollinators. One population showed evidence of strong pollinator limitation while the results from both populations indicated that diurnal pollinators are more important than nocturnal pollinators. These results are discussed in terms of specialization vs. generalization in the pollination biology of columnar cacti in tropical and extra-tropical deserts.

Vine species diversity across environmental gradients in northwestern México

The presence of vines has been described as a distinctive feature of tropical forests. However, vine species diversity exhibits trends across environmental gradients that are not well documented. Here we use a latitudinal and a rainfall gradient along the Pacific slope of México to explore the influence of environmental factors on vine species diversity. A total of 630 vines species were detected on the Pacific slope of México. Tropical deciduous forest (TDF) floras were composed of greater percentages of vines (5–16%) than desert floras (1–3%). Four families (Convolvulaceae, Fabaceae, Cucurbitaceae and Asclepiadaceae) composed 40–60% of the vines of the region. Changes in vine composition were gradual along the Pacific slope. The percentage of vines in floras declines with latitude. Annual rainfall and the minimum temperature of January were significantly associated with the latitudinal decline in the percentage of vines. A total of 43 species, mostly herbaceous vines, were detected along a rainfall gradient in northwestern México. Along the rainfall gradient, the number of vine species increased from 3 to 28 as summer rainfall, plant cover and canopy stature increased. Vine species richness and diversity increased from the desert to the TDF, especially along streams. Leaf area (LA) ranged from 0.6 to 284 cm2 and specific leaf area (SLA) from 80 to 904 cm2/g among the most common vine species. Community averages of LA and SLA decreased toward drier sites. These results are discussed within the context of our current knowledge about the role of the environment in limiting the distribution of vines.

Geographic differentiation in the pollination system of the columnar cactus Pachycereus pecten -aboriginum

The pollination biology of the cactus Pachycereus pecten-aboriginum was studied in a tropical location in western Mexico (ca. 18° N latitude) to compare with data from a northern population (ca. 28° N latitude). Throughout this range, the nectar-feeding bat Leptonycteris curasoae is resident within the tropics but migratory in its northern range. The hypothesis was tested that if a predictable bat presence has been an important force in the evolution of pollination systems in columnar cacti, P. pecten-aboriginum will have a specialized pollination system within the tropics and a generalized pollination system in northern populations. In both areas, pollination experiments showed that P. pecten-aboriginum has a self-incompatible, hermaphroditic breeding system. In the tropical area, flowers open at night and close early in the morning. Nectar is secreted only during the night, and flowers are exclusively pollinated by three species of nectar-feeding bats, indicating a specialized pollination system. In contrast, anthesis and nectar secretion in northern populations occur during the night and day, allowing visitation and effective pollination by both nocturnal and diurnal pollinators. This study provides evidence of divergence mediated by pollinator predictability (resident vs. migrant), through shifts from short to long anthesis and nectar production periods from southern to northern populations.

Wood biomechanics and anatomy of PACHYCEREUS PRINGLEI.

We report the longitudinal, biomechanical, and anatomical trends observed for tissue samples drawn from the parallel aligned, prismatic woody vascular bundles running the length of a Pachycereus pringlei plant measuring 5.22 m in height. The main vertical stem of this plant was cut into five segments (labeled A through E in the acropetal direction) measuring ∼1.02 m in length. Four of the 14 vascular bundles in each segment were surgically removed to obtain 20 vascular bundle segments that were tested in bending to determine their stiffness measured in the radial E(R) and tangential E(T) direction. We also determined the lignin content of representative samples of wood.A nonlinear trend in stiffness was observed: E(R) and E(T) were highest in segments B or C (1.67 GN/m and 1.09 GN/m, respectively), lower in segment A (E(R) = 1.18 GN/m and E(T) = 0.35 GN/m), and lowest in segment E (E(R) = 0.03 GN/m and E(T) = 0.20 GN/m). Similar longitudinal trends were seen for axial tissue volume fraction and fiber wall thickness, which achieved their highest values in segment B (69.8% and 6.59 μm, respectively). Wood stiffness also correlated significantly with cell wall lignin content: with respect to segment B (which had the highest lignin content, and was thus used as the standard reference for percent lignin content), lignin content, was 15, 60, 85, and 43% in segments E, D, C, and A, respectively. Fiber cell length, which increased toward the base of the stem and toward the vascular cambium in the most proximal vascular bundle segment, did not correlate with E(R) or E(T).Basic engineering principles were used to calculate stem stresses resulting from self-loading and any wind-induced bending moment (produced by drag forces). Calculations indicated that the less stiff wood produced in segment A eliminates a rapid and potentially dangerous increase in stresses that would otherwise occur in segments B or C. The less stiff wood in segment A also reduces the probability of shear failure at the cellular interface between the wood and surrounding tissues in this portion of the stem.We conclude that P. pringlei wood stiffness is dependent on the volume fraction and lignification of axial tissues, less so on fiber wall thickness, and that wood development in this species is adaptively responsive to self-loading and differentially applied external mechanical forces.

INBREEDING EFFECTS IN A GYNODIOECIOUS POPULATION OF THE COLONIZING SPECIES TRIFOLIUM HIRTUM ALL.

The demographic consequences of one and two generations of selfing were examined in a gynodioecious population of rose clover (Trifolium hirtum). Seeds collected from a roadside population were used to create an experimental population that consisted of seeds of open‐pollinated individuals (S0), and seeds obtained after one (S1) and two (S2) generations of selfing. Seeds from the three groups were equally likely to germinate. However, inbreeding effects were observed in the vigor and survivorship of the seedlings. The proportion of the transplanted seedlings that reached the reproductive stage was 0.42, 0.31, and 0.14 for the S0, S1, and S2 groups, indicating a reduction of 26% and 66% by one and two generations of selfing, respectively. Inbreeding effects on reproductive components of fitness were nonsignificant. Groups were compared by multiplicative fitness functions based on germination, survival to reproduction, and fecundity. The relative fitness of the S1 and S2 groups were 0.64 and 0.23 relative to the S0 group, representing a reduction in lifetime fitness of 36% and 77%, respectively. The differences in relative fitness among the groups were caused mainly by differences in survivorship. Thus, inbreeding depression is apparently an important factor in the maintenance of the gynodioecious breeding system in rose clover.

DOES THE POLLINATOR ABUNDANCE HYPOTHESIS EXPLAIN GEOGRAPHIC VARIATION IN THE BREEDING SYSTEM OF PACHYCEREUS PRINGLEI

Previous studies have shown that the nectar‐feeding bat, Leptonycteris curasoae, is the major pollinator of Pachycereus pringlei (cardon), a columnar cactus whose populations are either gynodioecious or trioecious in the Sonoran Desert. On the basis of evidence of pollinator limitation in females and low flower visitation rates, a hypothesis has been proposed to explain geographic variation in the breeding system of P. pringlei. The pollinator abundance hypothesis predicts high frequencies of males and females in areas where L. curasoae is abundant and high frequency of hermaphrodites in areas where this bat is uncommon. We test this hypothesis by describing the following: first, the relationship between sex frequencies and bat abundance as revealed by nocturnal pollen deposition on stigmas of female plants and, second, the extent of pollinator limitation for fruit set in females and hermaphrodites in four populations of P. pringlei from coastal Sonora, Mexico. We also describe geographic variation in the relative fitness of females compared with hermaphrodites and outcrossing rates and inbreeding depression of self‐compatible hermaphrodites in these populations. Some of these parameters should vary with bat abundance according to this hypothesis. Sex‐frequency distributions showed a clear latitudinal pattern, with gynodioecious populations in the southern range and trioecious populations in the northern range. Large differences in nocturnal pollen deposition in females and abundance of L. curasoae were detected among populations but not as predicted by the pollinator abundance hypothesis. Fruit set was greater in females than in hermaphrodites in different pollination treatments, and no evidence of pollinator limitation was detected in either sex class. In general, the relative fitness of females was greater than hermaphrodites in most populations. Outcrossing rates in hermaphrodites were close to 1.0 in each population. Field and experimental data indicated no evidence of inbreeding depression. Our results do not support the hypothesis that the sex‐frequency distribution of P. pringlei is closely correlated with the abundance of L. curasoae but do demonstrate that females outperform hermaphrodites in gynodioecious and trioecious populations. Studies of the distribution of mtDNA variants are needed to understand geographic variation in the breeding system of P. pringlei.

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).