Genetic differentiation in the Agave deserti (Agavaceae) complex of the Sonoran desert

The Agave deserti complex, comprising A. deserti, A. cerulata and A. subsimplex, represents a group of species and subspecies with a near allopatric distribution and clear differences in morphology. Genetic differentiation and taxonomic status with respect to spatial distribution of 14 populations of the complex were analyzed in an effort to understand the evolution and speciation process within the genus. Allelic frequencies, levels of genetic variation, expected heterozygosity (H S ), proportion of polymorphic loci (P), and genetic differentiation (y and Nei's genetic distance) were estimated using 41 putative RAPD loci. All three species show high levels of genetic variation (H S ¼ 0.12-0.29, P ¼ 63.4-95.1), and low genetic differentiation between populations and species (y populations ¼ 0.1470.02 (SE); G st ¼ 0.1170.02). Accordingly, gene flow among populations was estimated as high by three different methods (N m ¼ 2.91-6.14). Nei's genetic distances between the three species were low compared to the values obtained from other Agavaceae, and there was no clear correlation with taxonomic divisions. In a UPGMA analysis, A. subsimplex and A. cerulata formed exclusive monospecific clusters, whereas the A. deserti populations appear in more than one cluster together with other species. The results were consistent with a pattern of genetic isolation by distance

Pollination Biology and Adaptive Radiation of Agavaceae, with Special Emphasis on the Genus Agave

Agavaceae are an American family that comprises nine genera and ca. 300 species distributed in arid and semiarid environments, mainly in Mexico. The family is very successful and displays a wide array of ecological, reproductive, and morphological adaptations. Many of its members play important roles as keystone species, because they produce abundant resources during the reproductive season. In this paper we analyze the current knowledge about the pollination ecology of the different genera in the family and the role that pollination systems have played in the ecological and phylogenetic success of the group. After providing an overview of each of the genera in the family, we discuss in detail aspects of the reproductive ecology of species in the genus Agave s.l., which is composed of ca. 208 species and includes subgenera of Agave (Agave and Littaea), Manfreda, Polianthes, and Prochnyanthes. Finally, we describe the results of analyses to test the hypothesis that there has been an adaptive radiation in the genus Agave. Using chloroplast and nuclear DNA sequences we estimate the age of the Agavaceae family and the genus Agave to be 12-26 millions of years ago (MYA) and 10 MYA, respectively, and show that mean rates of diversification were higher in the genus Agave than the genus Yucca. The values we report for rates of diversification in Agave s.l. are high when compared to other radiations in plants and animals. We suggest that the desertification of North America, which started ca. 15 MY A was critical in the radiation of agaves and that the generalist pollination system of Agave has been more successful in generating new species than the extreme specialization of Yucca

Light availability affects sex lability in a gynodioecious plant

PREMISE OF THE STUDY: Sex lability (i.e., gender diphasy) in plants is classically linked to the larger resource needs associated with the female sexual function (i.e., seed production) compared to the male function (i.e., pollen production). Sex lability in response to the environment is extensively documented in dioecious species, but has been largely overlooked in gynodioecious plants. METHODS: Here, we tested whether environmental conditions induce sex lability in the gynodioecious Geranium sylvaticum. We conducted a transplantation experiment in the field where plants with different sex expression were reciprocally transplanted between high light and low light habitats. We measured plants’ reproductive output and sex expression over four years. KEY RESULTS: Our results show that sex expression was labile over the study period. The light level at the destination habitat had a significant effect on sexual expression and reproductive output, because plants decreased their reproductive output when transplanted to the low light habitat. Transplantation origin did not affect any parameter measured. CONCLUSIONS: This study shows that sex expression in Geranium sylvaticum is labile and related to light availability. Sexually labile plants did not produce more seeds or pollen, and thus, there was no apparent fitness gain in sexually labile individuals. Sex lability in gynodioecious plants may be more common than previously believed because detection of sex lability necessitates data on the same individuals over time, which is rare in sexually dimorphic herbaceous plants

Reproductive ecology of the rare clonal cactus Stenocereus eruca in the Sonoran desert

Abstract. Stenocereus eruca is a clonal cactus with an extremely narrow distribution in Baja California, in which seedling recruitment has rarely been observed. Low seedling recruitment in clonal plants may be caused by low seed production as a consequence of pollinator limitation or if seed input is sufficient, by lack of favorable conditions or microsites for seedling establishment. In this paper, we study the reproductive ecology of S. eruca along four years in order to explore the proximate causes of the low seedling recruitment observed in its populations

Microenvironmental heterogeneity and space utilization by desert vines within their host trees

The three-dimensional biomass distribution and the microenvironments experienced by several desert vine species growing within the canopy of host trees were studied at the Centro Ecológico de Sonora in México. The light environment within the crown of the host tree Cercidium microphyllum showed a horizontal and vertical gradient from the base of the trunk to the edges of the canopy. Within this gradient total daily photosynthetically active radiation (PAR) varied from 47.8 mol m-2outside the crown to 4.6 mol m-2at the centre of the crown and close to the ground. Maximum air temperature was 3 °C lower beneath the crown than outside. Within the canopy, most vines experienced less than 50% of the daily available PAR outside the canopy. For most of the day, leaves of vines received 15% or less of the maximum available PAR. Our study shows that vines do not grow towards full sunlight but rather they exploit different habitat possibilities within their host tree crown. Leaves along the stems of vines experienced a wide range of light environments, showing coefficients of variation (CV) in total daily PAR from 36.4 to 94.6%. Daily courses of PAR also showed that leaves within the canopy experienced short-term temporal variation in the light environment. Differences in CV of daily PAR values and preferences in heterogeneous light microenvironments among species suggested that different vine species might be spatially separated in the canopy. We suggest that in desert habitats, conditions within the crown of host trees result in an important microhabitat that vines can exploit, allowing them to avoid the high light, temperature and water deficits found in the surrounding environment

Evidence of gynodioecy in Kallstroemia grandiflora (Zygophyllaceae): Microsporogenesis in hermaphrodite and female plants and lack of reproductive compensation

Understanding mechanisms involved in the maintenance of gynodioecy in natural populations requires evidence concerning the phenotypic expression and reproductive consequences of male sterility. We have recently discovered male sterility in populations of Kallstroemia grandiflora from northwestern Mexico. Here we describe the development of anther and pollen grains in hermaphrodite and female plants and explore whether females experience any compensatory advantage over hermaphrodites. The major difference in anther development between morphs was the early disintegration of tapetal cells in anthers of pistillate flowers. Abnormalities in developing pollen grains in pistillate flowers were seen in the microspore mother cells, microspore tetrads, and young and mature pollen grains. Biomass allocation to flowers, petals, and stamens were larger in hermaphrodites, while both sexes had the same number of ovules per flower. Hermaphrodite flowers were self-compatible. Pollination experiments revealed evidence of pollinator limitation for female plants but not for hermaphrodites. No evidence of reproductive compensation was detected, as flower and fruit production were similar for both sexes in two populations of K. grandiflora. We suggest alternative mechanisms that could maintain male sterility in natural populations of K. grandiflora