Male sterility in Bambusa tuldoides Munro.

Despite their great economic importance, relatively little is known about bamboo sexual reproduction because they usually spread through rhizomes and have long intervals between flowering periods. Bambusa tuldoides is no exception; the intervals between flowering periods are about 23 years and often do not result in successful caryopsis production. The aim of the present work was to characterize Bambusa tuldoides sexual reproduction at three stages of flower development and investigate possible male sterility. Pollen was cultured onto several types of culture medium in order to encourage germination, but not a single of the thousands of observed pollen germinated under any condition. Anthers and microspores were analyzed by scanning electron microscopy, transmission electron microscopy, and optical microscopy techniques. Anther dehiscence appeared to be normal when compared to other species. In contrast, microspores began to develop abnormally starting as early as the first flower development stage: retraction of the cytoplasm and rupture of the nuclear and mitochondria membrane. As the interior machinery of the microspores degenerated, starch accumulated within numerous amyloplasts during stages two to four of flower development. The sporoderms of these microspores were similarly incomplete: though they possessed an exine, they lacked an intine. The results here obtained suggest that the non-viability of these abnormal pollen grains prevents the development of Bambusa tuldoides caryopses.

Pollen grain development and male sterility in the perfect flowers of Maytenus obtusifolia Mart. (Celastraceae).

Perfect flowers of Maytenus obtusifolia have partial sterility of pollen grains, resulting in collapsed and developed free microspores. However, the cellular events resulting in partial male sterility have not been determined. In pistillate flowers of this species, male sterility is related to the premature programmed cell death (PCD) in tapetum and sporogenic cells. The process occurs through autophagy via macroautophagy and massive autophagy and is associated with sporophytic cytoplasmic male sterility (CMS). Here, we characterised the development of pollen grains and investigated the cellular events that result in tapetum cells and free microspores PCD in perfect flowers, using light and transmission electron microscopy combined with the TUNEL (Terminal deoxynucleotidyl transferase mediated dUDP end-Labeling) assay and the ZIO (Zinc iodide-osmium tetroxide) method. Pollen grain development in perfect flowers was divided into eight developmental stages based on the characteristics of the pollen grains. Tapetum cells undergo PCD at the free microspore stage, through a macroautophagic process, by formation of autophagosomes and by autophagosomes giving rise to lytic vacuoles at maturity. In the final stage of PCD, massive autophagy occurs by rupture of the tonoplast. The development of viable and inviable microspores diverges at the vacuolated microspore stage, when PCD occurs in some free microspores, causing interruption of pollen development through necrosis. These events result in the observed partial male sterility. Viable microspores undergo mitosis and develop into tricellular pollen grains. Male sterility in hermaphrodite individuals is here interpreted as gametophytic CMS.

Apomixis does not affect visitation to flowers of Melastomataceae, but pollen sterility does.

Apomixis is an asexual seed reproduction mechanism thorough which embryos are originated from material tissues inside the ovules, without precedent fertilisation. It allows plants to colonise new habitats, even in places where flower visitors are scarce or where plants are isolate. Apomixis seems to be related to pollen sterility and, in species with flowers that offer pollen as a reward for pollinators, the amount or quality of the pollen offered by these species may influence the amount of the visits and specific composition of the visitors. In order to test this hypothesis, we studied breeding systems of 16 species of Melastomataceae and their flower visitors, evaluating composition and abundance of the visits to apomictic and sexual species. Apomictic plants with no viable pollen or with pollen with low viability did not receive visits from pollinators, and consequently probably produce strictly apomictic fruits. On the other hand, apomictic and sexual plants with high pollen viability do receive visits; in this case, apomictic plants may produce fruits and seeds through both sexual and apomictic methods. The species composition of insects visiting Melastomataceae with high pollen viability was similar, regardless of whether the plants were apomictic or not. It seems that pollen viability levels are important to determine visits to the flowers irrespective of breeding system.

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.

Sterile flowers increase pollinator attraction and promote female success in the Mediterranean herb Leopoldia comosa.

BACKGROUND AND AIMS: Large floral displays have opposing consequences for animal-pollinated angiosperms: they attract more pollinators but also enable elevated among-flower self-pollination (geitonogamy). The presence of sterile flowers as pollinator signals may enhance attraction while allowing displays of fewer open fertile flowers, limiting geitonogamy. The simultaneous contributions of fertile and non-fertile display components to pollinator attraction and reproductive output remain undetermined. METHODS: The simultaneous effects of the presence of sterile flowers and fertile-flower display size in two populations of Leopoldia comosa were experimentally assessed. Pollinator behaviour, pollen removal and deposition, and fruit and seed production were compared between intact plants and plants with sterile flowers removed. KEY RESULTS: The presence of sterile flowers almost tripled pollinator attraction, supplementing the positive effect of the number of fertile flowers on the number of bees approaching inflorescences. Although attracted bees visited more flowers on larger inflorescences, the number visited did not additionally depend on the presence of sterile flowers. The presence of sterile flowers improved all aspects of plant performance, the magnitude of plant benefit being context dependent. During weather favourable to pollinators, the presence of sterile flowers increased pollen deposition on stigmas of young flowers, but this difference was not evident in older flowers, probably because of autonomous self-pollination in poorly visited flowers. Total pollen receipt per stigma decreased with increasing fertile display size. In the population with more pollinators, the presence of sterile flowers increased fruit number but not seed set or mass, whereas in the other population sterile flowers enhanced seeds per fruit, but not fruit production. These contrasts are consistent with dissimilar cross-pollination and autonomous self-pollination, coupled with the strong predispersal inbreeding depression exhibited by L. comosa populations. CONCLUSIONS: Sterile flowers enrich pollination quality by promoting pollen export and import, while limiting the mating costs of geitonogamy associated with large fertile displays.

ms17: a meiotic mutation causing partial male sterility in a corn silage hybrid.

Cytological analysis under light microscopy of the single hybrid P30R50 of silage corn revealed an abnormal pattern of microsporogenesis that affected the meiotic products. Meiosis progressed normally until diakinesis, but before migration to the metaphase plate, bivalents underwent total desynapsis and 20 univalent chromosomes were scattered in the cytoplasm. At this stage, meiocytes also exhibited a number of chromatin-like fragments scattered throughout the cell. Metaphase I was completely abnormal in the affected cells, and univalent chromosomes and fragments were distributed among several curved spindles. Anaphase I did not occur, and each chromosome or group of chromosomes originated a micronucleus. After this phase, an irregular cytokinesis occurred, and secondary meiocytes with several micronuclei were observed. Metaphase II and anaphase II also did not occur, and after the second cytokinesis, the genomes were fractionated into polyads, generating several unbalanced microspores, with various-sized nuclei. About 35% of the tetrads were abnormal in the hybrid. This spontaneous mutation had been previously reported in a USA maize line called ms17 and was found to cause male sterility.