The term "Geographical parthenogenesis" describes the phenomenon that asexual organisms usually occupy larger and more northern distribution areas than their sexual relatives, and tend to colonize previously glaciated areas. Several case studies on alpine and arctic plants confirm the geographical pattern, but the causal factors behind the phenomenon are still unclear. Research of the last decade in several plant families has shed light on the question and evaluated some of the classical evolutionary theories. Results confirmed, in general, that the advantages of uniparental reproduction enable apomictic plants to re-colonize faster in larger and more northern distribution areas. Associated factors like polyploidy seem to contribute mainly to the spatial separation of sexual and asexual cytotypes. Ecological studies suggest a better tolerance of apomicts to colder climates and temperate extremes, whereby epigenetic flexibility and phenotypic plasticity play an important role in occupying ecological niches under harsh conditions. Genotypic diversity appears to be of lesser importance for the distributional success of asexual plants. Classical evolutionary theories like a reduced pressure of biotic interactions in colder climates and hence an advantage to asexuals (Red Queen hypothesis) did not gain support from studies on plants. However, it is also still enigmatic why sexual outcrossing remains the predominant mode of reproduction also in alpine floras. Constraints for the origin of apomixis might play a role. Interestingly, some studies suggest an association of sexuality with abiotic stresses. Light stress in high elevations might explain why most alpine plants retain sexual reproduction despite other environmental factors that would favor apomixis. Directions for future research will be given.

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We used light and confocal microscopy to visualize bacteria in leaf and bract cells of more than 30 species in 18 families of seed plants. Through histochemical analysis, we detected hormones (including ethylene and nitric oxide), superoxide, and nitrogenous chemicals (including nitric oxide and nitrate) around bacteria within plant cells. Bacteria were observed in epidermal cells, various filamentous and glandular trichomes, and other non-photosynthetic cells. Most notably, bacteria showing nitrate formation based on histochemical staining were present in glandular trichomes of some dicots (e.g., Humulus lupulus and Cannabis sativa ). Glandular trichome chemistry is hypothesized to function to scavenge oxygen around bacteria and reduce oxidative damage to intracellular bacterial cells. Experiments to assess the differential absorption of isotopic nitrogen into plants suggest the assimilation of nitrogen into actively growing tissues of plants, where bacteria are most active and carbohydrates are more available. The leaf and bract cell endosymbiosis types outlined in this paper have not been previously reported and may be important in facilitating plant growth, development, oxidative stress resistance, and nutrient absorption into plants. It is unknown whether leaf and bract cell endosymbioses are significant in increasing the nitrogen content of plants. From the experiments that we conducted, it is impossible to know whether plant trichomes evolved specifically as organs for nitrogen fixation or if, instead, trichomes are structures in which bacteria easily colonize and where some casual nitrogen transfer may occur between bacteria and plant cells. It is likely that the endosymbioses seen in leaves and bracts are less efficient than those of root nodules of legumes in similar plants. However, the presence of endosymbioses that yield nitrate in plants could confer a reduced need for soil nitrogen and constitute increased nitrogen-use efficiency, even if the actual amount of nitrogen transferred to plant cells is small. More research is needed to evaluate the importance of nitrogen transfer within leaf and bract cells of plants.

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Asexual organisms usually have larger, and in the Northern Hemisphere, more northern distributions than their sexual relatives. This phenomenon, called geographical parthenogenesis, has been attributed to predispositions in certain taxa, advantages of polyploidy and/or hybrid origin, advantages of uniparental reproduction, introgression of apomixis into sexuals, niche differentiation of clones, and biotic interactions. Here we focus on the role of uniparental reproduction in colonisation, and the importance of different developmental pathways, i.e. autonomous apomixis which does not require pollination and fertilisation of endosperm nuclei for successful seed set, and pseudogamous apomixis which does. A literature survey suggests that geographical parthenogenesis occurs frequently in species with autonomous apomixis, while the correlation with pseudogamy is poorly documented. However, taxonomic patterns (e.g. predominance of Asteraceae) and also methodological bias may influence estimates of frequencies of geographical parthenogenesis. We demonstrate that a flow cytometric seed screen (FCSS) is a powerful method for assessing pseudogamous vs. autonomous apomixis. We show that population genetic studies provide insights into the genetic diversity of apomicts, but do not give strong support for uniparental reproduction being the only explanation of geographical parthenogenesis. Molecular studies help elucidate the evolutionary and biogeographical history of apomictic complexes, and we conclude that multidisciplinary studies are needed to understand fully the phenomenon of geographical parthenogenesis.

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A reinvestigation of the aerial parts of EUPATORIUM ALTISSIMUM afforded, in addition to several, previously isolated sesquiterpene lactones, five new ones. The structures of these were elucidated by highfield (1)H-NMR spectroscopy.

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