A first assessment of genetic variation in Welwitschia mirabilis Hook. (1/8)
Welwitschia mirabilis is a monotypic member of the family Welwitchiaceae which, along with Ephedra and Gnetum species, comprises the gymnospermous order Gnetales. While the monophyly of this order is now widely accepted, the relationship of the Gnetales to other seed plants is still contentious. Despite the unique phylogenetic position of W. mirabilis and its extraordinary physiological and anatomical adaptations, little is known about the plant's phylogeny or its current distribution in isolated locations throughout the Namib Desert. As a preliminary step in the design of an more extensive phylogeographic study, we analyzed 37 random amplified polymorphic DNA (RAPD) loci from 59 plants distributed among five sites separated by distances of 6-440 km. Cluster analysis and analysis of molecular variance (AMOVA) revealed significant levels of variation within and between populations and little evidence of inbreeding. Genetic differences between populations reflect the geographic distances separating them. Three of the populations formed discernable genetic clusters, suggesting that little gene flow occurs between populations separated by > or = 18 km. In contrast, gene flow is occurring between two populations separated by only 6 km, supporting previous observations that pollen dispersal is primarily local and that seeds are not readily wind-born over the large distances separating most W. mirabilis populations. As a working hypothesis, we propose that W. mirabilis had a continuous distribution across its current range as much as 105 million years ago, and that as a consequence of subsequent drying trends and physical disturbance, populations became progressively isolated, accounting for their current distribution. (+info)High levels of genetic variation exist in Aspergillus niger populations infecting Welwitschia mirabilis hook. (2/8)
Aspergillus niger is an asexual, haploid fungus which infects the seeds of Namibia's national plant, Welwitschia mirabilis, severely affecting plant viability. We used 31 randomly amplified polymorphic DNA markers to assess genetic variation among 89 A. niger isolates collected from three W. mirabilis populations in the Namib Desert. While all isolates belonged to the same vegetative compatibility group, 84% were unique genotypes, and estimates of genotypic evenness and Simpson's index of diversity approached 1.0 in the three populations. Analysis of molecular variance revealed that 78% of the total variation sampled was among isolates from individual W. mirabilis plants. Lower, but significant, amounts of variation detected among isolates from different plants (12%) and different sites (10%) also indicated some site- and plant-level genetic differentiation. Total gene diversity (H(T) = 0.264) was mostly attributable to diversity within populations (H(S) = 0.217); the relatively low level of genetic differentiation among the sites (G(ST) = 0.141) suggests that gene flow is occurring among the three distant sites. Although sexual reproduction has never been observed in this fungus, parasexuality is a well-known phenomenon in laboratory strains. We thus attribute the high levels of genetic variation to parasexuality and/or wind-facilitated gene flow from an as of yet undocumented broader host range of the fungus on other desert vegetation. Given the apparent ease of transmission, high levels of genetic diversity, and potentially broad host range, A. niger infections of W. mirabilis may be extremely difficult to control or prevent. (+info)Former diversity of Ephedra (Gnetales): evidence from Early Cretaceous seeds from Portugal and North America. (3/8)
BACKGROUND AND AIMS: The extant species of the seed plant group Gnetales (Ephedra, Gnetum and Welwitschia) have been considered a remnant of a much greater, now extinct, diversity due to the pronounced differences in form and ecology among the genera. Until recently, this hypothesis has not been supported by evidence from the fossil record. This paper adds to the expanding information on Gnetales from the Early Cretaceous and describes coalified seeds from Barremian-Albian localities in Portugal and USA. METHODS: The fossils were extracted from sediment samples by sieving in water. Adhering mineral matrix was removed by chemical treatment. Seeds were investigated using light and scanning electron microscopy. Morphology and anatomy of the seeds were documented and compared with those of extant species. KEY RESULTS: The fossils share characters with extant Ephedra, for example papillae on the inner surface of the seed envelope and in situ polyplicate pollen grains that shed the exine during germination. They differ from extant Ephedra seeds in morphological and anatomical details as well as in their smaller size. Two new species of Ephedra are described together with one species assigned to a new genus of Gnetales. Other Ephedra-like seeds, for which pollen and critical morphological details are currently unknown, are also present in the samples. CONCLUSIONS: These Cretaceous seeds document that key reproductive characters and pollen germination processes have remained unchanged within Ephedra for about 120 million years or more. There is sufficient variety in details of morphology to suggest that a diversity of Ephedra and Ephedra-like species were present in the Early Cretaceous flora. Their presence in Portugal and eastern North America indicates that they were widespread on the Laurasian continent. The fossil seeds are similar to seeds of Erdtmanithecales and this supports the previously suggested relationship between Erdtmanithecales and Gnetales. (+info)The complete plastid genome sequence of Welwitschia mirabilis: an unusually compact plastome with accelerated divergence rates. (4/8)
(+info)A new early Cretaceous relative of Gnetales: Siphonospermum simplex gen. et sp. nov. from the Yixian formation of northeast China. (5/8)
(+info)The position of gnetales among seed plants: overcoming pitfalls of chloroplast phylogenomics. (6/8)
(+info)Systematic error in seed plant phylogenomics. (7/8)
(+info)Chengia laxispicata gen. et sp. nov., a new ephedroid plant from the Early Cretaceous Yixian Formation of western Liaoning, Northeast China: evolutionary, taxonomic, and biogeographic implications. (8/8)
(+info)Gnetophyta is a division of seed plants that includes three living genera: Gnetum, Welwitschia, and Ephedra. These plants are characterized by their vascular structure, which is similar to that of angiosperms (flowering plants) and gymnosperms (conifers and related plants). They also have specialized leaves and pollen-bearing structures. Gnetophytes are often considered to be a sister group to the angiosperms, although their exact phylogenetic relationship is still a matter of debate. Some researchers have suggested that gnetophytes may have evolved certain traits independently as a result of convergent evolution, rather than sharing a common ancestor with the angiosperms. Despite their small number of extant species, gnetophytes have a fossil record dating back to the early Mesozoic Era, making them an important group for understanding the evolutionary history of seed plants.