Molecular evolution of pteridophytes and their relationship to seed plants: Evidence from complete 18S rRNA gene sequences

Complete 18S ribosomal RNA sequence data from representatives of all extant pteridophyte lineages together with RNA sequences from different seed plants were used to infer a molecular phylogeny of vascular plants that included all major land plant lineages. The molecular data indicate that lycopsids are monophyletic and are the earliest diverging group within the vascular land plants, whereasPsilotum nudum is more closely related to the seed plants than to other pteridophyte lineages. The phylogenetic trees based on maximum likelihood, parsimony and distance analyses show substantial agreement with the evolutionary relationships of land plants as interpreted from the fossil record.     

Biogeography and genome size evolution of the oldest extant vascular plant genus,Equisetum (Equisetaceae)

Background and AimsExtant plant groups with a long fossil history are key elements in understanding vascular plant evolution. Horsetails (Equisetum, Equisetaceae) have a nearly continuous fossil record dating back to the Carboniferous, but their phylogenetic and biogeographic patterns are still poorly understood. We use here the most extensive phylogenetic analysis to date as a framework to evaluate their age, biogeography and genome size evolution.MethodsDNA sequences of four plastid loci were used to estimate divergence times and investigate the biogeographic history of all extant species of Equisetum. Flow cytometry was used to study genome size evolution against the framework of phylogenetic relationships in Equisetum.Key ResultsOn a well-supported phylogenetic tree including all extant Equisetum species, a molecular clock calibrated with multiple fossils places the node at which the outgroup and Equisetum diverged at 343 Mya (Early Carboniferous), with the first major split among extant species occurring 170 Mya (Middle Jurassic). These dates are older than those reported in some other recent molecular clock studies but are largely in agreement with a timeline established by fossil appearance in the geological record. Representatives of evergreen subgenus Hippochaete have much larger genome sizes than those of deciduous subgenus Equisetum, despite their shared conserved chromosome number. Subgenus Paramochaete has an intermediate genome size and maintains the same number of chromosomes.ConclusionsThe first divergences among extant members of the genus coincided with the break-up of Pangaea and the resulting more humid, warmer climate. Subsequent tectonic activity most likely involved vicariance events that led to species divergences combined with some more recent, long-distance dispersal events. We hypothesize that differences in genome size between subgenera may be related to the number of sperm flagellae.     

木贼类营养器官凯氏带类型的新观察

该研究用石蜡切片法比较观察了5种木贼科植物营养器官的内皮层及凯氏带,首次报道了2层内皮层及其凯氏带的形态特征及分布规律,并讨论各种类型的凯氏带及其与厚壁组织的协作防御机制。结果表明:(1)5种木贼的地下茎和根都只有1条凯氏带,其中4种木贼的地上茎有2条凯氏带。(2)木贼类营养器官具有3种凯氏带类型,即2层公共内皮层上各具有1条凯氏带、1层散生内皮层上的1条凯氏带、1层公共内皮层上的1条凯氏带。(3)木贼类地下茎和根都有发达的厚壁组织或致密的表皮。(4)问荆地上茎外侧内皮层具有复合内皮细胞。研究认为,木贼类植物凯氏带数量不能作为分类的依据;地下茎和根虽然只有1条凯氏带,但地下部分都有发达的厚壁组织或(和)与其紧密相连的表皮,推测厚壁组织或(和)表皮可能具有与凯氏带相同的功能;3种类型凯氏带的防御能力由强到弱依次是:2层公共内皮层上的凯氏带 1层散生内皮层上的凯氏带 1层公共内皮层上的凯氏带。     

Partial mycoheterotrophy is common among chlorophyllous plants with Paris-type arbuscular mycorrhiza

Background and AimsAn arbuscular mycorrhiza is a mutualistic symbiosis with plants as carbon providers for fungi. However, achlorophyllous arbuscular mycorrhizal species are known to obtain carbon from fungi, i.e. they are mycoheterotrophic. These species all have the Paris type of arbuscular mycorrhiza. Recently, two chlorophyllous Paris-type species proved to be partially mycoheterotrophic. In this study, we explore the frequency of this condition and its association with Paris-type arbuscular mycorrhiza.MethodsWe searched for evidence of mycoheterotrophy in all currently published ¹³C, ²H and ¹⁵N stable isotope abundance patterns suited for calculations of enrichment factors, i.e. isotopic differences between neighbouring Paris- and Arum-type species. We found suitable data for 135 plant species classified into the two arbuscular mycorrhizal morphotypes.Key ResultsAbout half of the chlorophyllous Paris-type species tested were significantly enriched in ¹³C and often also enriched in ²H and ¹⁵N, compared with co-occurring Arum-type species. Based on a two-source linear mixing model, the carbon gain from the fungal source ranged between 7 and 93 % with ferns > horsetails > seed plants. The seed plants represented 13 families, many without a previous record of mycoheterotrophy. The ¹³C-enriched chlorophyllous Paris-type species were exclusively herbaceous perennials, with a majority of them thriving on shady forest ground.ConclusionsSignificant carbon acquisition from fungi appears quite common and widespread among Paris-type species, this arbuscular mycorrhizal morphotype probably being a pre-condition for developing varying degrees of mycoheterotrophy.