Taxonomic publications: past and future - Senckenberg Museum

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Burckhardt & Mühlethaler (eds): 8 th GfBS Annual Conference Abstracts 44 New insights in the evolution of liverworts stimulated by symbiotic fungi M. Nebel, I. Kottke & M. Preußing Liverworts are the sister group to all the other landplants (mosses, hornworts, ferns and flowering plants). Liverworts, like algae, lack the specific introns which occur in all other landplants. A fossil record (spore tetrads) from the Ordovician (460 my) appeared to belong to basal liverworts. Several studies on molecular phylogeny of liverworts were published, recently. Projecting our results of symbiotic fungi in liverworts on these molecular phylogenetic trees we found the following scenario: Glomeromycota are restricted to basal groups of liverworts (Haplomitriopsida), complex thalloid Marchantiopsida, and the basal group of the simple thalloid liverworts (Fossombroniales sensu Heinrichs). These liverworts grow mainly on mineral soil as do the Glomeromycota. Molecular and fossil data support the position of the Glomeromycota as the most ancient terrestrial fungi. We, therefore, suggest that the symbiosis with Glomeromycota was established in liverworts, first, long before Rhyniales. Later, the more derived liverworts (Metzgeriales, Jungermanniales, Porellales sensu Heinrichs) lost the symbiosis with Glomeromycota, probably at a common event. New forms of symbiosis were then established, twice and independently, by the phylogenetic younger liverwort groups: at one hand by the simple thalloid Metzgeriales associating with the basidiomycotean genus Tulasnella, at the other hand within the leafy liverworts (Jungermanniales, Porellales) forming symbiosis with the genus Sebacina (Basidiomycota) and the genus Hymenoscyphus (Ascomycota). Basidio- and Ascomycota are phylogenetic younger terrestrial fungi. Loss of the symbiosis with glomeromycotan fungi was probably connected to a change of the terrestrial habitat to epiphytism. Likely, at that time, the increase of angiosperm tree species lead to a fundamental shift in ecosystem conditions. In denser forests, light on the forest floor was reduced but angiospermean tree bark improved the conditions for an epiphytic life style. Epiphytism was correlated with the loss of the soil dependent Glomeromycota. The new fungal symbionts belonging to the genera Sebacina, Tulasnella, and Hymenoscyphus are typical colonizers of rotten wood and humus. Thus liverworts obtained access to new nutrient resources. The symbiosis, thus, pushed the evolution of new taxa. If the loss of Glomeromycota and the regain of modern terrestrial fungi were the result of fundamental habitat changes, the symbiotic status can be used as an indicator for the development of former ecosystems. Org. Divers. Evol. 5, Electr. Suppl. 13 (2005)
Burckhardt & Mühlethaler (eds): 8 th GfBS Annual Conference Abstracts 45 Nucleotide frequency biases as a problem in phylogenetic analyses using mitochondrial gene sequences Lars Podsiadlowski Sequence data from mitochondrial genes are widely used in phylogenetic studies of metazoa, ranging from population level up to the major lineages of metazoa. Some of these major-level phylogenetic studies led to controversial results, e.g. the aggregation of all bony and cartilaginous fishes in one exclusive clade, a sistergroup relationship between Monotremata and Marsupialia or the separate origin of Collembola and the remaining insects with different sistergroups among Crustacea. Several problems connected with mitochondrial data sets are discussed in the literature. Some aspects of mitochondrial genome evolution are still speculative and in need of further study to set the basics for appropiate evolutionary models in phylogenetic analyses. I will focus here on two kinds of nucleotide frequency bias commonly observed in mitochondrial data sets and illustrate the problems using new mitogenomic data from Chelicerata. 1. Overall (AT/GC)-bias While almost all mitochondrial genomes exhibit an higher content of (A+T) versus (G+C) the proportion of (A+T) between different taxa in a phylogenetic study may vary in a considerable amount (between 55% and 85%). The causes of (A+T) bias are still not well understood. Missing or less effective repair mechanisms after uracil incorporation may be one factor affecting (A+T) content. 2. Strand-specific bias A second kind of nucleotide frequency bias is found between L- and H-strand in mitochondrial genomes: the L-strand is (A+C)-rich, the H-strand is (T+G)-rich. As replication in mitochondria is an assymetric process the causes for this nucleotide bias may lie in an assymetric mutation rate between the H-strand which is singlestranded for comparably long time during the replication process and the L-strand which is double-stranded almost all the time. In some taxa gene translocation events have moved coding regions from one strand to the other. Therefore these genes exhibit reversals in nucleotide biases, leading to long branches and/or homoplastic changes in distantly related taxa which independently exhibit gene translocation events. Both kinds of nucleotide frequency biases may severely affect phylogenetic studies using nucleotide sequences. In protein-coding genes most of the variability is thought to be due to third codon position nucleotides, therefore not affecting amino-acid sequences. But in fact first codon positions and amino-acid sequences as well change in considerable amounts with differences in nucleotide frequency. Careful analyses of nucleotide frequencies are required to prevent misleading phylogenetic statements obtained from mitochondrial datasets. Org. Divers. Evol. 5, Electr. Suppl. 13 (2005)
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