Plant Evolutionary History Research Articles

George Ledyard Stebbins (1906–2000)

George Ledyard Stebbins - the scientist whose work on the genetics and evolutionary history of plants was central to formulating the modern synthesis of evolution.

Plant evolution in disturbed habitats: An introduction

In this era with a loss of biodiversity to an unprecedented extent, one would expect that disturbance, responsible for considerable habitat destruction and fragmentation, causes the extinction of taxa and prevents evolutionary processes. Many recent studies address the spatiallyand ecologically-disturbed landscape infrastructure, which has many negative effects on the evolutionary and ecological perspectives of many species. In several reported cases, population complexes are broken up, ultimately resulting in the breakdown of the metapopulation. This causes losses of genetic variation through drift and inbreeding, which are not compensated for by gene flow. In combination with reduced reproductive success this genetic erosion clearly lowers the population viability of such taxa. This process ultimately may lead to regional or total extinction (ELLSTRAND & ELAM 1993, YOUNG et al. 1996). Human activities do not only destruct habitats, but they lead together with climate change to the spread of species far beyond their natural ranges. Alien species may threaten the indigenous flora, completely change the character of the place they invade, cause diseases and be pest organisms. The problem of biological invasions is growing in severity as global trade and travel accelerate. Habitat disturbance increases, and global climatic change is to be expected. Habitat disturbances and biological invasions create contact zones between conand heterospecific populations which were isolated by distance and/or by the environment. Natural hybridization might be the outcome. This has been viewed as deleterious for biodiversity. Numerous examples have been cited where rarer species are considered "threatened" by consequences of hybridizations with a more common, related taxon, as for instance outbreeding depression and genetic assimilation. On the other hand, hybridization is undoubtedly part of the evolutionary history of plants which is especially documented by molecular markers. It becomes more and more obvious that hybrid zones are focal points of genetic interactions between plant species, and may provide the raw material for adaptive evolution in rapidly changing environments (ARNOLD 1997, RiESEBERG & CARNEY 1998). Many years ago, the principle of hybridization of the habitat was introduced to describe situations where, due to human influence, two ecologicallyand spatially-separated habitats

Sorting by Reversals: Genome Rearrangements in Plant Organelles and Evolutionary History of X Chromosome

Sequence comparison in computational molecular biology is a powerful tool for deriving evolutionary and functional relationships between genes. However, classical alignment algorithms handle only local mutations (i.e., insertions, deletions, and substitutions of nucleotides) and ignore global rearrangements (i.e., inversions and transpositions of long fragments). As a result, applications of sequence alignment in analyzing highly rearranged genomes (i.e., herpes viruses or plant mitochondrial DNA) are very limited and may lead to contradictions in molecular phylogeny studies since different genes give rise to different evolutionary trees. The paper addresses the problem of genome comparison versus classical gene comparison and presents algorithms to analyze rearrangements in genomes evolving by inversions. In the simplest form the problem corresponds to sorting by reversals, that is, sorting of an array using reversals of arbitrary fragments. We describe algorithms to analyze genomes evolving by inversions and discuss applications of these algorithms in molecular evolution.

Occurrence and phylogenetic significance of glucose utilization by charophycean algae: glucose enhancement of growth in Coleochaete orbicularis

Two critical innovations had a profound influence upon the evolutionary history of plants: the nutritionally dependent embryo and apoplastic phloem loading processes. Both depend upon the ability of the plant cell membrane to transport sugars. The evolutionary origin of sugar transport by plants is, therefore, of special phylogenetic importance. Recent evidence suggests that hexoses such as glucose are the main form of sugar transported apoplastically across the placental junction between gametophyte and sporophyte of the moss Polytrichum (Renault et al., 1992, Plant Physiology 100: 1815–1822). There is also considerable evidence that hexose transport may be involved in apoplastic phloem loading in Arabidopsis and other flowering plants. Results of numerous molecular, biochemical, immunofluorescence, and ultrastructural studies indicate that bryophytes and charophycean algae are related to the ancestors of vascular plants. This report demonstrates that the charophyte Coleochaete orbicularis exhibits enhanced growth in the presence of glucose under conditions in which inorganic carbon sources are limiting. Computer image analysis was used to demonstrate that nonaerated cultures of C. orbicularis grown for 7 weeks in an inorganic medium supplemented with 1% glucose produce as much as 13.6 times the biomass of clonal cultures grown for the same length of time in the same medium without glucose. Furthermore, addition of 1% glucose to nonaerated cultures yielded chlorophyll a concentration 20 times higher after 4 weeks growth than cultures grown under the same conditions without added glucose. This and other evidence derived from comparative growth measurements strongly suggest that Coleochaete takes up (or loads) glucose. Experimental results also suggest that hexose uptake by charophytes (such as Coleochaete) that typically grow in low alkalinity waters might provide a supplementary source of organic carbon when dissolved carbon dioxide levels are low. This capability could have served as an evolutionary precursor to hexose import by plant embryos, as well as leptom and phloem loading in bryophytes and vascular plants.

Inferring plant evolutionary history from molecular data

Abstract The rapid development of molecular methods during the 1980s has had a profound effect on the study of plant evolution. Molecular data have accumulated very rapidly, and this abundance of data poses new problems for data analysis. We illustrate three problem areas that arise in plant evolutionary inference. The first problem concerns the use of molecular data to analyse closely related plant species linked through reticulate evolution. The second problem arises from our ability to sample DNA sequences from alleles of a single genetic locus within plant species. We show how cumulative information on selection and random genetic drift can be extracted from such data. The third problem area concerns the limitations of current algorithms for phylogenetic inference when confronted with large sets of DNA sequence data. Based on a consideration of these problem areas, we conclude that: (1) asymmetric transmission of cpDNA markers is useful in resolving the parentage of hybrid plant taxa; (2) simple clustering algorithms can provide useful information on cultivar or variety relationships, despite intervarietal hybridisation, if genetic similarities are averaged over sufficient loci; (3) samples of complete DNA sequence data from plant nuclear genes can provide a new dimension of information on historical effective population sizes and on the mechanisms that generate allelic diversity; (4) analyses of the chloroplast gene rbcL, sampled from across the monocotyledon class, reveal large variation in relative rates of nucleotide substitution—these variations, in turn, have important consequences for phylogeny estimation algorithms; and (5) the combined use of algorithms like parsimony and maximum likelihood may represent a more efficient approach to the phylogenetic analysis of very large (>100) sets of DNA sequences.

Chloroplast gene sequences and the study of plant evolution.

A large body of sequence data has accumulated for the chloroplast-encoded gene ribulose-1,5-biphosphate carboxylase/oxygenase (rbcL) as the result of a cooperative effort involving many laboratories. The data span all seed plants, including most major lineages from the angiosperms, and as such they provide an unprecedented opportunity to study plant evolutionary history. The full analysis of this large data set poses many problems and opportunities for plant evolutionary biologists and for biostatisticians.