Molecular Systematics

Alessandra Selbach Schnadelbach & Cássio van den Berg

Systematics is the branch of science that studies this biological diversity and organizes this information into a classification. The first classification appeared in Ancient Greece, notably with Aristotle. In 1758, Linnaeus created the hierarchical system of classification still employed in taxonomy. With the advent of Darwin’s theory of evolution in the 19th century, biological diversity began being explained as the result of the divergence of species from a common ancestor. The phylogenetic systematics, established by Hennig between the decades of 1950 and 1960, incorporated this paradigm, changing the principles of biological classification. Taxonomy started to reflect ancestry, and the systems of classification to accept only monophyletic taxa (groups of organisms with a common and exclusive ancestor) .

Morphology was largely used to reconstruct the evolutionary history of organisms. Morphological characters have been useful to the recognition of large groups, as well as to the description of families, genera and species, constituting the basis of any classification. However, with the advent of molecular biology, countless techniques to access genetic material were developed, among them the immunological assays, the electrophoresis of enzymes and proteins, the hybridization of DNA and the DNA polymerase chain reaction (PCR). More recently, the sequencing of specific regions of DNA has opened new opportunity to access DNA information, allowing comparisons between individuals representing different taxonomic levels.

The term molecular phylogeny refers to ancestral relationships inferred from molecular data. Molecular characters are extremely useful for phylogenetic inferences, composing an almost unlimited set of intrinsic data. Since then, molecular characters along with morphological ones have subsidized the systematicians in refining the classification of the living beings.

The first broad angiosperm (flowering plants) phylogenetic study based on DNA sequences became a landmark for plant systematics. Sequences of rbcL representing hundreds of species were compared and the taxonomic delimitation of many angiosperm groups was modified. Since then, several studies have sought to amplify and deepen the understanding of phylogenetic relationships in angiosperms at different levels, significantly refining their taxonomy. The Laboratory of Molecular Systematics of Plants (LAMOL, see also Chapter 22) at UEFS is inserted in this context.



Molecular Phylogenetic Studies of Plants at LAMOL

Molecular phylogeny studies developed at LAMOL involve the following stages:

1) collection of material for DNA extraction (leaves or flowers, preferably). Every sample contains a voucher material that is housed in the herbarium, guaranteeing the reliability of the identifications; 2) extraction of total DNA; 3) evaluation of quality and quantity of the DNA extracted; 4) amplification of specific regions of the DNA through PCR reaction; 5) automated sequencing of the amplified regions through PCR and; 6) comparative analyses of sequences to reconstruct phylogenies with methods of inference, such as maximum parsimony, maximum likelihood and Bayesian analyses.

Studies on plant molecular phylogeny are still rare in Brazil, especially with plant groups from the Semi-arid. The need for sophisticated equipment, the high cost and the difficulties to acquire reagents and equipments, beside the lack of specialists in several angiosperm families are the main obstacles in the implementation of molecular phylogeny studies. The molecular phylogeny projects developed at UEFS, in addition to refining the understanding of the phylogenetic relationships of plants that occur in Brazilian Semi-arid, also contribute to the formation of trained staff in many angiosperm families. Nowadays, there are several projects on molecular phylogeny of plants being carried out at LAMOL (see list), especially species-rich groups in the Semi-arid, such as orchids and legumes.



The DNA Bank

LAMOL includes also a DNA bank. Nowadays, this bank keeps around 3,000 samples representing more than 1,000 species, 500 genera and 130 families. The most representative families in the DNA bank are Leguminosae, Orchidaceae, Eriocaulaceae, Boraginaceae and Poaceae. These samples comprise even species not yet described, such as a new species of bamboo endemic to the Atlantic Rainforest of southern Bahia (Raddia sp.), taxa endemic to the semi-arid region, such as Calliandra leptopoda and C. macrocalyx var. aucta, and even endemic genera such as the recently published Adamantinia, an orchid endemic to the Chapada Diamantina, and a genus of Leguminosae endemic to the caatinga.

Percentage of total samples per family in LAMOL’s DNA bank.

The DNA bank constitutes a rich source of material for plant systematics. It allows the improvement of the graduate programs (Chapters 23 and 26) as well as represents a treasure for the community as a whole, since it guarantees the preservation of the genetic patrimony of several rare plants, many of them endangered species.

Chase, M.W. et al. 1993. Phylogenetics of seed plants: an analysis of nucleotide sequences from the plastid gene rbcL. Annals of the Missouri Botanical Garden 80: 528-580.

NT – rbcL is a plastid gene that encodes the larger subunit of the ribulose 1,5-bisphosphate carboxilase-oxigenase enzyme.

The ribosomal nuclear region ITS (Internal Transcribed Spacers) and the plastid regions, gene matK and trnL-trnF spacer are examples.