Modell Systems

Boechera

Boechera (Bocher’s rock cress; formerly Arabis) is a perennial member of the Brassicaceae that is distributed throughout North America and Greenland.  The genus is monophyletic, has a basic chromosome number x = 7, and is characterized by diploid sexual, as well as diploid, aneuploid, and polyploid (mostly 2n=3x=21) apomictic forms.  Polyploidy has arisen multiple times in geographically and genetically distinct populations.  Boechera species have also been used for comparative genomic analysis, including partial genome sequencing , genetic map construction and transcriptome sequencing, and the entire genomes of B. stricta and B. divaricarpa are being sequenced (DOE Joint Genome Institute; www.jgi.doe.gov).  

St John‘s wort

St. John’s wort Hypericum perforatum reproduces mainly through facultative aposporic parthenogenesis, and hence a single individual can produce both sexual and apomictic seed, although this can range naturally from nearly obligate apomixis to complete sexuality. Embryo sacs may be either reduced (meiotic) or unreduced (aposporic) and both types of egg cells may be either fertilized (gamic) or develop partenogenetically (agamic), resulting in four possible categories of progeny. Sexual reproduction via outcrossing or selfing is also possible in H. perforatumHypericum perforatum is characterized by a relatively small genome size (1C=0.650 pg), it has a basic chromosome number equal to 8, and its populations are composed mainly of tetraploids (2n=4x=32), although diploid and hexaploid chromosome numbers have also been reported. 

The genus Ranunculus

The Eurasian Ranunculus auricomus polyploid complex is a natural model system for studying hybridization and alterations of reproductive systems. Most of the species are tetraploid, aposporous and pseudogamous. Apospory is usually facultative, and even in apomicts the megaspore mother cells undergo meiosis and form a megaspore tetrad, but megaspores usually degenerate because of developmental problems. Endosperm formation requires fertilization (pseudogamy), and deviations from the optimal maternal : paternal genomic balance in the endosperm are tolerated. The diploid and autotetraploid sexual species of the complex are self-incompatible, which may promote interspecific hybridization, whereas allopolyploid pseudogamous cytotypes are self-compatible, which may promote dispersal of apomicts.  All evidence available supports the hypothesis of a hybrid origin of apomictic taxa.  

Poa pratensis

Kentucky bluegrass (Poa pratensis L.) is an important forage and turf grass adapted to a wide range of environmental conditions in continental Europe and America. More than 500 species have been described in the genus Poa, and chloroplast DNA analyses employed to study the diversity between and within these groups have proposed Eurasia as center of diversity for this genus. Hybridization and polyploidization have played a prominent role in the evolution of this complex genus, 86% taxa of which are polyploids. Poa has a versatile mode of reproduction, ranging from obligate apomixis to complete sexuality. Poa pratensis can reproduce via pseudogamous facultative apospory, sex, and occasional adventitious embryony and vivipary have also been documented. Despite the widespread distribution and cultivation of P. pratensis, little is known also about the evolution of the species still less about genome composition and phylogenetic relationships between different populations.