Models
Lotus japonicus
The wild perennial legume Lotus japonicus along with Medicago truncatula have become the models for legume research. This is due to a number of important feautures (Handberg and Stougaard 1992);
- Lotus japonicus is a diploid
- it has a relatively short generation time 3 months
- it has a small genome size (400 Mb)
- it has an ability to form nodules with the model microsymbiont Sinorhizobium meliloti
- it can be easily transformed using Agrobacterium-based protocols
As with Medicago, the adoption of Lotus as a model species has resulted in the international use of these species by researchers. This has further developed the scientific resources available to researchers. Resources have been established to facilitate map-based cloning of Lotus genes. Two mapping populations of recombinant inbred lines are available. An integrated genetic map of Lotus has been published (Hayashi et al. 2001). Large datasets of Lotus ESTs are available in public databases (http://www.kazusa.or.jp/en/plant/lotus/EST/ ) and genome sequencing of the whole genome is in progress (Cyranoski D 2001, Sato et al. 2001, Nakamura et al. 2002). Large insert genomic libraries (BAC and TAC) of Lotus ecotypes Gifu and MG20 are available (Sato et al. 2001, Nakamura et al. 2002).
Medicago trunculata
Medicago truncatula is one species of plant that has been adopted as a model plant for the study of legume biology. This species was chosen for a number of key criteria (Cook 1999):
- it has a small diploid genome (450 Mb)
- it is self fertile, has a short generation time and generates an abundance of seed
- it has high levels of natural diversity.
- it can be easily of transformed
- due to its establishment as a model species, a number of genetic tools and resources have been developed by the scientific community including the sequencing of its genome.
Brassica rapa
The Brassica genus belongs to the family Brassicaceae and includes a group of six inter-related species of worldwide economic importance. Brassica rapa (genome AA, 2n = 20) is used both as a vegetable crop (turnip, Chinese cabbage) and as an oil crop (turnip rape). B. nigra (BB, 2n = 16) is grown as a condiment (black mustard) and B. oleracea (CC, 2n = 18) contains numerous vegetable crops with a wide range of different morphologies including cabbage, cauliflower, kale, broccoli and Brussels sprouts. The amphidiploid hybrids B. juncea (AABB, 2n = 36, brown mustard) and B. napus (AACC, 2n = 38, oilseed rape) are important oilseed crops and B. carinata (BBCC, 2n = 34) is grown in Ethiopia as both a vegetable and oil crop (Ethiopian/Abyssinian mustard).
Members of the Brassica genus are closely related to the widely used model plant Arabidopsis thaliana with the divergence of Arabidopsis and Brassicas occurring 14.5 – 20.4 million years ago (Bowers et al. 2003). Due to a triplication event occurring in its ancestry the Brassica genome is much more complex than Arabidopsis. It is therefore often the case that three paralogous genes can be identified in a diploid Brassica genome for each gene in Arabidopsis (and up to six in the amphidiploid Brassicas where genomes are combined).
John E. Bowers, Brad A. Chapman, Junkang Rong and Andrew H. Paterson (2003) Unravelling angiosperm genome evolution by phylogenetic analysis of chromosomal duplication events.Nature 422, 433-438
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