The Spirodela polyrhiza genome reveals insights into its neotenous reduction fast growth and aquatic lifestyle.

The subfamily of the Lemnoideae belongs to a different order than other monocotyledonous species that have been sequenced and comprises aquatic plants that grow rapidly on the water surface. Here we select Spirodela polyrhiza for whole-genome sequencing. We show that Spirodela has a genome with no signs of recent retrotranspositions but signatures of two ancient whole-genome duplications, possibly 95 million years ago (mya), older than those in Arabidopsis and rice. Its genome has only 19,623 predicted protein-coding genes, which is 28% less than the dicotyledonous Arabidopsis thaliana and 50% less than monocotyledonous rice. We propose that at least in part, the neotenous reduction of these aquatic plants is based on readjusted copy numbers of promoters and repressors of the juvenile-to-adult transition. The Spirodela genome, along with its unique biology and physiology, will stimulate new insights into environmental adaptation, ecology, evolution and plant development, and will be instrumental for future bioenergy applications.

Mapping of the nucleolus organizer region on chromosome 4 in Arabidopsis thaliana.

In Arabidopsis thaliana the ribosomal RNA genes (rRNA genes or rDNA) are clustered in tandemly repeated blocks in two nucleolus organizer regions (NORs). Cytogenetic analysis has shown that the NORs are localized on chromosome 2 (NOR 2) and 4 (NOR 4). Recently the map position of NOR 2 was determined using a RFLP which was larger than 100 kb. In the course of a fingerprint analysis of different Arabidopsis ecotypes we have detected four rDNA polymorphisms between the ecotypes Landsberg (La) and Niederzenz (Nd). Mapping of these polymorphisms using established segregating F2 populations reveals that all polymorphisms detected are dominant. Three of them map to the locus on the second chromosome that has been shown to harbour the NOR 2. The fourth polymorphism can be unambiguously assigned to the upper arm of the fourth chromosome. This is the first polymorphism found which originates in the second rDNA cluster of Arabidopsis thaliana. It enables localization of NOR 4 and thus completes the mapping of rDNA genes in the NORs of Arabidopsis.