Ribosomal DNA organization patterns within the dinoflagellate genus Alexandrium as revealed by FISH: life cycle and evolutionary implications.
Dinoflagellates are a group of protists whose genome differs from that of other eukaryotes in terms of size (contains up to 250pg per haploid cell), base composition, chromosomal organization, and gene expression. But rDNA gene mapping of the active nucleolus in this unusual eukaryotic genome has not been carried out thus far. Here we used FISH in dinoflagellate species belonging to the genus Alexandrium (genome sizes ranging from 21 to 170 pg of DNA per haploid genome) to localize the sequences encoding the 18S, 5.8S, and 28S rRNA genes. The results can be summarized as follows: 1) Each dinoflagellate cell contains only one active nucleolus, with no hybridization signals outside it. However, the rDNA organization varies among species, from repetitive clusters forming discrete nuclear organizer regions (NORs) in some to specialized "ribosomal chromosomes" in other species. The latter chromosomes, never reported before in other eukaryotes, are mainly formed by rDNA genes and appeared in the species with the highest DNA content. 2) Dinoflagellate chromosomes are first characterized by several eukaryotic features, such as structural differentiation (centromere-like constrictions), size differences (dot chromosomes), and SAT (satellite) chromosomes. 3) NOR patterns prove to be useful in discriminating between cryptic species and life cycle stages in protists.
Aquatic Ecology, Biology Building, Lund University, 22362 Lund, Sweden; Instituto Español de Oceanografía (IEO), Subida a Radio Faro 50, 36390 Vigo (Spain). Electronic address: rosa.figueroa@biol.lu.se.
Rosa Isabel
RI
Figueroa
Universidad de Alcalá (UAH), Dpto de Biomedicina y Biotecnología, 28801 Alcalá de Henares (Spain).
Angeles
A
Cuadrado
Microbial Evolution Research Group, Department of Biosciences, University of Oslo, P.O. Box 1066, Blindern, 0316 Oslo (Norway).
Anke
A
Stüken
Instituto Español de Oceanografía (IEO), Subida a Radio Faro 50, 36390 Vigo (Spain).
Francisco
F
Rodríguez
Instituto Español de Oceanografía (IEO), Subida a Radio Faro 50, 36390 Vigo (Spain).
Santiago
S
Fraga
D001483:Base Sequence; D002875:Chromosomes / Q000235:genetics; D004275:DNA, Ribosomal / Q000235:genetics*; D004141:Dinoflagellida / Q000145:classification / Q000235:genetics* / Q000254:growth & development; D019143:Evolution, Molecular; D016678:Genome; D017404:In Situ Hybridization, Fluorescence; D008018:Life Cycle Stages; D018895:Microsatellite Repeats; D008969:Molecular Sequence Data; D010802:Phylogeny
D004275:DNA, Ribosomal
Chromosomes; Nuclear Organizer Regions (NORs); genome size; life cycle.; protists; ribosomal genes