Telomeric sequences of Asellus aquaticus (Crust. Isop.)

Molecular Cytogenetic Analysis in Freshwater Prawns of the Genus Macrobrachium (Crustacea: Decapoda: Palaemonidae)

Freshwater prawns of the genus Macrobrachium are one of the important components of circumtropical marine, estuarine, and freshwater environments. They have been extensively exploited for human consumption for many years. More than 250 species reflect the evolutionary success of this highly diversified group, with a complex and challenging taxonomy due to morphological variations and vast geographical distribution. Although genetic approaches have been used to clarify phylogenetic and taxonomic aspects of Macrobrachium species, cytogenetic information is still very scarce and mostly focused on chromosome number and morphology. Here, we present chromosome data for three species from the Neotropical region, M. carcinus,M. acanthurus, and M. amazonicum, and one species from the Oriental region, M. rosenbergii. Using conventional cytogenetic approaches and chromosome mapping of repetitive DNAs by fluorescence in situ hybridization (FISH), we identified numerical diversification of the diploid set, within and between both zoogeographic regions. These included M. acanthurus and M. amazonicum sharing diploid chromosomes of 98, while M. carcinus has 94, and M. rosenbergii has 118 chromosomes. Argentophilic sites are also variable in number, but they occur in a much higher number than 18S rDNA, representing two to 10 sites within the study species. Microsatellites repeat motifs are also abundant in the chromosomes, with a co-localization and uniform distribution along the chromosome arms, but completely absent in the AT-rich centromeric regions. As a whole, our study suggests that the 2n divergence was followed by a considerable rDNA diversification. The abundance of the exceptional amount of microsatellite sequences in the chromosomes also suggests that they are essential components of the Macrobrachium genome and, therefore, maintained as a shared feature by the species, the reason for which is yet unknown.

Comparative cytogenetic analysis of marine Palaemon species reveals a X1X1X2X2/X1X2Y sex chromosome system in Palaemon elegans

Background

The maintenance of species and the promotion of speciation are closely related to chromosomal rearrangements throughout evolution. Decapoda represents the most species-rich order among crustaceans and, despite its ecological and economic importance, little is known about decapod karyology. We aim at cytogenetically characterizing two sympatric prawn species.

Results

Analysis of mitotic metaphases and meiotic diakinesis of the common prawn Palaemon serratus and the rockpool prawn P. elegans, revealed considerable differences between their karyotypes including chromosome numbers and sex determination systems. The cytogenetic data for P. serratus showed a diploid number of 56 and the putative absence of heteromorphic sex chromosomes. However, the diploid chromosome number in P. elegans was 90 for females and 89 for males. The karyotype of the females consisted of the three largest acrocentric pairs and 42 submetacentric and metacentric pairs, while the karyotype of the males comprised a clearly identifiable large metacentric chromosome and two acrocentric pairs as well as the smaller 42 pairs. These results highlight the presence of the X₁X₁X₂X₂/X₁X₂Y multiple sex chromosome system in P. elegans, which constitute the only sexual system for Decapoda reported cytogenetically using modern techniques. The origin of this sex chromosome system is discussed. We hypothesize that the chromosome evolution within the genus could involve several fusion events giving rise to a reduction on the chromosome number in P. serratus. In both species, the major ribosomal genes were located in two chromosome pairs and hybridization signals of the telomeric sequences (TTAGGG)_n were visualized at the telomeres of all chromosomes. C-banding revealed that, when present, constitutive heterochromatin had a predominantly telomeric distribution and no centromeric constitutive heterochromatin was observed.

Conclusions

Although more comparative cytogenetic analyses are needed to clarify our hypotheses, the findings of this work indicate that the prawns of the genus Palaemon represent a promising model among Decapoda representatives to investigate the karyotype evolution and the patterns of sex chromosome differentiation.

Comparative cytogenetics in four species of Palinuridae: B chromosomes, ribosomal genes and telomeric sequences

The evolutionary pathway of Palinuridae (Crustacea, Decapoda) is still controversial, uncertain and unexplored, expecially from a karyological point of view. Here we describe the South African spiny lobster Jasus lalandii karyotype: n and 2n values, heterochromatin distribution, nucleolar organizer region (NOR) location and telomeric repeat structure and location. To compare the genomic and chromosomal organization in Palinuridae we located NORs in Panulirus regius, Palinurus gilchristi and Palinurus mauritanicus: all species showed multiple NORs. In J. lalandii NORs were located on three chromosome pairs, with interindividual polymorphism. In P. regius and in the two Palinurus species NORs were located on two chromosome pairs. In the two last species 45S ribosomal gene loci were also found on B chromosomes. In addition, the nature and location of telomeric repeats were investigated by FISH in J. lalandii, P. gilchristi, P. mauritanicus Palinurus elephas, and P. regius (Palinuridae, Achelata), and in Scyllarus arctus (Scyllaridae, Achelata): all these Achelata species showed the (TTAGG)n pentameric repeats. Furthermore, in J. lalandii these repeats occurred in all the telomeres and in some interstitial chromosomal sites, associated with NORs.

Telomeric DNA localization on dinoflagellate chromosomes: structural and evolutionary implications

Dinoflagellates are eukaryotic microalgae with distinct chromosomes throughout the cell cycle which lack histones and nucleosomes. The molecular organization of these chromosomes is still poorly understood. We have analysed the presence of telomeres in two evolutionarily distant and heterogeneous dinoflagellate species (Prorocentrum micans and Amphidinium carterae) by FISH with a probe containing the Arabidopsis consensus telomeric sequence. Telomere structures were identified at the chromosome ends of both species during interphase and mitosis and were frequently associated with the nuclear envelope. These results identify for the first time telomere structures in dinoflagellate chromosomes, which are formed in the absence of histones. The presence of telomeres supports the linear nature of dinoflagellate chromosomes.

Physical mapping of rDNA genes, (TTAGGG)n telomeric sequence and other karyological features in two earthworms of the family Lumbricidae (Annelida: Oligochaeta)

A cytogenetical study was carried out on the chromosomes and nuclear DNA amounts of the terrestrial earthworms Octodrilus complanatus and Eisenia foetida (Annelida: Oligochaeta: Lumbricidae). Chromosomes were studied using Giemsa staining, banding methods and fluorescent in situ hybridization (FISH) with two repetitive DNA probes [rDNA and (TTAGGG)n]. rDNA FISH and silver staining consistently identified one chromosome pair per spread in both species. The telomeric sequence (TTAGGG)n hybridized with termini of all the chromosomes in both earthworms. Flow cytometry DNA assays showed that O. complanatus and E. foetida had different nuclear DNA contents (2C value=1.72 and=1.40 pg, respectively) but very similar base composition in their genomes.

Cloning and physical mapping of Atlantic salmon (Salmo salar L.) telomeric sequences

We cloned and sequenced DNA containing telomeric sequences from Atlantic salmon (Salmo salar L.). At both low and high stringency conditions, a cloned probe containing (TTAGGG)n sequences binds only at the extreme termini of the chromosomes, not at interstitial sites. Blocks of interstitial telomere-like sequences were expected from the chromosome polymorphism because of Robertsonian translocations that exist in Atlantic salmon. Our results support the hypothesis that telomeric sequences were almost entirely lost during chromosome fusion events.

Localization of a vertebrate telomeric sequence in the chromosomes of two marine worms (phylum Annelida: class polychaeta)

Using the fluorescence in situ hybridization (FISH) technique, the presence of the vertebrate telomeric sequence (TTAGGG)n was found in the chromosomes of two marine polychaetes belonging to two separate orders: one errant, Platynereis dumerilii (family Nereidae), and the other sessile, Pomatoceros lamarckii (family Serpulidae). This sequence was exclusively present at the ends of the chromosomes in both species.