Molecular cytogenetic characterization of the Amazon River dolphin Inia geoffrensis

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.

Centromeric enrichment of LINE-1 retrotransposons and its significance for the chromosome evolution of Phyllostomid bats

Despite their ubiquitous incidence, little is known about the chromosomal distribution of long interspersed elements (LINEs) in mammalian genomes. Phyllostomid bats, characterized by lineages with distinct trends of chromosomal evolution coupled with remarkable ecological and taxonomic diversity, represent good models to understand how these repetitive sequences contribute to the evolution of genome architecture and its link to lineage diversification. To test the hypothesis that LINE-1 sequences were important modifiers of bat genome architecture, we characterized the distribution of LINE-1-derived sequences on genomes of 13 phyllostomid species within a phylogenetic framework. We found massive accumulation of LINE-1 elements in the centromeres of most species: a rare phenomenon on mammalian genomes. We hypothesize that expansion of these elements has occurred early in the radiation of phyllostomids and recurred episodically. LINE-1 expansions on centromeric heterochromatin probably spurred chromosomal change before the radiation of phyllostomids into the extant 11 subfamilies and contributed to the high degree of karyotypic variation observed among different lineages. Understanding centromere architecture in a variety of taxa promises to explain how lineage-specific changes on centromere structure can contribute to karyotypic diversity while not disrupting functional constraints for proper cell division.