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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 X 1 X 1 X 2 X 2 /X 1 X 2 Y 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 cytogenetic analysis of marine Palaemon species reveals a X1X1X2X2/X1X2Y sex chromosome system in Palaemon elegans