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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.

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