Scientists have studied the chromosomes of three freshwater stingrays from the family Potamotrygonidae which occur in the middle Rio Negro, Amazonas, Brazil.
The team analysed the chromosomes of three species of potamotrygonid ray, including Paratrygon aiereba, Potamotrygon motoro and Potamotrygon orbignyi, and found differences between the two genera, and the two Potamotrygon species.
The authors wrote: "Paratrygon aiereba, Potamotrygon motoro and Potamotrygon orbignyi. Paratrygon aiereba presented 2n = 90 chromosomes and 4M+2SM+10ST+74A.
"Both species of Potamotrygon presented 2n = 66 chromosomes and differed in their chromosomal formulas: P. motoro had 18M+12SM+10ST+26A and P. orbignyi had 22M+10SM+8ST+26A. No sex heteromorphism was detected.
"Paratrygon aiereba presented only four sites on the short arms of two chromosomal pairs, both in terminal regions.
"Potamotrygon motoro presented seven sites, on the long and short arms, all in terminal regions of non-homologous chromosomes; P. orbignyi presented eight sites on the long arms, all in terminal regions, of non-homologous chromosomes."
For more information see the paper: de Valentim FC, Falco JN, Porto JI, Feldberg E (2006) - Chromosomes of three freshwater stingrays (Rajiformes Potamotrygonidae) from the Rio Negro basin, Amazon, Brazil. Genetica. 2006 Sep ; 128(1-3): 33-9