One of my main interests when it comes to fish are the freshwater stingrays of south america. A few years ago I had two adult P. motoro rays and a adult of a unidentified species, both from south america. The pair of motoro rays were one female and one male and they had baby stingrays a few times during their time in my posession. The baby i had for the longest was a female. On the outher appearance of stingrays and other Chondrichthyes it is very simple to distinguish between male and females, males have one clasper on each pelvic fin and females have nothing but their normal fins. However what cytological differences underlie the sexes of the freshwater stingrays of south america?
My two old adult P. motoro rays with the male in the background.
This question Vanessa Paes da Cruz and colleages answered in a paper published Neotropical Ichthyology march 31 this year. They collected 30 P. aff. motoro and 34 P. falkneri specimens from different rivers in Brazil.
They found that in both species females have a diploid number of 66 chromosomes while males have 65 chromosomes. This is interesting, males have an uneven number of chromosomes! When they looked further into what chromosomes that differed between males and females it appeared to be the sex chromosomes that differed. They found that the sex chromosome system in these two species consisted of this arrangement: X1X1X2X2/X1X2Y. Possible differences in number of chromosomes have previously been proposed in other Potamotrygon species and the reason for this difference they attribute to a ancestral system similar to the one seen in humans XX/XY where the Y once during evolution have fused to an autosomal chromosome. This then resulted in a new Y chromosome and the X2 chromosome. This later resulting in the "new" X1X1X2X2/X1X2Y system seen in these two species.
One thing that came into my mind while reading this article was if this system also could be the result of a hybridization of two Potamotrygon species a long time ago. Many of the species of the Potamotrygon genus have to my knowledge been known to hybridize in aquarium. However the reason for this arrangement it is certainly interesting!
Vanessa Paes da Cruz, Cristiane Kioko Shimabukuro-Dias, Claudio Oliveira and Fausto Foresti, (2011), Karyotype description and evidence of multiple sex chromosome system X1X1X2X2/X1X2Y in Potamotrygon aff. motoro and P. falkneri (Chondrichthyes: Potamotrygonidae) in the upper Paraná River basin, Brazil., Neotropical Ichthyology, 9(1):201-208