Repetitive DNA Mapping Reveals Multiple Sex Chromosomes X1X1X2X2/X1X2Y in Pseudotylosurus microps (Günther 1866) (Beloniformes, Teleostei) from the Amazon.

Introduction: Needlefish (Belonidae family) comprises 44 known species distributed worldwide. These species are predominantly marine but include estuarine representatives and 12 freshwater species. Among the recognized species, eight are endemic to South American rivers. Cytogenetic studies of Belonidae are scarce and mostly limited to describing the diploid chromosome number (2n) and karyotypic structure.

Methods: We used classical and molecular cytogenetic markers to karyotypically characterize Pseudotylosurus microps to understand the evolutionary processes of Belonidae species in the Amazon basin.

Results: P. microps exhibited different diploid numbers between males (2n=47, 3m+3sm+41st/a FN=53) and females (2n=48, 4m+4sm+40st/a FN=56). Our study revealed the first case of multiple sex chromosomes in the Belonidae family.

Conclusions: These findings describe a multiple sex chromosome system of the type X1X1X2X2/X1X2Y. The C-banding pattern and 5S rDNA mapping suggest that this system likely resulted from a tandem fusion between a homolog of pair 1 and a homolog of pair 3, producing a large acrocentric Y chromosome. We propose that karyotypic changes due to internal chromosomal rearrangements, as observed in P. microps, can lead to species diversification and, in some cases, the emergence of a heteromorphic and multiple sex chromosome system.