Neo-X-Linked Chromosome Polymorphism: Cytogenetic Insights from Passalites nemorivagus (Mammalia, Cervidae)

Chromosomal instability plays a significant role in karyotype evolution and speciation in mammalian groups with notable intraspecific chromosomal variation. The Cervidae family, known for its rapid karyotypic evolution due to chromosomal fragility, shows substantial chromosomal diversity, making it a focal point for studies on chromosomal evolution, particularly with respect to conservation and taxonomic classification. The Amazon gray brocket deer (<i>Passalites nemorivagus</i>) exhibits pronounced chromosomal polymorphism, including two distinct sex chromosome systems: the ancestral XX/XY system and a new system due to an X-autosome fusion (neo-X), where males present XY1Y2. This variation is intriguing, especially given that the effects on hybrids have not been previously reported. This study uses bovine whole-chromosome painting (WCP) and BAC probes to document karyotypic variation in <i>P. nemorivagus</i>. A male with the XY system and a heterozygous autosomal Robertsonian fusion was paired with a female with neo-X chromosomes, and the resulting female offspring displayed an X-autosome fusion in heterozygosity. The females in this study, hybrids for the sex system, exhibited estrus, copulated, and both gave birth to offspring. This characterization is the first step in investigating the effects of sex chromosome system variation on hybrid viability and fertility, and provides insights into the reproductive biology of Neotropical deer.