Karyotype evolution of suliformes and description of a ♂Z1Z1Z2Z2/♀Z1Z2W multiple sex chromosome system in boobies (Sula spp.).

Abstract

Our comprehension of avian karyotypes still needs to be improved, especially for Suliform birds. To enhance understanding of chromosomal evolution in this order, we conducted conventional and molecular cytogenetic analysis in five species, named Sula dactylatra, Sula leucogaster, Sula sula (Sulidae), Fregata magnificens (Fregatidae), and Nannopterum brasilianum (Phalacrocoracidae). The diploid chromosome number for S. dactylatra and S. leucogaster was established as 2n = 76 in males, and 2n = 75 in females, but S. sula displayed a karyotype of 2n = 76 chromosomes in males. The disparity in diploid chromosome numbers between male and female Sula is due to a multiple sex chromosome system of the Z₁Z₁Z₂Z₂/Z₁Z₂W type. We propose that the emergence of this multiple-sex chromosome system resulted from a Robertsonian translocation involving the W chromosome and the smallest microchromosome. Fregata magnificens exhibited a diploid number 76 (2n = 76), while N. brasilianum displayed a diploid number of 74 (2n = 74) in both sexes. The ribosomal cluster was located in one microchromosome pair in S. dactylatra, S. leucogaster, S. sula, and F. magnificens and in four pairs in N. brasilianum. Our findings provide evidence of a conserved multiple-sex chromosome system within the Sula genus, shedding light on the high karyotype diversity in Suliformes.