Isolation and Genomic Analysis of the Cetacean Y-chromosome

Abstract

The male-specific mammalian Y-chromosome represents a powerful tool for studying malemediated
gene flow and genome evolution. Here it was possible to identify 7 polymorphic
microsatellites for the first time in an odontocete species, using a combination of cell culture,
cytogenetics and molecular approaches. Initially, the development of an efficient and
repeatable methodology for obtaining a growing lymphocyte culture that facilitated the
isolation of individual chromosomes is described. Flow karyotypic characterization and
isolation of individual chromosomes via flow sorting or microdissection is reported for the killer
whale (Orcinus orca). Microdissected Y-chromosomes from the killer whale and bottlenose
dolphin (Tursiops truncatus) were screened for sequences containing microsatellite motifs. 15
and 10 male-specific microsatellites were identified from the killer whale and bottlenose
dolphin, respectively. Additional microsatellite loci were identified from previously published
fin whale Y-chromosome sequence. 6 markers designed from heterologous sequences
amplified from sperm whales (Physeter macrocephalus), were also screened for variation. All
31 markers were monomorphic in the bottlenose dolphin, only 2 loci showed 2 variants in the
killer whale and 7 were polymorphic in the sperm whale. In addition 162 anonymous regions of
the Y-chromosome, isolated from the delphinid species were used to characterize the
comparative composition of the ‘Y’ relative to the autosomes in these species. Characteristics
are discussed in the context of the genome as a whole, species-specific history and with
reference to the expected patterns of mammalian Y-chromosome evolution.