title: The effect of heterochromatin on synapsis of the sex chromosomes of Peromyscus (Rodentia, Cricetidae).
abstract: The pairing behavior of the sex chromosomes in male and female individuals representing seven species of Peromyscus was analyzed by electron microscopy of silver-stained zygotene and pachytene configurations. Six species possess submetacentric or metacentric X chromosomes with heterochromatic short arms. Sex-chromosome pairing in these species is initiated during early pachynema at an interstitial position on the X and Y axes. Homologous synapsis then progresses in a unidirectional fashion towards the telomeres of the X short arm and the corresponding arm of the heterochromatic Y chromosome. The distinctive pattern of synaptic initiation allowed a late-synapsing bivalent in fetal oocytes to be tentatively identified as that of the X chromosomes. In contrast to the other species, Peromyscus megalops possesses an acrocentric X chromosome and a very small Y chromosome. Sex-chromosome pairing in this species is initiated at the proximal telomeric region during late zygonema, and then proceeds interstitially towards the distal end of the Y chromosome. These observations suggest that the presence of X short-arm heterochromatin and corresponding Y heterochromatin interferes with late-zygotene alignment of the pairing initiation sites, thereby delaying XY synaptic initiation until early pachynema. The pairing initiation sites are conserved in the vicinity of the X and Y centromeres in Peromyscus, and consequently the addition of heterochromatin during sex-chromosome evolution essentially displaces these sites to an interstitial position.
authors.affiliation: Department of Biology, Texas A & M University, College Station 77843.
authors.forename: D W
authors.initials: DW
authors.lastname: Hale
authors.forename: M C
authors.initials: MC
authors.lastname: Hedin
authors.forename: S A
authors.initials: SA
authors.lastname: Smith
authors.forename: P D
authors.initials: PD
authors.lastname: Sudman
authors.forename: I F
authors.initials: IF
authors.lastname: Greenbaum
mesh_terms: D000818:Animals; D002871:Chromosome Banding; D005260:Female; D006570:Heterochromatin / Q000502:physiology*; D008297:Male; D010542:Peromyscus / Q000235:genetics*; D012730:Sex Chromosomes / Q000502:physiology* / Q000648:ultrastructure; D013573:Synaptonemal Complex
chemical_list: D006570:Heterochromatin