Abstract
The patterns of chromosomal pairing and chiasma distribution were analyzed in deer mice (Peromyscus sitkensis, P. maniculatus, and offspring of the crosses P. m. borealis X P. m. bairdii and P. maniculatus x P. polionotus) heterozygous for terminally positioned pericentric inversions and for the presence of heterochromatic short arms. Analysis of silver and tungsten stained synaptonemal complexes in zygotene and pachytene nuclei from these individuals revealed an extremely low frequency of inversion loop formation and unpaired autosomal segments resulting from heterozygosity. Furthermore, analysis of diakinesis/metaphase I cells indicated that crossing over had not occurred within the heterosynapsed inverted segments. These observations indicate that the synaptic pattern in Peromyscus apparently precludes the reduced reproductive fitness usually associated with pericentric inversion heterozygosity and averts the meiotic disturbances that could result from heterozygosity for the presence of heterochromatic short arms. It may therefore play a crucial role in facilitating the incorporation of these chromosomal mutations in the karyotypic evolution of deer mice. Analysis of sex chromosome behavior during prophase I revealed a unique pattern of pairing during pachynema. Synapsis was initiated at an interstitial position, and progressed towards the telomeres of the pairing region. In diakinesis/metaphase I cells, the XY bivalent exhibited a distinct interstitial chiasma. The observation of a substantial pairing region and chiasma formation between the X and Y chromosomes of deer mice indicates homology within the segments that synapse during early pachynema. The sex chromosomes and one or two autosomes were present as univalents in some of the pachytene and diakinesis/metaphase I cells obtained from the hybrid individuals. This univalency is probably the consequence of genetic differentiation and decreased homology between the homologous chromosomes of the parental taxa. It appears that primary spermatocytes with univalent chromosomes were eliminated from the spermatogenic lineage prior to the first meiotic division; despite the potential for nondisjunction, all metaphase II cells examined possessed normal haploid complements.
Hale, David Wayne (1988). Cytogenetic analysis of chromosomal heterozygosity in the Peromyscus maniculatus species group. Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -794026.