Une inversion p�ricentrique � l'origine d'un polymorphisme chromosomique non-robertsonien dans une population de mastomys. (Rodentia-Murinae)

Cytogenetical study in Spalax leucodon Nordm. in Moldavia (Rumania)

The karyotype of Spalax leucodon (2n = 56) from Moldavia (Rumania) shows 27 pairs of autosomes: two metacentrics, three submetacentrics, eight suhtelocentrics and 14 acrocentrics. The X chromosomes are the longest subtelocentrics and the Y chromosome is a submetacentric of medium size. In metaphase I we have found a partial homology between the short arm of the X and the Y chromosome.

The population of S. leucodon from Moldavia shows considerable differences of karyotype as compared with other populations of the same species from the south of Europe. This species presents a high degree of chromosomal polymorphism in different populations.

Heterosynapsis and suppression of chiasmata within heterozygous pericentric inversions of the Sitka deer mouse

The patterns of chromosomal pairing and chiasma distribution were analyzed in male Sitka deer mice (Peromyscus sitkensis) polymorphic for terminally positioned pericentric inversions of chromosomes 6 and 7. G- and C-banding of somatic metaphases indicated that the inversions involved 30% and 40% of chromosomes 6 and 7, respectively. Analysis of silver-stained synaptonemal complexes in surface-spread zygotene and pachytene nuclei from heterozygous individuals revealed that inversion loops were not formed. The inverted segments proceeded directly to heterosynapsis without an intervening homosynaptic phase, and the heteromorphic bivalents remained straight-paired throughout pachynema. C-banded pachytene nuclei corroborated the occurrence of heterosynapsis, as the heteromorphic bivalents exhibited nonaligned centromeres. Analysis of diplonema and diakinesis indicated that crossing over had not occurred within the heterosynapsed inverted segments. The observation of chiasma suppression within the inversions indicates that pericentric inversion heterozygosity does not lead to the production of unbalanced gametes. Heterosynapsis of the inverted segments during zygonema and pachynema and the resulting chiasma suppression therefore represent a meiotic mechanism for the maintenance of pericentric inversion polymorphisms in this population of P. sitkensis.

Chromosomal Polymorphism in the White-Throated Sparrow, Zonotrichia albicollis (Gmelin)

In a study of 35 white-throated sparrows five distinct karyotypes were observed. A chromosomal polymorphism is present which involves at least two pairs of macrochromosomes. This species is phenotypically polymorphic with selective breeding occurring between morphs. Phenotype is related to chromosomal constitution, and selective breeding appears to maintain heterozygosity within the population.