Producing and detecting paracentric chromosomal inversions in mice

Replicated anterior zeugopod (raz): a polydactylous mouse mutant with lowered Shh signaling in the limb bud

A unique limb phenotype is described in a radiation-induced mutant mouse resulting from an inversion of a proximal segment of chromosome 5. The limb phenotype in the homozygous mutant presents with two anterior skeletal elements in the zeugopod but no posterior bone, hence the name replicated anterior zeugopod, raz. The zeugopod phenotype is accompanied by symmetrical central polydactyly of hand and foot. The chromosomal inversion includes the Shh gene and the regulatory locus, located ∼1 Mb away, within the Lmbr1 gene. In homozygous mutants, the expression of Shh mRNA and Shh protein is severely downregulated to about 20% of wild-type limb buds, but Shh expression appears normal throughout the remainder of the embryo. Correspondingly, Gli3 expression is upregulated and posteriorly expanded in the raz/raz limb bud. We propose that the double anterior zeugopod and symmetrical central polydactyly are due to an increased and uniform concentration of the Gli3 repressor form because of lowered Shh signaling.

Sex-specific differences in meiotic chromosome segregation revealed by dicentric bridge resolution in mice

The meiotic properties of paracentric inversion heterozygotes have been well studied in insects and plants, but not in mammalian species. In essence, a single meiotic recombination event within the inverted region results in the formation of a dicentric chromatid, which usually breaks or is stretched between the two daughter nuclei during the first meiotic anaphase. Here, we provide evidence that this is not the predominant mode of exchange resolution in female mice. In sharp contrast to previous observations in other organisms, we find that attempts to segregate the dicentric chromatid frequently result not in breakage, stretching, or loss, but instead in precocious separation of the sister centromeres of at least one homolog. This often further results in intact segregation of the dicentric into one of the meiotic products, where it can persist into the first few embryonic divisions. These novel observations point to an unusual mechanism for the processing of dicentric chromosomes in mammalian oogenesis. Furthermore, this mechanism is rare or nonexistent in mammalian spermatogenesis. Thus, our results provide additional evidence of sexual dimorphism in mammalian meiotic chromosome behavior; in "stressful" situations, meiotic sister chromatid cohesion is apparently handled differently in males than in females.

Patchy fur, a mouse coat mutation associated with X-Y nondisjunction, maps to the pseudoautosomal boundary region

Patchy fur is a semidominant X-linked mutation in the mouse, resulting in a sparse coat. The Paf mutation also alters the normal segregation of the X and the Y chromosomes during male meiosis by causing nondisjunction at anaphase I. Analysis of 1139 female meioses from an intersubspecific backcross using 15 PCR-based markers localizes Paf to an approximately 0.2-cM interval that includes the pseudoautosomal boundary. The meiotic nondisjunction phenotype may result from a chromosomal rearrangement that includes pseudoautosomal sequences and affects XY pairing.

The hairy ears (Eh) mutation is closely associated with a chromosomal rearrangement in mouse chromosome 15

The mouse mutation hairy ears (Eh) originated in a neutron irradiation experiment at Oak Ridge National Laboratory. Subsequent linkage studies with Eh and other loci on Chr 15 suggested that it is associated with a chromosomal rearrangement that inhibits recombination since it shows tight linkage with several loci occupying the region extending from congenital goiter (cog) distal to caracul (Ca). We report here (1) linkage experiments confirming this effect on recombination and (2) meiotic and mitotic cytological studies that confirm the presence of a chromosomal rearrangement. The data are consistent with the hypothesis of a paracentric inversion in the distal half of Chr 15. The effect of the inversion extends over a minimum of 30 cM, taking into account the genetic data and the cytologically determined chromosomal involvement extending to the region of the telomere.

Use of an inversion to test for induced X-linked lethals in mice

A long X-chromosomal inversion in the mouse was used to suppress crossing-over and thereby to scan 85% of the X-chromosome, or 5% of the genome, for recessive lethal mutations induced by radiation. After a fractionated absorbed dose of 500 + 500 rad X-rays 24 h apart to spermatogonia, 2/536 irradiated and 0/529 control X-chromosomes carried a confirmed lethal. This corresponds to a rate for recessive lethals of 1.9 x 10(-6)/rad/X-chromosome for single exposures (allowing for the enhancing effect of fractionation). This is believed to be the first demonstration of the induction of transmissible X-linked lethals in mammals. The results are consistent with previous findings by other methods and indicate the relatively low rate of induction of lethals and the value of inversions in detecting them.

Frequency distribution of histocompatibility-2 antigenic specificities in the Japanese wild mouse genetically remote from the European subspecies

Fifty-eight Japanese wild mice, Mus musculus molossinus, collected from twenty-eight localities were surveyed for twelve H-2 antigens using the haemagglutination method. Significantly higher frequencies of H-2.3 and H-2.5 specifities and relatively lower frequencies of the other public specificities were observed. This was confirmed by examining four specificities, H-2.3, 5, 13 and 23, in 370 mice. Quantitative absorption of the alloantisera by erythrocytes and spleen lymphocytes of molossinus mouse revealed definite absorption of H-2.3, 5 and 8 antigens, though their antigenic strength was apparently weaker than the controls, B10 congenic mice. Comparative assay of the strength of H-2.5 antigen in either homozygous and heterozygous conditions distinctly eliminated the possibility of gene-dose effect for the reduced strength of the public antigens in molossinus mouse. To explain this, divergence time between molossinus and domesticus was computed based on the allelic frequency data already reported on ten loci in both subspecies. It is roughly 1.5 x 10(6) years, which could allow for the occurrence of considerable genetic changes in many public antigens, except those conserved through selection.

Sperm size abnormalities in homozygous and heterozygous In(5)9Rk mice

Sperm abnormalities were scored in In(5)9Rk homozygotes, heterozygotes and in the inbred strains, C57BL/6J, C3H/HeJ, DBA/2J and JUFaCt, used to produce the geterozygotes. The sperm abnormalities of the inversion heterozygotes were remarkable, about 40% were either double-headed or abnormally large in size. The double-headed sperm had head dimensions similar to those of normal-sized sperm, but the large sperm heads were 1.2 times longer and 1.3 times wider and presumably had twice the usual nuclear volume. This observation suggests that the anaphase bridge formed in inversion heterozygotes resulting from the paracentric inversion may constitute an impediment to cytokinesis and may lead to two genomes being packaged into a single sperm. If this is a general phenomenon, sperm morphology may provide a useful screen for paracentric inversions.

Structural rearrangements of chromosomes in the domestic chicken: experimental production by X-irradiation of spermatozoa

In order to produce chicks heterozygous for structural aberrations of chromosomes, 67 hens were inseminated with semen that had been exposed to 1200 R of X-rays. A sample of 204 chicks was hatched and survived. Among these, 18 (8.9%) contained rearrangements comprising 19 translocations and one pericentric inversion. All 10 males and eight females heterozygous for rearrangements were fertile and transmitted these rearrangements to approximately half their hatched progeny. Each of the major chromosomes of the chicken karyotype, except number 6, was involved in one or more of the translocations. The pericentric inversion was of a segment of chromosome number 2.

Isolation of type-C viruses from the Asian feral mouse Mus musculus molossinus

N-tropic and xenotropic type-C viruses have been isolated from the wild Asian mouse subspecies M. musculus molossinus. By host range, morphologic and some immunologic criteria these viruses appear closely related to the previously studied murine type-C viruses isolated from highly inbred laboratory strains of mice.

Assignment of four linkage groups to chromosomes in Mus musculus and a cytogenetic method for locating their centromeric ends

The mitotic chromosomes in primary cultured cells from mouse embryos of the translocation stocks, T(5;18)26H, T(5;11)7Ca, and T(14;17)264Ca, have been identified by their distinctive fluorescent-banding patterns after staining with quinacrine mustard. In this way, linkage group (LG) V has been assigned to chromosome 2, LGXVIII to chromosome 9, and LGXIV and LGXVII to either of chromosomes 3 or 13. The assignment of LGXI to chromosome 6 has been confirmed. The centromeres of chromosomes 1 (LGXIII), 2 (LGV), 9 (LGXVIII), and 16 (LGIX) have been located at the ends nearest the genes fuzzy (fz), Danforth's short tail (Sd), nervous (nr), and T of their respective linkage groups by a cytogenetic method. The centromere of the X (LGXX) has been tentatively assigned to the end nearest to the scurfy (sf) locus.

Meiotic disjunction in mouse translocations and the determination of centromere position

If heterozygotes for a reciprocal translocation are intercrossed, some of their viable balanced progeny result from the fusion of unbalanced gametes with complementary duplications and deficiencies of the translocated segments. Therefore, if one parent in such an intercross is homozygous for a genetic marker on one of the segments concerned, some homozygous offspring will be produced even if the other parent does not have the marker. The expected frequency of such exceptional offspring among live-born is one-sixth if the marker is on the distal (non-centromeric) side of the point of exchange and single chiasmata normally occur in each interstitial segment. Much lower frequencies are expected if the marker is on the centromeric side, since duplications and deficiencies of proximal segments occur only as a consequence of adjacent-2 disjunction, in which homologous centromeres proceed to the same pole. This is rarer than normal disjunction. Thus, by comparing the frequencies of offspring homozygous for markers on one or other side of the point of exchange, it is possible (i) to determine which marker is in the centromeric segment, (ii) to estimate the frequency of adjacent-2 disjunction, given information on the nature of meiotic configurations in the translocation concerned.

By this method, it is shown that the frequency of adjacent-2 disjunction is similar in heterozygotes for mouse translocations(T5;18)26H, T(13; ?) 70HandT(14;17)264Ca, averaging 13%. Centromeres were located at theSdend of linkage group V (confirming previous findings), thefzend of XIII and the bg end of XIV.