The spatial relationship of the X and Y chromosomes during meiotic prophase in mouse spermatocytes

The ultrastructure of whole X-Y pairs has been reconstructed by serial sectioning and model building. Seven X-Y pairs were completely reconstructed and the lengths of the cores of the sex chromosomes were measured. These X-Y pairs corresponded to zygonema, early, middle and late pachynema. Special regions of the X-Y pair were reconstructed from thinner sections. — It has been shown that two cores exist in the sex pair during the cited stages, and that their lengths and morphology are rather constant in specific stages. The long core averages 8.9 in length and the short core is 3.5 long. Both cores have a common end region in which a synaptonemal complex is formed from zygonema up to midpachynema. This synaptonemal complex shortens progressively up to mid-pachynema and at late pachynema becomes obliterated. Each core has a free end touching the nuclear membrane. During mid-pachynema an anomalous synaptonemal complex is developed on most of the length of the long core. This complex is asymmetric and disappears at late pachynema. The meaning of the cores and the complexes are discussed, and the existence of a homologous region in the X-Y pair of the mouse is interpreted to be proved.     

Gene flow of unique sequences between Mus musculus domesticus and Mus spretus

Allelic diversity has been examined from a variety of Mus musculus subspecies and Mus spretus strains by sequencing at a 453-bp unique sequence locus. One M. m. domesticus classic inbred strain, C57BL/KsJ, contained a sequence identical to that in the M. spretus wild-derived inbred strain SEG, and other wild M. spretus isolates. Such a result should have been precluded by the expected divergence between the species unless there has been interspecies gene flow. Examination of C57BL/KsJ for M. spretus-specific repetitive sequences shows that it is neither a mis-identified spretus strain nor a domesticus/spretus hybrid. Thus, in addition to the previously reported presence of small amounts of Mus spretus-specific repetitive DNA in M. m. domesticus, there is a detectable flow of unique sequence between the two species. There was also ancestral polymorphism observed among the spretus alleles. The difficulty of distinguishing ancestral polymorphism from horizontal transfer is discussed. Received: 14 May 1999 / Accepted: 5 November 1999     

Fertility in intersubspecific hybrids of laboratory mice (Mus musculus domesticus) and molossinus mice (M. m. molossinus)

We studied the reproductive performance of F1 and F2 hybrids of laboratory mice (C57BL/6, B6 and BALB/c) and molossinus mice (MOM and Mol-A). The F1 x F1 crosses were fully fertile. In the F2 x F2 crosses, the copulation rate was slightly lower and the pregnancy rate was markedly depressed: only 5 out of 18 copulated females (27.8%) became pregnant in the F2 hybrids derived from the reciprocal crosses of B6 x MOM, and in the F2 hybrids from BALB/c x Mol-A crosses, the pregnancy rate was 51.4% (18/35). This low fertility was attributed mainly to the F2 females, because there was a much lower pregnancy rate (56.5%; 26/46) in the (B6 x MOM)F2 female x B6 male crosses compared with the B6 female x (B6 x MOM)F2 male crosses (80.6%; 26/32). On the other hand, the pregnant F2 females were judged to have normal reproductive ability, based on observations of the numbers of corpora lutea, implantations and live fetuses at day 14 of pregnancy. Apparently there is segregation of fertile and sterile females at the F2 generation, but it remains to be determined how the loss of fertility is brought about in the sterile F2 females.     

Natural hybridization between 2 sympatric species of mice, Mus musculus domesticus L. and Mus spretus Lataste

Using protein loci and DNA markers, we show by a multilocus genetic analysis that certain populations of the two sympatric mouse species Mus musculus domesticus and Mus spretus show clear signs of partial introgression. Given the sterility of F1 males and the known partial genetic incompatibilities between the genomes of the two species, our finding does not invalidate the biological species complex, but allows to think that very limited genetic exchanges remain possible even long after the divergence of taxa. This may have some consequences on the dynamics of certain kinds of invasive or advantageous DNAs like transposable elements or pathogen resistance genes.     

Cytological and molecular characterization of centromeres in Mus domesticus and Mus spretus

We have applied EM in situ hybridization (EMISH) and pulsed field gel electrophoresis (PFGE) to samples from diploid primary cell cultures and an established cell line to examine in detail the relative organization of the major and minor satellite DNAs and telomere sequences in the genomes of Mus domesticus and Mus spretus. EMISH localizes the Mus domesticus minor satellite to a single site at the centromere-proximal end of each chromosome. Double label hybridizations with both minor satellite and telomere probes show that they are in close proximity and possibly are linked. In fact, PFGE of M. domesticus DNA digested with Sal I and Sfi I reveals the presence of fragments which hybridize to both probes and is consistent with the physical linkage of these two sequences. The M. domesticus minor satellite is the more abundant satellite in Mus spretus. Its distribution in M. spretus is characterized by diffuse labeling with no obvious concentration near chromosome ends. In addition to this repeat the M. spretus genome contains a small amount of DNA that hybridizes to a M. domesticus major satellite probe. Unlike the M. domesticus minor satellite, it is not telomere proximal but is confined to a domain at the border of the centromere and the long arm. Thus, although both species possess all three sequences, except for the telomeres, their distribution relative to one another is not conserved. Based on the results presented, we propose preliminary molecular maps of the centromere regions of Mus domesticus and Mus spretus.     

Genome differentiation in Magonoliaceae as revealed from meiotic pairing in interspecific and intergeneric hybrids

The cross compatibility within and between Yulania Spach and Michelia L.(Magnoliaceae) is relatively good and various such hybrids,obtained by conventional artificial hybridization,are available.The aim of the present study was to determine the extent of genome differentiation between the species involved in these crosses through the observation of chromosome pairing during meiosis in pollen mother cells (PMCs) of the hybrids.Chromosome pairing behavior was studied in five species (2n =38) and two interspecific hybrids of Michelia,eight species (2n =38,76 and 114) and 10 interspecific hybrids of Yulania,and three intergeneric hybrids between Michelia and Yulania.The results showed that chromosome pairing was normal with bivalent formation in diploid parental species and in interspecific hybrids.In addition to bivalents,multivalents were encountered in polyploid parental species and polyploid interspecific hybrids.In the intergeneric hybrids between a tetraploid Yulania and two diploid Michelia,19 chromosomes,most likely originating from Michelia,were unable to synapse from zygotene to metaphase I.Meiotic chromosome pairing indicated a high degree of homology between species within Michelia and Yulania and less homology between the genomes of these two genera.The differentiation of morphological characters and the distinctness of natural distribution also support the conclusion that these two genera are likely independent monophyletic groups.This suggests that the two genera were split at early evolution of Magnoliaceae and the overlapping characteristics in external morphology and internal structures of the two genera may be the result of parallel evolution or ancient common ancestry.     

Systematic identification of LINE-1 repetitive DNA sequence differences having species specificity between Mus spretus and Mus domesticus

LINE-1 is a family of repetitive DNA sequences interspersed among mammalian genes. In the mouse haploid genome there are about 100,000 LINE-1 copies. We asked if the subspecies Mus spretus and Mus domesticus have developed species-specific LINE-1 subfamilies. Sequences from 14 M. spretus LINE-1 elements were obtained and compared to M. domesticus LINE-1 sequences. Using a molecular phylogenetic tree we identified several differences shared among a subset of young repeats in one or the other species as candidates for species-specific LINE-1 variants. Species specificity was tested using oligonucleotide probes complementary to each putative species-specific variant. When hybridized to genomic DNAs, single-variant probes detected an expanded number of elements in the expected mouse. In the other species these probes detected a smaller number of matches consistent with the average rate of random divergence among LINE-1 elements. It was further found that the combination of two species-specific sequence differences in the same probe reduced the detection background in the wrong species below our detection limit.     

On the nature and extent of XY pairing at meiotic prophase in man

Evidence is presented that pairing between the human X and Y chromosomes could be more extensive at early pachytene than has previously been supposed and could involve even the entire euchromatic portion of the Y chromosome. Following desynapsis over the major part of the X and Y axes, a small paired segment of Xp and Yp remains into late pachytene. Association between the distal tips of Xq and Yq can also be observed in about one half of the spermatocytes examined. A hypothesis linking meiotic pairing to early replicating sites along the chromosomes is proposed.     

Food neophobia in wild and laboratory mice (Mus musculus domesticus)

In a conditioned taste aversion procedure we were specifically interested in the topic of food neophobia. Wild and laboratory mice were individually presented with a novel drink (0.1 % saccharin solution). Compared with the daily water consumption, the intake of this was lower. This decrease was greater:(1) in wild than in tame populations ; (2) in random-bred (Swiss-albinos) than in inbred (C57 B1/6, BALB/c) strains ; (3) in F₁-hybrids (either wild x tame or inbred x inbred) than in the parental strains.These results are discussed:(1) in terms of a selective pressure linked to man's fight against rodents, leading to increased neophobia in wild mice ; and (2) by stressing the heterosis an inbreeding depression effects, which suggest that food neophobia is a component of Darwinian fitness.     

Genetic variation and correlations between genotype and locomotor physiology in outbred laboratory house mice (Mus domesticus).

Laboratory strains of house mice (Mus domesticus) are increasingly used as model organisms in evolutionary physiology, so information on levels of genetic variation is important. For example, are levels of genetic variation comparable to those found in populations of wild house mice? We studied allozymes to estimate genetic variation in outbred Hsd:ICR mice, which have been used in several studies with evolutionary emphasis. The physiological significance of allozyme variation remains obscure. Several workers have reported relationships between multi-locus heterozygosity and metabolic traits, but endotherms have not been studied. Therefore, we also measured mice for basal metabolic rate (BMR), maximal oxygen consumption during forced treadmill exercise (VO2max), and 12 other traits related to locomotor physiology, before genotyping them for 10 allozyme loci. Four of these loci were polymorphic, all were in Hardy-Weinberg equilibrium, and inbreeding coefficients were not significantly different from zero. Average heterozygosities were 11%, similar to values reported for wild populations of house mice. Fourteen percent of the associations between single-locus genotype and physiological traits were statistically significant. Multi-locus heterozygosity was not significantly related to VO2max, but was positively correlated with BMR, a result opposite to the negative correlation between standard metabolic rate and heterozygosity reported in many ectotherms. Therefore, the proposed mechanisms for the effect of multi-locus heterozygosity on metabolic rate in ectotherms may not apply to endotherms.     

Superovulation and in vitro oocyte maturation in three species of mice (Mus musculus, Mus spretus and Mus spicilegus)

Mouse oocytes can be obtained via superovulation or using in vitro maturation although several factors, including genetic background, may affect response. Our previous studies have identified various mouse species as models to understand the role of sexual selection on the evolution of sperm traits and function. In order to do comparative studies of sperm-oocyte interaction, we sought reliable methods for oocyte superovulation and in vitro maturation in mature females of three mouse species (genus Mus). When 5IU pregnant mare's serum gonadotrophin (PMSG) and 5IU human chorionic gonadotrophin (hCG) were injected 48h apart, and oocytes collected 14h post-hCG, good responses were obtained in Mus musculus (18+/-1.3oocytes/female; mean+/-S.E.M.) and Mus spretus (12+/-0.8), but no ovulation was seen in Mus spicilegus. Changes in PMSG or hCG doses, or longer post-hCG intervals, did not improve results. Use of PMSG/luteinizing hormone (LH) resulted in good responses in M. musculus (19+/-1.2) and M. spretus (12+/-1.1) but not in M. spicilegus (5+/-0.9) with ovulation not increasing with higher LH doses. Follicular puncture 48h after PMSG followed by in vitro maturation led to a high oocyte yield in the three species (M. musculus, 23+/-0.9; M. spretus, 17+/-1.1; M. spicilegus, 10+/-0.9) with a consistently high maturation rates. In vitro fertilization of both superovulated and in vitro matured oocytes resulted in a high proportion of fertilization (range: 83-87%) in the three species. Thus, in vitro maturation led to high yields in all three species. These results will allow future studies on gamete interaction in these closely related species and the role of sexual selection in gamete compatibility.     

Segregation distortion of X-linked marker genes in interspecific crosses between Mus musculus and M. spretus

An interspecific cross was made between females of the C3H/HeHa.Pgk-1 a inbred laboratory strain of Mus musculus and males of the separate species Mus spretus. The F1 males are sterile but the F1 females are fertile and they were backcrossed to both C3H and spretus males. Evidence is presented from the segregation of X-linked marker genes that the interspecific F1 female has a genetically deleterious effect on the C3H X chromosome that is expressed as a male-lethal effect with the spretus Y chromosome but not with the musculus Y chromosome of C3H.     

Analysis of sex-chromosome aneuploidy in interspecific backcross progeny between the laboratory mouse strain C57BL/6 and Mus spretus.

Sex-chromosomal aneuploidy was identified in four female progeny of 200 interspecific backcrosses between laboratory mice (C57BL/6Ros) and Mus spretus. The progeny included two 39,XO monosomy mice resulting from a backcross with M. spretus, as well as a 41,XXX trisomic mouse and a 40,XX/41,XXX mosaic mouse resulting from two separate backcrosses with C57BL/6 mice. The parental origin and meiotic stage of the aneuploidies was determined for each of the mice using a series of markers that identified allelic differences in the parental X-chromosome genes present in the hybrid female. Two of the probes identified differences in repeated elements between the M. spretus and laboratory mouse X chromosomes, whereas the remaining sites involved restriction fragment length differences of single-copy genes detectable by Southern analysis. These markers indicated that the aneuploidies were most likely of maternal origin and that the trisomy resulted from a nondisjunction at the second meiotic division. In contrast, the mosaic female could have originated either from a trisomic embryo that had lost a single X in a portion of its cells or from a mitotic nondisjunction during early embryogenesis that resulted in XXX and XO daughter cells, with subsequent loss of the XO cells.     

Activities of X-linked enzymes in spermatocytes of mice rendered sterile by chromosomal alterations

Spermatocytes in the late prophase of first meiotic division isolated from sterile males retain higher activities for three X-linked enzyme⁵, phosphoglycerate kinase (PGK)-1, glucose-6-phosphate dehydrogenase (G6PD), and hypoxanthine-guanine phosphoribosyl transferase (HGPRT) than those of fertile males. The sterilty of the male is presumed to be owing to the rearrangement of X-chromosome material and the possibility of abnormal meiotic X-chromosome inactivation is discussed.     

Comparison of linkage maps of mouse chromosome 12 derived from laboratory strain intraspecific and Mus spretus interspecific backcrosses

Progeny from an interspecific backcross between laboratory mice and Mus spretus were typed for inheritance of eight genetic markers on chromosome 12. Marker order determined by segregation analyses of 115 meiotic events was in good agreement with that determined previously using intraspecific laboratory strain backcrosses. Two additional markers, D12Nyu5 and Lamb-1, previously not ordered, were located in the middle of the interval between D12Nyu12 and D12Nyu1. Marker spacing was reduced in the interspecific cross relative to that observed in intraspecific crosses. Furthermore, the interspecific cross was characterized by marked deviation from 1:1 segregation in the recombinant chromosomes and very strong positive interference. These data suggest that comparisons of different mouse crosses may facilitate the understanding of underlying mechanisms that govern recombination events in complex genomes.     

Oxytocin inhibits infanticide in female house mice (Mus domesticus)

Between 60 and 90% of female house mice spontaneously kill unrelated young. A previous report indicated that subcutaneous administration of oxytocin significantly reduced the frequency of infanticide by virgin and pregnant females. However, in this study a distinction could not be made between an action of oxytocin on the CNS versus a secondary effect such as an enhanced release of prolactin by oxytocin. In the current experiment, oxytocin administered intracerebroventricularly was equally as effective at inhibiting infanticide as sc oxytocin. There was no difference in the effectiveness of oxytocin between groups of infanticidal females that were gonadally intact, ovariectomized, or estrogen treated. Pretreatment of infanticidal females with the prolactin inhibitors, bromocriptine and cysteamine, was also without effect on the ability of oxytocin to inhibit infanticide. Last, prolactin-inhibiting drugs had no significant effect on spontaneous parenting behavior by female mice. These data suggest that oxytocin acts directly on the CNS to alter behavior toward pups and that prolactin may not play a role in the maternal behavior of the house mouse.     

Absence of satellite DNA synthesis during meiotic prophase in mouse and human spermatocytes

Mouse spermatocytes were labelled in situ with ³H-thymidine at successive stages of meiosis. Isolated mouse as well as human spermatocytes were similarly labelled under in vitro conditions. DNA synthesis was followed either by tracking radioactivities in Cs₂SO₄ gradients or by measuring reassociation kinetics. Mouse satellite DNA and the 3 satellites of human DNA are labelled during S-phase but not during pachytene. In the mouse genome, there is a preferential labelling of regions containing foldbacks (human spermatocytes were not analyzed in this respect). The absence of detectable pachytene synthesis in satellite DNA is consistent with genetic evidence on the absence of crossing-over in constitutive heterochromatin.     

Water and sodium balances and metabolic physiology of house mice (Mus domesticus) and short-tailed mice (Leggadina lakedownensis) under laboratory conditions

A laboratory study investigated the metabolic physiology, and response to variable periods of water and sodium supply, of two arid-zone rodents, the house mouse (Mus domesticus) and the Lakeland Downs short-tailed mouse (Leggadina lakedownensis) under controlled conditions. Fractional water fluxes for M. domesticus (24 ± 0.8%) were significantly higher than those of L. lakedownensis (17 ± 0.7%) when provided with food ad libitum. In addition, the amount of water produced by M. domesticus and by L. lakedownensis from metabolic processes (1.3 ± 0.4 ml · day⁻¹ and 1.2 ± 0.4 ml · day⁻¹, respectively) was insufficient to provide them with their minimum water requirement (1.4 ± 0.2 ml · day⁻¹ and 2.0 ± 0.3 ml · day⁻¹, respectively). For both species of rodent, evaporative water loss was lowest at 25 °C, but remained significantly higher in M. domesticus (1.1 ± 0.1 mg H₂O · g^−0.122 · h⁻¹) than in L. lakedownensis (0.6 ± 0.1 mg H₂O · g^−0.122 · h⁻¹). When deprived of drinking water, mice of both species initially lost body mass, but regained it within 18 days following an increase in the amount of seed consumed. Both species were capable of drinking water of variable saline concentrations up to 1 mol · l⁻¹, and compensated for the increased sodium in the water by excreting more urine to remove the sodium. Basal metabolic rate was significantly higher in M. domesticus (3.3 ± 0.2 mg O₂ · g^−0.75 · h⁻¹) than in L. lakedownensis (2.5 ± 0.1 mg O₂ · g^−0.75 · h⁻¹). The study provides good evidence that water flux differences between M. domesticus and L. lakedownensis in the field are due to a requirement for more water in M. domesticus to meet their physiological and metabolic demands. Sodium fluxes were lower than those observed in free-ranging mice, whose relatively high sodium fluxes may reflect sodium associated with available food. Accepted: 16 August 1999