The Mouse Genome Database (MGD): from genes to mice--a community resource for mouse biology

The Mouse Genome Database (MGD) forms the core of the Mouse Genome Informatics (MGI) system (http://www.informatics.jax.org), a model organism database resource for the laboratory mouse. MGD provides essential integration of experimental knowledge for the mouse system with information annotated from both literature and online sources. MGD curates and presents consensus and experimental data representations of genotype (sequence) through phenotype information, including highly detailed reports about genes and gene products. Primary foci of integration are through representations of relationships among genes, sequences and phenotypes. MGD collaborates with other bioinformatics groups to curate a definitive set of information about the laboratory mouse and to build and implement the data and semantic standards that are essential for comparative genome analysis. Recent improvements in MGD discussed here include the enhancement of phenotype resources, the re-development of the International Mouse Strain Resource, IMSR, the update of mammalian orthology datasets and the electronic publication of classic books in mouse genetics.

Does one gene determine whether a C57BL/6J-Y(POS) mouse will develop as a female or as an hermaphrodite?

Two studies were conducted to further our understanding of the inherited condition in mice known as C57BL/6J-Y(POS) (B6-Y(POS)) sex reversal. One study determined what proportion of B6 XY(POS) mice develop as females or hermaphrodites. We found that 75% develop as females and the remainder develop as hermaphrodites regardless of whether the analysis is conducted at 14.5-16 days of embryonic development (based on gonad phenotype) or at weaning (based on the appearance of external genitalia and presence of mammary-associated yellow pigmented hair). We also found that 75 % of the gonads in B6 XY(POS) mice develop as ovaries and the remainder develop as ovotestes; none develop as a testis. We conclude that if any testicular tissue develops, sufficient testosterone is produced to cause at least some masculinization of the external genitalia. The second study tested the hypothesis that development of testicular tissue in B6 XY(POS) mice is due to the presence of a POS-derived gene, whereas B6 homozygosity of this gene guarantees ovarian development. The results did not support the POS gene theory. Therefore, we conclude it is a matter of chance that 75 % of B6 XY(POS) mice develop as females and 25 % develop as hermaphrodites.

C57BL/6J-T-associated sex reversal in mice is caused by reduced expression of a Mus domesticus Sry allele

C57BL/6J-T-associated sex reversal (B6-TAS) in XY mice results in ovarian development and involves (1) hemizygosity for Tas, a gene located in the region of Chromosome 17 deleted in T(hp) and T(Orl), (2) homozygosity for one or more B6-derived autosomal genes, and (3) the presence of the AKR Y chromosome. Here we report results from experiments designed to investigate the Y chromosome component of this sex reversal. Testis development was restored in B6 T(Orl)/+ XY(AKR) mice carrying a Mus musculus Sry transgene. In addition, two functionally different classes of M. domesticus Sry alleles were identified among eight standard and two wild-derived inbred strains. One class, which includes AKR, did not initiate normal testis development in B6 T(Orl)/+ XY mice, whereas the other did. DNA sequence analysis of the Sry ORF and a 5' 800-bp segment divided these inbred strains into the same groups. Finally, we found that Sry is transcribed in B6 T(Orl)/+ XY(AKR) fetal gonads but at a reduced level. These results pinpoint Sry as the Y-linked component of B6-TAS. We hypothesize that the inability of specific M. domesticus Sry alleles to initiate normal testis development in B6 T(Orl)/+ XY(AKR) mice results from a biologically insufficient level of Sry expression, allowing the ovarian development pathway to proceed.

Defects in the cappuccino (cno) gene on mouse chromosome 5 and human 4p cause Hermansky-Pudlak syndrome by an AP-3-independent mechanism

Defects in a triad of organelles (melanosomes, platelet granules, and lysosomes) result in albinism, prolonged bleeding, and lysosome abnormalities in Hermansky-Pudlak syndrome (HPS). Defects in HPS1, a protein of unknown function, and in components of the AP-3 complex cause some, but not all, cases of HPS in humans. There have been 15 inherited models of HPS described in the mouse, underscoring its marked genetic heterogeneity. Here we characterize a new spontaneous mutation in the mouse, cappuccino (cno), that maps to mouse chromosome 5 in a region conserved with human 4p15-p16. Melanosomes of cno/cno mice are immature and dramatically decreased in number in the eye and skin, resulting in severe oculocutaneous albinism. Platelet dense body contents (adenosine triphosphate, serotonin) are markedly deficient, leading to defective aggregation and prolonged bleeding. Lysosomal enzyme concentrations are significantly elevated in the kidney and liver. Genetic, immunofluorescence microscopy, and lysosomal protein trafficking studies indicate that the AP-3 complex is intact in cno/cno mice. It was concluded that the cappuccino gene encodes a product involved in an AP-3-independent mechanism critical to the biogenesis of lysosome-related organelles. (Blood. 2000;96:4227-4235)

Defective mesonephric cell migration is associated with abnormal testis cord development in C57BL/6J XY(Mus domesticus) mice

During the critical period of mouse sex determination, mesenchymal cells migrate from the mesonephros into the adjacent developing testis. This process is thought to initiate cord development and is dependent on Sry. The presence of Sry, however, does not always guarantee normal testis development. For example, transfer of certain Mus domesticus-derived Y chromosomes, i.e., M. domesticus Sry alleles, onto the C57BL/6J (B6) inbred mouse strain results in abnormal testis development. We tested the hypothesis that mesonephric cell migration was impaired in three cases representing a range of aberrant testis development: B6 XY(AKR), B6 XY(POS), and (BXD-21 x B6-Y(POS))F1 XY(POS). In each case, mesonephric cell migration was abnormal. Furthermore, the timing, extent, and position of migrating cells in vitro and cord development in vivo were coincident, supporting the hypothesis that mesonephric cells are critical for cord development. Additional experiments indicated that aberrant testis development results from the inability of Sry(M. domesticus) to initiate normal cell migration, but that downstream signal transduction mechanisms are intact. These experiments provide new insight into the mechanism of C57BL/6J-Y(M. domesticus) sex reversal. We present a model incorporating these findings as they relate to mammalian sex determination.

Sry induces cell proliferation in the mouse gonad

Sry is the only gene on the Y chromosome that is required for testis formation in mammals. One of the earliest morphological changes that occurs as a result of Sry expression is a size increase of the rudimentary XY gonad relative to the XX gonad. Using 5′-bromo-2′-deoxyuridine (BrdU) incorporation to label dividing cells, we found that the size increase corresponds with a dramatic increase in somatic cell proliferation in XY gonads, which is not detected in XX gonads. This male-specific proliferation was observed initially in the cells of the coelomic epithelium and occurred in two distinct stages. During the first stage, proliferation in the XY gonad was observed largely in SF1-positive cells and contributed to the Sertoli cell population. During the second stage, proliferation was observed in SF1-negative cells at and below the coelomic epithelium and did not give rise to Sertoli cells. Both stages of proliferation were dependent on Sry and independent of any other genetic differences between male and female gonads, such as X chromosome dosage or other genes on the Y chromosome. The increase in cell proliferation began less than 24 hours after the onset of Sry expression, before the establishment of male-specific gene expression patterns, and before the appearance of any other known male-specific morphological changes in the XY gonad. Therefore, an increase in cell proliferation in the male coelomic epithelium is the earliest identified effect of Sry expression.

Three new allelic mouse mutations that cause skeletal overgrowth involve the natriuretic peptide receptor C gene (Npr3)

In 1979, a BALB/cJ mouse was identified with an exceptionally long body. This phenotype was found to be caused by a recessive mutation, designated longjohn (lgj), that mapped to the proximal region of chromosome 15. Several years later, a mouse with a similarly elongated body was identified in an outbred stock after chemical mutagenesis with ethylnitrosourea. This phenotype also was caused by a recessive mutation, designated strigosus (stri). The two mutations were found to be allelic. A third allele was identified in a DBA/2J mouse and was designated longjohn-2J (lgj(2J)). Analysis of skeletal preparations of stri/stri mice indicated that the endochondral ossification process was slightly delayed, resulting in an extended proliferation zone. A recent study reported that mice overexpressing brain natriuretic peptide, one of the members of the natriuretic peptide family, exhibit a skeletal-overgrowth syndrome with endochondral ossification defects. The Npr3 gene coding for type C receptor for natriuretic peptides (NPR-C), which is mainly involved in the clearance of the natriuretic peptides, mapped in the vicinity of our mouse mutations and thus was a candidate gene. The present study reports that all three mutations involve the Npr3 gene and provides evidence in vivo that there is a natriuretic-related bone pathway, underscoring the importance of natriuretic peptide clearance by natriuretic peptide type C receptor.

Migration of mesonephric cells into the mammalian gonad depends on Sry

In mammals, the primary step in male sex determination is the initiation of testis development which depends on the expression of the Y-linked testis determining gene, Sry. The mechanisms by which Sry controls this process are unknown. Studies showed that cell migration from the adjacent mesonephros only occurs into XY gonads; however, it was not known whether this effect depended on Sry, another Y-linked gene, or the presence of one versus two X chromosomes. Here we provide genetic proof that Sry is the only Y-linked gene necessary for cell migration into the gonad. Cell migration from the mesonephros into the differentiating gonad is consistently associated with Sty's presence and with testis cord formation, suggesting that cell migration plays a critical role in the initiation of testis cord development. The induction of cell migration represents the earliest signaling pathway yet assigned to Sry.

Mouse homologues of the human AZF candidate gene RBM are expressed in spermatogonia and spermatids, and map to a Y chromosome deletion interval associated with a high incidence of sperm abnormalities

An RNA-binding motif (RBM) gene family has been identified on the human Y chromosome that maps to the same deletion interval as the 'azoospermia factor' (AZF). We have identified the homologous gene family (Rbm) on the mouse Y with a view to investigating the proposal that this gene family plays a role in spermatogenesis. At least 25 and probably >50 copies of Rbm are present on the mouse Y chromosome short arm located between Sry and the centromere. As in the human, a role in spermatogenesis is indicated by a germ cell-specific pattern of expression in the testis, but there are distinct differences in the pattern of expression between the two species. Mice carrying the deletion Yd1, that maps to the proximal Y short arm, are female due to a position effect resulting in non-expression of Sry ; sex-reversing such mice with an Sry transgene produces males with a high incidence of abnormal sperm, making this the third deletion interval on the mouse Y that affects some aspect of spermatogenesis. Most of the copies of Rbm map to this deletion interval, and the Yd1males have markedly reduced Rbm expression, suggesting that RBM deficiency may be responsible for, or contribute to, the abnormal sperm development. In man, deletion of the functional copies of RBM is associated with meiotic arrest rather than sperm anomalies; however, the different effects of deletion are consistent with the differences in expression between the two species.

Sex-determining genes on mouse autosomes identified by linkage analysis of C57BL/6J-YPOS sex reversal

A powerful approach for identifying mammalian primary (gonadal) sex determination genes is the molecular genetic analyses of sex reversal conditions (that is, XX individuals with testicular tissue and XY individuals with ovarian tissue). Here we determined the number and chromosomal location of autosomal and X-linked genes that cause sex reversal in C57BL/6J (B6) mice carrying a Y chromosome of Mus domesticus poschiavinus origin (YPOS). B6 XYPOS mice develop either as females with exclusively ovarian tissue or as true hermaphrodites with ovarian and testicular tissue. In contrast, the YPOS chromosome is fully masculinizing on most other inbred strain backgrounds. B6-YPOS sex reversal appears to result from the incompatibility of the Sry (sex determining region, Y chromosome) allele carried on the YPOS chromosome with B6-derived autosomal or X-linked loci. We found strong evidence for the location of one gene, designated tda1 (testis-determining, autosomal 1), at the distal end of Chromosome (Chr) 4 and a second gene, tda2, in the central region of Chr 2. A third gene, tda3, on Chr 5 is implicated, but the evidence here is not as strong. We suggest that B6 alleles at these loci predispose XYPOS fetuses to ovarian tissue development, but no single locus or combination of loci is necessary and sufficient to cause sex reversal. The TDA proteins may regulate Sry expression or form complexes with the SRY protein to regulate other genes, or the tda genes may be activated or repressed by the SRY protein.

Sex reversal in C57BL/6J-YPOS mice corrected by a Sry transgene

C57BL/6J mice carrying a Mus domesticus poschiavinus Y chromosome (YPOS) develop as females with ovarian tissue or as hermaphrodites with ovarian and testicular tissue. We tested the hypothesis that the Y-linked component of this inherited sex reversal is caused by the M. d. poschiavinus Y-linked testis determining gene (symbolized Tdy or Sry) by examining gonadal development in C57BL/6J XYPOS mice carrying a M. musculus allele of Sry as a transgene. We found that in the presence of the transgene, XYPOS mice developed exclusively testicular tissue. This result indicates that the Sry allele carried on the YPOS chromosome is responsible for development of ovarian tissue in the C57BL/6J inbred strain background. We discuss this finding in light of DNA polymorphisms present in Sry alleles carried by various M. domesticus and M. musculus Y chromosomes. In addition, we present a hypothesis concerning the timing of expression of the testicular and ovarian determining genes in the developing fetal gonad based on the organization of ovarian and testicular tissue in ovotestes.

Hypervariable yellow (Ahvy), a new murine agouti mutation: Ahvy displays the largest variation in coat color phenotypes of all known agouti alleles

A new coat color mutation, which occurred spontaneously in the C3H/HeJ strain, has been identified. The original C3H/HeJ male mouse carrying the mutation was unusual because its coat color appeared mostly yellow, in contrast to the wild-type agouti coat normally exhibited by mice of the C3H/HeJ strain. Genetic crosses showed that the mutant phenotype was inherited as a single autosomal dominant gene. The mutation was backcrossed onto a C57BL/6J background and tested for allelism with the agouti locus. The results showed that the mutation, named hypervariable yellow (Ahvy), is a new allele of the agouti locus. Ahvy is unique because mice carrying the mutation can display a range of coat color from pure yellow to almost pure black. The Ahvy mutation is responsible for the largest range of coat color phenotypes yet identified for any single agouti mutation.

Meiotic abnormalities in hybrid mice of the C57BL/6J x Mus spretus cross suggest a cytogenetic basis for Haldane's rule of hybrid sterility

Light- and electron-microscopic analyses of chromosomal pairing and recombination in F1 and first-backcross generation mice of the C57BL/6J x Mus spretus cross revealed a variety of meiotic irregularities that could contribute to meiocyte loss and infertility. Pachytene anomalies included univalency, partially paired bivalents, homolog-length inequalities, nonhomologous pairing, and associations of asynapsed autosomal segments with the X chromosome. These phenomena were most prevalent in F1 males, which are invariably sterile. Although F1 females were qualitatively fertile, breeding data indicated significant reproductive impairment. Molecular analyses of X-linked and pseudoautosomal loci in sterile and fertile backcross males revealed that the failure of X-Y pairing and recombination is correlated with heterozygosity within the pseudoautosomal regions of the X and Y chromosomes. In addition to impairing fertility, the synaptic disturbances (such as localized asynapsis and nonhomologous pairing) observed in F1 individuals can potentially alter recombinational patterns, thereby contributing to the genetic-map distortion observed with this interspecific cross. Together, the cytogenetic and reproductive data suggest that sex-related differences in the gametogenic process, quantitative differences in the incidence of synaptic irregularities in female and male meiosis, and phenomena associated with the X and Y chromosomes comprise the etiological basis of the sex-biased F1 sterility. The differential gender-related effects of these cytogenetic phenomena may constitute the underlying basis of Haldane's rule in mammals.

Disorganization is a completely dominant gain-of-function mouse mutation causing sporadic developmental defects

Disorganization (Ds) is an exceptional mutation because of its diverse and profound developmental effects. Although other mouse mutations produce similar congenital defects, extreme pleiotropism, random occurrence, developmental independence of multiple defects, and type of anomaly make Ds unique. Examples of developmental defects include cranioschisis, rachischisis, thoracoschisis, exencephaly, hamartomas, and anomalies of appendages, digestive, genital and urinary tracts, sense organs, limbs and girdles, tail and pharynx. No other mutation in the mouse has such broad effects. Ds is therefore an important model for studying not only the genetic control of lineage determination and pattern formation, but also the occurrence of sporadic congenital defects. To characterize the effects of gene dosage, we examined the viability and phenotype of Ds homozygotes and the phenotype of +/+/Ds trisomic fetuses. Occurrence of homozygotes was tested by intercrossing Ds/+ heterozygotes, typing genetic markers that flank Ds, and examining homozygotes for morphological abnormalities. Not only were Ds homozygotes found in their expected frequency, homozygotes were not more severely affected than heterozygotes. Trisomies provide a direct test for determining whether Ds is a gain-of-function mutation. Trisomic fetuses were derived by crossing Ds/Ds homozygous mice to hybrid mice that were heterozygous for two related Robertsonian translocations. Two trisomic fetuses had developmental defects characteristic of Ds mice. Together these results demonstrate that Ds is a completely dominant, gain-of-function mutation.

Telomere-related markers for the pseudoautosomal region of the mouse genome

The pseudoautosomal (PA) region of the mammalian genome is the region of the X and Y chromosomes that shares extensive DNA sequence homology and is of special interest because it may play an essential role during male meiosis. We have identified three telomere-related restriction fragments from the PA region of the mouse genome, using an oligonucleotide probe composed of the mammalian telomere consensus sequence TTAGGG. PA assignment of two C57BL/6J-derived fragments was initially suggested by analysis of DNAs from progeny sired by C57BL/6J males carrying the rearranged Y chromosome, Y*: the hybridization intensity of both fragments was concordant with the sex-chromosome complement of the offspring. Further analysis indicated that both fragments were present in female and male F1, mice regardless of the sex of their C57BL/6J parent--a criterion for autosomal or PA linkage. Both fragments were closely linked to each other and located on the X chromosome distal to amelogenin (Amg)--in agreement with X or PA linkage. Confirmation of the PA derivation of these fragments was accomplished by following their segregation in a cross involving XY* males mated to DBA/2J females. A similar experiment identified a third PA-derived restriction fragment of LT/SvEi origin. Identification of PA-derived telomere-related restriction fragments will enable further genetic analysis of this region of the mouse genome.

The mouse Y* chromosome involves a complex rearrangement, including interstitial positioning of the pseudoautosomal region

Cytological analysis of the mouse Y* chromosome revealed a complex rearrangement involving acquisition of a functional centromere and centromeric heterochromatin and attachment of this chromosomal segment to the distal end of a normal Y* chromosome. This rearrangement positioned the Y* short-arm region at the distal end of the Y* chromosome and the pseudoautosomal region interstitially, just distal to the newly acquired centromere. In addition, the majority of the pseudoautosomal region was inverted. Recombination between the X and the Y* chromosomes generates two new sex chromosomes: (1) a large chromosome comprised of the X chromosome attached at its distal end to all of the Y* chromosome but missing the centromeric region (XY*) and (2) a small chromosome containing the centromeric portion of the Y* chromosome attached to G-band-negative material from the X chromosome (YX). Mice that inherit the XY* chromosome develop as sterile males, whereas mice that inherit the Y*X chromosome develop as fertile females. Recovery of equal numbers of recombinant and nonrecombinant offspring from XY* males supports the hypothesis that recombination between the mammalian X and Y chromosomes is necessary for primary spermatocytes to successfully complete spermatogenesis and form functional sperm.

Inheritance of T-associated sex reversal in mice

We previously identified a primary sex-determining locus, Tas, on mouse Chr 17 that causes ovarian tissue development in C57BL/6J Thp/+ and TOrl/+ individuals if the AKR/JY chromosome is present. We hypothesized that Tas is located within the region of Chr 17 deleted by Thp and TOrl and that C57BL/6J carries a diagnostic Tas allele, based on the observation that ovarian tissue develops in XY mice when Thp is on a C57BL/6J inbred strain background, whereas normal testicular development occurs when Thp is on a C3H/HeSnJ inbred strain background. To test this hypothesis, we mated (C57BL/6J x C3H/HeSnJ)F1 females to C57BL/6J Thp/+ hermaphrodites. As expected, half of the XY Thp/+ offspring developed ovarian and testicular tissue while half developed exclusively testicular tissue. Unexpectedly, the inheritance of selected Chr 17 molecular loci was independent of gonadal development, as half of the male and hermaphroditic offspring inherited C3H/HeSnJ-derived Chr 17 loci and half inherited C57BL/6J-derived Chr 17 loci. We conclude that for ovarian tissue to develop in an XY Thp/+ or XY TOrl/+ individual (1) Tas must be present in a hemizygous state, which is accomplished by heterozygosity for the Thp or TOrl deletions; (2) the AKR/J-derived Y chromosome must be present; and (3) an additional locus involved in primary sex determination must be present in a homozygous C57BL/6J state. This newly identified gene may be one of the previously defined loci, tda-1 or tda-2.

Normal testis determination in the mouse depends on genetic interaction of a locus on chromosome 17 and the Y chromosome

We previously described a locus on chromosome (Chr) 17 of the mouse that is critical for normal testis development. This locus was designated "T-associated sex reversal" (Tas) because it segregated with the dominant brachyury allele hairpin tail (Thp) and caused gonads of C57BL/6J XY, Thp/+ individuals to develop as ovaries or ovotestes rather than as testes. To clarify the inheritance of Tas, we investigated the effects of T-Orleans (TOrl), another brachyury mutation, on gonad development. We found that gonads of C57BL/6J XY, Thp/+ and TOrl/+ mice develop ovarian tissue if the Y chromosome is derived from the AKR/J inbred strain, whereas normal testicular development occurs in the presence of a Y chromosome derived from the C57BL/6J inbred strain. From these observations we conclude that: (1) Tas is located in a region on Chr 17 common to the deletions associated with Thp, and TOrl, and (2) the Y-linked testis determining gene, Tdy, carried by the AKR/J inbred strain differs from that of the C57BL/6J inbred strain. We suggest that in mammals Tdy is not the sole testis determinant because autosomal loci must be genetically compatible with Tdy for normal testicular development.

Male sterility caused by p6H and qk mutations is not corrected in chimeric mice

It is not known if the male sterility caused by the pleiotropic mutations p6H (pink-eyed 6H) and qk (quaking) is intrinsic or extrinsic to spermatogenic cells. This question was addressed by juxtaposing mutant and normal cells in the testes of chimeric mice and determining whether the mutant germ cells could form functional sperm. Twenty-one male chimeras consisting of normal cells and p6H/p6H or qk/qk cells were analyzed. For each, breeding productivity and testicular and sperm morphology were determined. Karyotypes and isozyme analyses were performed to identify the two cellular components of each chimera. All male chimeras that contained p6H/p6H, XY cells were sterile. Although some chimeras with a qk/qk, XY mutant component were fertile, none produced offspring from the homozygous qk component. Spermatids of the sterile chimeras showed abnormalities characteristic of the mutations. We conclude from this study that the presence of normal XY germ and somatic cells in the testis did not rescue the male sterile phenotype of homozygous p6H or qk XY germ cells. Therefore, the action of these mutant genes in causing sperm abnormalities and sterility is autonomous to the germ cells.

Inherited sex reversal in mice: identification of a new primary sex-determining gene

We present evidence that the inbred mouse strain C57BL/6J (B6) differs from Mus domesticus (DOM) at a locus involved in gonad determination and development as well as at the testis-determining locus on the Y chromosome (Tdy). This newly identified gene is named testis-determining, autosomal-1 (tda-1). Although the inheritance of tda-1 suggests it is located on an autosome, its location on the X and Y chromosome in their pairing-recombination region is not excluded. When mice are homozygous for the C57BL/6J-derived tda-1 allele (tda-1B6) and carry the Y chromosome from M. domesticus, they develop as true hermaphrodites or, more rarely, as females. In contrast, when they are homozygous Tda-1DOM/Tda-1DOM or heterozygous Tda-1/DOMtda-1B6 and carry a B6-derived or M. domesticus-derived Y chromosome they develop as normal males. A simple genetic model is presented that integrates proposed steps in gonad determination and the results from genetic experiments reported here. In addition, arguments are presented for the existence of an ovary-determining gene.

Sex reversal in XY mice caused by dominant mutation on chromosome 17

A Y-linked gene(s) is undoubtedly a prerequisite for testicular development in mammals. However, exceptional cases suggest that the mere presence of the Y chromosome or the Y-linked testis-determining gene does not totally control the fate of the bipotential gonad. For example, in the mouse, a major as yet unmapped autosomal locus is necessary for normal testicular development and at least one additional autosomal locus is implicated. We present here a third autosomal sex-determining locus. This dominantly inherited trait, tentatively named T-associated sex reversal (gene symbol Tas), is closely linked to or a part of the T/t complex on chromosome 17 of the mouse. Gonads of chromosomally XY individuals who inherit Tas on the C57BL/6J inbred strain background differentiate as ovaries or ovotestes.

H-Y typing of karyotypically abnormal mice

It has been proposed that the male-specific transplantation antigen H-Y is the trigger for testis formation (Ohno's hypothesis). We have tested this hypothesis by examining the H-Y status of a number of mice with abnormal or aberrant karyotypes relative to their gonad development. This analysis includes the discussion of published results of XX males carrying the Sxr mutation and XO females, and the presentation of unpublished data from XY females carrying the YPOS or YORB chromosome, XY females carrying Thp on chromosome 17, and a series of mice carrying X-Y recombination products derived from the Y* chromosome. The XX Sxr males have testes and are H-Y transplantation antigen positive; XO females have ovaries and are H-Y negative; mice with X-Y reciprocal recombinant chromosomes resulting from an abnormal Y chromosome show concordance between the presence of testes and the presence of the H-Y antigen. All of these findings are in accord with Ohno's hypothesis. In contrast are our findings that three types of C57BL/6J XY females are H-Y positive, two inheriting a Y chromosome from the mouse species Mus domesticus, either YPOS or YORB, and the other a mutation associated with Thp. Genetic analysis of the inheritance of this paradoxical phenotype indicates that normal testis differentiation involves the interaction of two or three genes, and is not simply a function of a Y-linked gene product. We conclude that H-Y is not the sole Y-linked testis determining gene.

A cytogenetic investigation of inherited true hermaphroditism in BALB/cWt mice

A predictably high incidence of true hermaphroditism (3%) among fetal BALB/cWt mice provided the opportunity for studying the cause of inherited hermaphroditism. G-banded liver metaphase chromosomes were analyzed from 20 fetal hermaphrodites and from 15 normal female and male littermate controls. All hermaphrodites and seven males were chromosomal mosaics, i.e. XO/XY or XO/XY/XYY. In each mosaic fetus there were more XO than XYY cells. From these findings we hypothesize that BALB/cWt hermaphrodites arise from non-disjunction and probable loss of the Y-chromosome during mitosis. Results from breeding experiments indicated that a defect in the BALB/cWt Y-chromosome per se accounts for its nondisjunction and loss. A significant relationship was found between the amount of ovarian tissue and the percentage of XO cells in mosaic fetuses. If all chromosomally mosaic (XO/XY and XO/XY/XYY) individuals are considered, ovarian tissue was formed if there was 24% or more XO cells in an individual. If only hermaphroditic individuals are considered, ovarian tissue was formed if there were 44% or more XO cells. Finally, we found that the distribution of ovarian and testicular tissue within an ovotestis of BALB/cWt hermaphrodites was not random: ovarian tissue was situated at both ends or only at the cranial end; in only one case was ovarian tissue situated in the center flanked by testicular tissue. We suggest that such nonrandom distribution of ovarian and testicular tissue argues against the hypothesis that the organization of testicular tissue is dependent only on the presence of H-Y antigen.

Colinearity in the mouse genome: a study of chromosome 2

The cytologic positions (determined by G-banding) of the breakpoints on mouse chromosome 2 of a series of ten reciprocal translocations were compared with their most probable genetic positions on the linkage map, as determined by studies on recombination with known chromosome 2 (= linkage group V) markers. The most probable proximaldistal orders of the genetic and cytologic breakpoints were found to be the same; i.e., the two sets of breakpoints were colinear. However, there was no close correspondence between these two measures of the distance apart of adjacent breakpoints, since some translocation breaks which were well separated in G-band positions seemed close together in terms of the linkage map, and vice versa. This helps to confirm LYON'S conclusion that in certain mouse chromosomes, including No. 2, the distribution of chiasmata is nonrandom.

Assignment of genes to regions of mouse chromosomes

A genetic mapping procedure, called the duplication-deficiency method, is described. This method permits the genetic location of a translocation to be determined within a linkage group without the use of recombination. By utilizing the duplication-deficiency method to define the genetic breakpoints for a series of translocations involving a given chromosome and integrating this information with their cytological breakpoints, obtained by Giemsa banding, a genetic map of the chromosomes is constructed whereby groups of loci are assigned to banded regions. Duplication-deficiency mapping and Giemsa banding analysis of the T(X;7)1Ct and T(7;19)145H translocations together with information from the c25H deletion have permitted mouse chromosome 7 to be divided into six and chromosome 19 into two definable genetic regions.

Hormonal regulation of lobulo-alveolar growth, functional differentiation and regression of whole mouse mammary gland in organ culture

The entire second thoracic mammary glands of 4-week-old BALB/c female mice primed with oestradiol plus progesterone were cultivate in organ culture medium containing the "growth-promoting" hormone combinations: insulin, prolactin, growth hormone, oestradiol, progesterone and aldosterone or insulin, prolactin and aldosterone. Full lobulo-alveolar development was induced after 5-6 days of incubation and could be maintained for 15-16 days in organ culture in medium containing either hormone combination. After the initial 5-6 days in the "growth-promoting" medium, subsequent cultivation of the glands in a medium with the "lactogenic hormones", insulin, prolactin plus cortisol, led to accumulation of "milk-like" secretory material in the ductal and alveolar lumina. Incubation of the lobulo-alveolar gland in medium with insulin alone for 7-9 days resulted in complete regression of the alveoli leaving only a ductal parenchyma. Incubation in insulin, prolactin, growth hormone or insulin plus the steriod hormones for 7-9 days led to considerable alveolar degeneration without a complete regression. The results indicate that both pituitary and steroid hormones are essential for development and maintenance of mammary alveoli; insulin can only sustain the basal ductal structure.