A mouse Y chromosome pseudogene is related to human ubiquitin activating enzyme E1

Natural exceptions to normal gonad development in mammals

Gonads are the only organs with 2 possible developmental pathways, testis or ovary. A consequence of this unique feature is that mutations in genes controlling gonad development give rise not only to gonadal malformation or dysfunction but also to frequent cases of sex reversal, including XY females, XX males and intersexes. Most of our current knowledge on mammalian sex determination, the genetic process by which the gonadal primordia are committed to differentiate as either testes or ovaries, has derived mainly from the study of sex-reversed mice obtained by direct genetic manipulation. However, there are also numerous cases of natural exceptions to normal gonad development which have been described in a variety of mammals, including both domestic and wild species. Here, we review the most relevant cases of: (1) natural, non-induced sex reversal and intersexuality described in laboratory rodents, including Sxr and B6-Y(DOM) mice; (2) sex reversal in domestic animals, including freemartinism in bovids and pigs, XX sex reversal in pigs, goats and dogs, XY sex reversal in the horse, and sex chromosome chimerism and sex reversal in the cat, and (3) sex reversal in wild mammals, including the generalised true hermaphroditism described in talpid moles, XY sex reversal in Akodon, Microtus and Dicrostonyx species, males lacking a Y chromosome and SRY in Ellobius lutescens, the X* chromosome of Myopus schisticolor, and sex chromosome mosaicism and X0 females in Microtus oregoni. These studies are necessary to elucidate particular aspects of mammalian gonad development in some instances and to understand how the genetic mechanisms controlling gonad development have evolved.

A candidate subspecies discrimination system involving a vomeronasal receptor gene with different alleles fixed in M. m. domesticus and M. m. musculus

Assortative mating, a potentially efficient prezygotic reproductive barrier, may prevent loss of genetic potential by avoiding the production of unfit hybrids (i.e., because of hybrid infertility or hybrid breakdown) that occur at regions of secondary contact between incipient species. In the case of the mouse hybrid zone, where two subspecies of Mus musculus (M. m. domesticus and M. m. musculus) meet and exchange genes to a limited extent, assortative mating requires a means of subspecies recognition. We based the work reported here on the hypothesis that, if there is a pheromone sufficiently diverged between M. m. domesticus and M. m. musculus to mediate subspecies recognition, then that process must also require a specific receptor(s), also sufficiently diverged between the subspecies, to receive the signal and elicit an assortative mating response. We studied the mouse V1R genes, which encode a large family of receptors in the vomeronasal organ (VNO), by screening Perlegen SNP data and identified one, Vmn1r67, with 24 fixed SNP differences most of which (15/24) are nonsynonymous nucleotide substitutions between M. m. domesticus and M. m. musculus. We observed substantial linkage disequilibrium (LD) between Vmn1r67 and Abpa27, a mouse salivary androgen-binding protein gene that encodes a proteinaceous pheromone (ABP) capable of mediating assortative mating, perhaps in conjunction with its bound small lipophilic ligand. The LD we observed is likely a case of association rather than residual physical linkage from a very recent selective sweep, because an intervening gene, Vmn1r71, shows significant intra(sub)specific polymorphism but no inter(sub)specific divergence in its nucleotide sequence. We discuss alternative explanations of these observations, for example that Abpa27 and Vmn1r67 are coevolving as signal and receptor to reinforce subspecies hybridization barriers or that the unusually divergent Vmn1r67 allele was not a product of fast positive selection, but was derived from an introgressed allele, possibly from Mus spretus.

Conjugation of the 15-kDa interferon-induced ubiquitin homolog is distinct from that of ubiquitin

The biological effect of type 1 interferons is proposed to arise in part from the conjugation of ubiquitin cross-reactive protein (UCRP), the ISG15 gene product, to intracellular target proteins in a process analogous to that of its sequence homolog ubiquitin, a highly conserved 8.6-kDa polypeptide whose ligation marks proteins for degradation via the 26 S proteasome. Inclusion of CoCl2 during the purification of recombinant UCRP blocks the proteolytic inactivation of the polypeptide occurring by cleavage of the carboxyl-terminal glycine dipeptide required for activation and subsequent ligation. Intact UCRP supports a low rate of ubiquitin-activating enzyme (E1)-dependent ATP:PPi exchange but fails to form a stoichiometric E1-UCRP thiol ester or undergo transfer to ubiquitin carrier protein (E2). The binding affinity of E1 for UCRP is significantly diminished relative to that of ubiquitin. These results suggest that UCRP conjugation proceeds through an enzyme pathway distinct from that of ubiquitin, at least with respect to the step of activation. This was confirmed for an in vitro conjugation assay in which 125I-UCRP could be ligated in an ATP-dependent reaction to proteins present within an A549 human lung carcinoma cell extract and could be competitively inhibited by excess unlabeled UCRP but not ubiquitin. Other results demonstrate that 125I-UCRP conjugation is significantly increased in cell extracts after 24 h of incubation in the presence of interferon-beta, consistent with the late induction of UCRP conjugating activity. Thus, interferon-responsive cells contain a pathway for UCRP ligation that is parallel but distinct from that of ubiquitin.

Estimating the intensity of male-driven evolution in rodents by using X-linked and Y-linked Ube 1 genes and pseudogenes

Using sequence data from the last introns of ZFX and ZFY genes, we previously estimated the male-to-female ratio (alpha) of mutation rate to be close to 6 in higher primates and 1.8 in rodents. As the mutation rate may vary among different regions of the mammalian genome, it is interesting to see whether sequence data from other regions will give similar estimates. In this study, we have determined the partial genomic sequences of the ubiquitin-activating enzyme E1 genes (Ube 1x and Ube 1y for the X-linked and Y-linked homologues, respectively) of mice and rats and two mouse Ube 1y pseudogenes. From the intron sequences of the Ube 1 genes, we calculated the divergence of the Y-linked genes (Y = 0.161) and that of the X-linked genes (X = 0.107) between mouse and rat, and found the Y/X ratio to be 1.50. This ratio led to an estimate of alpha = 2.0 with a 95% confidence interval of (1.0, 3.9). Similar estimates of alpha were obtained if mouse Ube 1y pseudogenes were used instead of the mouse Ube 1y functional gene. These estimates are consistent with our previous estimate for rodents and suggest that the sex ratio of mutation rate in rodents is approximately only one-third of that in higher primates. Our estimate of the divergence time between Ube 1x and Ube 1y supports the view that the two genes separated before the eutherian radiation.

The human pseudoautosomal GM-CSF receptor alpha subunit gene is autosomal in mouse

The gene encoding the granulocyte macrophage colony stimulating factor receptor alpha subunit (CSF2RA) has previously been mapped to the pseudoautosomal region of the human sex chromosomes. In contrast, we report that the murine locus, Csf2ra, maps to an autosome in the laboratory mouse. By in situ hybridization and genetic mapping, Csf2ra maps at telomeric band D2 of mouse chromosome 19. This first instance of a pseudoautosomal locus in human being autosomal in mouse, indicates incomplete conservation between the human and mouse X chromosomes and suggests that the genetic content of the pseudoautosomal region may differ between species of eutherian mammals due to chromosomal rearrangements.

Maintenance of X inactivation of the Rps4, Zfx, and Ube1 genes in a mouse in vitro system

Several genes, including RPS4X (ribosomal protein subunit 4), ZFX (zinc finger on the X chromosome), and UBE1 (ubiquitin-activating enzyme), have been shown to be expressed from the inactive X chromosome of cultured human cells. By contrast, these genes are subject to X-chromosome inactivation in tissues from adult mice. We have now examined the inactivation status of these genes in cultured mouse cells to determine whether the differences in X-chromosome inactivation between species is due to an intrinsic difference between human and mouse X-chromosome genes or whether it is a function of gene reactivation in cell culture per se. The expression of three mouse X-chromosome genes, Rps4, Zfx, and Ube1 was examined by reverse transcriptase polymerase chain reaction (RT-PCR) in heterozygous cultured cells from a cross of a laboratory mouse by Mus spretus, which were selected to uniformly express the X chromosome from the laboratory mouse parent. No expression of the M. spretus alleles of these genes was observed in the cell line (Hobmski), which is consistent with the patterns of expression previously observed in mouse in vivo and indicates that these genes remain stably inactivated in an immortalized mouse cell line. By cytogenetic and RT-PCR analyses the Hobmski cell line was shown to retain a late-replicating X chromosome from M. spretus, which expressed the M. spretus allele of the X (inactive) specific transcript (Xist). The Hobmski cell line will be a useful resource for studying the features that maintain X-chromosome genes inactive.