Disruption of FEM1C-W gene in zebra finch: evolutionary insights on avian ZW genes

Sex chromosome genes control sex determination and differentiation, but the mechanisms of sex determination in birds are unknown. In this study, we analyzed the gene FEM1C which is highly conserved from Caenorhabditis elegans to higher vertebrates and interacts with the sex determining pathway in C. elegans. We found that FEM1C is located on the Z and W chromosome of zebra finches and probably other Passerine birds, but shows only Z linkage in other avian orders. In the zebra finch, FEM1C-W is degraded because of a point mutation and possibly because of loss of the first exon containing the start methionine. Thus, FEM1C-W appears to have degenerated or been lost from most bird species. FEM1C-Z is expressed in a cytoplasmic location in zebra finch fibroblast cells, as in C. elegans. FEM1C represents an interesting example of evolutionary degradation of a W chromosome gene

Dosage compensation is less effective in birds than in mammals

<p>Abstract</p> <p>Background</p> <p>In animals with heteromorphic sex chromosomes, dosage compensation of sex-chromosome genes is thought to be critical for species survival. Diverse molecular mechanisms have evolved to effectively balance the expressed dose of X-linked genes between XX and XY animals, and to balance expression of X and autosomal genes. Dosage compensation is not understood in birds, in which females (ZW) and males (ZZ) differ in the number of Z chromosomes.</p> <p>Results</p> <p>Using microarray analysis, we compared the male:female ratio of expression of sets of Z-linked and autosomal genes in two bird species, zebra finch and chicken, and in two mammalian species, mouse and human. Male:female ratios of expression were significantly higher for Z genes than for autosomal genes in several finch and chicken tissues. In contrast, in mouse and human the male:female ratio of expression of X-linked genes is quite similar to that of autosomal genes, indicating effective dosage compensation even in humans, in which a significant percentage of genes escape X-inactivation.</p> <p>Conclusion</p> <p>Birds represent an unprecedented case in which genes on one sex chromosome are expressed on average at constitutively higher levels in one sex compared with the other. Sex-chromosome dosage compensation is surprisingly ineffective in birds, suggesting that some genomes can do without effective sex-specific sex-chromosome dosage compensation mechanisms.</p

Targeted overexpression of a golli–myelin basic protein isoform to oligodendrocytes results in aberrant oligodendrocyte maturation and myelination

Recently, several in vitro studies have shown that the golli–myelin basic proteins regulate Ca2+ homoeostasis in OPCs (oligodendrocyte precursor cells) and immature OLs (oligodendrocytes), and that a number of the functions of these cells are affected by cellular levels of the golli proteins. To determine the influence of golli in vivo on OL development and myelination, a transgenic mouse was generated in which the golli isoform J37 was overexpressed specifically within OLs and OPCs. The mouse, called JOE (J37-overexpressing), is severely hypomyelinated between birth and postnatal day 50. During this time, it exhibits severe intention tremors that gradually abate at later ages. After postnatal day 50, ultrastructural studies and Northern and Western blot analyses indicate that myelin accumulates in the brain, but never reaches normal levels. Several factors appear to underlie the extensive hypomyelination. In vitro and in vivo experiments indicate that golli overexpression causes a significant delay in OL maturation, with accumulation of significantly greater numbers of pre-myelinating OLs that fail to myelinate axons during the normal myelinating period. Immunohistochemical studies with cell death and myelin markers indicate that JOE OLs undergo a heightened and extended period of cell death and are unable to effectively myelinate until 2 months after birth. The results indicate that increased levels of golli in OPC/OLs delays myelination, causing significant cell death of OLs particularly in white matter tracts. The results provide in vivo evidence for a significant role of the golli proteins in the regulation of maturation of OLs and normal myelination

Region-Specific Myelin Pathology in Mice Lacking the Golli Products of the Myelin Basic Protein Gene

The myelin basic protein (MBP) gene encodes two families of proteins, the classic MBP constituents of myelin and the golli-MBPs, the function of which is less well understood. In this study, targeted ablation of the golli-MBPs, but not the classic MBPs, resulted in a distinct phenotype unlike that of knock-outs (KOs) of the classic MBPs or other myelin proteins. Although the golli KO animals did not display an overt dysmyelinating phenotype, they did exhibit delayed and/or hypomyelination in selected areas of the brain, such as the visual cortex and the optic nerve, as determined by Northern and Western blots and immunohistochemical analysis with myelin protein markers. Hypomyelination in some areas, such as the visual cortex, persisted into adulthood. Ultrastructural analysis of the KOs confirmed both the delay and hypomyelination and revealed abnormalities in myelin structure and in some oligodendrocytes. Abnormal visual-evoked potentials indicated that the hypomyelination in the visual cortex had functional consequences in the golli KO brain. Evidence that the abnormal myelination in these animals was a consequence of intrinsic problems with the oligodendrocyte was indicated by an impaired ability of oligodendrocytes to form myelin sheets in culture and by the presence of abnormal Ca^(2+) transients in purified cortical oligodendrocytes studied in vitro. The Ca^(2+) results reported in this study complement previous results implicating golli proteins in modulating intracellular signaling in T-cells. Together, all these findings suggest a role for golli proteins in oligodendrocyte differentiation, migration, and/or myelin elaboration in the brain

Karyotypic polymorphism of the zebra finch Z chromosome

We describe a karyotypic polymorphism on the zebra finch Z chromosome. This polymorphism was discovered because of a difference in the position of the centromere and because it occurs at varying frequencies in domesticated colonies in the USA and Germany and among two zebra finch subspecies. Using DNA fluorescent in situ hybridization to map specific Z genes and measurements of DNA replication, we show that this polymorphism is the result of a large pericentric inversion involving the majority of the chromosome. We sequenced a likely breakpoint for the inversion and found many repetitive sequences. Around the breakpoint, there are numerous repetitive sequences and several copies of PAK3 (p21-activated kinase 3)-related sequences (PAK3Z) which showed testes-specific expression by RT-PCR. Our findings further suggest that the sequenced genome of the zebra finch may be derived from a male heterozygote for the Z chromosome polymorphism. This finding, in combination with regional differences in the frequency of the polymorphism, has important consequences for future studies using zebra finches

Molecular cloning and characterization of the germline-restricted chromosome sequence in the zebra finch

The zebra finch (Taeniopygia guttata) germline-restricted chromosome (GRC) is the largest chromosome and has a unique system of transmission in germ cells. In the male, the GRC exists as a single heterochromatic chromosome in the germline and is eliminated from nuclei in late spermatogenesis. In the female, the GRC is bivalent and euchromatic and experiences recombination. These characteristics suggest a female-specific or female-beneficial function of the GRC. To shed light on the function of GRC, we cloned a portion of the GRC using random amplified polymorphic DNA–polymerase chain reaction and analyzed it using molecular genetic and cytogenetic methods. The GRC clone hybridized strongly to testis but not blood DNA in genomic Southern blots. In fluorescent in situ hybridization analysis on meiotic chromosomes from synaptonemal complex spreads, the probe showed hybridization across a large area of the GRC, suggesting that it contains repetitive sequences. We isolated a sequence homologous to the GRC from zebra finch chromosome 3 and a region of chicken chromosome 1 that is homologous to zebra finch chromosome 3; the phylogenetic analysis of these three sequences suggested that the GRC sequence and the zebra finch chromosome 3 sequence are most closely related. Thus, the GRC sequences likely originated from autosomal DNA and have evolved after the galliform–passeriform split. The present study provides a foundation for further study of the intriguing GRC

Karyotypic polymorphism of the zebra finch Z chromosome.

We describe a karyotypic polymorphism on the zebra finch Z chromosome. This polymorphism was discovered because of a difference in the position of the centromere and because it occurs at varying frequencies in domesticated colonies in the USA and Germany and among two zebra finch subspecies. Using DNA fluorescent in situ hybridization to map specific Z genes and measurements of DNA replication, we show that this polymorphism is the result of a large pericentric inversion involving the majority of the chromosome. We sequenced a likely breakpoint for the inversion and found many repetitive sequences. Around the breakpoint, there are numerous repetitive sequences and several copies of PAK3 (p21-activated kinase 3)-related sequences (PAK3Z) which showed testes-specific expression by RT-PCR. Our findings further suggest that the sequenced genome of the zebra finch may be derived from a male heterozygote for the Z chromosome polymorphism. This finding, in combination with regional differences in the frequency of the polymorphism, has important consequences for future studies using zebra finches

Molecular cloning and characterization of the germline-restricted chromosome sequence in the zebra finch

The zebra finch (Taeniopygia guttata) germline-restricted chromosome (GRC) is the largest chromosome and has a unique system of transmission in germ cells. In the male, the GRC exists as a single heterochromatic chromosome in the germline and is eliminated from nuclei in late spermatogenesis. In the female, the GRC is bivalent and euchromatic and experiences recombination. These characteristics suggest a female-specific or female-beneficial function of the GRC. To shed light on the function of GRC, we cloned a portion of the GRC using random amplified polymorphic DNA–polymerase chain reaction and analyzed it using molecular genetic and cytogenetic methods. The GRC clone hybridized strongly to testis but not blood DNA in genomic Southern blots. In fluorescent in situ hybridization analysis on meiotic chromosomes from synaptonemal complex spreads, the probe showed hybridization across a large area of the GRC, suggesting that it contains repetitive sequences. We isolated a sequence homologous to the GRC from zebra finch chromosome 3 and a region of chicken chromosome 1 that is homologous to zebra finch chromosome 3; the phylogenetic analysis of these three sequences suggested that the GRC sequence and the zebra finch chromosome 3 sequence are most closely related. Thus, the GRC sequences likely originated from autosomal DNA and have evolved after the galliform–passeriform split. The present study provides a foundation for further study of the intriguing GRC