Linkage mapping and partial sequencing of 10 cDNA loci in the chicken

Progress from chicken genetics to the chicken genome

The chicken has a proud history, both in genetic research and as a source of food. Here we attempt to provide an overview of past contributions of the chicken in both arenas and to link those contributions to the near future from a genetic perspective. Companion articles will discuss current poultry genetics research in greater detail. The chicken was the first animal species in which Mendelian inheritance was demonstrated. A century later, the chicken was the first among farm animals to have its genome sequenced. Between these firsts, the chicken remained a key organism used in genetic research. Breeding programs, based on sound genetic principles, facilitated the global emergence of the chicken meat and egg industries. Concomitantly, the chicken served as a model whose experimental populations and mutant stocks were used in basic and applied studies with broad application to other species, including humans. In this paper, we review some of these contributions, trace the path from the origin of molecular genetics to the sequence of the chicken genome, and discuss the merits of the chicken as a model organism for furthering our understanding of biology.

Development and characterization of expressed sequence tags for the turkey (Meleagris gallopavo) genome and comparative sequence analysis with other birds

Twenty-one randomly selected clones from a turkey (Meleagris gallopavo) pituitary complementary DNA (cDNA) library were sequenced to develop expressed sequence tags (ESTs) for this economically important avian species whose genome is among the least understood. Primers specific for the ESTs were used to produce amplicons from the genomic DNA of turkey, chicken (Gallus gallus), guinea fowl (Numidia meleagris), pigeon (Columba domestica), and quail (Corturnix japonica). The amplicons were sequenced and analyzed for sequence variation within- and similarity among-species and with GenBank database sequences. The proportion of shared bases between the turkey sequence and the consensus sequence from each of the other species ranged from 72% to 93% between turkey and pigeon and quail and between turkey and chicken, respectively. The total number of single nucleotide polymorphisms (SNPs) observed ranged from 3 in quail to 18 in chicken out of 4898 and 5265 bases analyzed, respectively. The most frequent nucleotide variation observed was a C-->T transition. Linkage analysis of one such SNP in the backcross progeny of the East Lansing reference DNA panel, localized TUS0005, the chicken sequence derived from primers specific for turkey TUT2E EST, to chromosome 4. The ESTs reported, as well as the SNPs may provide a useful resource for ongoing efforts to develop high utility genome maps for the turkey and chicken. The primers described can also be used as a tool in future investigations directed at further understanding the biology of the guinea fowl, pigeon and quail and their relatedness to the turkey.

Gene organisation determines evolution of function in the chicken MHC

Some years ago, we used our data for class I genes, proteins and peptide-binding specificities to develop the hypothesis that the chicken B-F/B-L region represents a "minimal essential MHC". In this view, the B locus contains the classical (highly expressed and polymorphic) class I alpha and class II beta multigene families, which are reduced to one or two members, with many other genes moved away or deleted from the chicken genome altogether. We found that a single dominantly expressed class I gene determines the immune response to certain infectious pathogens, due to peptide-binding specificity and cell-surface expression level. This stands in stark contrast to well-studied mammals like humans and mice, in which every haplotype is more-or-less responsive to every pathogen and vaccine, presumably due to the multigene family of MHC molecules present. In order to approach the basis for a single dominantly expressed class I molecule, we have sequenced a portion of the B complex and examined the location and polymorphism of the class I (B-F) alpha, TAP and class II (B-L) beta genes. The region is remarkably compact and simple, with many of the genes expected from the MHC of mammals absent, including LMP, class II alpha and DO genes as well as most class III region genes. However, unexpected genes were present, including tapasin and putative natural killer receptor genes. The region is also organised differently from mammals, with the TAPs in between the class I genes, the tapasin gene in between the class II (B-L) beta genes, and the C4 gene outside of the class I alpha and class II beta genes. The close proximity of TAP and class I alpha genes leads to the possibility of co-evolution, which can drive the use of a single dominantly expressed class I molecule with peptide-binding specificity like the TAP molecule. There is also a single dominantly expressed class II beta gene, but the reason for this is not yet clear. Finally, the presence of the C4 gene outside of the classical class I alpha and class II beta genes suggests the possibility that this organisation was ancestral, although a number of models of organisation and evolution are still possible, given the presence of the Rfp-Y region with non-classical class I alpha and class II beta genes as well as the presence of multigene families of B-G and rRNA genes.

The comparative genomic structure and sequence of the surfeit gene homologs in the puffer fish Fugu rubripes and their association with CpG-rich islands

The puffer fish Fugu rubripes (Fugu) has a compact genome approximately one-seventh the size of man, mainly owing to small intron size and the presence of few dispersed repetitive DNA elements, which greatly facilitates the study of its genes at the genomic level. It has been shown previously that, whereas the Surfeit genes are tightly clustered at a single locus in mammals and birds, the genes are found at three separate loci in the Fugu genome. Here, Fugu gene homologs of all six Surfeit genes (Surf-1 to Surf-6) have been cloned and sequenced, and their gene structure has been compared with that of their mammalian and avian homologs. The predicted protein products of each gene are well conserved between vertebrate species, and in most cases their gene structures are identical to their mammalian and avian homologs except for the Fugu Surf-6 gene, which was found to lack an intron present in the mouse gene. In addition, we have identified conserved regulatory elements at the 5' and 3' ends of the Surf-3/rpL7a gene by comparison with the mammalian and chicken Surf-3/rpL7a gene homologs, including the presence of a polypyrimidine tract at the extreme 5' end of this ribosomal protein gene. The Fugu Surfeit gene homologs appear to be associated with CpG-rich islands, like the Surfeit genes in higher vertebrates, but these Fugu CpG islands are similar to the nonclassical islands characteristic of other fish species. Our observations support the use of the Fugu genome to study vertebrate gene structure, to predict the structure of mammalian genes, and to identify vertebrate regulatory elements. [The sequence data described in this paper have been submitted to the data library under accession nos. Y15170 (Surf-2, Surf-4), Y15171 (Surf-3, Surf-1, Surf-6), and Y15172 (Surf-5.)]

Development and mapping of polymorphic microsatellite markers derived from a chicken brain cDNA library

Until now the genetic linkage map in chicken has ben based mainly on random genomic markers. The addition of expressed sequence tags (ESTs) to the genetic linkage maps is becoming more important because ESTs can form the basis for comparative mapping studies. This may be helpful for the detection of candidate genes for quantitative trait loci (QTLs). In our study we used a (TG)13 repeat as probe for the detection of microsatellites in a chicken brain cDNA library. After hybridization 0.15% of the cDNA clones gave a positive signal. The cDNA complexity of the library was high; of the 90 cDNA clones that were sequenced 60 occurred only once. For 29 clones primer sets for the polymerase chain reaction could be developed. Twenty-one microsatellites were polymorphic on one or more of the test panels and 15 markers could be mapped on either or both of the international reference families. Because sequence homology between chicken and mammalian cDNAs is sometimes low it was difficult to assess the level of sequence homology that indicated a true homologous transcript. In our study seven cDNA cones, of which three could be mapped, showed a relatively high percentage of sequence homology with sequences found in other species. Because sequencing and mapping of expressed sequence tags in human and mouse is progressing very rapidly, it is predicted that further information will soon be readily available. Therefore, increasing the number of expressed sequences on the chicken genetic linkage map will be of value for comparative mapping studies in the near future.

CpG islands of chicken are concentrated on microchromosomes

The chicken karyotype comprises 39 chromosome pairs of which at least 29 are 'microchromosomes'. Microchromosomes account for about 25% of the genomic DNA, but they are cytologically indistinguishable from one another (1). Due to technical limitations there is a strong bias of mapped genes within the chicken genome database ChickGBASE (2) towards macrochromosomes 1-6 and Z, with specific assignments to only one microchromosome (3,4). Several genes have, however, been assigned to the microchromosome group as a whole (3,5-9), demonstrating that these tiny chromosomes do not represent genetically inert DNA. To determine the overall chromosomal distribution of genes, as well as to provide a mapping resource, we prepared a CpG island library from chicken using differential binding to a methyl-CpG chicken using differential binding to a methyl-CpG binding column before and after de novo methylation (10). Surprisingly, we found that chicken CpG islands are highly concentrated on the microchromosomes, whereas macrochromosomes 1-6 are comparatively gene-poor by this assay. Our results raise the possibility that gene density on chicken microchromosomes approaches the maximum value known for vertebrates.