Association and in silico assignment of sequences from turkey BACs

Large scale single nucleotide polymorphism discovery in unsequenced genomes using second generation high throughput sequencing technology: applied to turkey

BACKGROUND

The development of second generation sequencing methods has enabled large scale DNA variation studies at moderate cost. For the high throughput discovery of single nucleotide polymorphisms (SNPs) in species lacking a sequenced reference genome, we set-up an analysis pipeline based on a short read de novo sequence assembler and a program designed to identify variation within short reads. To illustrate the potential of this technique, we present the results obtained with a randomly sheared, enzymatically generated, 2-3 kbp genome fraction of six pooled Meleagris gallopavo (turkey) individuals.

RESULTS

A total of 100 million 36 bp reads were generated, representing approximately 5-6% (approximately 62 Mbp) of the turkey genome, with an estimated sequence depth of 58. Reads consisting of bases called with less than 1% error probability were selected and assembled into contigs. Subsequently, high throughput discovery of nucleotide variation was performed using sequences with more than 90% reliability by using the assembled contigs that were 50 bp or longer as the reference sequence. We identified more than 7,500 SNPs with a high probability of representing true nucleotide variation in turkeys. Increasing the reference genome by adding publicly available turkey BAC-end sequences increased the number of SNPs to over 11,000. A comparison with the sequenced chicken genome indicated that the assembled turkey contigs were distributed uniformly across the turkey genome. Genotyping of a representative sample of 340 SNPs resulted in a SNP conversion rate of 95%. The correlation of the minor allele count (MAC) and observed minor allele frequency (MAF) for the validated SNPs was 0.69.

CONCLUSION

We provide an efficient and cost-effective approach for the identification of thousands of high quality SNPs in species currently lacking a sequenced genome and applied this to turkey. The methodology addresses a random fraction of the genome, resulting in an even distribution of SNPs across the targeted genome.

Genome evolution in Reptilia: in silico chicken mapping of 12,000 BAC-end sequences from two reptiles and a basal bird

BACKGROUND

With the publication of the draft chicken genome and the recent production of several BAC clone libraries from non-avian reptiles and birds, it is now possible to undertake more detailed comparative genomic studies in Reptilia. Of interest in particular are the genomic events that transformed the large, repeat-rich genomes of mammals and non-avian reptiles into the minimalist chicken genome. We have used paired BAC end sequences (BESs) from the American alligator (Alligator mississippiensis), painted turtle (Chrysemys picta) and emu (Dromaius novaehollandiae) to investigate patterns of sequence divergence, gene and retroelement content, and microsynteny between these species and chicken.

RESULTS

From a total of 11,967 curated BESs, we successfully mapped 725, 773 and 2597 sequences in alligator, turtle, and emu, respectively, to sites in the draft chicken genome using a stringent BLAST protocol. Most commonly, sequences mapped to a single site in the chicken genome. Of 1675, 1828 and 2936 paired BESs obtained for alligator, turtle, and emu, respectively, a total of 34 (alligator, 2%), 24 (turtle, 1.3%) and 479 (emu, 16.3%) pairs were found to map with high confidence and in the correct orientation and with BAC-sized intermarker distances to single chicken chromosomes, including 25 such paired hits in emu mapping to the chicken Z chromosome. By determining the insert sizes of a subset of BAC clones from these three species, we also found a significant correlation between the intermarker distance in alligator and turtle and in chicken, with slopes as expected on the basis of the ratio of the genome sizes.

CONCLUSION

Our results suggest that a large number of small-scale chromosomal rearrangements and deletions in the lineage leading to chicken have drastically reduced the number of detected syntenies observed between the chicken and alligator, turtle, and emu genomes and imply that small deletions occurring widely throughout the genomes of reptilian and avian ancestors led to the ~50% reduction in genome size observed in birds compared to reptiles. We have also mapped and identified likely gene regions in hundreds of new BAC clones from these species.

Characterization and evolutionary landscape of AmnSINE1 in Amniota genomes

Discovery of a large number of conserved non-coding elements (CNEs) in vertebrate genomes provides a cornerstone to elucidate molecular mechanisms of macroevolution. Extensive comparative genomics has proven that transposons such as short interspersed elements (SINEs) were an important source of CNEs. We recently characterized AmnSINE1, a SINE family in Amniota genomes, some of which are present in CNEs, and demonstrated that two AmnSINE1 loci play an important role in mammalian-specific brain development by functioning as an enhancer (Sasaki et al. Proc. Natl. Acad. Sci. USA 2008). To get more information about AmnSINE1s, we here performed a multi-species search for AmnSINE1, and revealed the distribution and evolutionary history of these SINEs in amniote genomes. The number of AmnSINE1 regions in amniotes ranged from 160 to 1200; the number in the eutherians were under 500 and the largest was that in chicken. Phylogenetic analysis established that each AmnSINE1 locus has evolved uniquely, primarily since the divergence of mammals from reptiles. These results support the notion that AmnSINE1s were amplified as an ancient retroposon in a common ancestor of Amniota and subsequently have survived for 300 Myr because of functions acquired by mutation-coupled exaptation prior mammalian radiation. On the basis of sequence homology and conserved synteny, we detected the orthologs of AmnSINE1 for candidates of further enhancer analysis, which are more conserved than two loci that were shown to have been involved in mammalian brain development. The present work provides a comprehensive data set to test the role of AmnSINE1s, many of which were exapted and contributed to mammalian macroevolution.