YOC, A new strategy for pairwise alignment of collinear genomes

Inferring synteny between genome assemblies: a systematic evaluation

BACKGROUND

Genome assemblies across all domains of life are being produced routinely. Initial analysis of a new genome usually includes annotation and comparative genomics. Synteny provides a framework in which conservation of homologous genes and gene order is identified between genomes of different species. The availability of human and mouse genomes paved the way for algorithm development in large-scale synteny mapping, which eventually became an integral part of comparative genomics. Synteny analysis is regularly performed on assembled sequences that are fragmented, neglecting the fact that most methods were developed using complete genomes. It is unknown to what extent draft assemblies lead to errors in such analysis.

RESULTS

We fragmented genome assemblies of model nematodes to various extents and conducted synteny identification and downstream analysis. We first show that synteny between species can be underestimated up to 40% and find disagreements between popular tools that infer synteny blocks. This inconsistency and further demonstration of erroneous gene ontology enrichment tests raise questions about the robustness of previous synteny analysis when gold standard genome sequences remain limited. In addition, assembly scaffolding using a reference guided approach with a closely related species may result in chimeric scaffolds with inflated assembly metrics if a true evolutionary relationship was overlooked. Annotation quality, however, has minimal effect on synteny if the assembled genome is highly contiguous.

CONCLUSIONS

Our results show that a minimum N50 of 1 Mb is required for robust downstream synteny analysis, which emphasizes the importance of gold standard genomes to the science community, and should be achieved given the current progress in sequencing technology.

Comparative Genomics for Prokaryotes

Bacteria and archaea, collectively known as prokaryotes, have in general genomes that are much smaller than those of eukaryotes. As a result, thousands of these genomes have been sequenced. In prokaryotes, gene architecture lacks the intron-exon structure of eukaryotic genes (with an occasional exception). These two facts mean that there is an abundance of data for prokaryotic genomes, and that they are easier to study than the more complex eukaryotic genomes. In this chapter, we provide an overview of genome comparison tools that have been developed primarily (sometimes exclusively) for prokaryotic genomes. We cover methods that use only the DNA sequences, methods that use only the gene content, and methods that use both data types.