Skip-mers: increasing entropy and sensitivity to detect conserved genic regions with simple cyclic q-grams

Abstract

10Bioinformatic analyses and tools make extensive use of k-mers (fixed contiguous strings of k nucleotides) as an informational unit. K-mer analyses are both useful and fast, but are strongly affected by singlenucleotide polymorphisms or sequencing errors, effectively hindering direct-analyses of whole regions and decreasing their usability between evolutionary distant samples. Q-grams or spaced seeds, subsequences generated with a pattern of used-and-skipped nucleotides, overcome many of these limitations but introduce larger complexity which hinders their wider adoption. We introduce a concept of skip-mers, a cyclic pattern of used-and-skipped positions of k nucleotides spanning a region of size S ≥ k, and show how analyses are improved by using this simple subset of q-grams as a replacement for k-mers. The entropy of skip-mers increases with the larger span, capturing information from more distant positions and increasing the specificity, and uniqueness, of larger span skip-mers within a genome. In addition, skip-mers constructed in cycles of 1 or 2 nucleotides in every 3 (or a multiple of 3) lead to increased sensitivity in the coding regions of genes, by grouping together the more conserved nucleotides of the protein-coding regions. We implemented a set of tools to count and intersect skip-mers between different datasets, a simple task given that the properties of skip-mers make them a direct substitute for k-mers. We used these tools to show how skip-mers have advantages over k-mers in terms of entropy and increased sensitivity to detect conserved coding sequence, allowing better identification of genic matches between evolutionarily distant species. We then show benefits for multi-genome analyses provided by increased and better correlated coverage of conserved skip-mers across multiple samples.

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