IndelFreeAligner: A Streaming Aligner for Comprehensive Gapless Alignment Against Terabase-Scale References

Abstract

The comparison of short sequences to massive reference databases is a cornerstone of modern genomics, but it presents a significant scalability challenge. Traditional alignment tools rely on time- and memory-intensive pre-indexing of the reference, creating a substantial bottleneck for applications involving small query sets against terabase-scale data, such as CRISPR spacer analysis. Here we present IndelFreeAligner, a streaming, indel-free alignment tool that eliminates the preprocessing bottleneck. It operates in two modes: an indexed mode for larger query sets and a brute-force mode for maximum speed on small query sets. By processing reference sequences on-the-fly, IndelFreeAligner supports user-specified mismatch thresholds up to the full query length and maintains memory usage independent of total reference size. A novel MinHitsCalculator component uses Monte Carlo simulation to determine adaptive seed-hit thresholds for indexed mode. Benchmarks against Bowtie1 and BLAST+ demonstrate that IndelFreeAligner aligns a single query against a 4 Gbp reference in 1.7 seconds versus 17 minutes for Bowtie1 (including index construction at optimal thread count), a 607-fold speedup. Against RefSeq Bacteria (560 GB compressed), IndelFreeAligner completes a 10-query search in 12 minutes using 8 GB of RAM, while BLAST+ requires 3 hours and 17 minutes to build its database alone and 506 GB of RAM to query it. Indexed mode achieves 0% false negatives through 4 substitutions (99.84-99.90% mapped at 8-16); brute-force mode is exhaustive by design. IndelFreeAligner provides a scalable and efficient solution for alignment tasks that were previously computationally prohibitive. It is distributed open-source as part of BBTools (Bushnell, 2014).

9 pages, 2 figures, 7 tables

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