Diminishing Return for Increased Mappability with Longer Sequencing Reads: Implications of the k-mer Distributions in the Human Genome

TL;DR

This study analyzes how increasing read length in sequencing improves genome mappability, revealing diminishing returns beyond 200 basepairs and identifying regions that remain challenging to sequence reliably.

Contribution

It provides a quantitative analysis of k-mer distributions in the human genome, showing limited gains in mappability with longer reads and highlighting persistent sequencing challenges.

Findings

Non-singleton k-mers decrease slowly with increasing k

Read lengths over 200 bp yield limited mappability improvements

Certain genomic regions remain difficult to sequence reliably

Abstract

The amount of non-unique sequence (non-singletons) in a genome directly affects the difficulty of read alignment to a reference assembly for high throughput-sequencing data. Although a greater length increases the chance for reads being uniquely mapped to the reference genome, a quantitative analysis of the influence of read lengths on mappability has been lacking. To address this question, we evaluate the k-mer distribution of the human reference genome. The k-mer frequency is determined for k ranging from 20 to 1000 basepairs. We use the proportion of non-singleton k-mers to evaluate the mappability of reads for a corresponding read length. We observe that the proportion of non-singletons decreases slowly with increasing k, and can be fitted by piecewise power-law functions with different exponents at different k ranges. A faster decay at smaller values for k indicates more limited gains for read lengths > 200 basepairs. The frequency distributions of k-mers exhibit long tails in a power-law-like trend, and rank frequency plots exhibit a concave Zipf's curve. The location of the most frequent 1000-mers comprises 172 kilobase-ranged regions, including four large stretches on chromosomes 1 and X, containing genes with biomedical implications. Even the read length 1000 would be insufficient to reliably sequence these specific regions.