Genome Assembly - An Exchange between Two Experts

Genome Assembly - An Exchange between Two Experts

Both Rayan Chikhi (an author of Minia assembler) and Anton Korobeynikov (an author of SPAdes assembler) commented in our earlier post on a recent PLOS One paper that benchmarked a number of low-memory genome assembly programs. The comments are quite informative for those not familiar with the intricate details of such programs, and therefore we decided to present them in a new commentary.

Anton said:

The results are definitely misleading. People should stop comparing to Velvet as single gold reference assembler. GAGE-B clearly shown that state-of- the-art assemblers can easily beat Velvet by 20x in terms of N50. E.g. for R. sphaeroides dataset from GAGE-B Velvet achieved N50 of 24kb (3 kbp in the paper with plenty of misassemblies), MaSuRCA achieved 130 kbp and both Ray and SPAdes were able to produce contigs of N50 with more than 500 kbp (Ray was not included into GAGE-B, this is our internal run with parameter tuning like in GAGE-B).

So really, the results of the paper need to be redone using both recent data and recent assemblers.


There is the usual time / memory / quality tradeoff. However, for me, the paper looks like the indirect way propagating two their own approaches (DiMA, ZeMA, etc.), rather than compare the assemblers properly.

For example, the authors claim that Our experiments prove that it is possible to generate draft assemblies of reasonable quality on conventional multi- purpose computers with very limited available memory by choosing suitable assembly methods

This is not true. The quality of of the results (look for N50 and misassemblies in Tables 3-5) is below the level of acceptance these days. They effectively reported the results 10 times lower than in GAGE study and ignored all the results from GAGE in order to deduce the conclusions. Note also the differences in the methodologies GAGE tried to derive best assemblers selecting the best parameters. The authors simply fixed k-mer length to 31 and thats it.

Combing the authors provided tables and GAGE tables, we can instead easily say Our experiments prove that it is NOT possible to generate draft assemblies of reasonable quality on conventional multi-purpose computers with very limited available memory by choosing suitable assembly methods. In order to achieve the reasonable quality the proper assembly methods requiring more memory are necessary.

So, the conclusions of the paper would be the opposite! However, judging from how they worked with GAGE results and methodologies, I wont trust the authors anymore, instead I demand all the work to be thoroughly redone, and the true data to be put in the tables.

Perhaps, we can ask Rayan to run Minia on GAGE / GAGE-B data and report the results? :) This way we at least will be sure that Minia results are of GAGE- level quality.

Rayan replied -

All right Anton, good idea! Heres my informal Minia assembly of the GAGE chromosome 14 dataset.

Minia version: 1.5938

Reads taken: all the raw reads, i.e. fragments and short jump and long jump.

Parameters k=53 and min_abundance=5 were given by Kmergenie (command line: ./kmergenie list_reads).

Command line: minia list_reads 53 5 100000000 chr14_k53_m5

Peak mem: 147 MB

Time: 73 mins

Excerpt from Quast output:

contigs (>= 0 bp) 82433

Total length (>= 0 bp) 87199867

Largest contig 27533

N50 3054

NG50 1965

NA50 3043

NGA50 1954

misassemblies 18

local misassemblies 17

All those metrics look much better than in Table 5..

Here is what differs from their assembly: 1) parameters are optimized 2) that new Minia version yields better genome coverage than older ones. 3) I used all the GAGE reads, they used only the fragments.

A few more notes on the paper in general:

1) Many of the low-memory assemblers tested in the PLOS One paper do not include a scaffolder nor a gap-filler. Those tested in GAGE and GAGE-B at least have a scaffolder. Thus it is not surprising that the GAGE/GAGE-B contiguity stats look much better.

2) GAGE used the best possible error-corrected read dataset for each organism. The Plos One paper apparently used the raw data, which in my experience gives worse assemblies.

3) GAGE also picked reasonable k-mer sizes for each dataset separately.

In summary, the benchmarks in this paper are fair, in the sense that they ran all the programs in similar conditions. But its too bad that those conditions led to assemblies that were either very poor (e.g. for chr14), or that looked poor in comparison to GAGE, because of the lack of tuning.

By the way, Anton, not sure if the following statement is an accurate comparison:

E.g. for R. sphaeroides dataset from GAGE-B Velvet achieved N50 of 24kb (3 kbp in the paper with plenty of misassemblies), MaSuRCA achieved 130 kbp and both Ray and SPAdes were able to produce contigs of N50 with more than 500 kbp

That 24 kbp N50 figure for Velvet/GAGE-B is for the MiSeq data, it drops to 13 kbp with the HiSeq data. But, is the R. sphaeroides Hiseq dataset from GAGE-B the same as from GAGE? In my experience, the raw GAGE dataset is quite challenging to assemble. Also for the MiSeq dataset, Spades N50 is at 118 kbp in the GAGE-B paper. A newer version can do 500 kbp? :)

When reading such exchanges, readers should not assume that one person is right and therefore the other has to be wrong. A genome assembly problem is multi-faceted and both of them can be right in covering different aspects. Given that the exchange is based on GAGE vs PLOS One comparison, the same can be said about various benchmarking papers. There are so many variables in comparing two assemblers that asking which assembler is the best is too simplistic, unless one qualifies it with machine type, RAM size, library type and a number of other parameters. On the other hand, it is often impossible to test for all those parameters and still come up with a useful metric. Especially the ‘machine type’ variable is the hardest to quantify, because it includes disk speed and RAM size, which can change the speed of execution quite a bit. Throw in different read sizes and different types of libraries (PacBio?) and you are in a complete mess.

The exchange should rather be taken as the types of things a bioinformatician should keep in mind in working on his problem. Often conventional wisdom fails, as we describe below.

a) Hardware Limitations:

A few days back, we were trying to benchmark DSK, a k-mer counting program written by Rayan. We ran it with default options and the program finished in 3 hours. The machine happened to have 32 cores and fairly big RAM. So, we thought of running it with multi-threading option with lot of RAM to finish the run in fraction of that time. Lo and behold, the program continued to run for 8 hours with no result. What was going on?

With more resources, the processor cores, RAM and disk started to compete with each other to move data from here to there, and spent less time on actual computation (which is minimal for k-mer counting). So, essentially we had Craig Barrett’s breakfast problem described earlier. Too many cooks were opening and closing the fridge, and very few actually got time to fry omelettes.


We write RAM separately, because in many servers it is the biggest limiting factor. Amazon charges you an arm and a leg, if you want to rent a computer with high RAM. What can you do? You can design your data structure more efficiently, but beyond that the only solutions are - (i) crashing, (ii) shifting data between RAM and hard-drive during assembly. The speed of the second solution can be increased quite a bit by replacing the hard disk with SSD, and SSD storage does not cost that much these days. After all, you can design a server with one smaller SSD storage for computation and another slower disk for permanent storage. That is small increase in cost with huge added benefit.

c) Multi-threading:

Many bioinformatics programs are written to use multiple cores, but their scale up is not always linear as we explained earlier. How do you figure out what is going on? You may have to think about what the program is doing and break it down into smaller problems to identify the bottleneck.

**d) Cache Size - Problem with Small Pieces: **

A few days back, we were trying to benchmark BWA-MEM. To understand the performance of its various steps, we decided to replace the human genome with a tiny genome. At that point, everything started to behave abnormally and we got results much faster than we expected. What happened?

In our usual model of computing, a processor gets its data from the RAM. However, when the genome is too small, the processor fitted the entire memory block into its cache memory (memory within the chip) and did not need to access RAM any more. Very few bioinformatics programs use this aspect of processing, even though considerable speed gain can be achieved from using the cache effectively.

Those are just four aspects in producing machine-to-machine difference in performance of bioinformatics programs and we have not gone into algorithmic difference, library differences or contig assembly vs scafold assembly.

Written by M. //