The ANI between species within a same genus is notably low

When annotating my self-assembled MAG, I found that my MAG was annotated as ‘d__Bacteria; p__Bacillota_A; c__Clostridia; o__Lachnospirales; f__Lachnospiraceae; g__Bilifractor; s__Bilifractor sp902797025’ with an ANI of 99.55 to ‘GCA_902797025.1’ (MAG from another project).
However, in NCBI, there is only one species ‘s__Bilifractor porci’ under ‘g__Bilifractor.’
So, I wanted to know how similar my MAG is to ‘Bilifractor porci’ (GCF_009696005.1), and the result is 75%.
Furthermore, I compared ANI between ‘GCA_902797025.1’ and ‘GCF_009696005.1’, and the similarity is only 70%.
Is it reasonable to have such low similarity between species within the same genus?
Or could it be an error that ‘GCA_902797025.1’ is classified under ‘g__Bilifractor’?
I used GTDBtk version 2.3.2, database V214.
I’d appreciate it if someone could help me with my confusion.

In fact, I want to culture my MAG from the sample;
first, I need a method to detect whether this MAG is present in the sample.
I thought about extracting 16S rRNA from the MAG and designing primers to amplify it.
However, I used Barrnap (version 0.9), and couldn’t find any 16S sequence in my MAG or GCA_902797025.1.
Do you have any good suggestions? Thank you.



Hi Chentianrong,

GTDB defines genera using relative evolutionary divergence (RED) and not ANI. RED takes into account varying rates of evolution across the tree. The genus Bilifractor has an RED of 0.912 which is typical for a GTDB genus: GTDB - R214 Statistics


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Hello Donovan

Thanks for answering my question. It is helpful.