There
has been some confusion about Blastocystis "subtypes" and "genotypes".
Often, these
two terms have been used interchangeably. While “subtype” refers to a distinct
ribosomal lineage (which in the case of Blastocystis may very well be a
distinct species), “genotype” denotes variation WITHIN subtypes.
Currently,
there is no clear definition of genotypes in Blastocystis. Based on
phylogenetic analysis of barcode sequences of ST4, the existence of two
genotypes in ST4 has been mentioned (Stensvold et al., 2011).
Based on markers in the
mitochondrion-like organelle of Blastocystis, we recently developed MLST assays
for ST3 and ST4 and published data on intra-subtype variation in these two
subtypes (Stensvold et al., 2012). While 58 sequence types (SQTs) were found among 81 ST3 isolates, only
5 SQTs were found among 50 ST4 isolates.
By comparing SQTs with
barcode sequences, we discovered that barcode sequences belonging to the same subtype may display intra-subtype diversity, and we found out that barcode sequences can be seen as valid proxies
for SQTs. We have chosen to use the term "allele" to enable denotation of variation in
barcode sequences. Currently, we have discovered 38 ST3 alleles (i.e. 38
different ST3 barcode sequences) as opposed to 8 different ST4 alleles. There
are still no published data on ST1 and ST2 SQTs, but given the fact that 22
different alleles have been discovered so far for each of these two subtypes,
we may expect a substantial number of SQTs.
The world of Blastocystis terminology and subtyping, etc. may seem a bit overwhelming and at times confusing, but believe me, - much has improved since 2006, when Blastocystis terminology was completely up in the air!
For more information or further clarification, please don't hesitate to contact me.
Cited literature:
1. Stensvold CR, Alfellani M, Clark CG. Levels of genetic diversity vary
dramatically between Blastocystis subtypes. Infect Genet Evol. 2012 Mar;
12 (2) :263-73. PubMed PMID:22116021.
2. Stensvold CR, Christiansen DB, Olsen KE, Nielsen HV. Blastocystis sp.
subtype 4 is common in Danish Blastocystis-positive patients presenting
with acute diarrhea. Am J Trop Med Hyg. 2011 Jun; 84 (6) :883-5. PubMed
PMID:21633023; PubMed Central PMCID: PMC3110361.
