Internal transcribed spacer
Internal transcribed spacer (ITS) refers to a piece of non-functional RNA situated between structural ribosomal RNAs (rRNA) on a common precursor transcript. Read from 5' to 3', this polycistronic rRNA precursor transcript contains the 5' external transcribed sequence (5' ETS), 18S rRNA, ITS1, 5.8S rRNA, ITS2, 28S rRNA and finally the 3' ETS. During rRNA maturation, ETS and ITS pieces are excised and as non-functional maturation by-products rapidly degraded. Genes encoding ribosomal RNA and spacers occur in tandem repeats that are thousands of copies long, each separated by regions of non-transcribed DNA termed intergenic spacer (IGS) or non-transcribed spacer (NTS). Sequence comparison of the ITS region is widely used in taxonomy and molecular phylogeny because it a) is easy to amplify even from small quantities of DNA (due to the high copy number of rRNA genes), and b) has a high degree of variation even between closely related species. This can be explained by the relatively low evolutionary pressure acting on such non-functional sequences.
The ITS region is the most widely sequenced DNA region in molecular ecology of fungi and has been recommended as the universal fungal barcode sequence. It has typically been most useful for molecular systematics at the species level, and even within species (e.g., to identify geographic races). Because of its higher degree of variation than other genic regions of rDNA (for small- and large-subunit rRNA), variation among individual rDNA repeats can sometimes be observed within both the ITS and IGS regions. In addition to the standard ITS1+ITS4 primers used by most labs, several taxon-specific primers have been described that allow selective amplification of fungal sequences (e.g., see Gardes & Bruns 1993 paper describing amplification of basidiomycete ITS sequences from mycorrhiza samples).
ITS and organismal life history
There are some indications suggesting that ITS sequences are conserved irrespective of the life history evolution in plants. First confirmation was reported on marine green algae, Monostroma latissimum, in which ITS sequence data was identical between sexual (heteromorphic alternation) and asexual (no alternation) strains. In other words, this study reported that the "two different life cycles in the eukaryote belong to the same species" with identical ITS sequences.
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