Lots of ‘junk’ in the genome of a small aquatic invertebrate

The genomes of animals and plants usually contain more DNA than required for organismic function. Therefore, genome size may vary greatly among species. Here we investigated genome size variation among several closely related rotifer species of the Brachionus plicatilis complex, which differed up to four-fold in genome size. We found that species with larger genomes contained proportionally higher amounts of repetitive DNA, and that this DNA could account for up to half the genome. Repetitive DNA consisted mainly of transposable elements, a form of 'junk' DNA often found in animals and plants. However, such high proportions of repetitive DNA are rather unusual for a small genome, like in rotifers. Our study contradicts the naïve expectation that small genomes are streamlined, or less complex, and that large variations in DNA content between closely related species are due to polyploidy (duplications of the genome). 


Science Flash Blommaert

Fig. 1: The rotifer Brachionus plicatilis (here: a female with egg) can be found worldwide in the zooplankton of small water bodies.


Blommaert J, Riss S, Hecox-Lea B, Mark Welch DB, Stelzer CP (2019) Small, but surprisingly repetitive genomes: transposon expansion and not polyploidy has driven a doubling in genome size in a metazoan species complex. BMC Genomics  20:466



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