Largest genome of any insect discovered in rare grasshopper from Alps

Researchers from the German Leibniz Institute for the Analysis of Biodiversity Change (LIB) and the Czech Academy of Sciences recently discovered the largest genome of any insect in a grasshopper.

Found in the speckled buzzing grasshopper (Bryodemella tuberculata), the genome is also seven times the size of the human genome. This disproves the "idea" that insect genomes are small and less complex, said a press release.

Bryodemella tuberculata is known as the most "conspicuous" grasshoppers of Central Europe and is among the rarest. Most of the population is extinct; the rest live on the banks of rivers in the Alps.

The study is published in PLOS ONE.

Buzzing grasshopper replaced the Asian desert cricket

How did an insect have such a large genome?

While the study is the latest in a series of publications on the evolution of genome sizes in insects, not one published paper has answered the question. In fact, most insect genomes, like that of fruit flies, have genomes whose size is not more than a sixth of that of the human genome.

For now, data on genome sizes are only available for 1,345 out of the more than a million known species of insects. And all the largest genomes have been found in grasshoppers and crickets.

The size of genomes not only varies between different groups of animals, but sometimes they even differ within groups. Scientists are looking for the reason behind the variability as the complete genome must be duplicated during every cell division.

Researchers measured the genomes of 50 grasshopper species employing flow cytometry which investigated the variability in related species. The largest genome was found in the speckled buzzing grasshopper, replacing the previous record holder, the Asian desert cricket (Deracantha onos).

According to Oliver Hawlitschek, head of the Hamburg LIB genetics lab, more detailed sequence-based genomic analysis can help learn about the evolutionary mechanisms determining the sizes of genomes. "I am confident that studying these extremes will also provide us with many insights about the function of our human genomes," he said.

Study Abstract:

Animal genomes vary widely in size, and much of their architecture and content remains poorly understood. Even among related groups, such as orders of insects, genomes may vary in size by orders of magnitude–for reasons unknown. The largest known insect genomes were repeatedly found in Orthoptera, e.g., Podisma pedestris (1C = 16.93 pg), Stethophyma grossum (1C = 18.48 pg) and Bryodemella holdereri (1C = 18.64 pg). While all these species belong to the suborder of Caelifera, the ensiferan Deracantha onos (1C = 19.60 pg) was recently found to have the largest genome. Here, we present new genome size estimates of 50 further species of Ensifera (superfamilies Gryllidea, Tettigoniidea) and Caelifera (Acrididae, Tetrigidae) based on flow cytometric measurements. We found that Bryodemella tuberculata (Caelifera: Acrididae) has the so far largest measured genome of all insects with 1C = 21.96 pg (21.48 gBp). Species of Orthoptera with 2n = 16 and 2n = 22 chromosomes have significantly larger genomes than species with other chromosome counts. Gryllidea genomes vary between 1C = 0.95 and 2.88 pg, and Tetrigidae between 1C = 2.18 and 2.41, while the genomes of all other studied Orthoptera range in size from 1C = 1.37 to 21.96 pg. Reconstructing ancestral genome sizes based on a phylogenetic tree of mitochondrial genomic data, we found genome size values of >15.84 pg only for the nodes of Bryodemella holdereri / B. tuberculata and Chrysochraon dispar / Euthystira brachyptera. The predicted values of ancestral genome sizes are 6.19 pg for Orthoptera, 5.37 pg for Ensifera, and 7.28 pg for Caelifera. The reasons for the large genomes in Orthoptera remain largely unknown, but a duplication or polyploidization seems unlikely as chromosome numbers do not differ much. Sequence-based genomic studies may shed light on the underlying evolutionary mechanisms.