Research

Grasshoppers are one of the most ubiquitous groups of insects in the world, found everywhere from grasslands to tropical rainforests to isolated mountain ranges to sandy deserts.

And now, thanks to a decade-long analysis of grasshoppers’ genetic relationships, scientists have the clearest picture yet of the evolutionary pathways grasshoppers have followed to attain such diversity–and the findings put the birthplace of the broadest lineage of grasshoppers in South America, not Africa, as previously thought. These findings were published in the latest issue of Insect Systematics and Diversity.

Led by associate professor Dr. Hojun Song, researchers at Texas A&M and the Museo de La Plata in Argentina gathered grasshopper specimens from 22 countries and extracted DNA samples. During the study, the researchers analyzed nucleotide sequences of both nuclear and mitochondrial genomes from 142 grasshopper species to learn how they are related to each other.

“We used the differences in nucleotides among different species to infer the relationships,” Song said. “For example, closely related species will share similar stretches of nucleotides because they share a common ancestor, but distantly related species will have more different nucleotides between them.”

The resulting phylogeny of the family Acrididae, which is the largest taxonomic family of grasshoppers, gives science a new, more nuanced understanding of how grasshoppers have evolved. It shows that grasshoppers within Acrididae descended and diversified from one common ancestor, but many of the currently recognized subfamilies are deemed “paraphyletic,” meaning they couldn’t be narrowed down to their own single common ancestor on the Acrididae family tree.

The taxonomy has been very difficult to understand due to convergent evolution, but Song and his group said that their genetic analysis offers a new lens through which taxonomists may look to revisit grasshopper classification.

“There are some subfamilies, such as Catantopinae and Hemiacridinae, that have been considered taxonomic dumping ground for many decades,” Song says. “This means that a lot of unrelated groups have accumulated in these artificial groupings. Showing the paraphyletic nature of these groups is the first step to reclassify taxonomy, and we foresee that there would be some major shifts in grasshopper classification in the near future.”

The most significant of those shifts is the determination that the common ancestor of grasshoppers in the Acrididae family lived in South America, not Africa. The researchers also studied fossil specimens to calibrate the age of certain grasshopper subfamilies, and they found that the earliest diverging lineage within the Acrididae family is also primarily found in South America.

“These relationships collectively point to the South American origin of this cosmopolitan family,” Song says. “Our time-calibrated tree shows that Acrididae originated in the Paleocene of the Cenozoic period, 59.3 million years ago.”

At that point in history, Song said that the continents of South America and Africa were already separated but closer compared to their current positions, and northern Africa was covered in tropical rainforests, much like the Amazonian region in South America.

Song and colleagues propose that Acrididae’s single ancestor first branched off from its relatives in South America and then traversed the Atlantic sometime around 57 million years ago. Those grasshopper “colonists” found suitable habitat in Africa and then rapidly radiated and diversified across Africa and into Europe and Asia. After that, the genetic analysis points to at least three subsequent recolonization events in which grasshoppers traversed back to North America, furthering their global spread and diversification.

Given grasshoppers’ iconic status in the insect realm, Song said he was surprised that no one had previously attempted to build a phylogeny of Acrididae through molecular genetic techniques. The new effort was made possible by grants, dating back to 2008, from the National Science Foundation for Song’s research into the evolution of Orthoptera, the insect order comprising grasshoppers and their relatives such as locusts, crickets, and katydids. Before such a phylogenetic project can even begin, years of field work is necessary to collect the broad range of sample species needed, aided by contributions from international collaborators, as well.

“It is not an overstatement to say that this study took 10 years to complete,” Song said. “This type of research requires extensive taxon sampling to appropriately represent the known diversity, which is probably the most challenging–but also the most exciting–aspect of any large-scale phylogenetic study.”

Acrididae is known to contain approximately 6,700 species. While the new genetic analysis is the most detailed yet on the grasshopper family, the species it sampled constitute only 2 percent of Acrididae’s full diversity. Song and his colleagues are eager to further build out the grasshopper family tree.

“We plan to increase the taxon sampling in the future and use more phylogenetic markers to build a more comprehensive phylogeny. At the same time, we plan to reclassify major groups within the family so that the classification would reflect monophyletic groups,” he says.

The paper can be viewed at https://academic.oup.com/isd/article/2/4/3/5052737 and via Entomology Today at https://entomologytoday.org/2018/07/24/grasshoppers-new-analysis-redraws-evolutionary-tree-acrididae-family-insect-systematics-diversity/