Where Do Shield-back Katydids Fit within the Katydid Evolutionary Tree?
By John P. Roche
With more than 7,000 species globally, the katydids are the second most diverse group within the insect order Orthoptera, after the grasshoppers. Katydids are members of the taxonomic family Tettigoniidae, and though familiar to people throughout the world for their musical mating call, the specifics of many aspects of the evolutionary relationships within the katydid group remain a mystery.
Jeffrey Cole, an entomologist at Pasadena City College in California, became interested in using mating calls to identify orthopteran species in California that are difficult to separate by their morphology. One category of katydids is known as the shield-back katydids, members of the subfamily Tettigoniinae. Shield-back katydids are named for the enlarged upper section of the front part of their thorax, which looks like a shield. One subgroup within the shield-back katydids is called the Nedubini. Cole found that the Nedubini was probably more diverse than present estimates indicate, and that the group was a prime candidate for a phylogenetic revision. As a result, Cole and collaborator Bo Huey Chiang from the University of California, San Diego conducted a study, just published in the Annals of the Entomological Society of America, that seeks an answer to the following question: Where do the Nedubini and other shield-back katydids fit within the katydid evolutionary tree?
Orthopterans are highly diverse along the Pacific coast of North America. There are many species in California, and about half of them are found only in that state. The shield-back katydids in North America are made up of multiple evolutionary groups (that is, they do not share just one direct evolutionary ancestor). North American shield-back katydids consist of the following: 1) North American members of the Nedubini, a group which originated in the southern continents, and 2) many other groups of shield-back katydids, which originated in the northern continents.
To pursue their question about the katydid evolutionary tree, Cole and Chiang performed a phylogenetic study of katydids in western North America. From the wild, they collected katydids from three genera in the Nedubini and from 17 non-Nedubini genera. They extracted DNA from the femurs of sampled individuals and sequenced five gene regions. They also retrieved gene sequences from GenBank from 50 katydid groups. Then they performed analyses of evolutionary relatedness using phylogenetic methods known as Bayesian and likelihood-analysis tools. Members of four, non-shield-back katydid orthopterans were used as outgroup samples for the analysis.
As Jeffrey Cole commented in an interview, “This project had a routine beginning, but it led to a surprising result.”
Using their phylogenetic analysis, Cole and Chiang created a detailed hypothesis about the evolutionary relationships among all of the shield-back katydids (i.e., the family Tettigoniidae). Recall that North American shield-back katydids consist of Nedubini that originated in the southern continents, and other groups of shield-back katydids that originated in the northern continents. Cole and Chiang discovered that the Nedubini in North America is a basal group that is a sister group to all of the other groups within the katydids. They also discovered that the second category of shield-back katydids in North America, those originating in northern continents, received strong support as an evolutionary group in this study, a finding also seen in recent evolutionary studies by Joseph Mugleston and colleagues (2013) and Hojun Song and colleagues (2015).
Cole and Chiang estimate that the North American Nedubini group shared a common ancestor with all other katydids approximately 175 million years ago. This is consistent with the what we know about the shift of continents over geological time. Somewhere between 180 million years ago and 140 million years ago, a giant supercontinent called Pangea split into two land masses, Gondwana and Laurasia. Gondwana became the continents in the southern hemisphere plus India, and Laurasia became the continents in the northern hemisphere. The Nedubini lived in Gondwana and the other katydids lived in Laurasia.
The North American shield-back katydids that originated in northern continents are believed to have diversified in the Miocene epoch, from about 23 million years ago to 5 million years ago. It was during the Miocene when a Mediterranean climate arose in California, and when deserts formed in western North America in the dry rain shadows to the immediate east of western mountain ranges. These conditions created extensive habitats for shield-back katydids.
When asked about the most important next steps for this research, Cole said that the single biggest challenge is sampling from different orthopteran taxonomic groups.
“Many orthopterans are rare, locally distributed, or difficult to collect for a variety of reasons,” he said. “The taxonomy is poorly known, and some of the diversity in orthopterans is cryptic, identifiable only through songs or other behavior, and will therefore not be evident in museum specimens.”
Because less sampling has been conducted there, collection of specimens from southern continents will be particularly important. Also, Cole observed, some often-studied orthopteran morphological features such as song-producing structures are under intense natural selection, making it hard to infer phylogeny from them. And orthopterans have huge genomes with a lot of “junk” DNA that does not code for proteins. The junk DNA presents problems for molecular analyses, particularly when junk regions are magnified along with functional coding regions. The junk DNA presents a problem because the rates of evolution in junk and functional DNA are so different, and thus the resulting data can be misleading phylogenetically.
The discovery that the North American Nedubini are the most basal katydid lineage in the world is, as the authors state, “of considerable evolutionary significance.” Also important was Cole and Chiang’s finding that the other category of North American shield-back katydids, which originated in the northern continents, received support as a definite evolutionary grouping. Their work hypothesizes a revised evolutionary tree for the shield-back katydids, but Cole and Chiang suggest that a formal revision should wait for further sampling. They suggest that the North American members of the Nedubini, however, could be elevated to a subfamily: the Nedubinae. This study is just one step in an ongoing trajectory of challenging taxonomic research on katydids, but it is an important step. New sampling and additional phylogenetic analyses offer promise to further elucidate the evolution of the katydids.
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John P. Roche is a science writer and author with a PhD and a postdoctoral fellowship in the biological sciences. He has served as editor-in-chief of periodicals at Indiana University and Boston College, as a senior scientist at Boston College, and as a science writer at Indiana University and the University of Massachusetts Medical School. He has published more than 150 articles, and has written and taught extensively about science. Dr. Roche also directs Science View Productions™, which provides writing and editing services for clients in academia and business. For more information about Dr. Roche’s writing, visit http://authorjohnproche.com.