Phylogenomics and Fossilized Birth-Death Dating Reveals Extensive Post-Cretaceous Worldwide Diversification of Cicadidae (Hemiptera, Auchenorrhyncha)

By integrating phylogenomic data from 490 nuclear loci and mitochondrial genomes with a fossilized birth-death dating model, this study reveals that while the Cicadidae family originated in the Cretaceous, four of its five subfamilies underwent extensive worldwide diversification shortly after the Cretaceous-Paleogene mass extinction.

The Gist

Imagine the history of life on Earth as a massive, tangled family tree. For a long time, scientists thought they knew when the branches of the cicada family (Cicadidae) started growing. They looked at the oldest known cicada fossils and saw that the first clear ones appeared just after a giant asteroid hit the Earth 66 million years ago, wiping out the dinosaurs. This suggested that cicadas were like survivors who waited for the dust to settle before throwing a massive party and multiplying.

But this new study says: "Wait a minute. The party started earlier, but the guests just didn't show up to the photo until later."

Here is the story of what the researchers found, explained simply:

1. The Detective Work: Building a Better Family Tree

In the past, scientists tried to build the cicada family tree using just a few genetic clues (like trying to solve a puzzle with only 50 pieces). It was often blurry and confusing.

In this study, the team acted like super-sleuths. They gathered 490 different genetic "clues" (nuclear genes) from 160 different types of cicadas, covering almost every major branch of the family. They also used a special technique called Anchored Hybrid Enrichment, which is like using a high-tech magnet to fish out specific, important DNA strands from a messy bucket of genetic soup.

They didn't just look at the DNA; they also looked at the fossils. But instead of just using the fossils to mark a spot on the tree (like a sticky note), they used a sophisticated computer model called the Fossilized Birth-Death (FBD) model.

• The Analogy: Imagine trying to guess when a family started having kids.
  • Old way: You only look at the oldest baby photo you have and say, "They must have started having kids right before this photo."
  • New way (FBD): You look at every photo you have (even the blurry ones of great-grandparents), you know how many kids the family usually has, and you know how many kids usually die young. The computer then calculates the most likely time the family actually started, even if the first clear photo is missing.

2. The Big Surprise: The "Ghost" Timeline

The results were a shocker. The study found that the cicada family actually originated in the Early Cretaceous period (about 135 million years ago), long before the asteroid hit.

However, for about 70 million years, there were almost no cicada fossils.

• The Metaphor: Think of this as a "Phylogenetic Fuse." Imagine a fuse is lit (the family starts existing), but it burns very slowly and invisibly for a long time. The family was there, evolving and splitting into different groups, but they were like ghosts in the fossil record. They didn't leave behind many bones or leaves in the rock until the very end of that period.

3. The K-Pg Asteroid: The Great Filter

When the asteroid hit 66 million years ago (the K-Pg extinction), it killed off about 76% of all species, including the dinosaurs.

The study shows that while the cicada family started long before this event, the explosion of diversity happened right after.

• The Analogy: Imagine a crowded concert hall where everyone is fighting for space. Suddenly, half the audience leaves (the extinction event). The remaining people suddenly have a ton of empty space, no one is fighting them, and they can spread out and have fun.
• What happened to cicadas: Four out of the five major subgroups of cicadas (the "tribes" or "clans") went through a massive boom in numbers and variety right after the dinosaurs died. They filled the empty ecological niches left behind.

4. The One Group That Didn't Party

There was one small group of cicadas called Derotettiginae that didn't join the party. They stayed small and didn't diversify much. The researchers think this group might have been out-competed by other cicadas that were better at surviving in the new, changing world.

5. Why This Matters

This study changes how we see the history of life. It proves that just because we don't see fossils from a certain time, it doesn't mean the animals weren't there. They might just be hiding in the "dark ages" of the fossil record.

It also shows that mass extinctions, while tragic, can act as a reset button for evolution. By clearing out the competition, they allow surviving groups (like cicadas) to explode into new forms and dominate the world we see today.

In a nutshell: Cicadas are ancient survivors that existed long before the dinosaurs died, but they waited until the asteroid cleared the stage before they really started their global takeover. The researchers used a massive amount of DNA and a smart computer model to finally hear their story.

Technical Summary

1. Problem Statement

The study addresses two primary challenges in understanding the evolutionary history of the insect family Cicadidae (cicadas):

• Phylogenetic Resolution: Previous studies using limited gene sets (e.g., 5 genes) failed to resolve deep relationships within the family, particularly the polytomy between the subfamilies Tettigomyiinae, Cicadettinae, and Cicadinae. Additionally, many tribal relationships remained unresolved, and taxonomic classifications were often inconsistent with molecular data.
• Divergence Timing and the K-Pg Event: There is debate regarding the origin of Cicadidae relative to the Cretaceous-Paleogene (K-Pg) mass extinction (~66 Ma). While the oldest unambiguous crown-group fossils date to the Paleocene, suggesting a post-extinction radiation, some morphological studies suggest older origins. Previous molecular dating studies relied on node-calibration methods, which can introduce bias by subjectively assigning calibration densities to specific nodes and ignoring the full fossil record.

2. Methodology

The authors employed a comprehensive phylogenomic and Bayesian dating framework to overcome previous limitations.

• Taxon Sampling: The study expanded upon a previous dataset (Owen et al., 2022) by adding 60 new taxa. The final dataset includes 160 taxa (158 ingroup Cicadidae + 2 outgroup Tettigarctidae), representing all five subfamilies, 82% of tribes, and 25% of genera. Sampling was specifically enhanced for underrepresented regions (Afrotropical and Neotropical).
• Data Generation:
  • Nuclear Data: Used Anchored Hybrid Enrichment (AHE) to sequence 490 nuclear loci.
  • Mitochondrial Data: Assembled 114 mitochondrial genomes from off-target AHE "bycatch" reads, supplemented by GenBank sequences, totaling 120 taxa.
  • Data Processing: Utilized a custom pipeline involving SPAdes for assembly, BLASTN for locus extraction, MAFFT for alignment, and HMMCleaner for removing misaligned regions. Strict filtering removed taxa with <100 loci and identified/removed contaminated samples.
• Phylogenetic Inference:
  • Maximum Likelihood (ML): Constructed concatenated trees using IQ-TREE with ModelFinder for partitioning.
  • Species Tree Estimation: Employed the Multispecies Coalescent (MSC) model using ASTRAL (from gene trees) and SVDQuartets to account for incomplete lineage sorting (ILS).
• Divergence Time Estimation:
  • Model: Used a Fossilized Birth-Death (FBD) model within a Bayesian framework (StarBeast3 in BEAST v2.7.5). This approach treats fossils as tips in the tree rather than just node calibrations.
  • Strategy: Due to computational constraints of applying FBD to large datasets, a "divide and conquer" approach was used. A backbone tree (20 taxa) was estimated first, and its posterior distributions served as priors for subtree analyses (subfamilies and tribes).
  • Fossil Integration: Incorporated 44 fossil taxa (both stem and crown groups) directly into the analysis to calibrate the tree without subjective node priors.

3. Key Contributions

• Resolution of Deep Polytomies: The study definitively resolved the relationship between the three major subfamilies, identifying Tettigomyiinae + Cicadinae as a clade sister to Cicadettinae.
• Taxonomic Revisions: Provided molecular evidence for reclassifying several genera. Notably, it confirmed the placement of the tribe Hovanini within Tettigomyiinae and identified several African genera (e.g., Neomuda, Koranna, Malgotilia, Masupha) currently classified in Cicadettinae as belonging to Tettigomyiinae. It also clarified the non-monophyly of the tribe Zammarini, suggesting Adusella requires a new tribal classification.
• Methodological Advancement: Demonstrated the utility of the FBD tip-dating approach combined with the MSC model for large-scale phylogenomic datasets, showing improved precision over traditional node-dating methods.

4. Key Results

• Phylogeny: All analytical methods (Concatenated ML, ASTRAL, SVDQuartets, and mitochondrial ML) were largely congruent. The study recovered a robust topology for the family, with high support for most deep nodes.
• Divergence Times:
  • Cicadoidea Origin: Estimated at the Early Jurassic (~170 Ma).
  • Cicadidae Origin: Estimated at the Early Cretaceous (~135 Ma).
  • Subfamily Diversification: Four of the five subfamilies (Tibicininae, Cicadettinae, Tettigomyiinae, and Cicadinae) underwent their major crown diversification shortly after the K-Pg boundary (Late Cretaceous to Early Paleocene, ~65–72 Ma).
  • Derotettiginae: The fifth subfamily did not show a similar post-K-Pg radiation, likely due to competition and habitat changes.
• FBD vs. Node-Dating: The FBD approach provided more precise age estimates (narrower Highest Posterior Density intervals) for many clades compared to previous node-dating studies. While the FBD model suggested an older crown age for Cicadidae (implying a ~70 Myr "ghost lineage" before the first fossil appearance), the authors argue this is biologically plausible given the preservation bias toward stem-group fossils in the Mesozoic.

5. Significance

• Impact of Mass Extinctions: The study provides strong evidence that the K-Pg mass extinction acted as a catalyst for the global diversification of Cicadidae. While the family originated in the Cretaceous, the extinction event cleared ecological niches, allowing four of the five subfamilies to radiate rapidly in the Cenozoic.
• Evolutionary Patterns: The findings align with patterns seen in other taxa (birds, mammals, frogs), where stem lineages survived the K-Pg event, but the major crown-group radiations occurred in the aftermath.
• Taxonomic Stability: The results offer a stable phylogenetic framework for future evolutionary, biogeographic, and ecological studies of cicadas, correcting long-standing taxonomic ambiguities regarding African and Neotropical lineages.
• Methodological Benchmark: The successful application of the FBD model to a large phylogenomic dataset (490 loci, 160 taxa) via a divide-and-conquer strategy sets a precedent for dating other large, rapid radiations where standard Bayesian methods are computationally prohibitive.