Study reveals evolution of blow flies and flesh flies

Large-scale mitogenomics data suggest that this group of insects, called Schizophora, emerged 22 million years ago, at the same time as browsing and grazing mammals.

By Karina Toledo  |  Agência FAPESP – At the same time as the dinosaurs began to disappear – during the transition from the Cretaceous to the Paleocene over 60 million years ago – a group of insects called Schizophora began to flourish.

The clade Schizophora, which belongs to the Brachycera suborder of Diptera (the order of true flies), is an adaptive radiation, a term used in evolutionary biology to refer to lineages that diversified rapidly in a given period. The clade comprises thousands of species, representing approximately 3% of the Earth’s animal diversity, surpassing even the number of all terrestrial species of vertebrates.

Blow flies and flesh flies (from families such as Calliphoridae and Sarcophagidae) date from 22 million years ago. Their appearance on earth coincided with that of the browsing and grazing mammals, and millions of years later some species became parasites of such mammals. An example is the New World screw-worm fly Cochliomyia hominivorax.

These are some of the findings of a study published in Scientific Reports, an online journal that belongs to Nature Publishing Group. The authors are researchers at the University of Campinas (UNICAMP) and the University of São Paulo (USP) at Ribeirão Preto in Brazil, the University of Virginia in the US, and Nanyang Technological University in Singapore.

To investigate the evolutionary histories of these insects, the scientists adapted techniques from large-scale mitogenomics, the simultaneous sequencing of mitochondrial DNA in hundreds of samples. The study was conducted with support from FAPESP as part of Ana Carolina Martins Junqueira’s postdoctoral research under the supervision of Ana Maria Lima de Azeredo-Espin, a professor at UNICAMP.

“Mitogenomics is a variant of genomics that analyzes mitochondrial nucleotide sequences instead of cell nucleus DNA,” explained Junqueira, currently a researcher at Nanyang Technological University’s Singapore Center for Environmental Life Sciences Engineering (SCELSE).

Mitochondrial genome sequencing is relatively fast, so this is the most commonly used marker in evolutionary studies. “The technology was originally developed for research on vertebrates and has been intensively used in studies of human migration,” Junqueira said. “But it’s been underused in the case of insects, even though they represent the greatest animal diversity on the planet.”

To develop large-scale mitogenomics in insects, the group chose the Schizophora radiation as a model. Thirty-two species of flies from different families were collected in the US, Australia and Singapore, as well as São Paulo State and Amazonas State.

The insects were macerated, and the total genomic DNA contained in the samples was extracted. Next, bioinformatics tools were used to select only the data referring to mitochondrial nucleotide sequences, which corresponded to between 1% and 5% of the total. Almost 100 mitochondrial genomes are detailed in the article published by Scientific Reports.

“Based on nucleotide diversity, it’s possible to estimate the time taken for each species or family to diverge during the geological timescale and to clarify phylogenetic relationships,” Junqueira said. “We used the few invertebrate fossils available as a benchmark. In the case of insects, fossils are dated according to the geological layer in which they were found. We added the molecular data to this information to estimate the period in which one species diverged from another.”

According to Azeredo-Espin, the analysis focused on flies that cause myiasis, the parasitic infestation of the body of a live mammal by fly larvae that grow inside the host while feeding on its tissue.

“Our line of research seeks to understand the evolution of parasitism in this group of flies, which are considered livestock pests and cause significant economic losses,” she said. “Additional knowledge can be useful for control strategies. The findings suggest ectoparasitism arose in flesh flies and blow flies between 2.3 million and 6.9 million years ago.”

The group’s hypothesis is that the Schizophora radiation began diversifying intensely after a period of severe worldwide drought gave rise to treeless dwarf vegetation and grasslands, to which herbivorous mammals adapted.

“At that time, the continents were closer together,” Junqueira said. “East Asia and North America were connected by the Bering land bridge, allowing animals and plants to disperse to the New World. There was an abundance of food, and they began to diversify. Herbivores became more common, increasing the diversification of carnivores. As a result, there was an increase in biomass, which was essential to the diversification of these species of sacrophagous insects that feed on dead or decaying organic matter.”

According to Azeredo-Espin, more generalist species emerged first, with habits that were not necessarily associated with the presence of other animals. After millions of years of coexistence with grazing mammals, insects with specialized habits appeared.

“Examples include the human botfly Dermatobia hominis and the secondary screw-worm Cochliomyia macellaria, which complete their reproductive cycle by laying eggs in a living warm-blooded host,” she said.

Population analysis

For two of the species included in the study – the Oriental latrine blow fly Chrysomya megacephala and the house fly Musca domestica – population analyses were also performed. These species are mechanical vectors of disease and hence are important to physicians and veterinarians.

Metagenomic datasets were generated for 64 specimens of these two species collected on three continents, with the aim of measuring genetic variability and gene flows from one location to another.

The results suggested that while house flies appeared to be grouped in distinct geographical zones with low levels of gene flow among the different populations, blow flies were practically uniform on all continents.

“The genetic variability observed in Chrysomya megacephala was very small, practically residual, on the scale expected only for species in extinction, yet this species displays significant invasive and adaptive capabilities,” Junqueira said.

To enhance their understanding of the phenomenon, the group plan to perform the same type of analysis using data from the nuclear genomes of the flies collected, according to Azeredo-Espin.

“We want to find out whether loss of genetic variation occurs only in the mitochondrial genome or is also present in the nuclear genome. Depending on what we discover, there could be a different explanation,” she said.

Given the impact of flies on environmental and human health, she added, this information could be highly strategic for the development of vector control programs in urban areas, where flies are major carriers of viruses and bacteria.

The article “Large-scale mitogenomics enables insights into Schizophora (Diptera) radiation and population diversity” (doi: 10.1038/srep21762) can be read at www.nature.com/articles/srep21762.