(Editor's Note: Geoffrey Attardo, assistant professor, UC Davis Department of Entomology and Nematology, published this piece July 29, 2020 on The Conversation website. This article is republished from The Conversation under a Creative Commons license. Read the original article.)
Bloodthirsty tsetse flies nurse their young, one live birth at a time – understanding this unusual strategy could help fight the disease they spread
Tsetse flies are bloodthirsty. Natives of sub-Saharan Africa, tsetse flies can transmit the microbe Trypanosoma when they take a blood meal. That's the protozoan that causes African sleeping sickness in people; without treatment, it's fatal, and millions of people are at risk due to the bite of a tsetse fly.
My entomology research focuses on insects that feed on the blood of people and animals. From a human health standpoint, understanding what makes all these bugs tick is key to developing ways to control them and prevent transmission of the diseases they carry, such as malaria, dengue, Lyme disease, West Nile virus and many others.
Tsetse flies stand out from their blood-feeding cousins the mosquitoes and ticks because of their unique reproductive biology. They give birth to live young and, even more unusual, the mother lactates and provides milk for her offspring. Here's how it all works – and why their unusual reproduction strategy might be a key to controlling tsetse flies and the parasite they carry once and for all.
From egg to larva
Scientists know of other flies that hold onto their eggs in their reproductive tract until they hatch into young larvae, with each brood consisting of dozens of offspring. The mother then tries to find a suitable source of nutrition in the environment, deposits the larvae and leaves them to survive on their own. The mother does not provide any nutrition for her young.
That's the standard fly way of life. Tsetse flies take a different approach.
Female tsetse flies develop just one single egg at a time. When the egg is complete, the mother moves it from her ovaries into her uterus in a process called ovulation. Once in the uterus, the egg is fertilized with sperm the female has stored in an organ called the spermatheca. While females can mate multiple times, they obtain all the sperm they need for their lifetime from a male fly during a single mating event.
After fertilization, the female keeps the egg in her uterus for five days while an embryo develops within the egg. When the embryo is ready, the egg hatches in the uterus of the female and the tsetse fly larva begins its life living inside its mother's uterus.
Milk meals for baby
Here's where tsetse flies dramatically diverge from most other insects.
Attached to the mother's uterus is a specialized gland that makes a milk-like substance. The organ is called the milk gland, and it produces a rich mixture of fats and particular proteins that provide the larva with all the nutrition it needs to develop into an adult.
Just like in mammals, the milk also transfers beneficial bacteria from the mother to the offspring. These bacteria are essential for tsetse flies, and without them adult female flies are unable to reproduce.
After five or six days of developing and feeding on milk, the larva is fully grown and ready to enter the world. The mother finds a safe spot and gives birth. The larva immediately burrows underground to avoid predators and parasites.
Once buried, the outer surface of the larva's skin hardens and turns black, forming a protective shell. This is called the pupal stage and it lasts for around three weeks. During this time, the pupa transforms into an adult fly.
It then emerges from the pupa, climbs out of the ground, and begins its life as an adult tsetse fly looking for hosts to blood-feed on and other tsetse flies to mate with.
Why live birth?
Why would an insect evolve this slow and resource-intensive way to reproduce?
One idea is that this method provides a defensive advantage relative to free-living larvae against parasites and predation. Larvae on their own have few (if any) ways to defend against these threats. But keeping larvae in the mother's uterus provides shelter and a guaranteed food source. While this strategy is much slower, scientists think the extra maternal care results in higher larval survival rates. It's a matter of quality over quantity.
A result of this reproductive strategy is that tsetse fly populations are small and slow to recover from control efforts, relative to more prolific insects like mosquitoes.
My colleagues and I hope that we can parlay our understanding of the molecular processes that regulate tsetses' milk production and mating behavior into new environmentally friendly, cost-effective and tsetse-specific control strategies for these insects.
The sleeping sickness tsetse flies spread is a potential issue for millions of people in 36 sub-Saharan countries, though the number of annual cases has decreased drastically thanks to major control efforts – including trapping flies, applying insecticides and releasing sterile males to the environment where they mate with wild females but don't produce offspring. Ultimately, we'd like to contribute to the World Health Organization's goal of eliminating African sleeping sickness by 2030 with a new way to prevent the transmission of disease-causing trypanosomes to people and animals./h2>/h2>/h2>/figcaption>/h1>
And what's the canceled 105th annual UC Davis Picnic without virtual insects?
The Department of Entomology and Nematology annually hosts dozens of insect-themed Picnic Day events at Briggs Hall and at the Bohart Museum of Entomology. But this year, the insects went virtual due to the coronavirus pandemic precautions.
The campuswide Picnic Day Committee hosted a virtual tour of some of the planned events, and posted this link: https://picnicday.ucdavis.edu/virtual/
The spotlight paused on the Bohart Museum, which houses nearly eight million insect specimens; the seventh largest insect collection in North America; the California Insect Survey, a storehouse of the insect biodiversity; and a live “petting zoo” comprised of Madagascar hissing cockroaches, walking sticks, tarantulas and the like. It also is the home of a gift shop, stocked with T-shirts, sweatshirts, books, jewelry, posters, insect-collecting equipment and insect-themed candy.
Directed by UC Davis entomology professor Lynn Kimsey for 30 years, the museum is named for noted entomologist Richard Bohart (1913-2007). The Bohart team includes senior museum scientist Steve Heydon; Tabatha Yang, education and outreach coordinator; and entomologist Jeff Smith, who curates the Lepidoptera (butterflies and moths section).
If you browse the Bohart Museum site, you'll find fact sheets about insects, written by Professor Kimsey.
But if you want to see the Bohart Museum's virtual tours, be sure to watch these videos:
- Director Lynn Kimsey giving a Bohart Museum introduction
- Tabatha Yang, education and outreach coordinator, presenting an arthropod virtual tour
- Diane Ullman, professor of entomology and former chair of the department, presenting a view of the Lepidodpera section.
Also on the UC Davis Virtual Picnic Day site, you'll learn “How to Make an Insect Collection," thanks to project coordinator James R. Carey, distinguished professor, UC Davis Department of Entomology and Nematology; and "Can Plants Talk to Each Other?" a TED-Ed Talk featuring the work of ecologist Rick Karban, professor, UC Davis Department of Entomology and Nematology.
"Female tsetse flies carry their young in an adapted uterus for the entirety of their immature development and provide their complete nutritional requirements via the synthesis and secretion of a milk like substance," he says. PBS featured his work in its Deep Look video, “A Tsetse Fly Births One Enormous Milk-Fed Baby,” released Jan. 28, 2020. (See its accompanying news story.)
PBS also collaborated with the Attardo lab and the Chris Barker lab, UC Davis School of Veterinary Medicine, for a PBS Deep Look video on Aedes aegypti, the mosquito that transmits dengue fever and Zika. The eggs are hardy; "they can dry out, but remain alive for months, waiting for a little water so they can hatch into squiggly larvae," according to the introduction. Watch the video, "This Dangerous Mosquito Lays Her Armored Eggs--in Your House."
In the meantime, the UC Davis Picnic Day leaders are gearing up for the 106th annual, set for April 17, 2021. What's a picnic without insects?
The UC Davis Department of Entomology and Nematology's seminar on Wednesday, Feb. 26 will feature six “Faculty Flash Talks.”
The seminar, set from 4:10 to 5 p.m. in 122 Briggs Hall, will include Joanna Chiu, Jason Bond, Geoffrey Attardo, Rachel Vannette, Julia Fine, and Arathi Seshadri.
Associate professor Joanna Chiu, vice chair of the department, will present “results from a project in which we study the mechanisms by which insects sense environmental changes (temperature and photoperiod) to regulate their seasonal physiology. Our lab has identified a protein that can track seasonal changes in temperature and photoperiod to promote winter physiology. Without this protein, insects don't know winter is coming!”
Her laboratory research interests include molecular genetics of animal behavior, Circadian rhythm biology, and posttranslational regulation of proteins.
Jason Bond, Schlinger Chair in Insect Systematics, is a global expert on spiders. His research interests include systematics, taxonomy, and evolution of terrestrial arthropods with an emphasis on arachnids and myriapods. "We employ molecular, morphological, and ecological approaches to study questions related to evolutionary diversification at multiple hierarchical levels (populations – higher taxa)," he says. (See recent grant.)
Geoffrey Attardo, a medical entomologist/geneticist, focuses his research on insect disease vectors, insect reproduction, vector/parasite interactions, reproductive physiology, male seminal secretions, symbiosis, lactation, nutrition, lipid metabolism, transcriptional regulation, comparative genomics, transcriptomics, proteomics and metabolomics. His research on tsetse flies was recently featured on KQED's Deep Look (see news story on Deep Look). (See news story on landmark research.)
Rachel Vannette, community ecologist and assistant professor who coordinates the department's seminars, says: "All plants are colonized by microorganisms that influence plant traits and interactions with other species, including insects that consume or pollinate plants. I am interested in the basic and applied aspects of microbial contributions to the interaction between plants and insects. I also use these systems to answer basic ecological questions, such as what mechanisms influence plant biodiversity and trait evolution." (See recent research)
Arathi Seshadri and Julia Fine, who recently joined the USDA-ARS lab on Bee Biology Road, UC Davis, aim to improve honey bee survival and beekeeping sustainability in California and nationwide. They collaborate with federal, university, non-governmental and industry partners. (See news story on opening of the facility.)
Seshadri, a pollination biologist with expertise in honey bee behavior and plant reproductive strategies, is working with beekeepers and farmer stakeholders to develop projects aimed at finding solutions to the ongoing pollination challenges. Also trained as an evolutionary biologist, she has applied principles of plant-pollinator mutualism, specifically the impact of phytochemicals in pollen and nectar on honey bee health and colony performance. Her contributions to pollinator conservation include enhancing the sustainability of all pollinators, including native bees on farms and urban areas. She also has expertise in agroecosystem-based approaches and citizen science programs to promote pollinator diversity and abundance.
Fine, an entomologist with expertise in insect toxicology, honey bee physiology, reproduction and development, focuses her research on identifying how stressors impact honey bee behavior, health and fecundity. She uses both established and novel laboratory techniques. Her previous projects involved investigating how agrochemical and viral stressors interact to affect the development and survival of honey bee brood and how nutritional stress affects honey bee queen fecundity. In engaging with beekeepers and growers, Fine is researching how realistic biotic and abiotic stressors affect honey bee reproduction, longevity and pollination services, and she is identifying techniques and strategies to overcome these effects.
The seminar is open to all interested persons. For more information, contact Vannette at email@example.com.
In humans, the disease is commonly known as sleeping sickness: the parasite invades the central nervous system and disrupts the sleep cycle. If not treated, the disease can result in progressive mental deterioration, coma, systemic organ failure and death.
The newly published research in the journal Genome Biology compares and analyzes the genomes of six species of tsetse flies and could lead to better insights into disease prevention and control. “It was a behemoth project, spanning six to seven years,” said Attardo, an assistant professor in the Department of Entomology and Nematology. “This project represents the combined efforts of a consortium of 56 researchers throughout the United States, Europe, Africa and China.”
Tsetse flies, Glossina sp., are of great medical and economic importance, wrote Attardo and co-authors Adly M. M. Abd-Alla of the Insect Pest Control Laboratory, Division of Nuclear Techniques in Food and Agriculture, Vienna, Austria, and Serap Aksoy of the Yale School of Public Health, New Haven, Conn. They related that since the implementation of surveillance and record-keeping in the 20th century, “millions of people in sub-Saharan Africa” have died from sleeping sickness.
Tsetse flies, which resemble house flies, are distinguished from other Diptera by unique adaptations, including lactation and the birthing of live young, a vertebrate blood-specific diet by both sexes, and obligate bacterial symbiosis. The scientists targeted six Glossina genomes representing three sub-genera: Morsitans (G. morsitans morsitans, G. pallidipes, G. austeni), Palpalis (G. palpalis, G. fuscipes), and Fusca (G. brevipalpis) which represent different habitats, host preferences, and vectorial capacity.
“The aim of these studies,” the authors wrote, “was to generate and mine the genomic sequences of six species of tsetse flies with different ecological niches, host preferences, and vectorial capacities. The goals of the analyses performed here are to identify the novel genetic features specific to tsetse flies and to characterize the differences between the Glossina species to correlate the genetic changes with phenotypic differences in these divergent species.”
“Expanded genomic discoveries reveal the genetics underlying Glossina biology and provide a rich body of knowledge for basic science and disease control,” the scientists concluded. “They also provide insight into the evolutionary biology underlying novel adaptations and are relevant to applied aspects of vector control such as trap design and discovery of novel pest and disease control strategies.”
Attardo, who joined the UC Davis faculty in 2017 after serving 13 years with the Yale School of Public Health, said the massive research project involved “the complete sequencing and assembly of six Glossina species, including the two primary vectors of human African tryapnosomiasis, three major vectors of animal trypanosomiasis and one ancestral tsetse species which demonstrates some resistance to the species of trypanosomes responsible for human and some animal forms of the disease.”
The research, titled "Comparative Genomic Analysis of Six Glossina Genomes, Vectors of African Trypanosomes," offers:
- A clearer definition of the Glossina phylogenetic tree and placement of a controversial species.
- Identification of rapidly evolving regions of the tsetse genome relative to Drosophila.
- Identification of Glossina specific genes and their functions as well as expansions and contractions of gene families in tsetse relative to other flies.
“We discuss the functional implications of these changes and how they relate to tsetses' physiological adaptations and evolutionary history,” Attardo noted.
“We discovered that the rhodopsin gene family which is associated with vision/color detection shows conservation in motion detection and tracking associated genes.” Attardo said. “However, the gene coding for the protein that detects blue wavelengths is divergent relative to houseflies and shows the highest variance between Glossina species of all the rhodopsin genes. This is significant as the color blue is used as an attractant to bring tsetse into the traps used for control. It suggests that different species may be tuned/attracted to different wavelengths of blue.”
They also analyzed the genes associated with tsetse immunity and the relative differences in comparison with houseflies and fruit flies. “We see many immune genes missing in Glossina and increased copy numbers of genes associated with negative regulation of immune function. We think this may be associated with the evolution of obligate symbiosis as a way to protect their symbionts.”
“We also found extreme conservation of milk proteins between all sequenced species,” the UC Davis medical entomologist said. “On the flip side, male reproductive proteins (seminal proteins) appear to be very rapidly evolving relative to the rest of the genome. The copy numbers of these genes also change significantly between species.”
The scientists also found an overall reduction of olfactory associated genes and protein modifications specific to salivary proteins in the two species that vector human trypanosomiasis.
In 1995, the World Health Organization (WHO) estimated that 60 million people were at risk of sleeping sickness, with an estimated 300,000 new cases per year in Africa, and fewer than 30,000 cases diagnosed and treated. Due to increased control, only 3796 cases were reported in 2014, with less than 15,000 estimated cases, according to WHO statistics.
The parasitic disease “mostly affects poor populations living in remote rural areas of Africa,” according to WHO. “Untreated, it is usually fatal. Travelers also risk becoming infected if they venture through regions where the insect is common. Generally, the disease is not found in urban areas, although cases have been reported in suburban areas of big cities in some disease endemic countries.”
Several National Institutes of Health (NIH) grants, awarded to Attardo and Aksoy, funded the research. They also drew funding from the McDonnell Genome Institute at Washington University School of Medicine; the National Research Foundation, the Swiss National Science Foundation, and the Slovak Research and Development Agency.
The event is free and open to the public and will be hosted by Jared Shaw of the UC Davis College of Letters and Science.
“It is actually going to be a very basic talk aimed at lay audiences and kids,” Attardo says. “I'll be talking about my background, how I became an entomologist and how I ended up working on tsetse flies. Then I am going to discuss the life history of tsetse flies, where they can be found, why they are of medical importance and how their reproductive biology differs so dramatically from other flies that people are familiar with. My plan is to go over their reproductive cycle, how they develop intrauterine larvae, the reproductive adaptations that allow them to perform this feat and then go over what we know about tsetse milk secretions and how they compare to mammalian milk in terms of nutritional content.”
“The aim is for it to be very informal, with very little scientific jargon and to be discussion-oriented so that there is lots of questions and answers. I am also bringing some items from the lab that can be passed around the audience for show and tell (homemade tsetse cages, the blood feeding system we use to feed the flies and some tsetse flies preserved in alcohol).
Attardo focuses his research on numerous aspects of the physiology of tsetse fly reproduction, with the goal to identify and understand key aspects of its reproductive biology. He joined the UC Davis Department of Entomology and Nematology in 2017 from the Yale University School of Public Health, New Haven, Conn., where he researched tsetse flies in the lab of Serap Aksoy.
Attardo considers the tsetse fly "one of the champions of the insect world."
"In addition to being vectors of a deadly disease, Trypanosomiasis, these flies have undergone amazing alterations to their physiology relative to other insects," he says. "Some examples of this are their ability feed exclusively on blood, their obligate relationship with a bacterial symbiont, the fact that they lactate and that they give birth to fully developed larval offspring."
Attardo is the co-author of Adenotrophic Viviparity in Tsetse Flies: Potential for Population Control and as an Insect Model for Lactation, published in January 2015 in the Annual Review of Entomology.