A trap of Culex pipiens mosquitoes from France, provided by EID (Entente Interdépartementale Démoustication) in Montpellier, France. Credit: Julie Reveillaud

WOODS HOLE, Mass. -- Controlling mosquito-borne illnesses, such as Dengue or West Nile virus, has historically been difficult due to a lack of effective vaccines and concerns about the environmental impact of insecticides. Thus, scientists have turned to manipulating Wolbachia, a parasitic bacterium within mosquitoes, as a way to control the reproductive fitness of mosquito populations that transmit human disease.

In a new study in Nature Communications, an international team including scientists from the Marine Biological Laboratory (MBL) and the University of Chicago identified a new mobile DNA element in Wolbachia, which may contribute to improved control strategies for mosquito vectors of disease.

Led by former MBL scientists Julie Reveillaud of INRA, France, and Sarah Bordenstein of Vanderbilt University, the researchers reconstructed near-complete genomes of Wolbachia isolated from individual ovaries of four Culex pipiens mosquitoes. In the process, they identified a novel plasmid — a circular piece of DNA that can replicate independently of the chromosomes. Because a plasmid is a mobile DNA element, it can transfer from one cell to another and can have great implications for the fitness and evolution of a microbial species. Mobile genetic elements that can spread through different Wolbachia cells, and thus across a Wolbachia population, hold promise for controlling mosquito populations that may carry disease.

“Our data show that this novel plasmid is widespread across natural Wolbachia populations that infect C. pipiens mosquitoes throughout the world, which implies it has an essential role. The idea that it may enable transformation of the Wolbachia populations is simply very exciting,” says the senior author of the study, A. Murat Eren (Meren), an assistant professor of medicine at the University of Chicago and an MBL Fellow.

Wolbachia is transmitted from mother to offspring and can influence the reproductive behavior of its mosquito host. Wolbachia can modify sperm so if an infected male mates with an uninfected female, or with one that carries a different strain of Wolbachia, embryos cannot develop. The mechanism behind this embryonic killing lies within another mobile genetic element in the Wolbachia genome.

“Wolbachia populations do not lend themselves to direct genetic modification. While a natural plasmid sounds very promising to bypass those limitations, we do not yet have any evidence to suggest the possibility or efficacy of any transformation through this plasmid,” Meren says. ”We just discovered its existence, and the time will tell how useful it is.”

“Other exciting questions for which we do not have yet clear answers include whether there is an ancestral relationship between the plasmid and other mobile genetic elements of Wolbachia, and whether these interact or not,” adds Reveillaud.

The Wolbachia bacterium (colorized purple). Credit: Dennis Kunkel, Sarah Bordenstein and Robert Brucker

This collaboration began at MBL in 2011, when Meren and Reveillaud brought their computational and molecular evolution backgrounds together to explore questions in microbial ecology in the MBL's Bay Paul Center. (See Reveillaud’s blog post about the collaboration.) Years later, they began examining Wolbachia genomic diversity by focusing on individual mosquitoes captured in the wild, rather than using laboratory mosquito strains. With the help of former MBL scientists Sarah Bordenstein and Seth Bordenstein of Vanderbilt University, who have expertise in Wolbachia, they began unraveling the intricacies of the Wolbachia genome.

“Wolbachia is particularly difficult to study because, unlike free-living bacteria, it cannot be cultured in cell-free media. Through this interdisciplinary collaboration, we were able to tease apart the genomes of the mosquito host, Wolbachia, and its mobile elements,” says Sarah Bordenstein.

While fragments of this novel plasmid had appeared in previous Wolbachia sequencing studies, these fragments had never been assembled into a complete circular piece and its extrachromosomal nature was not recognized, most likely due to computational limitations. The team was able to overcome these limitations by employing a combination of cutting-edge strategies, such as genome-resolved metagenomics and long-read sequencing.

“The key bioinformatics platform that enabled us to study mosquito metagenomes and Wolbachia pangenomes in this study, and the connections between scientists with distinct skills that made this discovery possible, were developed at the MBL,” Meren says. “Almost everyone in this paper is somehow linked to the MBL.”

Written by Jennifer Tsang

Homepage photo: Surface sterilizing a Culex pipiens mosquito sample before dissection. Credit: Julie Reveillaud

Citation:

Reveillaud, Julie and Bordenstein, Sarah et al (2019) The Wolbachia mobilome in Culex pipiens includes a putative plasmid. Nature Communications, DOI: 10.1038/s41467-019-08973-w

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The Marine Biological Laboratory (MBL) is dedicated to scientific discovery – exploring fundamental biology, understanding marine biodiversity and the environment, and informing the human condition through research and education. Founded in Woods Hole, Massachusetts in 1888, the MBL is a private, nonprofit institution and an affiliate of the University of Chicago.

A female Brachionus manjavacas rotifer, carrying four large eggs. The hair-like cilia at the top are used for swimming and filter feeding, while the tail-like “foot,” ending in two small toes, is used for defense and attachment. Photo credit: Michael Shribak and Kristin Gribble

WOODS HOLE, Mass. – Advanced maternal age at the time of giving birth is known to decrease how long the offspring will live and their fecundity. However, why this occurs is unknown, and it remains unclear if maternal age also alters how offspring respond to interventions to aging.

Researchers at the Marine Biological Laboratory (MBL) are working on answers. In a study published in Scientific Reports, they demonstrate that maternal age affects how well offspring respond to dietary interventions that are known to increase lifespan.

“There has been evidence for well over a century, from experiments done in a wide variety of animal species and from data in humans, that offspring from older mothers have shorter lifespans and lower [rates of] reproduction,” says MBL scientist Kristin Gribble. “But it wasn’t well understood how a mother’s age might affect other aspects of her offspring’s health or response to interventions.”

Using rotifers (microscopic, water-dwelling animals), Gribble and her team studied the effects of maternal age on offspring aging and their response to dietary changes. In their experiments, they fed mother rotifers a regular diet. They then studied the offspring from young (about three days old), middle-aged, and advanced-aged (about nine days old) mothers. The offspring were fed one of three different diets: constant high food, constant low food, or alternating between high food and fasting every other day.

“These calorie-restricted and intermittent-fasting diets are known to significantly increase lifespan in rotifers and many other species,” says Gribble.

“Our study confirmed that offspring from older mothers have shorter lifespans and lower reproductive rates than offspring of younger mothers,” Gribble says. “However, offspring of older mothers, we found, have a greater increase in lifespan in response to caloric restriction than do young-mother offspring.”

In the offspring born from older mothers, the decreased lifespan seemed to be due to an earlier onset of aging. This early onset was delayed when those offspring were subject to caloric restriction.

Even though offspring from older mothers responded more positively to caloric restriction, it did not improve their overall fitness. In evolutionary terms, “fitness” takes into consideration both lifespan and rates of reproduction. For old-mother offspring on caloric restriction or full food diets, the window for reproduction was shortened and they had half as many offspring – only 14-15, instead of the average of 25 to 30 for young-mother offspring. Caloric restriction did not rescue reproduction.

“It is an important finding that a mother’s age can affect not only her offspring’s lifespan and reproduction, but also other aspects of her offspring’s health, body size, and responses to environmental conditions,” says Gribble. “This could have implications for how drugs, diet, exercise, or other interventions might differently affect children from young mothers and from older mothers.”

In their next steps, Gribble and her lab will probe the molecular mechanisms controlling maternal age effects in rotifers. Given that rotifers and humans share many genes, they think their findings may have implications for human health, and may be used to develop therapies to treat human aging.

“Rotifers are quite different than people, of course. But there are already many studies in humans that suggest that advanced maternal age negatively impacts children’s lifespans, and increases the chance for neurodegenerative and other diseases,” says Gribble. “I think it is likely that maternal age may also determine the response to environmental conditions, lifestyle, and medical treatments in people. A lot more work will need to be done to determine if our findings hold in people and if they are caused by similar molecular mechanisms.”

Written by Stephanie McPherson

Homepage photo: A female Brachionus manjavacas rotifer, carrying four large eggs. Credit: Michael Shribak and Kristin Gribble

Citation: Brock, Martha J. et al (2019) Maternal age alters offspring lifespan, fitness, and lifespan extension under caloric restriction. Scientific Reports, DOI: 10.1038/s41598-019-40011-z

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The Marine Biological Laboratory (MBL) is dedicated to scientific discovery – exploring fundamental biology, understanding marine biodiversity and the environment, and informing the human condition through research and education. Founded in Woods Hole, Massachusetts in 1888, the MBL is a private, nonprofit institution and an affiliate of the University of Chicago.