Integrity of hereditary material—the genome —is critical for species survival. Genomes need protection from agents that can cause mutations affecting DNA coding, regulatory functions, and duplication during cell division. DNA sequences called transposons, or jumping genes (discovered by Carnegie’s Barbara McClintock,) can multiply and randomly jump around the genome and cause mutations. About half of the sequence of the human and mouse genomes is derived from these mobile elements.  RNA interference (RNAi, codiscovered by Carnegie’s Andy Fire) and related processes are central to transposon control, particularly in egg and sperm precursor cells.  

The Bortvin lab, with colleagues, identified a key protein that suppresses jumping genes in mouse sperm and found that the protein is vital to sperm formation. It had been known previously that a similar element did this in the fruit fly. The protein, called Maelstrom, is old evolutionarily and found in many organisms. It is implicated in transposon silencing in flies and mice by means of specialized small RNAs known as Piwi-interacting RNAs (or piRNAs).  

Bortvin’s group showed the critical role of transposon silencing for normal fertility of male mice. But only recently did they discover the impact of transposons on the mammalian egg precursor.  The group found that mouse oocytes repress transposons inefficiently. Because of this poor transposon silencing, every oocyte stores this potent mutagen. Safia Malki in the lab correlated transposon abundance with oocyte viability and oocyte cell division reliability. She found that a burst of activity of a single transposon in transgenic mice increased oocyte death. Most strikingly, Malki improved oocyte viability and prevented errors in chromosome segregation by blocking the ability of the transposon to copy itself using a drug that blocks multiplication of HIV, the AIDS-causing virus.


This unique mode of transposon control in mouse oocytes sheds light on two puzzles—prenatal death of most oocytes and the age-related increase in chromosome errors, such as those that cause Down syndrome. Malki and Bortvin speculate that the lax control of transposons in mice, and perhaps human oocytes, causes the elimination of oocytes with either highly active transposons or those incapable of more stringent transposon control.

The surviving oocytes may prevent excessive transposon alterations to the genomes and be better suited to support the healthy development of the next generation. The Malki and Bortvin findings also suggest that an ovary of a newborn girl already contains “good” oocytes as well as those predisposed for chromosomal errors. It may be the case that “good” oocytes are ovulated during first two decades of a female’s reproductive life, while “bad” ones are ovulated later.

Bortvin received his Ph. D. in genetics from Harvard and was a postdoctoral fellow at the Whitehead Institute before joining the Carnegie staff in 2004. For more information see Bortvin lab


Explore Carnegie Science

Experimental zebrafish larvae, courtesy Navid Marvi.
August 7, 2020

Baltimore, MD—New work led by Carnegie’s Meredith Wilson and Steve Farber identifies a potential therapeutic target for clogged arteries and other health risks that stem from an excess of harmful fats in the bloodstream.  Their findings are published by PLOS Genetics. 

“Cardiovascular disease occurs when lipids from the blood plasma are deposited in the walls of blood vessels, ultimately restricting blood flow,” explained Farber, who specializes in elucidating how cells process lipids. “This complex disease affects about a third of the world’s population, so improving our understanding of the mechanisms that regulate the levels of

Xenia in Carnegie's coral facility, courtesy Carnegie Embryology
June 17, 2020

Baltimore, MD— New work from a team of Carnegie cell, genomic, and developmental biologists solves a longstanding marine science mystery that could aid coral conservation. The researchers identified the type of cell that enables a soft coral to recognize and take up the photosynthetic algae with which it maintains a symbiotic relationship, as well as the genes responsible for this transaction.

Their breakthrough research is published in Nature.

Corals are marine invertebrates that build large exoskeletons from which reefs are constructed. But this architecture is only possible because of a mutually beneficial relationship between the coral and various species of

Yixian Zheng
March 11, 2020

Baltimore, MD— Carnegie’s Director of Embryology Yixian Zheng is one of 15 scientists awarded a grant from the Gordon and Betty Moore Foundation to support research on symbiosis in aquatic systems.

For the past two years, Zheng and her colleagues have been working to elucidate the molecular mechanisms of endosymbiosis in the relationships between coral and jellyfish and the photosynthetic algal species that they host. She has been building on Carnegie’s longstanding tradition of model organism development to begin revealing the genetics underlying the uptake and sustenance of symbiotic dinoflagellates by the soft coral species Xenia.

“I have always

Illustration courtesy of Navid Marvi and Andres Aranda-Diaz.
March 5, 2020

Baltimore, MD—Antibiotics can make easy work of infections. But how do they affect the complex ecosystems of friendly bacteria that make up our microbiome?

“When a doctor prescribes antibiotics, it sets up a multi-faceted experiment in your gastrointestinal system,” explains Carnegie’s Will Ludington “What can it teach us about the molecular principles of species interactions in nature?”

New work led by Ludington and Stanford University’s K.C. Huang set out to answer this challenging question and discovered a new form of antibiotic tolerance. Their findings, which have important health implications, are published by eLife.

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The Zheng lab studies cell division including the study of stem cells, genome organization, and lineage specification. They study the mechanism of genome organization in development, homeostasis—metabolic balance-- and aging; and the influence of cell morphogenesis, or cell shape and steructure,  on cell fate decisions. They use a wide range of tools and systems, including genetics in model organisms, cell culture, biochemistry, proteomics, and genomics.


The Spradling laboratory studies the biology of reproduction. By unknown means eggs reset the normally irreversible processes of differentiation and aging. The fruit fly Drosophila provides a favorable multicellular system for molecular genetic studies. The lab focuses on several aspects of egg development, called oogenesis, which promises to provide insight into the rejuvenation of the nucleus and surrounding cytoplasm. By studying ovarian stem cells, they are learning how cells maintain an undifferentiated state and how cell production is regulated by microenvironments known as niches. They are  also re-investigating the role of steroid and prostaglandin hormones in controlling

The Marnie Halpern laboratory studies how left-right differences arise in the developing brain and discovers the genes that control this asymmetry. Using the tiny zebrafish, Danio rerio, they explores how regional specializations occur within the neural tube, the embryonic tissue that develops into the brain and spinal cord.

The zebrafish is ideal for these studies because its basic body plan is set within 24 hours of fertilization. By day five, young larvae are able to feed and swim, and within three months they are ready to reproduce. They are also prolific breeders. Most importantly the embryos are transparent, allowing scientists to watch the nervous system develop and to

The Gall laboratory studies all aspects of the cell nucleus, particularly the structure of chromosomes, the transcription and processing of RNA, and the role of bodies inside the cell nucleus, especially the Cajal body (CB) and the histone locus body (HLB).

Much of the work makes use of the giant oocyte of amphibians and the equally giant nucleus or germinal vesicle (GV) found in it. He is particularly  interested in how the structure of the nucleus is related to the synthesis and processing of RNA—specifically, what changes occur in the chromosomes and other nuclear components when RNA is synthesized, processed, and transported to the cytoplasm.

Brittany Belin joined the Department of Embryology staff in August 2020. Her Ph.D. research involved developing new tools for in vivo imaging of actin in cell nuclei. Actin is a major structural element in eukaryotic cells—cells with a nucleus and organelles —forming contractile polymers that drive muscle contraction, the migration of immune cells to  infection sites, and the movement of signals from one part of a cell to another. Using the tools developed in her Ph.D., Belin discovered a new role for actin in aiding the repair of DNA breaks in human cells caused by carcinogens, UV light, and other mutagens.

Belin changed course for her postdoctoral work, in

Evolutionary geneticist Moises Exposito-Alonso joined the Department of Plant Biology as a staff associate in September 2019. He investigates whether and how plants will evolve to keep pace with climate change by conducting large-scale ecological and genome sequencing experiments. He also develops computational methods to derive fundamental principles of evolution, such as how fast natural populations acquire new mutations and how past climates shaped continental-scale biodiversity patterns. His goal is to use these first principles and computational approaches to forecast evolutionary outcomes of populations under climate change to anticipate potential future

Staff Associate Kamena Kostova joined the Department of Embryology in November 2018. She studies ribosomes, the factory-like structures inside cells that produce proteins. Scientists have known about ribosome structure, function, and biogenesis for some time. But, a major unanswered question is how cells monitor the integrity of the ribosome itself. Problems with ribosomes have been associated with diseases including neurodegeneration and cancer. The Kostova lab investigates the fundamental question of how cells respond when their ribosomes break down using mass spectrometry, functional genomics methods, and CRISPR genome editing.

Kostova received a B.S. in Biology from the

Sally June Tracy applies cutting-edge experimental and analytical techniques to understand the fundamental physical behavior of materials at extreme conditions. She uses dynamic compression techniques with high-flux X-ray sources to probe the structural changes and phase transitions in materials at conditions that mimic impacts and the interiors of terrestrial and exoplanets. She is also an expert in nuclear resonant scattering and synchrotron X-ray diffraction. She uses these techniques to understand novel behavior at the electronic level.  Tracy received her Ph.D. from the California Institute of