Evolutionary déjà vu

This article is part of our series, “Evolution in action”. To view the entire collection, click here.

Bats, whales, and humans have more in common than you’d expect. In fact, if we were to rewind the clock far enough, we’d find that all life on Earth can be traced back to Luca, or the Last Universal Common Ancestor. From this single point of origin, the tree of life has sprawled out over hundreds of millions of years into the striking diversity that surrounds us today.

Surprisingly, this journey towards survival has unfolded similarly across species, a sort of evolutionary déjà vu: passing down of some traits to the next generation, fine-tuning of others, and the emergence, repression, or complete abandonment of a few. Researchers at Whitehead Institute are particularly interested in studying reproductive, developmental, and regenerative mechanisms that are preserved between generations and across multiple species. These processes, they hope, will offer a window into human health. 

The underappreciated chromosome

Genes play a key role in determining which traits develop within a species and get passed down from one generation to the next. These genes are arranged along strands of DNA, like beads on strings, known as chromosomes. During reproduction, each parent contributes a set of chromosomes, allowing the offspring to inherit a unique combination of traits from both parents.

Whitehead Institute Member David Page has dedicated his career to studying a specific pair of chromosomes, X and Y, which determine an organism’s sex across many species. These chromosomes carry genes essential for sexual development, reproduction, and survival.

Typically, males possess one X and one Y sex chromosome, while females have two X chromosomes. Previously, one of the two X chromosomes in females was considered 'inactive.' But, researchers at the Page lab have discovered that the inactive X chromosome can, in fact, modulate gene expression in the active-type X chromosome.

This overlooked role of the inactive X chromosome in gene expression may explain differences in the incidence rates and symptoms of many diseases between males and females.

Healing factor

Whitehead Institute Member Peter Reddien is focused on studying an organism with a superpower — the planarian.  With its flattened, ribbon-like body, and googly eyes, this flatworm might look ordinary but has mastered the craft of regeneration; not only does a planarian reproduce by tearing itself in half, it’s able to restore anything from an eye or a tail to a head.

This remarkable feat is made possible by stem cells, known as neoblasts, which can give rise to a wide array of mature cell types. But how these stem cells select their biological fate — whether they will become an eye, a head, or a tail — has intrigued scientists for decades. 

Researchers from the Reddien lab answered this longstanding question in a recent study using cutting-edge cell imaging technology that allowed the team to simultaneously visualize the choices of hundreds of individual stem cells.

Surprisingly, they discovered that stem cells operate with little spatial organization. Much like rolling a dice, they can choose to become an eye or skin or gut cell, regardless of the decisions made by nearby cells. This process unfolds in a seemingly chaotic, haphazard manner, with cells of different destinies jumbled together.

While the exact molecular mechanisms behind this disorganization remain unclear, these findings have significant implications for our understanding of tissue repair and regeneration in humans, who also rely on stem cells for these processes.

“The more we understand about the mechanisms underlying regeneration in animals that naturally can accomplish it, the better we will understand the basis for the limitations of our own regenerative capacity and whether we might improve upon it,” Reddien says.

Seeds of tomorrow

From ancient hunter-gatherer societies to modern-day food systems, human survival hinges on food security. But with increasingly erratic growing seasons and more frequent extreme weather events, such as droughts and floods, a global food crisis is on the horizon.

Work from the lab of Whitehead Institute Member and David Baltimore Chair Mary Gehring has revealed that essential processes governing plant reproduction and seed development are controlled by the epigenetic activation and deactivation of genes. This means, by manipulating which genes get turned “on” and “off” and to what degree, the researchers can alter seed traits and potentially engineer stress-resistant, more nutritious, and higher yielding plants capable of withstanding changing climatic conditions. 

Gehring and lab members are using these insights to enhance resiliency of underappreciated agricultural crops like pigeon pea. But their work doesn’t only apply to plants; the same pathways — DNA methylation, demethylation, and chromatin remodeling — regulate gene expression and function in mammals too, helping scientists advance their understanding of human biology.  

As the labs of Gehring, Page, and Reddien continue studying mechanisms underlying reproduction, development, and regeneration — many of which are shared across species — they hope to discover insights into human health and disease.