Cheating Chromosomes Vault Past Competitors to Reach Egg Cell First

Each of your cells contains two copies of 23 chromosomes, one inherited from your father and one from your mother. Theoretically, when you create a gamete – a sperm or an egg – each copy has a 50-50 shot at being passed on. But the reality isn’t so clear cut.

Studies by scientists at the University of Pennsylvania have now shown how, during female gamete formation, certain genes, or “selfish elements” can actually improve their chances of being passed into the egg cell, through a process known as meiotic drive. Essentially, female gamete formation is not simply a game of chance.

“Usually we think about selfish genes at the level of natural selection and selection of the fittest,” comments Michael Lampson, Ph.D., associate professor of biology at Penn’s School of Arts and Sciences. “That might mean a gene that makes you live longer or reproduce more or kill your enemies is more likely to be passed on. But we can also think about selfishness at the level of the gene itself. In that context, genes are competing with each other to get into the gamete.”

This vying for position is particularly relevant to female gamete formation, because the meiotic cell division results in formation of a large egg and a smaller polar body, which is degraded. This means that only those chromosomes that make it into the egg can feasibly be passed on to the next generation—assuming the egg is fertilized. “Female meiosis provides a clear opportunity for selfish elements to cheat because only chromosomes that segregate to the egg can be transmitted to offspring,” the researchers write in their published paper in Science (“Spindle Asymmetry Drives Non-Mendelian Chromosome Segregation”).

This bias in chromosome segregation during female gamete formation isn’t a new observation. Selfish elements preferentially attach to the “egg” side of the meiotic spindle, a structure of microtubules that pulls the chromosomes to opposite poles of the dividing cell. “While we had evidence that this could happen, we didn’t really understand how it did happen,” Lampson adds. “If we understand how these selfish elements are exploiting the mechanics of meiosis, then we’ll understand more deeply how that process works in the first place.”

The University of Pennsylvania team reasoned that there must be something about the mechanics of cell division that underpins meiotic drive. Studying mouse oocytes, they showed signaling from the cell cortex led to asymmetric tyrosination of the spindle microtubules, with the egg side of the cell demonstrating less tyrosination, closer to the cortex. The asymmetry was only evident at the stage in meiosis when the spindle moves from the middle of the cell toward the cortex. “That told us that whatever signal is setting up the tyrosine modification is coming from the cortex,” Lampson said.

The team already had some data on the expression of signaling factors on the cortical side of the cell, including a molecule called CDC242. Their results, using a light-sensitive assay system devised by Lampson and colleague David Chenoweth, Ph.D., an associate professor in the department of chemistry, indicated that CDC42 was at least in part responsible for inducing the asymmetric tyrosination that  resulted in spindle asymmetry in the dividing cell.

To find out how this all relates to chromosomal cheating, the team then focused on the chromosomes’ centromeres, which attach each chromosome to the spindle. The researchers generated experimental mice that possessed two different types of centromeres – one large and one small – in each cell. Their earlier work had shown that larger centromeres were more likely to move toward the egg-side pole of the cell. The new studies found preventing spindle asymmetry by mutating CDC42 made this bias in centromere orientation disappear. “That connects the spindle asymmetry to the idea of chromosomes or centromeres actually cheating,” Lampson notes.

What the scientists didn’t yet know, however, was at what point the centromere orientation became biased. The spindle is initially symmetrical and sited in the middle of the cell, and the centromeres initially attach in an unbiased manner. Spindle asymmetry and biased centromere attachment occur at a later time point, but how?

Live imaging of mouse oocytes suggested that the stronger centromeres could actually “flip” to change their orientation when the spindle became asymmetric. “Thus, the bias arose from reorientation or flipping of stronger centromeres from the cortical to the egg side of the spindle while it was cortically positioned and asymmetric,” the authors write. Effectively, these stronger centromeres were capable of detaching from the spindle, if they were orientated to the cortical side of the cell and flip to reorient themselves towards the egg pole of the cell. “If you’re a selfish centromere and you’re facing the wrong way, you need to let go so you can face the other way,” Lampson says. “That’s how you ‘win’.” In contrast, the weaker centromeres didn’t show any propensity to detach, and had no preference for one side of the cell or the other.

“Here, we have shown that asymmetry within the spindle is essential for meiotic drive,” the authors conclude.“Because signals from the cell cortex regulate MTs [microtubules] to induce spindle asymmetry and the cortical side ultimately ends up in the polar body, our findings explain how spindle asymmetry is consistently oriented relative to cell fate, providing spatial cues to guide the segregation of selfish elements.”

The researchers hope to investigate which characteristics make centromeres either weak or strong. “This work gave us some good information about biased transmission of centromeres, but it also brings up a ton of other questions,” Lampson acknowledges. “Why do our centromeres look the way they do, and how do they evolve to win these competitions? These are fundamental biological questions that we still don’t know a lot about.”