Tag Archives: chromosomes

How These All-Female Lizards Are Able to Reproduce and Thrive Without the Help of Any Males

As far back as the 1960s, scientists were aware that a number of whiptail lizards in Mexico and the southwestern United States were made up entirely of females.

The most notable of these species, the New Mexico whiptail lizard, is able to reproduce healthy, well-bred offspring without the aid of male fertilization.

Whiptails aren’t the only species that reproduce asexually. In fact, there are 70 other vertebrate species that can do it. But the New Mexico whiptail may have unlocked the secret as to how it’s possible for a species that produces exclusively asexually to thrive.

Komodo dragons are among the vertebrate species that are able to reproduce asexually

Peter Baumann works at the Stowers Institute for Medical Research in Kansas City, Missouri. He co-authored a study on the lizards that was published in the journal Nature back in 2010.

Baumann explains that parthenogenteic species (species that reproduce without fertilization), are genetically isolated because they only inherit the DNA of one parent.

This means that any genetic weaknesses, like susceptibility to a disease or physical mutation, can’t be “overridden” by healthy genes from a second parent. The shallower the gene pool, the more likely it is to produce sick or mutated offspring.

To deal with this issue, the all-female whiptail lizard species have evolved to start the reproductive process with twice as many chromosomes as their sexually-producing lizard relatives.

New Mexico whiptail lizards were actually the result of two different species of lizard (the western whiptail and little striped whiptail) interbreeding to form a hybrid species. Because of this, these all-female lizards are equipped with a very diverse gene pool.

Left: little striped whiptail. Middle: New Mexico whiptail. Right: tiger whiptail. Click to enlarge

Instead of combining homologous chromosomes (like sexual species do, getting one set from each parent), the lizards pair recombined sister chromosomes instead. This maintains heterozygosity in the offspring.

Here’s a more simple way to think about it. Every one one us has DNA from generations and generations of our ancestors. When we reproduce, we combine our DNA with our partner’s- the resulting offspring’s genetic codes contains parts of both parents’ DNA.

But since we have such vast genetic diversity from all of our ancestors, the exact coding of the genes we pass along when we reproduce isn’t always the same, which is why brothers and sisters don’t all look the same.

A basic way to visualize how genetic information is passed on in sexual reproduction. Note that the “marbles” passed on by each individual parent are different for the two children. Click to enlarge

So, rather than combining its genetic code with that of a male, the whiptail lizard combines two different versions of its own DNA code, ensuring that each pairing of sister chromosomes will have multiple alleles (different forms of a gene), which gives the offspring the genetic diversity it needs to be healthy.

This discovery means that,

“these lizards have a way of distinguishing sister from homologous chromosomes,”

says Baumann. How do they do this? The researchers aren’t sure yet, but it’s the next question they will be investigating, along with the question of how they evolved to start reproduction with double the normal amount of chromosomes.

Female whiptail lizards perform courtship rituals with one another to stimulate ovulation. The top lizard will lay smaller eggs while the one on the bottom will lay larger eggs. They switch spots every mating season. Click to enlarge

Though it may seem like asexual reproduction would eventually hurt a species in the long run, Baumann also pointed that,

“You’re greatly increasing the chances of populating a new habitat if it only takes one individual.”

It seems to be working pretty well for these lizard ladies.

Read the original story from the Scientific American here.

Watching A Cell Divide Under An Electron Microscope Is Mesmerizing (GIF)

Mitosis, or cell division, is the process by which your body grows and/or repairs itself by producing more cells.

In the first stage of mitosis, known as prophase, the cell condenses, and the chromosomes inside the cell’s nucleus replicate. The membrane which encases the nucleus also disappears.

In the second stage, metaphase, these chromosomes align along the center of the cell, held in place by structures known as microtubules. Then in the third phase, anaphase, the chromatids which make up each chromosome are pulled apart to opposite ends of the cell.

In the last phase, telophase, the microtubules disappear, new membranes form around each set out chromosomes, and the cell completes its division. The incredible gif below (courtesy of Nikon’s MicroscopyU) shows a cell going through mitosis:

This gif is sped up though. Mitosis can take anywhere from a few minutes to years or even decades, depending on the animal and what type of cell is being replicated.

For example, human skin cells can replicate in about 20-24 hours, which is why we are able to heal relatively quickly after a cut or scrape.

Human liver cells, on the other hand, take a year or more to replicate, which is why the livers of alcoholics are often destroyed by heavy alcohol consumption over an extended period of time.

The diagram below illustrates the phases of mitosis and gives a little more detail about what is happening in each phase.

Click to enlarge

How Unemployment Is Literally Shortening Men’s Lives

A team of researchers from the Imperial College of London and the University of Oulu in Finland recently conducted a study where they examined the DNA of 5,620 men and women born in Finland in 1966.

The researchers were measuring the subjects’ telomeres. Telomeres are the structures at the ends of our chromosomes which protect our genetic code. They naturally wear away and shorten as we age, so scientists are able to use them as a biological marker for aging.

Telomeres and DNA structure

The researchers examined DNA samples from 1997 when the subjects were all 31. What they found was that men who had been unemployed for more than 2 of the 3 years before the samples were taken were more than twice as likely to have shorter telomeres compared to men who were employed continuously.

The study also accounted for a number of other, “social, biological and behavioural factors” to rule out the possibility that the short telomeres were a result of other causes.

This trend was not observed in the women in the study, however.

Read the full story here.