A pair of matching sex chromosomes could mean a longer life

A new study finds that animals with a pair of matching sex chromosomes have a greater life expectancy.

In humans and other mammals, females are homogametic, meaning they have two X chromosomes. The males are heterogametic, which means they have two different sex chromosomes — an X chromosome and one “smaller” Y chromosome.

A new report from researchers at the University of New South Wales in Sydney, Australia, suggests that having a pair of the same sex chromosomes may be part of the reason females live longer.

The authors find that across the animal kingdom, those with two identical sex chromosomes live an average of 17.6% longer than those with two different kinds of sex chromosomes.

The paper appears in The Royal Society journalBiology Letters.

Diverse and yet the same

Mammals are not the only animals in which a pair of matching chromosomes have associations with a particular sex.

Also, not all females are homogametic. Male birds and some reptiles, for example, have ZZ chromosomes, while females are heterogametic, having a ZW chromosome combination.

Some animals do not have a second sex chromosome at all, including most male arachnids.

Using existing data from scientific books, papers, and online databases, the researchers studied the longevity records of both sexes in 229 animal species, including 99 families, 38 orders, and eight classes.

Says lead author ecologist Zoe Xirocostas, “I thought it was really cool how, across insects and fish, we’re all showing the same sort of response.”

The researchers found that those species in which the females have a pair of matching sex chromosomes see a more significant boost in longevity than homogametic males do.

In species such as mammals, insects, fish, and some reptiles — where females are the homogametic sex — females live 20.9% longer. In species where the males are homogametic, such as birds and butterflies, the increase in lifespan is just 7.1%.

What explains the difference?

It is not clear whether this disparity reflects a greater capacity for long lives in homogametic females, or whether it reveals that male longevity is adversely affected by other factors, regardless of whether they have matched sex chromosomes.

An expert who was not involved in the study, evolutionary biologist Gabriel Marais from Claude Bernard University Lyon, France, suggests two such possible factors.

First, the energy spent by males in attracting females takes a toll on them.

Males tend to sport exaggerated physical traits, especially homogametic males — male birds, for example, are more brightly colored than females.

Special, energy-consuming mating behaviors are also more common among males, as with the dramatic mating dance of the bird of paradise.

“We know that sexual selection is stronger in males,” Marais says, adding that perhaps males “pay the cost of this sexual selection by faster aging, and they will die younger.”

There is also the “unguarded X” hypothesis, which the researchers wanted to test. The Y chromosome is smaller than the X chromosome, and in some cases, totally absent.

Because of this, the hypothesis considers the Y chromosome too small to cover an X chromosome that has harmful mutations, which leaves the individual exposed to possible health threats.

A tendency toward the steady accumulation of life-shortening mutations may be present in individuals of either sex with one reduced or missing chromosome.

However, in males, the impact of this may be more profound since it compounds the already noted effects involved in sex selection.

Marais considers the study a good start in a largely unexplored field with many questions. For example, scientists do not yet know whether the relatively smaller size of a second, non-matching chromosome has any effect on longevity.

There’s also a tremendous amount of diversity between species that scientists will want to explore further. The authors of the report found that, for example, the XX female Blattella germanica — the German cockroach — outlives her male counterparts by significantly more than the 20.9% of other homogametic females.

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