Crazy ants have wacky genetics thanks to their never-before-seen way of reproducing

It turns out that yellow crazy ants (Anoplolepis gracilipes), named after their erratic walking style, live up to their name in more ways than one.

Scientists have for the first time, discovered that males of this notoriously invasive species are “chimeras” – a phenomenon which is usually just a developmental accident, but in this case is due a mode of reproduction previously unknown to science.

Dr Hugo Darras, Assistant Professor at Johannes Gutenberg University Mainz (JGU) in Germany says the discovery was “highly unexpected.”

“The results of previous genetic analyses of the yellow crazy ant have shown that the males of this species have two copies of each chromosome. Males usually develop from unfertilised eggs in ants, bees, and wasps – and therefore should only have one maternal copy of each chromosome,” he says.

The extraordinary results are described in a new paper in Science.

A yellow crazy ant with long, thin legs and antennae
A male yellow crazy ant (Anoplolepis gracilipes). Credit: Hugo Darras

Until now, it had been assumed that the males carried the same two sets of chromosomes in all cells of their bodies like other normal multicellular animals including humans. All the cells of our body (except gametes, sperm and eggs) contain copies of both our maternal and paternal chromosomes, so we’re diploid.

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A longitudinal section of the brain of a chimeric male yellow crazy ant with maternal (pink) and paternal (blue) genomes in situ hybridisation: The male tissue consists of large cell clusters carrying only maternal or paternal genomes. Credit: Hugo Darras

Instead, the researchers found that the cells in the male crazy ant’s body are actually a mixture of two different lineages – some contain copies of maternal chromosomes, while others contain the paternal ones, so they’re haploid.

“We discovered that the male ants have maternal and paternal genomes in different cells of their body and are thus chimeras. To put it another way, all males have two genomes, but each cell of their bodies contains only one or the other of the two genomes,” explains Darras, who is lead author of the study.

How is this possible?

Male crazy ants develop from fertilised eggs where the two parental gametes (the sperm and egg) do not fuse. Instead, they continue to divide and multiply separately within the same egg, so that the resulting adult males have both parents’ DNA, but in different cells in the body.

If the gametes were to fuse, this would result in either a queen or a worker developing. The mechanism that determines whether this fusion takes place or not is not yet known.

Chimeras are individual organisms whose cells contain different genetic materials. Normally, this occurs naturally in some species where separate individuals can merge to become one – like corals and angler fish.

They can also be found in humans and other placental mammals. During gestation small-scale exchanges of cells can occur between mother and fetus, so that the offspring usually has a few cells that contain the same genome as the mother, or between twins in the womb.

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