The Ants with Three Sexes

While making the recent BBC Radio 4 series, Sexual Nature: A Brief Natural History of Sex, I came across some research on North American ants with an extremely weird mating system.    They have three sexes  - sort of.

The strange sexual antics are going in at least two populations of seed harvester ant, Pogonomyrmex, which live in southwestern United States and northern Mexico.   'Sexual Nature' presenter Adam Rutherford interviewed Joel Parker who’d done some work on them a few years ago. 

Unfortunately the interview we recorded with Joel, at Plattsburgh University in remote New York State, didn’t make it into the final cut of the series.  But it is one of those Aren't-Ants-Amazing stories, deserving of more time than we had in the 'Sexual Nature' slot.   So I've posted a link to the audio of Joel's interview below. 

Before getting stuck in, it helps to be armed with the basics of who's who in a regular ant colony, and vanilla reproduction and parentage in ants. 

There is the queen ant of the colony who is the one reproductive female in the family.  She is the result of a mating between a male and female.

Pogonomyrmex queen

There are the workers who are also female but they are sterile.   They don't reproduce.  They  gather food for the colony, defend the nest, do nursery duties etc.  They are queen's daughters - the results of mating between female and male parents.

Pogonomyrmex workers

Third, there are males.  They have just one parent - a queen.  They hatch from her unfertilised eggs.  No fatherly input.  Males are made merely for their sperm and are only produced at mating time.  They don't live very long.

Male Pogonomyrmex being killed by a spider

To recap, in a regular colony, queens and workers are the result of matings between a female ant and a male ant.

But in these two strange Pogonomyrmex populations - one is red, the other is black -  Joel et al discovered that the queens need to mate with one kind of male to produce future queens and another kind of male to produce the workers.  The survival and future of an ant colony needs both queens and the worker caste so you can argue that the two types of males amount to two sexes.

What makes this situation even more intriguing is the two populations are dependent on each other.    Red queens mate with black males to make workers, and black queens need red males to make black workers.  New queens are made through red-on-red and black-on-black sex.  So the whole system actually needs four sexes. 

Here's Joel Parker talking about all this and how it might have come about.

Also, read more in this article by Joel Parker, published in 2004 in 'Trends in Ecology and Evolution'.  This piece from the Public Library of Science (Biology) is also good on the number of sexes in nature.