Female Ants Go After Foreign Sex

Photo: Micheal Herrmann & Sara Helms Cahan

Inter-genomic sexual conflict drives antagonistic coevolution in harvester ants.

In a fascinating paper published by the Royal Society a few days ago, two researchers have shown how the opposed interests of males and females in hybridization can lead to different competing strategies. Like many species of social insects such as ants and wasps, the females mate once a year, or even sometimes only once in a lifetime.

The harvester ants of the closely related Pogonomyrmex genus mate each year after the monsoon rains and the females mate with males of not only their own species but with males of related species too. In fact, they need to mate with the males of other species because their eggs which are fertilised by foreign sperm develop into the sterile female workers. Only the eggs fertilised by her own species will develop into the next generation of queens and drones.

Abstract

The reproductive interests of males and females are not always aligned, leading to sexual conflict over parental investment, rate of reproduction and mate choice. Traits that increase the genetic interests of one sex often occur at the expense of the other, selecting for counter-adaptations leading to antagonistic coevolution. Reproductive conflict is not limited to intraspecific interactions; interspecific hybridization can produce pronounced sexual conflict between males and females of different species, but it is unclear whether such conflict can drive sexually antagonistic coevolution between reproductively isolated genomes. We tested for hybridization-driven sexually antagonistic adaptations in queens and males of the socially hybridogenetic ‘J’ lineages of Pogonomyrmex harvester ants, whose mating system promotes hybridization in queens but selects against it in males. We conducted no-choice mating assays to compare patterns of mating behaviour and sperm transfer between inter- and intra-lineage pairings. There was no evidence for mate discrimination on the basis of pair type, and the total quantity of sperm transferred did not differ between intra- and inter-lineage pairs; however, further dissection of the sperm transfer process into distinct mechanistic components revealed significant, and opposing, cryptic manipulation of copulatory investment by both sexes. Males of both lineages increased their rate of sperm transfer to high-fitness intra-lineage mates, with a stronger response in the rarer lineage for whom mating mistakes are the most likely. By contrast, the total duration of copulation for intra-lineage mating pairs was significantly shorter than for inter-lineage crosses, suggesting that queens respond to prevent excessive sperm loading by prematurely terminating copulation. These findings demonstrate that sexual conflict can lead to antagonistic coevolution in both intra-genomic and inter-genomic contexts. Indeed, the resolution of sexual conflict may be a key determinant of the long-term evolutionary potential of host-dependent reproductive strategies, counteracting the inherent instabilities arising from such systems.

Michael Herrmann and Sara Helms Cahan;
Inter-genomic sexual conflict drives antagonistic coevolution in harvester ants
Proc. R. Soc. B December 22, 2014 281 1797 20141771; doi:10.1098/rspb.2014.1771 1471-2954

So, while this is in the interests of the females, it is certainly not in the interests of the males whose genes get no benefit from the arrangement. The return on their investment in seeking out a mate and using their supply of sperm is zero, but the females get a supply of workers to help rear their fertile offspring which carry their genes into the next generation.

So to minimise this loss, males attempt to break off copulation once they discover their mistake; females, however need to mate long enough to get a supply of sperm sufficient to supply her with all the workers she needs for the coming year, so they cling on to the males until they've had their fill, so to speak.

Now, what intelligent designer would come up with something so unutterably stupid? Females needing a supply of sperm which serves no purpose other than to be wasted when anyone with half an ounce of intelligence could have used the same method that other ants and wasps use and simply have unfertilized eggs develop into sterile workers. And males being tricked into wasting their sperm and so failing to use it for the purpose it was evolved to serve!

It's hard to think of anything less intelligent.

In evolutionary terms however, it all makes perfect sense as an arms race between males and females each with different reproductive strategies. In effect, females are sperm parasites on the males while the male tactic of promiscuity has rebounded on them.

There is no morality, no sentimentality, no planning, no forethought and no intelligence in evolution.

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