In the vast majority of ants, females are produced from fertilized eggs (they are diploid, containing 2 sets of chromosomes or 2n) and males from unfertilized eggs (haploid, 1n) (see Hymenoptera Sex Determination for details). However, in a handful of species diploid males are known. These diploid males generally show low viability, or fail to produce viable offspring as they are unable to mate properly or because they are sterile. Additionally, while some diploid males can produce viable sperm, their sperm is diploid rather than haploid and the resulting offspring are triploid and sterile.
In some species, for example Lasius sakagamii (Yamauchi et al., 2001), body size of diploid males is, on average, larger than that of the haploid males but overall external and internal morphology is similar to normal, haploid males. In other taxa haploid and diploid males are essentially identical and inseparable morphologically.
In Hypoponera opacior, inbreeding occurs between wingless sexuals, which mate within the mother nest, while winged sexuals outbreed during mating flights earlier in the season. Wingless males mate with queen pupae and guard their mating partners. Kureck et al. (2013) found that these males mate randomly with respect to relatedness, indicating that males do not avoid mating with close kin. These frequent sib-matings lead to the production of diploid males, which are able to produce sterile triploid offspring. They compared mating activity and lifespan of haploid and diploid wingless males and found they resemble each other in all investigated traits. Thus, while diploid males cannot produce fertile offspring, they keep up with haploid males in their lifetime mating success. Moreover, by fathering viable triploid workers, they contribute to the colonies’ workforce. They found that the lack of inbreeding avoidance leads to frequent sib-matings of wingless sexuals, which in turn results in the regular production of diploid males. However, in contrast to many other Hymenopteran species, diploid males exhibit normal sexual behaviour and sire viable, albeit sterile daughters.
The occurrence of diploid males within a species ranges from less than 1% to as high as 80%, but in most cases diploids are less than 10% of males. See Cournault et al. (2009) for the most recent survey.
It is difficult to access how common diploid males are within the ants. This is because determining if a male is diploid requires examination of either the karyotype or the amount of DNA present (it will be twice as much as in a haploid male). Both methods require complex sample preparation and analysis.
It seems likely that diploid males are more common than current understanding would suggest. Known diploid males occur across four of the largest subfamilies (Dolichoderinae, Formicinae, Myrmicinae, Ponerinae) and 12 diverse genera. These taxa show a range of life history attributes and have relatively few in common. This suggests that the presence of diploid males is not tied to a specific life history trait such as inbreeding but may potentially occur in any taxon. Only detailed surveys will determine the true extent of the occurrence of these males within the ants.
Taxa Known to have Diploid Males
- Acromyrmex heyeri
- Amoimyrmex striatus
- Atta sexdens
- Cardiocondyla obscurior
- Formica aquilonia
- Formica lugubris
- Formica polyctena
- Formica pressilabris
- Formica rufa
- Formica truncorum
- Harpagoxenus sublaevis
- Hypoponera opaciceps
- Lasius sakagamii
- Leptothorax acervorum
- Leptothorax kutteri
- Leptothorax muscorum
- Proformica longiseta
- Pseudolasius emeryi Forel
- Solenopsis invicta
- Tapinoma erraticum
- Temnothorax ambiguus
- Temnothorax nylanderi
- Temnothorax stumperi
- Armitage, S., Boomsma, J., Bear, B. 2010. Diploid male production in a leaf-cutting ant. Ecological Entomology 35, 175–182 (doi:10.1111/j.1365-2311.2009.01167.x).
- Cournault, L., Aron, S. 2009. Diploid males, diploid sperm production, and triploid females in the ant Tapinoma erraticum. Naturwissenschaften 96: 1393–1400 (doi:10.1007/s00114-009-0590-1).
- Cowan, D.P., Stahlhut, J.K. 2004. Functionally reproductive diploid and haploid males in an inbreeding hymenopteran with complementary sex determination. PNAS 101: 10374–10379 (doi:10.1073/pnas.0402481101).
- Elias, J., Mazzi, D., Dorn, S. 2009. No need to discriminate? Reproductive diploid males in a parasitoid with complementary sex determination. PLoS ONE 4, e6024 (doi:10.1371/journal.pone.0006024).
- Harpur, B.A., Sobhani, M., Zayed, A. 2013. A review of the consequences of complementary sex determination and diploid male production on mating failures in the Hymenoptera. Entomologia Experimentalis et Applicata 146, 156–164 (doi:10.1111/J.1570-7458.2012.01306.X).
- Krieger, M.J.B., K.G. Ross, C.W.Y. Chang and L. Keller, 1999. Frequency and origin of triploidy in the fire ant Solenopsis invicta. Heredity 82: 142–150.
- Kureck, I.M., Nicolai, B., Foitzik, S. 2013. Similar performance of diploid and haploid males in an ant species without inbreeding avoidance. Ethology 119: 360–367 (doi:10.1111/eth.12073).
- Pamilo, P., Sundström, L., Fortelius, W., Rosengren, R. 1994. Diploid males and colony-level selection in Formica ants. Ethology Ecology, Evolution 6, 221–235 (doi:10.1080/08927014.1994.9522996).
- Ross, K.G., Fletcher, D.J.C. 1985. Genetic origin of male diploidy in the Fire Ant, Solenopsis invicta (Hymenoptera: Formicidae), and its evolutionary significance. Evolution 39: 888-903 (doi:10.2307/2408688).
- Yamauchi, K., Yoshida, T., Ogawa, T., Itoh, S., Ogawa, Y., Jimbo, S., Imai, H.T. 2001. Spermatogenesis of diploid males in the formicine ant, Lasius sakagamii. Insectes Sociaux 48 28–32.