Diploid male production in a leaf-cutting ant
1. In haplodiploid social insects where males are haploid and females
are diploid, inbreeding depression is expressed as the production of diploid males
when homozygosity at the sex-determining locus results in the production of diploid
individuals with a male phenotype. Diploid males are often assumed to have reduced
fitness compared with their haploid brothers.
2. While studying the reproductive biology of a leaf-cutting ant, Atta sexdens, in
Gamboa, Republic of Panama, we detected the presence of a larger male morph. Using
microsatellite markers we were able to confirm that the large male morph was diploid
in 87% of cases.
3. We infer that the Gamboa population of A. sexdens experiences inbreeding
depression because diploid males were found in three out of five mature colonies.
However, their frequencies were relatively low because queens were multiply mated
and our estimates suggest that many diploid male larvae may not survive to adulthood.
4. We measured two traits potentially linked to male reproductive success: sperm
length and sperm number, and showed that diploid males produced fewer but longer
sperm. These results provide indirect evidence that diploid male reproductive success
would be reduced compared with haploid males if they were able to copulate.
5. We conclude that diploid male production is likely to affect the fitness of
A. sexdens queens with a matched mating, as these males are produced at the cost of
workers and, if the colony survives to reach mature size, also gynes.
Atta sexdens, fitness reduction, haplodiploid, inbreeding depression,
matched mating, sperm.