Abstract
Social parasitism, one of the most intriguing phenomena in ants, has evolved to various levels, the most extreme form being parasites that have lost the worker caste and rely completely on the host's worker force to raise their brood. A remarkable feature of workerless social parasites is the small size of sexuals. It has been suggested that reduced size evolved as a means to take advantage of the host's caste-determination system, so that parasite larvae develop into sexuals with less food than is required to produce host workers. An important consequence of size reduction is that it might restrict the host workers' ability to discriminate between the brood of the social parasite and their own brood and might protect parasite sexuals from elimination. We found that sexuals of the workerless inquiline ant Plagiolepis xene were significantly smaller than the sexuals of their host Plagiolepis pygmaea, but remarkably similar to the host workers. The size variance of parasite sexuals was much lower than that of their host; this result possibly suggests that there is very stabilizing selection acting on size of the parasite sexuals. Comparison of the primary (egg) and secondary (adult) sex ratios of the parasite and host showed that miniaturization of P. xene sexuals has been accompanied by their ability to develop into sexuals even when the host P. pygmaea actively prevents production of its own sexuals. These results suggest that the inquiline's size and caste threshold have been reduced such that all individuals in a parasite brood will develop into sexuals. We also found that the adult sex ratio of P. xene was heavily female-biased. This bias probably stems from local mate competition that arises from sexuals mating within the nest. There was no significant difference between the proportion of haploid eggs and adult males produced; this observation indicates that a female-biased sex ratio is achieved by queens producing a higher proportion of diploid eggs rather than by a higher mortality of haploid males.
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Selected References
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- Alexander R. D., Sherman P. W. Local mate competition and parental investment in social insects. Science. 1977 Apr 29;196(4289):494–500. doi: 10.1126/science.196.4289.494. [DOI] [PubMed] [Google Scholar]
- Hamilton W. D. Extraordinary sex ratios. A sex-ratio theory for sex linkage and inbreeding has new implications in cytogenetics and entomology. Science. 1967 Apr 28;156(3774):477–488. doi: 10.1126/science.156.3774.477. [DOI] [PubMed] [Google Scholar]
- Herre E. A. Sex ratio adjustment in fig wasps. Science. 1985 May 17;228(4701):896–898. doi: 10.1126/science.228.4701.896. [DOI] [PubMed] [Google Scholar]
- Sundstrom L, Chapuisat M, Keller L. Conditional Manipulation of Sex Ratios by Ant Workers: A Test of Kin Selection Theory. Science. 1996 Nov 8;274(5289):993–995. doi: 10.1126/science.274.5289.993. [DOI] [PubMed] [Google Scholar]
- Trivers R. L., Hare H. Haploidploidy and the evolution of the social insect. Science. 1976 Jan 23;191(4224):249–263. doi: 10.1126/science.1108197. [DOI] [PubMed] [Google Scholar]
- Werren J. H. Sex ratio adaptations to local mate competition in a parasitic wasp. Science. 1980 Jun 6;208(4448):1157–1159. doi: 10.1126/science.208.4448.1157. [DOI] [PubMed] [Google Scholar]