Functionally reproductive diploid and haploid males in an inbreeding hymenopteran with complementary sex determination

Abstract

It has become a matter of orthodoxy that among wasps, ants, bees, and other insects in the order Hymenoptera, only uniparental haploid males that arise from unfertilized eggs are capable of reproduction. This idea is of interest because the best understood and perhaps most widespread sex determination system among these insects [known as single locus complementary sex determination (sl-CSD)] does not depend on ploidy alone and, paradoxically, consistently results in small numbers of diploid biparental males. To date, the reproductive potential of diploid males has been studied in 13 of the perhaps 200,000 hymenopterans world-wide; in each of these instances, the diploid males are genetic dead ends because they are inviable or sterile. The data from these species have resulted in a general conclusion that has been invoked for virtually all species with sl-CSD and has become the basis for assumptions regarding conservation biology, sex ratio analysis, and the evolution of social behavior. Here, we report that in the solitary vespid wasp Euodynerus foraminatus, both diploid and haploid males are fertile, which documents normal fertility in diploid males of a hymenopteran with sl-CSD. This wasp has high levels of inbreeding because of frequent brother-sister mating in nature; therefore, diploid males are more frequently produced and thus more likely exposed to selection favoring their fertility. Because inbreeding and diploid male production may be important features of the population biology of many hymenopterans, we sound a cautionary note regarding ideas about the evolutionary ecology of these insects.

Fig. 1.

Experimental (A) and control (B) breeding protocols. X₁, X₂,..., X₅ represent hypothetical distinct alleles at the sex locus. The experimental and control pedigrees differ in that biparental diploid F₂ males (male a) were used in the experimental lineage and uniparental haploid males (male b) were used in the control lineage. According to the current understanding of sl-CSD, if diploid males are fertile, their offspring (F₃ female c) should be triploid and sterile and no F₄ offspring should be produced.

Fig. 2.

Electropherograms of microsatellite fragments showing the parentage of a haploid male. The numbers next to each peak are the fragment lengths as measured by Beckman Coulter ceq 2000 fragment analysis software. Results for two loci in each individual are shown; diagrams for the Efo3 locus are on the left, and diagrams for the Efo4 locus are on the right. The mother is heterozygous at both the Efo3 and Efo4 loci. In this case, the haploid male has inherited allele 128 at the Efo3 locus and allele 213 at the Efo4 locus from his mother. The mother's mate's alleles are absent, indicating that the son is uniparental and haploid.

Fig. 3.

Electropherograms for the family of a diploid male showing (from top to bottom) the diploid male's mother, his father, the diploid male, the diploid male's mate, and two daughters of the diploid male. The Efo3 locus is on the left, and the Efo4 locus is on the right. In this case, the male is heterozygous (diploid/biparental) at both loci, having inherited allele 127 at the Efo3 locus and allele 230 at the Efo4 locus from his father. The electropherograms for the daughters show that they are heterozygous (diploid, not triploid), having inherited one distinctive allele from their father and one from their mother. Note that each daughter carries a different paternal allele.