There are a lot of different ways for animals to determine which individuals develop into boys and which ones become girls. You’re probably most familiar with the form of chromosomal sex determination that utilizes X and Y chromosomes — males are XY and females are XX. There are others, including ZW (males are ZZ, females are ZW) and environmental sex determination (e.g., sex can be determined by egg rearing temperature).
One of the most interesting sex determination systems — from an evolutionary perspective — involves differences in ploidy between males and females. Hymenoptera (ants, bees, and wasps) employ this method, known as haplodiploidy. Females are diploid and are the result of sperm from a male fertilizing an egg from a female. Males, however, are haploid and derive from unfertilized eggs. This leads to interesting genetic relationships between males and their female relatives, which partially explains the evolution of social structure in ants and bees.
A paper published in Science reports that in a strain of the species Nasonia vitripennis (a parasitic wasp) 10% of the unfertilized eggs develop into individuals with both male and female anatomical structures, or gynandromorphs. The researchers found that they could increase the frequency of gynandromorphs by rearing the embryos at a higher temperature or selecting for females that produce gynandromorphs. The gynandromorphs have female structures at the anterior end and male structures at the posterior. They range from having female-like antenna and male for the rest of the body to entirely female.
This is the first example of haploid females (they are haploid because they result from unfertilized eggs) in Hymenoptera. The authors point out, however, that less than 1% (2/281) of the haploid females were fertile, indicating that this is hardly a stable strategy. Though it does reveal insights into the molecular mechanism of sex determination in this particular haplodiploid species.
The authors hypothesize that mothers imprint on a feminizing gene which remains unaltered in the male germ ine. Diploids receive an imprinted copy from their mother and an unaltered copy from their father (the unaltered copy of the feminizing gene allow the diploids to develop into females). Haploids, on the other hand, only receive the imprinted gene from their mother. This prevents the gene from feminizing the haploids, allowing them to develop into males. Gynandromorphs and females resulting from unfertilized eggs are presumably due to incomplete imprinting on the feminizing gene by the mothers.
Beukeboom LW, Kamping A, Louter M, Pignacker LP, Katju V, Feree PM, Werren JH. 2007. Haploid females in the parasitic wasp Nasonia vitripennis. Science 315: 206. DOI:10.1126/science.1133388