Discovery of Intersex honeybee complicates current model stating unfertilized bee eggs become haploid males, fertilized bee eggs become diploid females. (Gender Showcase, 9-12)

The total number of chromosome sets determines whether bees become male, female, or a mix. This haplodiploidy system means all male bees are haploid and all females are diploid (see above). Male bees have half the number of chromosomes that female bees do.

Distinguish this from XO sex determination, where only the number of sex chromosomes are halved, not the autosomes. For example, males and females both receive the same number of autosomal chromosomes, but males only get O for their sex chromosome (1 chromosome) and females get XX for their sex chromosome set (2 chromosomes).

Fertilized eggs are either homozygous at the Sex Determination Locus (SDL) and differentiate into diploid males or are heterozygous and develop into females. The diploid males, however, don't survive in a bee colony as they are eaten by worker bees shortly after hatching from the egg. Fertile males are produced by the queen's unfertilized, haploid eggs that are hemizygous at SDL. (Gempe et al. 2009)

Image credit: Ian Alexander

Gempe et al. (2009) tested areas in the Apis mellifera sequence and manipulated the complementary sex determiner gene (csd in excerpt below) and the feminizer gene (fem in excerpt below). They tried different ways of suppressing and adding the influence of these genes. They discovered that female bee development requires fem activity and csd activity processes the heterozygous (female) state and not the homozygous or hemizygous (male) states.

We show that heterozygous csd is only required to induce the female pathway, while the feminizer (fem) gene maintains this decision throughout development. By RNAi induced knockdown we show that the fem gene is essential for entire female development and that the csd gene exclusively processes the heterozygous state. Fem activity is also required to maintain the female determined pathway throughout development, which we show by mosaic structures in fem-repressed intersexuals. We use expression of Fem protein in males to demonstrate that the female maintenance mechanism is controlled by a positive feedback splicing loop in which Fem proteins mediate their own synthesis by directing female fem mRNA splicing. The csd gene is only necessary to induce this positive feedback loop in early embryogenesis by directing splicing of fem mRNAs. Finally, fem also controls the splicing of Am-doublesex transcripts encoding conserved male- and female-specific transcription factors involved in sexual differentiation.

This means that fatal mutations automatically kill their haploid males, and double-diploid bees automatically get cannibalized by nurse bees when they hatch! But recently, researchers discovered an unusual intersex honeybee, shown below.

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Figure 1 (a-k). Individual of Euglossa melanotricha with male and female phenotypic characteristics. a Side view (right), b side view (left), c front view of the head, d scutellum with a scutellar tuft (as observed in the female of this species), e view of the abdomen showing the sting, f ventral view of the sting, g dorsal view, h metatibia (left) with the presence of slit, i metatibia (right) with corbicula, j mesotibia (left) with velvety area and basal and distal cushions, k mesotibia (right side).

Researchers discovered an orchid bee that had a blend of male and female body parts and genetics, though genetic analysis allowed them to conclude this bee is mostly feminine.

Suzuki et al. (2015) report:

Findings obtained through both morphological and genetic analyses of a gynander orchid bee (Euglossa melanotricha). For the genetic analysis, microsatellite markers were used to genotype the gynander bee. The morphological analysis revealed that the individual studied had a sting, and most parts of the insect body showed female phenotype, except for the three left legs. As in other reports on gynanders of orchid bees, the specimen of E. melanotricha analyzed herein was included in the category of mixed (or mosaic). From the seven microsatellite loci amplified, five were heterozygous for both male and female tissues, indicating that the organism analyzed is compatible with a diploid organism and not with a hemizygous or haploid one. Both the morphological and genetic characteristics of the gynander of E. melanotricha analyzed reveal that this specimen shows predominantly female characteristics.

Yet, Suzuki and colleagues suggest that this female-male labeling is not as clarifying as directly studying the mechanisms would be, and urge other researchers to look further into csd gene regulation:

In parallel, when considering the genetic uniformity of phenotypically different tissues (male and female) of this individual, the gynandromorph of E. melanotricha would be, in fact, an intersex bee.

In the current literature, there are over 100 reports of anomalous bees, showing both female and male phenotypes in the same individual, usually named gynander or gynadromorph (Wcislo et al. 2004; Michez et al. 2009). In light of the above scenario [of possible sampling bias discussed in omitted text], we suggest that future studies on gynander and intersex bees should give more emphasis to the understanding of the mechanisms involved in the csd gene regulation in an attempt to better elucidate how these anomalous organisms are generated.

Interested in haplodiploidy?

This sex determination system exists in all insects, Hymenoptera (bees, ants, wasps) and Thysanoptera (thrips), rotifers, Hemiptera (cicadas, aphids, leafhoppers), and Coeoptera (bark beetles).

References

• Gempe, T., Hasselmann, M., Schiøtt, M., Hause, G., Otte, M., & Beye, M. 2009. Sex Determination in Honeybees: Two Separate Mechanisms Induce and Maintain the Female Pathway. PLoS Biol. 2009 Oct; 7(10): e1000222.  doi: 10.1371/journal.pbio.1000222. PMID: 19841734.

• Hoff, M. 2009. Male or Female? For Honeybees, a Single Gene Makes All the Difference. PLoS Biol. 2009 Oct; 7(10): e1000186. doi: 10.1371/journal.pbio.1000186. PMID: 20076733.

• “The origin of the gynandromorphs has been attributed to genetic problems, and although different hypotheses have been raised to explain genetically the origin of the gynandromorphism in bees, the mechanisms that generate these abnormal individuals have not been elucidated.” Michez, D., Rasmont, P., Terzo, M., Vereecken, N.J. (2009) “A synthesis of gynandromorphy among wild bees (Hymenoptera: Apoidea), with an annotated description of several new cases.” Ann. Soc. Entomol. Fr. 45, 365–375

• Michez, D., Rasmont, P., Terzo, M., Vereecken, N.J. (2009) A synthesis of gynandromorphy among wild bees (Hymenoptera: Apoidea), with an annotated description of several new cases. Ann. Soc. Entomol. Fr. 45, 365–375

• Suzuki, K.M., Giangarelli, D.C., Ferreira, D.G. et al. (2015) “A scientific note on an anomalous diploid individual of Euglossa melanotricha (Apidae, Euglossini) with both female and male phenotypes”. Apidologie (2015) 46: 495. https://doi.org/10.1007/s13592-014-0339-5.

• Wcislo, W.T., Gonzalez, V.H., Arneson, L. (2004) A review of deviant phenotypes in bees in relation to brood parasitism, and a gynandromorph of Megalopta genalis (Hymenoptera: Halictidae). J. Nat. Hist. 38, 1443–1457.

For more about intersexuality in bees, see this comprehensive review papeR

• Narita, S., Pereira, R.A.S., Kjellberg, F., Kageyama, D. (2010) Gynandromorphs and intersexes: potential to understand the mechanism of sex determination in arthropods. Terr. Arthropod Rev. 3, 63–96.