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Brian Charlesworth
In 1845, the German apiarist J. Dzierzon proposed that male honeybees arise from unfertilized eggs, while females come from fertilized eggs. It is now known that this sex determination system (haplodiploidy) is probably common to all sexually reproducing members of the Hymenoptera (ants, bees, and wasps) and Thysanoptera (thrips), as well as being found sporadically in other orders of insects including beetles and Homoptera (scale insects and whiteflies), in the Acarina (ticks and mites), and in monogont rotifers (Bull, 1983). It is very hard to see how haplodiploidy could evolve from one of the two best-known sex-determining systems, a male-determining Y chromosome (as found in mammals), or X: autosome balance (as in Drosophila and Caenorhabditis). In the first case, it is impossible for a male to develop without the male-determining region of the Y, and for a female to develop in its presence. In the second case, there is no difference in X: autosome balance between haploids and diploids.
In the few cases (all in Hymenoptera) in which a detailed genetic analysis of haplodiploid sex determination has been performed, the mechanism involves what is known as complementary sex determination (Bull, 1983). Females are always heterozygous for a pair of distinct alleles at the sex-determining locus, whereas males are homozygous (if derived from a fertilized egg), or haploid (if derived from an unfertilized egg). Such a sex determination system can perfectly well exist without haplodiploidy if all eggs are fertilized, and evolutionary models of the conversion of diploid complementary sex determination into haplodiploidy can be constructed (Bull, 1983). This removes some of the mystery surrounding the origin of haplodiploidy.
In honeybees, the best-studied example of complementary sex determination, homozygous diploid males can be produced by inbreeeding, but are normally eaten by the workers. The lethality of diploid males means that there is a selection pressure to increase the number of functionally distinct alleles, and indeed in honeybees as many 12 alleles have been detected in a single population (Bull, 1983). This pressure to make the system highly polymorphic is similar that in the self-incompatibility loci of flowering plants and some fungal mating types, where a large number of alleles coexist and successful matings only occur between individuals carrying different alleles (Charlesworth, 2002; Casselton, 2002). Variability with respect to neutral markers at sites closely linked to the sex locus itself has been exploited by Beye et al. to identify, clone, and characterize the sex-determining gene of honeybees, csd, in a tour de force of positional cloning (Beye et al., 2003).
Earlier work had identified two genetic markers flanking the sex-determining locus, at distances of 1 and 7 cM. Using the closer marker, they isolated 70 kb of DNA containing the sex locus, using polymorphic markers to orientate a chromosome walk. They found a 13 kb region that was always heterozygous in females in the cross they used. cDNA analyses identified a transcript within this region, and they inferred that this was likely to be the sex-determining gene itself. Sequencing of the corresponding genomic DNA shows that csd consists of 1453 bases, with nine exons and a protein of 385 amino acids. The protein is a novel type of arginine-serine rich (SR) protein. Intriguingly, its C terminus has some sequence similarity with the protein coded by the tra locus of Drosophila, an important player in sex determination (Marín and Baker, 1998). Expression studies showed that csd is transcribed in both males and females, starting at 12 hr of development, so that differential expression plays no role in sex determination.
Sequence comparisons were carried out between four different sex-determining alleles, revealing an unusually high level of amino acid sequence differences between alleles, especially toward the C-terminal region. In addition to single substitutions, alleles typically differ with respect to insertions and deletions of amino acids, often in tracts of several at a time. Variation in a hypervariable region mainly involves repeats of asparagine and tyrosine residues. This abundance of amino acid sequence differences among alleles is also seen in self-incompatibility alleles, and presents a considerable problem for identifying sites of functional significance. The magnitude of these differences suggests that the alleles may be rather old, in terms of evolutionary origin; it would be of interest to look for trans-specific polymorphisms of the type sometimes found at self-incompatibility loci (Charlesworth, 2002).
This pattern of variation in itself strongly supports the inference that csd is the sex-determining locus. Further evidence is provided by functional studies, using RNA interference. Injection of csd dsRNA into developing eggs caused genetic females to develop as male larvae with high probability, but males were unaffected. This indicates that csd function is required in females, but not in males. In turn, this implies that its product is nonfunctional when csd is transcribed from a single allele. Again, this has some parallels with self-incompatibility and mating-type systems, in which pollination or development of a sexual fusion product cannot proceed if genetically similar partner cells are involved. The fact that RS proteins typically engage in protein-protein interaction suggests that csd may well be interacting with other proteins; in addition, some form of homodimerization is the simplest explanation for the effect of having only a single allele on its functionality. The details of its actions remain to be determined. While it is tempting to speculate that csd may be regulating dsx, the only known phylogenetically well-conserved component of the sex determination pathway in insects (Marín and Baker, 1998), at present this is just speculation.
In addition to these mechanistic questions, the characterization of csd opens the door to asking questions about the evolution of haplodiploidy in the Hymenoptera at large, e.g., was there a single origin or multiple origins of complementary sex determination? One problem for the single origin model is that complementary sex determination cannot be the only form of sex determination in Hymenoptera. Many species of Hymenopteran parasitoids are regular inbreeders, yet produce an excess of female over male offspring (Bull, 1983). It is clear that this cannot happen with complementary sex determination, since homozygosity for sex-determining alleles produces males. Genetic studies in the parasitoid wasp Nasonia vitripennis have confirmed that sex determination works quite differently in this inbreeder, and may involve some form of genomic imprinting (Dobson and Tanouye, 1998). This raises some awkward questions concerning what is going on in these species, and how it relates to the “standard” complementary sex-determining mechanism. Like most good pieces of science, a lot of interesting questions are raised by the characterization of csd.
Beye, M., Hasselmann, M., Fondrk, M.K., Page, R.E., and Omholt, S.W. (2003). Cell 114, 419-429. [Medline] [Summary] [Full Text]
Bull, J.J. (1983). Evolution of Sex Determining Mechanisms. (Menlo Park, CA: Benjamin Cummings),
Casselton, L.A. (2002). Heredity 88, 142-147. [Medline]
Charlesworth, D. (2002). Curr. Biol. 12, R424-R426. [Medline]
Dobson, S.L. and Tanouye, M.A. (1998). Genetics 149, 233-242. [Medline]
Marín, I. and Baker, B.S. (1998). Science 281, 1990-1994. [Medline]
