Driving the evolution of sexdetermining mechanisms

Sex-determination systems, the developmental processes that underlie the differentiation of male and female, are remarkably variable. This variability is paradoxical when it is considered that the core problem, making male and female, remains constant between taxa. All sexual insects make male and female, but they do so with a variety of systems controlling sex determination. Sxl, the major switch gene involved with sex determination in drosophilids, is not involved with sex determination in other flies (Schutt and Nothiger, 2000). msl-3, a gene involved with dosage compensation in Drosophila, is present in other fly species, but it is not involved with dosage compensation (Ruiz et al., 2000). At a grosser level, we can observe male heterogametic, female heterogametic and haplodiploid systems of sex determination in insects. Some groups, such as the bark beetles, appear to show transitions on a very regular basis.

This paradox has been subject of much debate, and passenger microorganisms have been proposed as one factor driving the changes. Hamilton (1993) speculates that haplodiploidy may arise as a consequence of conflict between bacteria that feminize or kill males by attacking or eliminating the Y chromosome. Hamilton conjectures that this selects for the movement of the male determining factor from the Y chromosome to a previous autosome as a mechanism of preventing the parasite from cueing in on sex, with the old Y chromosome no longer determining sex. As the symbiont cues in on the new chromosome, so the sex-determining locus moves, until males are completely haploid, and females diploid, and sex is determined by chromosome dosage (which the bacterium cannot detect before fertilization, at least). This theory has been extended to suggest male-killing bacteria as a driving force behind the evolution of this sexual system (Normark, 2004).

More widely, Werren and Beukeboom (1998) and Hurst and Werren (2001) suggest that changes in the structure of sex determination may be a result of reproductive conflicts. They point to the alteration in sex-determination system in the pill woodlouse, Armadillidium vulgare, which have been driven by the feminizing Wolbachia bacterium. Feminization first results in the fixation of the Z chromosome in this female heterogametic species, followed by selection for repression of the transmission of the feminizing factor, such that sex is now determined in some populations by presence/absence of Wolbachia combined with nuclear genes affecting Wolbachia transmission (Rigaud, 1997).

The applicability of this study to insects requires urgent resolution. One thing is certain. Male-killing bacteria and feminizers can produce very strong selection on the host to avoid the action and transmission of male-killing bacteria, and modifiers in the host sex-determination system that prevented recognition by the parasite would spread. What is uncertain is the feasibility of mutation to these modifiers: can a modified system still produce a functional male? Our feeling is that a coevolutionary process, in which mild changes in host sex-determining molecules is followed by 'catch-up' by the parasite, can result in shifts in the protein sequence or titre of sex-determining genes, and these may produce compensatory selection for changes in the structure of sex determination. More studies of the genetic basis of resistance in natural populations and model organisms are required.

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