Gradual molecular evolution of a sex determination switch through incomplete penetrance of femaleness

Martin Beye, Christine Seelmann, Tanja Gempe, Martin Hasselmann, Xavier Vekemans, M. Kim Fondrk, Robert Page

Research output: Contribution to journalArticlepeer-review

21 Scopus citations


Some genes regulate phenotypes that are either present or absent. They are often important regulators of developmental switches and are involved in morphological evolution. We have little understanding of the molecular mechanisms by which these absence/presence gene functions have evolved, because the phenotype and fitness of molecular intermediate forms are unknown. Here, we studied the sex-determining switch of 14 natural sequence variants of the csd gene among 76 genotypes of the honeybee (Apis mellifera). Heterozygous genotypes (different specificities) of the csd gene determine femaleness, while hemizygous genotypes (single specificity) determine maleness. Homozygous genotypes of the csd gene (same specificity) are lethal [1-6]. We found that at least five amino acid differences and length variation between Csd specificities in the specifying domain (PSD) were sufficient to regularly induce femaleness. We estimated that, on average, six pairwise amino acid differences evolved under positive selection [7-9]. We also identified a natural evolutionary intermediate that showed only three amino acid length differences in the PSD relative to its parental allele. This genotype showed an intermediate fitness because it implemented lethality regularly and induced femaleness infrequently (i.e., incomplete penetrance). We suggest incomplete penetrance as a mechanism through which new molecular switches can gradually and adaptively evolve.

Original languageEnglish (US)
Pages (from-to)2559-2564
Number of pages6
JournalCurrent Biology
Issue number24
StatePublished - Dec 16 2013

ASJC Scopus subject areas

  • General Agricultural and Biological Sciences
  • General Biochemistry, Genetics and Molecular Biology


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