Hasselmann Martin, Gempe Tanja, Schiøtt Morten, Nunes-Silva Carlos Gustavo, Otte Marianne, Beye Martin
Department of Genetics, Heinrich Heine University Duesseldorf, Universitaetsstrasse 1, 40225 Duesseldorf, Germany.
Nature. 2008 Jul 24;454(7203):519-22. doi: 10.1038/nature07052. Epub 2008 Jun 25.
Sex determination in honeybees (Apis mellifera) is governed by heterozygosity at a single locus harbouring the complementary sex determiner (csd) gene, in contrast to the well-studied sex chromosome system of Drosophila melanogaster. Bees heterozygous at csd are females, whereas homozygotes and hemizygotes (haploid individuals) are males. Although at least 15 different csd alleles are known among natural bee populations, the mechanisms linking allelic interactions to switching of the sexual development programme are still obscure. Here we report a new component of the sex-determining pathway in honeybees, encoded 12 kilobases upstream of csd. The gene feminizer (fem) is the ancestrally conserved progenitor gene from which csd arose and encodes an SR-type protein, harbouring an Arg/Ser-rich domain. Fem shares the same arrangement of Arg/Ser- and proline-rich-domain with the Drosophila principal sex-determining gene transformer (tra), but lacks conserved motifs except for a 30-amino-acid motif that Fem shares only with Tra of another fly, Ceratitis capitata. Like tra, the fem transcript is alternatively spliced. The male-specific splice variant contains a premature stop codon and yields no functional product, whereas the female-specific splice variant encodes the functional protein. We show that RNA interference (RNAi)-induced knockdowns of the female-specific fem splice variant result in male bees, indicating that the fem product is required for entire female development. Furthermore, RNAi-induced knockdowns of female allelic csd transcripts result in the male-specific fem splice variant, suggesting that the fem gene implements the switch of developmental pathways controlled by heterozygosity at csd. Comparative analysis of fem and csd coding sequences from five bee species indicates a recent origin of csd in the honeybee lineage from the fem progenitor and provides evidence for positive selection at csd accompanied by purifying selection at fem. The fem locus in bees uncovers gene duplication and positive selection as evolutionary mechanisms underlying the origin of a novel sex determination pathway.
与经过充分研究的黑腹果蝇性染色体系统不同，蜜蜂（西方蜜蜂）的性别决定由一个携带互补性别决定因子（csd）基因的单一位点的杂合性所控制。在csd位点杂合的蜜蜂为雌性，而纯合子和半合子（单倍体个体）为雄性。尽管在天然蜜蜂种群中已知至少有15种不同的csd等位基因，但将等位基因相互作用与性发育程序转换联系起来的机制仍不清楚。在这里，我们报告了蜜蜂性别决定途径的一个新成分，它编码于csd上游12千碱基处。雌性化基因（fem）是csd起源的祖传保守祖基因，编码一种SR型蛋白，含有富含精氨酸/丝氨酸的结构域。Fem与果蝇主要性别决定基因transformer（tra）具有相同的富含精氨酸/丝氨酸和脯氨酸结构域的排列，但除了一个仅与另一种果蝇地中海实蝇的Tra共有的30个氨基酸的基序外，缺乏保守基序。与tra一样，fem转录本也可选择性剪接。雄性特异性剪接变体包含一个提前终止密码子，不产生功能性产物，而雌性特异性剪接变体编码功能性蛋白质。我们表明，RNA干扰（RNAi）诱导的雌性特异性fem剪接变体的敲低导致雄蜂，这表明fem产物是整个雌性发育所必需的。此外，RNAi诱导的雌性等位基因csd转录本的敲低导致雄性特异性fem剪接变体，这表明fem基因实现了由csd位点杂合性控制的发育途径的转换。对五个蜜蜂物种的fem和csd编码序列的比较分析表明，csd在蜜蜂谱系中最近起源于fem祖基因，并为csd处的正选择以及fem处的纯化选择提供了证据。蜜蜂中的fem位点揭示了基因复制和正选择是新性别决定途径起源的进化机制。
