Department of Evolution and Ecology, University of California-Davis, Davis, CA, USA.
Basic Sciences Division, Fred Hutchinson Cancer Research Center, Seattle, WA, USA.
Dev Genes Evol. 2022 Dec;232(5-6):89-102. doi: 10.1007/s00427-022-00694-3. Epub 2022 Aug 8.
The origin, diversification, and secondary loss of sexually dimorphic characters are common in animal evolution. In some cases, structurally and functionally similar traits have evolved independently in multiple lineages. Prominent examples of such traits include the male-specific grasping structures that develop on the front legs of many dipteran insects. In this report, we describe the evolution and development of one of these structures, the male-specific "sex brush." The sex brush is composed of densely packed, irregularly arranged modified bristles and is found in several distantly related lineages in the family Drosophilidae. Phylogenetic analysis using 250 genes from over 200 species provides modest support for a single origin of the sex brush followed by many secondary losses; however, independent origins of the sex brush cannot be ruled out completely. We show that sex brushes develop in very similar ways in all brush-bearing lineages. The dense packing of brush hairs is explained by the specification of bristle precursor cells at a near-maximum density permitted by the lateral inhibition mechanism, as well as by the reduced size of the surrounding epithelial cells. In contrast to the female and the ancestral male condition, where bristles are arranged in stereotypical, precisely spaced rows, cell migration does not contribute appreciably to the formation of the sex brush. The complex phylogenetic history of the sex brush can make it a valuable model for investigating coevolution of sex-specific morphology and mating behavior.
性二型特征的起源、多样化和次生消失在动物进化中很常见。在某些情况下，结构和功能相似的特征在多个谱系中独立进化。这种特征的突出例子包括许多双翅目昆虫前腿上发育的雄性特有的抓握结构。在本报告中，我们描述了这些结构之一的进化和发育，即雄性特有的“性刷”。性刷由密集排列、不规则排列的改性刚毛组成，在果蝇科的几个远缘谱系中都有发现。使用来自 200 多种物种的 250 个基因进行的系统发育分析为性刷的单一起源随后多次次生消失提供了适度的支持；然而，不能完全排除性刷的独立起源。我们表明，所有带刷的谱系中，性刷的发育方式非常相似。刷毛的密集包装可以通过以下两种方式来解释：侧抑制机制允许的最大密度下的刚毛前体细胞的特异性，以及周围上皮细胞的减小。与雌性和原始雄性的情况不同，雌性和原始雄性的刚毛以刻板的、精确间隔的行排列，细胞迁移对性刷的形成没有显著贡献。性刷复杂的系统发育历史使其成为研究性别形态和交配行为协同进化的有价值模型。
