Yamamoto Yoshiyuki, Byerly Mardi S, Jackman William R, Jeffery William R
Department of Biology, University of Maryland, College Park, MD 20742, USA.
Dev Biol. 2009 Jun 1;330(1):200-11. doi: 10.1016/j.ydbio.2009.03.003. Epub 2009 Mar 11.
This study addresses the role of sonic hedgehog (shh) in increasing oral-pharyngeal constructive traits (jaws and taste buds) at the expense of eyes in the blind cavefish Astyanax mexicanus. In cavefish embryos, eye primordia degenerate under the influence of hyperactive Shh signaling. In concert, cavefish show amplified jaw size and taste bud numbers as part of a change in feeding behavior. To determine whether pleiotropic effects of hyperactive Shh signaling link these regressive and constructive traits, shh expression was compared during late development of the surface-dwelling (surface fish) and cave-dwelling (cavefish) forms of Astyanax. After an initial expansion along the midline of early embryos, shh was elevated in the oral-pharyngeal region in cavefish and later was confined to taste buds. The results of shh inhibition and overexpression experiments indicate that Shh signaling has an important role in oral and taste bud development. Conditional overexpression of an injected shh transgene at specific times in development showed that taste bud amplification and eye degeneration are sensitive to shh overexpression during the same early developmental period, although taste buds are not formed until much later. Genetic crosses between cavefish and surface fish revealed an inverse relationship between eye size and jaw size/taste bud number, supporting a link between oral-pharyngeal constructive traits and eye degeneration. The results suggest that hyperactive Shh signaling increases oral and taste bud amplification in cavefish at the expense of eyes. Therefore, selection for constructive oral-pharyngeal traits may be responsible for eye loss during cavefish evolution via pleiotropic function of the Shh signaling pathway.
本研究探讨了音猬因子（shh）在墨西哥丽脂鲤（Astyanax mexicanus）这种盲穴鱼中，以牺牲眼睛为代价来增加口咽构造特征（颌骨和味蕾）方面所起的作用。在穴鱼胚胎中，眼原基在过度活跃的Shh信号影响下退化。与此同时，穴鱼表现出颌骨尺寸增大和味蕾数量增加，这是摄食行为变化的一部分。为了确定过度活跃的Shh信号的多效性作用是否将这些退化和构造特征联系起来，我们比较了丽脂鲤表层栖息型（表层鱼）和洞穴栖息型（穴鱼）在发育后期的shh表达情况。在早期胚胎沿中线初步扩展后，穴鱼口咽区域的shh表达升高，随后局限于味蕾。shh抑制和过表达实验结果表明，Shh信号在口腔和味蕾发育中起重要作用。在发育的特定时间对注射的shh转基因进行条件性过表达表明，尽管味蕾直到很久以后才形成，但在同一早期发育阶段，味蕾扩增和眼睛退化对shh过表达敏感。穴鱼和表层鱼之间的遗传杂交揭示了眼睛大小与颌骨大小/味蕾数量之间的负相关关系，支持了口咽构造特征与眼睛退化之间的联系。结果表明，过度活跃的Shh信号以牺牲眼睛为代价增加了穴鱼的口腔和味蕾扩增。因此，在穴鱼进化过程中，通过Shh信号通路的多效性功能，对口咽构造特征的选择可能是导致眼睛丧失的原因。
