Hybridization underlies localized trait evolution in cavefish

A rapidly growing body of work has demonstrated that introgressive
hybridization often drives patterns of phenotypic evolution and may play
an integral role in the evolutionary processes of local adaptation and
speciation. Indeed, several of studies have shown that behavioral
variation can result from introgressive hybridization (e.g., song in
hybrid Darwin’s finches, mate choice in hybrid baboons, defensive behavior
in hybrid honey bees), providing new substrate for selection to act upon.
 A powerful model system for investigating the genetic and
evolutionary basis of trait development and behavior is the
Mexican tetra, Astyanax mexicanus. Cave populations have repeatedly
evolved numerous traits including eye loss, sleep loss, and albinism. Of
the 30 caves inhabited by A. mexicanus, the Chica cave is unique because
it contains several pool microenvironments inhabited by putative hybrids
between surface and cave populations, providing an opportunity to
investigate hybridization and its impact on complex trait evolution. We
demonstrate that hybridization between cave and surface populations
contributes to highly localized variation in pigmentation, eye
development, and sleep, traits that are thought to be associated with cave
evolution. Our findings suggest that hybridization drives highly-localized
behavioral and morphological evolution. Lastly, our analyses uncovered a
compelling example of convergent evolution in a core circadian clock gene
in multiple independent cavefish lineages and burrowing mammals,
suggesting a shared genetic mechanism underlying circadian disruption in
subterranean vertebrates. Together, our results provide insight into the
evolutionary mechanisms that generate adaptive genetic
variation.

日益增多的研究表明，渐渗杂交（introgressive hybridization）常驱动表型进化（phenotypic evolution）的模式，并可能在本地适应（local adaptation）与物种形成（speciation）的进化过程中发挥不可或缺的作用。事实上，多项研究已证实行为变异（behavioral variation）可由渐渗杂交引发——例如达尔文雀（Darwin’s finches）的鸣唱变异、杂交狒狒（baboons）的配偶选择行为、杂交蜜蜂（honey bees）的防御行为等，为自然选择提供了新的作用底物。
墨西哥丽脂鲤（Astyanax mexicanus）是探究性状发育与行为的遗传及进化基础的极具价值的模式系统（model system）。该物种的洞穴种群（cave populations）已多次独立演化出诸多特征，包括眼睛退化（eye loss）、睡眠缺失（sleep loss）与白化症（albinism）。在已被墨西哥丽脂鲤定居的30处洞穴中，奇卡洞穴尤为特殊：其内部存在多个池塘微生境（pool microenvironments），栖息着地表种群与洞穴种群的疑似杂交个体（putative hybrids），为研究渐渗杂交及其对复杂性状进化的影响提供了绝佳契机。
本研究证实，洞穴种群与地表种群的杂交会导致色素沉着（pigmentation）、眼睛发育与睡眠模式出现高度局域化的变异——这些性状均被认为与洞穴适应演化相关。研究结果表明，渐渗杂交可驱动高度局域化的行为与形态演化。此外，本研究还发现了一个极具说服力的趋同演化（convergent evolution）实例：在多个独立的洞穴鱼支系与穴居哺乳动物（burrowing mammals）中，核心生物钟基因（circadian clock gene）均发生了趋同变异，这暗示地下脊椎动物（subterranean vertebrates）的生物钟紊乱可能共享相同的遗传机制。
综上，本研究结果为解析产生适应性遗传变异（adaptive genetic variation）的进化机制提供了新的见解。