Phenotypic responses to temperature in the ciliate Tetrahymena thermophila

Understanding the effects of temperature on ecological and evolutionary processes is crucial for generating future climate adaptation scenarios. Using experimental evolution, we evolved the model ciliate Tetrahymena thermophila in an initially novel high temperature environment for more than 35 generations, closely monitoring population dynamics and morphological changes. We observed initially long lag phases in the high temperature environment that over about 26 generations reduced to no lag phase, a strong reduction in cell size and modifications in cell shape at high temperature. When exposing the adapted populations to their original temperature, most phenotypic traits returned to the observed levels in the ancestral populations, indicating phenotypic plasticity is an important component of this species thermal stress response. However, persistent changes in cell size were detected, indicating possible costs related to the adaptation process. Exploring the molecular basis of thermal adaptation will help clarify the mechanisms driving these phenotypic responses.

Methods
Experimental populations of T. thermophila were monitored with videos to estimate population abundances and obtain morphological measurements. The samples were placed in counting chambers and the videos were taken on a stereomicroscope (Leica M205 C) mounted with a digital CMOS camera (Hamamatsu Orca C11440, Hamamatsu Photonics, Japan) with 1.57X magnification. Each video comprised 125 frames in 5 seconds. The videos were processed using the R package BEMOVI version 1.0 (Pennekamp, Schtickzelle, and Petchey 2015), which extracts morphological information of all the moving cells in the field of view and allows for population density estimation.

解析温度对生态与进化过程的影响，是构建未来气候适应情景的核心前提。本研究采用实验进化手段，将模式纤毛虫嗜热四膜虫（Tetrahymena thermophila）置于初始为全新的高温环境中传代培养超过35代，并全程密切监测其种群动态与形态变化。我们观察到，初始阶段种群在高温环境中存在较长的滞缓期，经过约26代进化后，滞缓期完全消失；同时细胞尺寸显著缩小，高温环境下细胞形态也发生了改变。当将适应后的种群放回初始温度环境时，绝大多数表型性状恢复至祖先种群的观测水平，表明表型可塑性是该物种热胁迫响应的重要组成部分。然而，细胞尺寸的持续性变化仍被检测到，提示适应过程可能伴随相关适应性代价。后续解析热适应的分子基础，将有助于阐明调控这些表型响应的内在机制。

实验方法
通过视频录制监测嗜热四膜虫实验种群，以估算种群丰度并获取形态学测量数据。样本被置于计数池内，使用搭载数字CMOS相机（滨松Orca C11440，滨松光子学株式会社，日本）的体视显微镜（徕卡M205 C）进行视频录制，放大倍数为1.57X。每段视频时长5秒，包含125帧。视频处理采用R语言包BEMOVI 1.0版本（Pennekamp、Schtickzelle与Petchey，2015），该工具可提取视野内所有运动细胞的形态学信息，并实现种群密度估算。