Understanding causes and consequences of the extreme thermal sensitivity of male fertility using a model insect

雄性生育能力是一个关键的生物特征，对一个基本的非生物变量：温度非常敏感。我们早就知道，如果环境或睾丸温度上升，雄性哺乳动物就会出现不育，而我们只是在冷血物种中发现了同样的现象，它们的生理机能直接受到温度变化的影响。在线虫、果蝇和面粉甲虫中，我们现在知道，在模拟自然热浪的条件下，雄性生育力下降到仅仅由于雄性不育而不可能实现种群生存的程度。相比之下，在相同的温度下，女性的生育力不会受到影响。由于(A)雄性生育力使种群得以持续存在，(B)极端气候正在增加，以及(C)世界上大多数生物多样性是冷血动物，并以有性繁殖，我们建议了解热浪条件如何影响生殖。我们需要知道哪些男性阶段和特征如此敏感，他们恢复的速度有多快，以及男性生育能力是否能够适应或进化到更温暖的制度。为了提供答案，我们将使用一种模型昆虫：粉甲虫，它(1)我们可以培养并以实验方式复制到高水平，(2)有详细的测试来测量生育、繁殖和精子形态和功能，(3)具有较短的世代时间来进行实验进化。重要的是，我们已经确定，卡氏锥虫雄性育性对温度变化非常敏感：对年轻成虫进行模拟5天热浪的治疗(预计本世纪将增加10倍)，在高于种群生产力最佳水平的2.5至5摄氏度的温度下，雄性育性下降20%至50%。雄性不育温度高于最适温度7.5摄氏度，而雌性受精率不受这些处理的影响。这一背景意味着(I)温度变化限制了卡氏锥虫的生育能力，(Ii)我们可以进行高重复的对照实验，以确定温度对雄性生育能力的特定影响，它在实验室中的恢复，以及对种群生产力的影响，(Iii)我们可以在体外(通过精子形态和功能分析)和体内(通过后代和种群生产力)测量这些影响，以获得机械性和预测性的洞察力，以及(Iv)发育和世代时间使我们能够测量种群如何适应和进化到温度变化。因此，我们的目标是回答以下关键问题，以了解温度敏感型男性生育能力的个体原因、群体和进化后果：A.男性生育能力何时对热变化敏感，对精子发育、形态和功能有什么具体影响，恢复能力是什么，以及群体水平的影响是什么？-在这里，我们将通过分离生殖细胞、精子发生和/或成熟精子形态和功能的不同发育阶段的热影响来评估敏感窗口。我们将使用不同的精子退化检测方法来测量热处理(在不同的发育阶段)如何影响精子的数量和质量。治疗后对男性生育力的重新测试将衡量恢复的速度和程度，人口后果将使对自然环境的预测成为可能。B.男性生育能力和精子适应和适应新的热力机制的速度和程度是多少？-我们需要了解种群适应男性能力以应对更频繁的热应激的能力，因此在这里，我们将进行代内热硬化测试和为期3年的重复实验进化试验，这些试验将量化男性能育力对新热力机制的耐受程度和适应速度。C.雌性交配模式能缓解雄性不育症吗？-在这里，我们将测试代价高昂的雌性乱交和精子精选入库是否能提高种群生产力，以应对因体温过高而导致的男性不育。

Male fertility is a key biotic trait with exceptional sensitivity to a fundamental abiotic variable: temperature. We have long known that male mammals suffer infertility if ambient or testicular temperatures rise, and we are just recognising the same phenomenon in cold-blooded species, whose physiology is directly affected by temperature changes. In nematodes, fruitflies and flour beetles, we now know that after conditions mimicking a natural heatwave, male fertility decreases to a point where population viability is impossible solely due to male sterility. Female fertility, by contrast, is unaffected across the same temperatures. Because (a) male fertility allows populations to persist, (b) climate extremes are increasing, and (c) most of the world's biodiversity is cold-blooded and reproduces sexually, we propose to understand how heatwave conditions impact upon reproduction. We need to know which male stages and traits are so sensitive, how quickly they recover, and whether male fertility can adapt or evolve to warmer regimes. To provide answers, we will use a model insect: the flour beetle Tribolium castaneum which (1) we can culture and experimentally replicate to high levels, (2) has detailed assays for measuring fertility, reproduction and sperm form and function, and (3) has a short generation time for experimental evolution. Importantly, we have established that T. castaneum male fertility is characteristically sensitive to thermal change: treatments on young adults that mimic 5-day heatwaves (expected to increase 10-fold this century), create male fertility declines of 20 to 50% in temperatures 2.5C to 5C above the population productivity optimum. Males become sterile 7.5C above the optimum, whereas female fertility is unaffected by these treatments. This background means that (i) thermal change constrains T. castaneum fertility, (ii) we can run controlled experiments with high replication to determine the specific effects of temperature on male fertility, its recovery in the laboratory, and the consequences for population productivity, (iii) we can measure these effects in vitro (via sperm form and function assays) and in vivo (via offspring and population productivity) to gain mechanistic and predictive insight, and (iv) the development and generation times allow us to measure how populations acclimate and evolve to thermal change. We therefore aim to answer the following key questions to understand the individual causes, and population and evolutionary consequences, of thermosensitive male fertility: A. When is male fertility sensitive to thermal change, what are the specific effects on sperm development, form and function, what is the capacity for recovery, and what are the population-level effects?- Here, we will assess windows of sensitivity, by isolating thermal effects at separate developmental stages of germ cells, spermatogenesis and/or mature sperm form and function. We will measure how thermal treatment (at different developmental stages) affects sperm quantity and quality using different assays of sperm degeneration. Re-tests of male fertility after treatment will measure rate and extent of recovery, and population consequences will allow predictions for the natural environment. B. What is the rate and extent of acclimation and adaptation of male fertility and sperm to new thermal regimes?- We need to understand the capacity of populations to adapt male fertility to cope with more frequent thermal stress, so here we will conduct within-generation heat hardening tests and a 3-year replicated experimental evolution trial, which together will quantify the extent and rates of adaptation in the tolerance of male fertility to novel thermal regimes. C. Can female mating pattern mitigate against male infertility?- Here we will test whether costly female promiscuity, and sperm selection into storage, improve population productivity in the face of male subfertility because of hyperthermia.