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

男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：温度异常敏感。我们早就知道，如果环境温度或睾丸温度升高，雄性哺乳动物就会不育。我们刚刚认识到，冷血动物也有同样的现象，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，雄性的生育能力会下降到一个点，因为雄性不育，种群的生存能力是不可能的。相比之下，在相同的温度下，女性的生育能力不受影响。因为(a)男性的生育能力使种群得以延续，(b)极端气候正在增加，(c)世界上大多数生物多样性是冷血的，并且是有性繁殖的，我们建议了解热浪条件对繁殖的影响。我们需要知道男性的哪些阶段和特征是如此敏感，它们恢复的速度有多快，以及男性的生育能力是否能够适应或进化到更温暖的环境。为了提供答案，我们将使用一种模式昆虫：面粉甲虫Tribolium castaneum，它(1)我们可以培养和实验复制到高水平，(2)有详细的测量生育力、繁殖和精子形态和功能的分析，(3)实验进化的世代时间短。重要的是，我们已经确定，castaneum雄性生育力对热变化具有典型的敏感性：在模拟5天热浪（预计本世纪将增加10倍）的年轻成虫上处理，在高于种群生产力最佳温度2.5℃至5℃的温度下，雄性生育力下降20%至50%。雄性不育比最佳温度高出7.5摄氏度，而雌性的生育能力不受这些处理的影响。这一背景意味着(i)温度变化限制了castaneum的生育能力，（ii）我们可以进行高复制的对照实验，以确定温度对雄性生育能力的具体影响，其在实验室中的恢复，以及对种群生产力的影响，（iii）我们可以在体外（通过精子形式和功能分析）和体内（通过后代和种群生产力）测量这些影响，以获得机制和预测性的洞察力。（iv）发展和繁殖时间使我们能够测量种群如何适应和进化以适应热变化。因此，我们的目标是回答以下关键问题，以了解男性温敏性生育的个体原因、种群和进化后果：A.男性生育力何时对温度变化敏感，对精子发育、形态和功能的具体影响是什么，恢复能力是什么，以及种群水平的影响是什么？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效应来评估敏感性窗口。我们将测量热处理（在不同的发育阶段）如何影响精子数量和质量，使用不同的精子退化试验。治疗后对男性生育能力的重新测试将衡量恢复的速度和程度，人口后果将允许对自然环境进行预测。B.男性生殖能力和精子对新的温度制度的适应和适应的速度和程度是多少？-我们需要了解种群适应雄性繁殖力以应对更频繁的热应激的能力，因此我们将进行代内热硬化测试和3年重复的实验进化试验，这些试验将共同量化雄性繁殖力对新热体制的耐受性的适应程度和速度。C.女性交配模式能减轻男性不育吗？在这里，我们将测试昂贵的雌性乱交和精子选择储存是否能在面对雄性因高温而不育的情况下提高种群生产力。

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.