Genetic bottlenecks and the geographic distribution of sexual and asexual organisms

许多池塘动植物的生命岌岌可危，因为池塘经常干涸，受到污染，或者以各种方式无法居住。池塘的重新定居通常是通过几个殖民者到达一个空荡荡的池塘(或者对于植物来说，几个种子)。池塘中的这种殖民事件(以及必须在岛屿和被不适合物种栖息地包围的环境中的部分地区发生的类似事件)被称为“遗传瓶颈”。遗传瓶颈很可能在边缘或在某种程度上不适合某一物种的栖息地中特别频繁或严重，因为如果种群较少，则更有可能发生灭绝，这在边缘栖息地通常是正确的。在瓶颈之后，池塘或地块的居民将是殖民者的后代，如果殖民者很少，或者只有一只雌性产卵，后代将是亲戚。当它们交配时，它们的后代将被近亲繁殖，一个非常常见的后果是，与来自更大种群的正常动物相比，这些动物的存活率或繁殖力较低。这就是所谓的近亲繁殖抑郁。然而，对于像水蚤这样的动物来说，与其他动物交配并不是唯一的繁殖方式。自然界中的许多品系都会无性繁殖后代(克隆母大型蚤)。因此，这些后代不会遭受近亲繁殖抑郁症的困扰，因为他们就像他们的母亲一样。其结果是，无性繁殖的后代应该存活和繁殖得更好，因此能够在边缘栖息地胜过有性繁殖的后代，那里的遗传瓶颈很常见，有性繁殖的后代患有近亲繁殖抑制。这可以解释为什么无性生物通常更常见于不适宜居住的栖息地，如遥远的北方或高山中。目前尚不清楚这一模型在水蚤种群的真实世界中是否可信，因为我们不知道自然条件下近亲繁殖抑制是否很严重，也不知道它是否会影响与无性水蚤品系的竞争。为了测试这个模型，我们计划进行实验，在这些实验中，我们制作人工池塘(装满天然池塘水的水桶)，同时引入有性和无性的水蚤，并允许它们竞争。为了做到这一点，我们计划在一年中的某个时候(初夏)引入雌性动物，届时两种动物都会进行几代的无性繁殖。这意味着我们可以使用一种识别它们的遗传标记，在夏季期间计算每种类型的数量。这项实验涉及到大量水桶的定居，并经常采集产卵的样本。这些将被带到实验室进行评分。为了测试这个模型是否可信，实验将使用已经经历了不同大小的瓶颈的有性行为的雌性，这将导致不同程度的近亲繁殖。

The lives of many pond animals and plants are precarious, because ponds often dry up, become polluted, or in various ways become uninhabitable. Re-colonisation of ponds often occurs through just a few colonists reaching an empty pond (or a few seeds, in the case of plants). Such colonisation events in ponds (and similar events that must occur in islands, and in patches of environment that are surrounded by unsuitable habitat for a species) are called 'genetic bottlenecks'. Genetic bottlenecks are likely to be particularly frequent or severe in habitats that are marginal or in some way unsuitable for a species, because extinction is more likely to occur if a population is small, as is generally true in marginal habitats. After a bottleneck, the inhabitants of a pond or patch will be the descendants of the colonists, and if there are very few colonists, or just a single female laying eggs, the descendants will be relatives. When they mate, their offspring will be inbred, and a very common consequence is that these animals will have low survival or fertility, compared with normal animals from larger populations. This is called 'inbreeding depression'. In an animal like the water flea, Daphnia, however, mating with other animals is not the only means of reproducing. Many strains in nature, produce offspring asexually (cloning the mother Daphnia). These offspring do not, therefore, suffer from inbreeding depression, as they are just like their mothers. The result is that asexual offspring should survive and reproduce better and therefore be able to outcompete sexual offspring in in marginal habitats, where genetic bottlenecks are common and sexual offspring suffer from inbreeding depression. This could explain why, in general, asexual organisms are more commonly found in unhospitable habitats such as far up in the north or high up in the mountains. It is not clear whether this model is plausible in the real world of Daphnia populations, because we don't know whether inbreeding depression in natural conditions is strong, and whether it affects competition with asexual Daphnia strains. To test the model, we plan experiments in which we make artificial pools (buckets filled with water from natural ponds) and introduce both sexual and asexual Daphnia and allow them to compete. We plan to do this by introducing the female animals at a time of year (early summer) when both kinds reproduce asexually for several generations. This means that we can count the numbers of each type during the course of the summer, using a genetic marker that identifies them. The experiment involves colonising large numbers of buckets, and taking frequent samples of the eggs laid. These will be taken to the lab for the marker to be scored. To test whether the model is plausible, the experiments will use sexual females that have been put through bottlenecks of various sizes, which will lead to different degrees of inbreeding.