Evolution in Dryopteris: The Role of Hybrids in Polyploid Speciation

不同物种之间的杂交是大自然的偶然，在某些情况下可能会有一个非常重要的结果：新物种的起源。杂交种通常无法通过普通的有性方式繁殖，因为它们含有不相容的染色体，在减数分裂过程中不会配对，减数分裂产生单倍体细胞，最终产生卵子和精子。然而，第二个意外，染色体加倍(多倍体)可能会通过为每个染色体提供一个可以配对的完全相同的副本来恢复生育能力。杂交和多倍体这两个过程以这种方式结合在一起，产生了大量的新植物物种，称为异源多倍体。异源多倍体包括一些最重要的农作物，其中包括小麦、棉花、咖啡、花生和烟草。然而，令人惊讶的是，人们对产生异源多倍体的确切细胞机制知之甚少，也不知道成功建立异源多倍体后自然界的普遍情况。目前的建议旨在通过对鳞毛蕨属物种之间的自然杂交的详细研究来加强我们对异源多倍体起源的认识。鳞毛蕨属是北美东部常见的一种蕨类植物。鳞毛蕨属是一个理想的模式系统，原因如下：(1)鳞毛蕨属植物的减数分裂产物是可以直接生长到植物中的孢子，而不是开花植物的花粉粒或胚囊，这很难进一步研究；(2)鳞毛蕨属植物很大，产生大量孢子，为观察相对罕见的事件提供了更好的机会；(3)自然界中产生了许多杂交种，其中一些杂交种非常频繁，允许评估与个别种类的杂交种有关的过程之间的差异。这项研究将包括两个阶段，实验室阶段和实地阶段。在实验室中，随后将产生具有双染色体的孢子。这种孢子被认为是大的球形(“巨型”)孢子，与通常由杂交种产生的萎缩孢子的其余部分相区别。这些“巨型”孢子的遗传构成和生长潜力将通过将它们培育成幼龄蕨类植物(配子体)并使用分子和显微技术进行检测来评估。还将尝试通过有性和无性两种方式从配子体中产生成年蕨类植物(孢子体)。在田间，将对成群的杂交种进行研究，以确定它们是否真的通过多倍体孢子繁殖，即杂交种是否都是第一代，还是包括一些后代。将使用分子指纹技术来区分这两种可能性。这些田间和实验室相结合的研究结果将为杂交种产生异源多倍体物种的方法提供重要的新信息。

Hybridization between different species is an accident of nature that may in some instances have a very important outcome: the origin of new species. Hybrids are typically unable to reproduce through ordinary sexual means because they contain incompatible chromosomes that do not pair during meiosis, the reduction division that produces the haploid cells that ultimately produce eggs and sperms. However, a second accident, chromosome doubling (polyploidy) may restore fertility by providing each chromosome with an exact duplicate with which it can pair. These two processes, hybridization and polyploidy, have combined in this way to produce a vast number of new plant species, termed allopolyploids. Allopolyploids include some of our most important crops, among them wheat, cotton, coffee, peanut, and tobacco. Nonetheless, surprisingly little is known about the precise cellular mechanisms that operate to produce allopolyploids, or about the circumstances that prevail in nature when successful allopolyploids become established. The present proposal is designed to bolster our knowledge of the origin of allopolyploids through detailed studies of natural hybrids between species of Dryopteris, a genus of ferns common in eastern North America. Dryopteris is an ideal model system for the following reasons: (1) as a fern, the products of meiosis in Dryopteris are spores that can be grown directly into plants rather than the pollen grains or embryo sacs of flowering plants which are much less amenable to further study; (2) Dryopteris plants are large and produce numerous spores, offering an enhanced opportunity to observe relatively rare events; (3) numerous hybrids are produce in nature, some of them very frequently, allowing for the variation between processes that pertain to individual kinds of hybrids to be evaluated. This study will consist of two phases, a laboratory phase and a field phase. In the laboratory, the production of spores with doubled chromosomes will be followed. Such spores are believed to be distinguishable as large spherical ("giant") spores that stand out from the remainder of the shriveled spores ordinarily produced by the hybrid. The genetic constitution and growth potential of these "giant" spores will be evaluated by growing them into juvenile ferns (gametophytes) and examining these using molecular and microscopic techniques. Attempts will also be made to produce adult ferns (sporophytes) from the gametophytes by both sexual and asexual means. In the field, clusters of hybrids will be studied to determine whether they are actually reproducing through polyploid spores, i.e. whether the hybrids are all first generation, or include some later generations. Molecular fingerprinting techniques will be used that can distinguish between these two possibilities. The results from these combined field and laboratory studies will provide important new information on the means by which hybrids can give rise to allopolyploid species.