Department of Geosciences and Centre for Earth Evolution and Dynamics, University of Oslo, PO Box 1047, 0316 Oslo, Norway.
Proc Biol Sci. 2013 Aug 7;280(1768):20131708. doi: 10.1098/rspb.2013.1708. Print 2013 Oct 7.
Polyploidy (or whole-genome doubling) is a key mechanism for plant speciation leading to new evolutionary lineages. Several lines of evidence show that most species among flowering plants had polyploidy ancestry, but it is virtually unknown for conifers. Here, we study variability in pollen tetrad morphology and the size of the conifer pollen type Classopollis extracted from sediments of the Triassic-Jurassic transition, 200 Ma. Classopollis producing Cheirolepidiaceae were one of the most dominant and diverse groups of conifers during the Mesozoic. We show that aberrant pollen Classopollis tetrads, triads and dyads, and the large variation in pollen size indicates the presence of unreduced (2n) pollen, which is one of the main mechanisms in modern polyploid formation. Polyploid speciation may explain the high variability of growth forms and adaptation of these conifers to different environments and their resistance to extreme growth conditions. We suggest that polyploidy may have also reduced the extinction risk of these conifers during the End-Triassic biotic crisis.
多倍体（或全基因组加倍）是导致新进化谱系的植物物种形成的关键机制。有几条证据表明，开花植物中的大多数物种都有多倍体的祖先，但在针叶树中实际上是未知的。在这里，我们研究了花粉四分体形态的可变性以及从 2 亿年前三叠纪-侏罗纪过渡期的沉积物中提取的针叶树花粉类型 Classopollis 的大小。在中生代期间，产生 Cheirolepidiaceae 的 Classopollis 是最主要和最多样化的针叶树群之一。我们表明，异常花粉 Classopollis 四分体、三分体和二分体，以及花粉大小的巨大变化表明存在未减数（2n）花粉，这是现代多倍体形成的主要机制之一。多倍体物种形成可能解释了这些针叶树的生长形式的高度可变性以及它们对不同环境的适应能力，以及它们对极端生长条件的抵抗力。我们认为，在三叠纪末生物大灭绝期间，多倍体也可能降低了这些针叶树的灭绝风险。
