被子植物起源研究中几种观点的思考

Consideration on some viewpoints in researches of the origin of angiosperms

对被子植物起源研究中的几种观点进行了讨论。(1)由于被子植物存在着一组共同的性状,它们不可能是从不同祖先起源的,而是有着共同的祖先。被子植物是一个单源起源的类群。现存被子植物分类系统是依据包括形态学(广义)、分子系统学、古植物学和植物地理学等的综合性状建立的,只能表示出现存类群的亲缘关系并且追溯到它们最近的祖先。人们现在还不可能建立一个包括全部已绝灭的类群和现代生存类群的谱系发生系统。因此,现存被子植物分类系统只能看作是“亲缘”系统。(2)分析了用于推测被子植物起源时间的分子、化石和地理分布证据。我们认为,要确定被子植物起源时间,植物化石是一类重要证据,但化石只能说是植物本身可保存部分和当时当地所提供的化石条件的综合反映,它们不可能就是植物类群或种的起源时间。人们还必须考虑到化石本身的演化历史。应用分子钟也是一种手段,但误差比较大。如果我们除了利用上述两种资料之外,根据植物类群的现代分布格局及其形成,把植物的演化同地球的历史和板块运动联系起来,以推断它们起源的时间,这无疑会增加其可信度。通过对56个种子植物不同演化水平的重要科属地理分布的研究结果,我们曾提出被子植物的起源时间可能要追溯到早侏罗世,甚至晚三叠世。(3)分析了基于分子证据所提出的被子植物基部类群——ANITA成员(包括无油樟科Amborellaceae、睡莲科Nymphaeaceae、八角目Illiciales、早落瓣科Trimeniaceae、木兰藤科Austrobaileyaceae)的性质,讨论了ANITA成员在现代几个被子植物分类系统中的系统位置的不同观点,评价了它们的形态学(广义)性状。指出ANITA的成员由于包含大量的祖征,是属于原始的类群。但由于它们的共有衍征很少,如花粉球形,说明它们在被子植物演化早期就分道扬镳了,沿着不同的传代线分化。因此ANITA是一个源于不同传代线的复合群。

In this paper, some viewpoints on the origin of angiosperms are discussed. （1） The angiosperms share certain remarkably consistent character syndrome, and thus it is unlikely that they evolved from more than one ancestor, although they might have a common ancestor group. The angiosperms should be a group of unitary origin. The classification systems of extant angiosperms were constructed based on comprehensive evidence from morphological, molecular systematics, palaeobotanical, and phytogeographical studies, and could only express affinity among the extant angiosperms. At present it is impossible to construct a phylogenetic system that contains all extinct and extant angiosperm groups, and for this reason the classification systems of extant angiosperms can only be considered as “affinity” ones. （2） The evidence from molecular clocks, fossils and geographic distribution data on the origin time of angiosperms has been greatly accumulated in the past two decades. Fossil data are very important for determination of the origin time of angiosperms. However, fossil evidence is only the integrated embodiment for the preserved parts of plants and geological fossilization conditions, but is not, and unlikely to be, the indicator of the exact age of the groups or species. In addition, we have to consider the evolutionary history of the fossils. The application of molecular clocks is another approach, but it carries even greater errors. Besides the two lines of evidence, researches on modern distribution patterns, the formation of the plant groups, and combination of plant evolution with the earth history as well as the theory of plate tectonics, can undoubtedly improve the reliability in inferring angiosperm origin time. Analyses of 56 important spermatophyte （mostly angiosperms） families or genera at different evolutionary levels have suggested that the origin time of angiosperms could be dated back to the Early Jurassic or Late Triassic. （3） The nature of basal angiosperm groups, i.e., members of ANITA grade （incl. Amborellaceae, Nymphaeaceae, Illiciales, Trimeniaceae and Austrobaileyaceae）, is analyzed. Views on the systematic positions of ANITA members in modern classification systems of angiosperms are discussed, and their morphological characters （sensu lato） are evaluated. We consider that the ANITA members belong to the primitive groups because of their many plesiomorphies. But they only share few synapomorphies, such as globose pollen grains, indicating that they may have already diverged into different lineages during the early stages of angiosperm evolution. Therefore, ANITA is a complex group originated from different lineages.