生物谷：兰科植物是植物界种类最为丰富的花卉植物，但是关于它的起源问题一直没有弄清。美国科学家近日对一个含有兰花花粉化石的蜜蜂琥珀研究发现，兰科植物起源于大约8000万年前的共同祖先。8月30日的《自然》杂志以封面文章的形式刊载了这一研究成果。
研究兰花起源问题相当困难，因为兰花的化石非常罕见，这其中的原因多种多样。比如，它们的种子呈粉尘状，花粉很容易就溶解在用来提取花粉化石的酸溶液里。另外，兰花喜阴，这使得它们倒地后容易腐烂，很难形成化石。
这个特殊的蜜蜂琥珀帮助科学家解决了这一难题。美国哈佛大学的Santiago Ramírez和同事通过分析，确认了这只蜜蜂身上所带的正是兰花花粉化石，并根据这种花粉的结构，将其归进了斑叶兰亚族（subtribe Goodyerinae）。与现存的种类比较起来，这种花粉与在多米尼加发现的两种兰科植物花粉十分相像。
接下来，研究小组利用现存55个属的兰花的遗传信息建立了一个兰花进化族谱，确定了现存兰花种类之间的亲缘关系，从而推测它们可能的进化分歧时间。
经过测定，这块琥珀的年龄为1500万－2000万年。假设兰花的进化速率相对稳定，那么可以推测兰科植物的共同祖先大约生长在至少7600万年前，也就是白垩纪的晚期。Ramírez半开玩笑地说道：“恐龙很可能曾从兰花丛中漫步走过。”
纽约植物园的兰花专家Kenneth Cameron欣喜地说：“这简直太美妙了！这正是兰花学界期待以久的事情。”（科学网 梅进/编译）
原始出处：
Nature 448, 1042-1045 (30 August 2007) | doi:10.1038/nature06039; Received 17 January 2007; Accepted 21 June 2007
8月30日Nature封面
Dating the origin of the Orchidaceae from a fossil orchid with its pollinator
Santiago R. Ramírez1, Barbara Gravendeel2, Rodrigo B. Singer3, Charles R. Marshall1,4 & Naomi E. Pierce1
Museum of Comparative Zoology, Harvard University, 26 Oxford St., Cambridge, Massachusetts 02138, USA
Nationaal Herbarium Nederland, Universiteit Leiden, P.O. Box 9514, Leiden, The Netherlands
Depto Botânica, Instituto de Biociências, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, RS 91501-970, Porto Alegre, Brasil
Department of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, Massachusetts 02138, USA
Correspondence to: Santiago R. Ramírez1 Correspondence and requests for materials should be addressed to S.R.R. (Email: sramirez@oeb.harvard.edu).
Since the time of Darwin1, evolutionary biologists have been fascinated by the spectacular adaptations to insect pollination exhibited by orchids. However, despite being the most diverse plant family on Earth2, the Orchidaceae lack a definitive fossil record and thus many aspects of their evolutionary history remain obscure. Here we report an exquisitely preserved orchid pollinarium (of Meliorchis caribea gen. et sp. nov.) attached to the mesoscutellum of an extinct stingless bee, Proplebeia dominicana, recovered from Miocene amber in the Dominican Republic, that is 15–20 million years (Myr) old3. This discovery constitutes both the first unambiguous fossil of Orchidaceae4 and an unprecedented direct fossil observation of a plant–pollinator interaction5, 6. By applying cladistic methods to a morphological character matrix, we resolve the phylogenetic position of M. caribea within the extant subtribe Goodyerinae (subfamily Orchidoideae). We use the ages of other fossil monocots and M. caribea to calibrate a molecular phylogenetic tree of the Orchidaceae. Our results indicate that the most recent common ancestor of extant orchids lived in the Late Cretaceous (76–84 Myr ago), and also suggest that the dramatic radiation of orchids began shortly after the mass extinctions at the K/T boundary. These results further support the hypothesis of an ancient origin for Orchidaceae.
