Johnson S D, Steiner K E
Department of Botany, University of Cape Town, Rondebosch 7700, South Africa.
Compton Herbarium, National Botanical Institute, P. Bag X7, Claremont 7735, South Africa.
Evolution. 1997 Feb;51(1):45-53. doi: 10.1111/j.1558-5646.1997.tb02387.x.
Field studies in South Africa showed that floral spur length in the Disa draconis complex (Orchidaceae) varies enormously between populations in the southern mountains (means = 32-38 mm), lowland sandplain (mean = 48 mm), and northern mountains (means = 57-72 mm). We tested the hypothesis that divergence in spur length has resulted from selection exerted through pollinator proboscis length. Short-spurred plants in several southern mountain populations, as well as long-spurred plants in one northern mountain population, were pollinated by a horsefly, Philoliche rostrata (Tabanidae), with a proboscis length that varied from 22 to 35 mm among sites. Long-spurred plants on the sandplain were pollinated by the tanglewing fly, Moegistorynchus longirostris (Nemestrinidae), which has a very long proboscis (mean = 57 mm). Selection apparently favors long spurs in sandplain plants, as artificial shortening of spurs resulted in a significant decline in pollen receipt and fruit set, although pollinaria removal was not significantly affected. Fruit set in the study populations was limited by pollen availability, which further suggests that selection on spur length occurs mainly through the female component of reproductive success.
在南非进行的实地研究表明，龙血树兰属复合体（兰科）的花距长度在南部山区（平均值为32 - 38毫米）、低地沙质平原（平均值为48毫米）和北部山区（平均值为57 - 72毫米）的不同种群间差异极大。我们检验了这样一个假设，即花距长度的差异是由传粉者喙长所施加的选择导致的。几个南部山区种群中的短花距植株，以及一个北部山区种群中的长花距植株，是由一种马蝇（Philoliche rostrata，虻科）传粉的，该马蝇的喙长在不同地点从22毫米到35毫米不等。沙质平原上的长花距植株是由长喙缠翅蝇（Moegistorynchus longirostris，长足虻科）传粉的，这种蝇有非常长的喙（平均值为57毫米）。选择显然有利于沙质平原植株中的长花距，因为人工缩短花距会导致花粉接收量和结实率显著下降，尽管花粉块移除并未受到显著影响。研究种群中的结实率受花粉可利用性限制，这进一步表明对花距长度的选择主要通过繁殖成功的雌性部分发生。
