Department of Neurobiology and Behavior, Cornell University, 215 Tower Road, Ithaca, NY 14853, USA.
Ann Bot. 2014 Feb;113(3):533-44. doi: 10.1093/aob/mct279. Epub 2013 Dec 18.
Floral traits, such as floral volatiles, can contribute to pre-zygotic reproductive isolation by promoting species-specific pollinator foraging. When hybrid zones form, floral traits could also influence post-zygotic isolation. This study examined floral volatiles in parental species and natural hybrids in order to explore potential scent mediation of pre-zygotic and post-zygotic isolation.
Floral bouquets were analysed for the sister species Ipomopsis aggregata and I. tenuituba and their natural hybrids at two contact sites differing in both hybridization rate and temporal foraging pattern of hawkmoth pollinators. Floral volatiles were quantified in diurnal and nocturnal scent samples using gas chromatography-mass spectrometry.
The bouquets of parental species and hybrids showed qualitative overlap. All flowers emitted similar sets of monoterpenoid, sesquiterpenoid, aliphatic and benzenoid compounds, but separated into groups defined by multivariate analysis of quantitative emissions. The parental species differed most strikingly in the nitrogenous compound indole, which was found almost exclusively in nocturnal bouquets of I. tenuituba. Natural hybrid bouquets were highly variable, and showed emission rates of several compounds that appeared transgressive. However, indole emission rates were intermediate in the hybrids compared with rates in the parents. Volatile bouquets at the contact site with lower hybridization did not show greater species specificity in overall scent emission, but I. tenuituba presented a stronger indole signal during peak hawkmoth activity at that site.
The two species of Ipomopsis differed in patterns of floral bouquets, with indole emitted in nocturnal I. tenuituba, but not in I. aggregata. Natural hybrid bouquets were not consistently intermediate between the parents, although hybrids were intermediate in indole emission. The indole signal could potentially serve as a hawkmoth attractant that mediates reproductive isolation both before and after hybrid formation.
花的特征，如挥发性化合物，可以通过促进特定于物种的传粉者觅食来促进配子前生殖隔离。当杂交区形成时，花的特征也可能影响合子后隔离。本研究检测了亲种和自然杂种的花挥发性化合物，以探索前合子和合子后隔离中潜在的气味介导作用。
在两个接触点分析了姐妹种银胶菊（Ipomopsis aggregata）和短舌匹菊（I. tenuituba）及其自然杂种的花束，这两个接触点在天蛾传粉者的杂交率和时间觅食模式上存在差异。使用气相色谱-质谱联用仪对昼间和夜间的花香味样本进行了花挥发性化合物的定量分析。
亲种和杂种的花束表现出定性重叠。所有花朵都释放出类似的单萜、倍半萜、脂肪族和苯环化合物，但通过对定量排放的多元分析分为不同的组。亲种之间差异最大的是氮化合物吲哚，它几乎只存在于短舌匹菊的夜间花束中。自然杂种花束高度可变，并且表现出几种似乎超越的化合物的排放率。然而，与亲种相比，杂种的吲哚排放率处于中间水平。在杂交率较低的接触点，挥发性花束并没有在整体气味排放方面表现出更大的物种特异性，但在该地点，天蛾活动高峰期的短舌匹菊表现出更强的吲哚信号。
两种银胶菊在花束模式上存在差异，短舌匹菊在夜间释放吲哚，但银胶菊则不释放。自然杂种花束并不总是介于亲代之间，尽管杂种在吲哚排放方面处于中间水平。吲哚信号可能作为一种天蛾引诱剂，在杂种形成前后都能介导生殖隔离。
