Taylor Mackenzie L, Williams Joseph H
Department of Biology, Creighton University, 2500 California Plaza, Omaha, NE 68178, USA.
Sex Plant Reprod. 2012 Jun;25(2):83-96. doi: 10.1007/s00497-012-0183-6. Epub 2012 Feb 25.
Trithuria (Hydatellaceae; Nymphaeales) is unique among early-divergent angiosperms in that its species are extremely small and most have exceptionally short, annual life histories. Given the evolution of these extremes of size and development, we sought to understand whether post-pollination processes still varied predictably with breeding system in Trithuria. To address this question, we studied two Western Australian species, Trithuria austinensis (dioecious, obligately outcrossing) and Trithuria submersa (bisexual, highly selfing). To document developmental timing, carpels were hand-pollinated, collected at sequential time points, and examined with light and fluorescence microscopy. In both species, pollen tubes first entered ovules<1 h after pollination, but the pollen tube pathway of outcrossing T. austinensis was almost four times longer and its pollen tube growth rates were up to six times faster (≤2,166 vs. 321 μm/h) than those of T. submersa. T. austinensis also exhibited greater male investment, slower pollen germination, and greater pollen tube attrition. These differences in male gametophyte development are predicted for outcrossers versus selfers in phylogenetically derived angiosperms. These new data for Hydatellaceae reinforce the idea that an acceleration of pollen tube development occurred in the Nymphaeales stem lineage, before the origin of Hydatellaceae. We infer that a recent evolutionary transition to selfing in T. submersa has been accompanied by predictable modifications to reproductive development, which, because of the ancient relationship between Hydatellaceae and all other angiosperms, suggests that traits underlying the lability of flowering plant post-pollination biology were present early in their history.
水穗草属（水穗草科；睡莲目）在早期分化的被子植物中独具特色，其植株极小，且多数具有极短的一年生生活史。鉴于其在大小和发育方面的这些极端特征的演化，我们试图了解在水穗草属中，授粉后过程是否仍随繁育系统而呈现可预测的变化。为解决这个问题，我们研究了西澳大利亚的两个物种，奥斯汀水穗草（雌雄异株，专性异交）和沉水水穗草（两性花，高度自交）。为记录发育时间，对心皮进行人工授粉，在连续时间点采集样本，并通过光学显微镜和荧光显微镜进行观察。在这两个物种中，花粉管在授粉后不到1小时就首次进入胚珠，但异交的奥斯汀水穗草的花粉管路径几乎是沉水水穗草的四倍长，其花粉管生长速度比沉水水穗草快达六倍（分别为≤2166与321微米/小时）。奥斯汀水穗草还表现出更大的雄性投入、较慢的花粉萌发以及更高的花粉管损耗。在系统发育衍生的被子植物中，异交种和自交种在雄配子体发育上的这些差异是可预测的。水穗草科的这些新数据强化了这样一种观点，即在水穗草科起源之前，睡莲目茎系中就已出现花粉管发育加速的现象。我们推断，沉水水穗草最近向自交的进化转变伴随着生殖发育的可预测变化，由于水穗草科与所有其他被子植物之间的古老关系，这表明开花植物授粉后生物学易变的潜在性状在其历史早期就已存在。
