Wu Ling-Yun, Wang Bo, Schoen Daniel J, Huang Shuang-Quan
State Key Laboratory of Hybrid Rice, College of Life Sciences, Wuhan University, Wuhan 430072, China.
Department of Biology, McGill University, Montreal, QC, H3A 1B1 Canada.
Am J Bot. 2017 Aug;104(8):1232-1240. doi: 10.3732/ajb.1700189.
Documenting trait transitions among species with dimorphic flowers can help to test whether similar patterns of selection are responsible for divergence in floral traits among different species. Heterostyly is thought to promote outcrossing. Theory suggests that the evolutionary transition from heterostylous to homostylous flowers should be accompanied by a reduction in floral size in which pollen size and style length are expected to covary. Patterns of such correlated floral trait evolution have not, however, been widely examined.
The evolutionary history of heterostyly and homostyly was reconstructed from a molecular phylogeny of 13 species of Fagopyrum and two outgroups, based on one nuclear gene (nrITS) and three chloroplast regions (matK, atpB-rbcL, and psbA-trnH spacer). Floral traits of nine Fagopyrum species including pollen number and size, as well as stigma depth (length of the capitate stigma), were measured and ancestral characters of the herkogamic condition were reconstructed.
Three transitions from distyly to homostyly were observed. In two transitions, flower size (corolla diameter, pedicel length), herkogamy (stigma-anther distance) and pollen production decreased, but stigma depth and pollen size did not. Changes of anther height and style length were inconsistent. The predicted positive relationship between style length and pollen size in the two transitions to homostyly was not observed.
Floral evolution accompanying transitions to homostyly in Fagopyrum were found to be consistent with predictions of mating system evolution theory, and the correlation of traits in distylous vs. homostylous species revealed that pollen size generally correlates with stigma depth rather than style length.
记录具有二型花的物种间的性状转变，有助于检验相似的选择模式是否导致了不同物种间花部性状的分化。异型花柱被认为可促进异交。理论表明，从异型花柱花到同型花柱花的进化转变应伴随着花大小的减小，其中花粉大小和花柱长度预计会协同变化。然而，这种相关花部性状进化的模式尚未得到广泛研究。
基于一个核基因（nrITS）和三个叶绿体区域（matK、atpB - rbcL和psbA - trnH间隔区），从13种荞麦属植物及两个外类群的分子系统发育中重建异型花柱和同型花柱的进化历史。测量了9种荞麦属植物的花部性状，包括花粉数量和大小，以及柱头深度（头状柱头的长度），并重建了雌雄异位状况的祖先特征。
观察到三次从异型花柱到同型花柱的转变。在两次转变中，花大小（花冠直径、花梗长度）、雌雄异位（柱头 - 花药距离）和花粉产量下降，但柱头深度和花粉大小未下降。花药高度和花柱长度的变化不一致。在两次向同型花柱转变中，未观察到花柱长度与花粉大小之间预测的正相关关系。
发现荞麦属植物向同型花柱转变伴随的花部进化与交配系统进化理论的预测一致，异型花柱与同型花柱物种性状的相关性表明，花粉大小通常与柱头深度相关，而非与花柱长度相关。
