Pollen morphology of Waltheria L. (Malvaceae-Byttnerioideae) from Bahia, Brazil

ABSTRACT Waltheria encompasses about 60 species distributed in Brazil and Mexico. To improve the palynology of the genus, we analyzed 14 species occurring in the Brazilian State of Bahia. Pollen grains were acetolyzed and examined using light microscopy and scanning electron microscopy. Non-acetolyzed pollen grains were viewed by transmission electron microscopy. The results revealed two general pollen types based on exine ornamentation, such as reticulate and echinate, which are distinct with regard to (1) size and shape, (2) apertural type and number, (3) relative length of ectoapertures, and (4) thickening of the nexine compared to the sexine. An ultrastructural analysis of the pollen wall revealed a continuous foot layer and a compact and discontinuous endexine in most cases. The sexine consisted of heterogeneous columellae, which support a perforated semitectum (echinate type) or the semitectum was composed of perforations and muri-shaped thickening (reticulate type). These results confirm a high degree of pollen dimorphism in Waltheria associated with heterostyly.

摘要 沃氏草属（Waltheria）目前包含约60个物种，分布于巴西与墨西哥。为完善该属的孢粉学研究，我们对分布于巴西巴伊亚州的14个物种开展了分析：对花粉粒实施乙酰解处理后，采用光学显微镜与扫描电子显微镜开展观测；未经过乙酰解处理的花粉粒则通过透射电子显微镜进行观察。研究结果基于花粉外壁纹饰识别出两类主要花粉类型，即网状（reticulate）与刺状（echinate），二者在以下四方面存在显著差异：（1）花粉大小与形态；（2）萌发孔类型与数量；（3）外萌发孔相对长度；（4）相较于外壁外层（sexine），外壁内层（nexine）的增厚程度。对花粉壁开展的超微结构分析显示，多数样本中存在连续的基底层（foot layer）以及紧凑且不连续的外壁内层(endexine)。外壁外层由异质性柱状层（columellae）构成，该结构支撑具穿孔的半覆盖层（semitectum）以形成刺状花粉类型；而网状花粉类型的半覆盖层则由穿孔与脊状增厚结构（muri）组成。本研究结果证实，沃氏草属存在与花柱异长（heterostyly）相关的高度花粉二型性。